Compass: Magnetic and Gyro

This chapter discusses the major types of compasses available to the navigator, their operating principles, their capabilities, and limitations of their use. As with other aspects of navigation, technology is rapidly revolutionizing the field of compasses.

For much of maritime history the sole heading reference for navigators has been the magnetic compass. However, a great deal of effort and expense has gone into understanding the magnetic compass scientifically to make it as accurate as possible through research and development of elaborate compensation techniques.

Over time, technological advances like the development of more sophisticated means for obtaining accurate compass readings, such as the electro-mechanical gyrocompass, diminished traditional reliance upon the magnetic compass, relegating it to backup status in many large vessels. Later came the development of inertial navigation systems based on gyroscopic principles, but perturbations like the interruption of electrical power to the gyrocompass or inertial navigator, mechanical failure, and equipment deterioration have reminded navigators of the important reliability of the magnetic compass.

New technologies are both refining and replacing the magnetic compass as the primary heading reference and navigational tool. Even relatively new advances like the electro-mechanical gyrocompasses are being supplanted by far lighter, cheaper, and more dependable ring laser gyrocompasses. These devices do not operate on the principle of the gyroscope (which is based on Newton's laws of motion), but instead rely on the principles of electromagnetic energy and wave theory. Magnetic flux gate compasses, while relying on the Earth's magnetic field for reference, have no moving parts and can compensate themselves, adjusting for both deviation and variation to provide true heading, thus completely eliminating the process of compass correction.

Regardless of newer technologies, SOLAS regulations require that all ships (excluding fishing vessels and pleasure craft under 150 gross tons) to be fitted with a magnetic compass or other means to determine and display the vessel's heading independent of any power supply. Further, each magnetic compass required to be carried by the Regulations shall be properly adjusted and its table or curve of residual deviations available at all times. Magnetic compasses should be adjusted when: they are first installed; they become unreliable; the ship undergoes structural repairs or alterations that could affect its permanent and induced magnetism; electrical or magnetic equipment close to the compass is added, removed or altered; or, a period of two years has elapsed since the last adjustment and a record of compass deviations has not been maintained, or the recorded deviations are excessive or when the compass shows physical defects. Therefore, a basic understanding of magnetism and how it effects the magnetic compass is warranted.

Whatever type of compass being used for navigation, it is advisable to check it periodically against an error free reference to determine its error. This may be done when steering along any range during harbor and approach navigation, or by aligning any two charted objects to find the difference between their observed and charted bearings. When navigating offshore, the use of azimuths and amplitudes of celestial bodies is also an effective method; a subject covered in Chapter 16 - Sextant Altitude Corrections.

The fact that iron can be magnetized (given the ability to attract other iron) has been known for thousands of years, but the explanation of this phenomenon has awaited the recently acquired knowledge of atomic structure. According to present theory, the magnetic field around a current carrying wire and the magnetism of a permanent magnet are the same phenome non-fields created by moving electrical charges. This occurs whether the charge is moving along a wire, flowing with the magma of the Earth's core, encircling the Earth at high altitude as a stream of charged particles, or rotating around the nucleus of an atom.

It has been shown that microscopically small regions, called domains, exist in iron and other ferromagnetic substances. In each domain the fields created by electrons spin-

ning around their atomic nuclei are parallel to each other, causing the domain to be magnetized to saturation. In a piece of unmagnetized iron, the directions of the various domains are arranged in a random manner with respect to each other. If the substance is placed in a weak magnetic field, the domains rotate somewhat toward the direction of that field. Those domains which are more nearly parallel to the field increase in size at the expense of the more non parallel ones. If the field is made sufficiently strong, entire domains rotate suddenly by angles of as much as 90° or 180° so as to become parallel to that “crystal axis” which is most nearly parallel to the direction of the field. If the strength of the field is increased to a certain value depending upon individual conditions, all of the domains rotate into parallelism with the field, and the iron itself is said to be magnetically saturated. If the field is removed, the domains have a tendency to rotate more or less rapidly to a more natural direction parallel to some crystal axis, and more slowly to random directions under the influence of thermal agitation.

Magnetism which is present only when the material is under the influence of an external field is called induced magnetism. That which remains after the magnetizing force is removed is called residual magnetism. That which is retained for long periods without appreciable reduction, unless the material is subjected to a demagnetizing force, is called permanent magnetism.

Certain substances respond readily to a magnetic field. These magnetic materials are principally those composed largely of iron, although nickel and cobalt also exhibit magnetic properties. The best magnets are made of an alloy composed mostly of iron, nickel, and cobalt. Aluminum and some copper may be added. Platinum and silver, properly alloyed with other material, make excellent magnets, but for ordinary purposes the increased expense is not justified by the improvement in performance. Permanent magnets occur in nature in the form of lodestone, a form of magnetite (an oxide of iron) possessing magnetic properties. A piece of this material constitutes a natural magnet.

In some alloys of iron, the crystals can be so arranged and internally stressed that the domains remain parallel to each other indefinitely, and the metal thus becomes a permanent magnet. Such alloys are used for the magnets of a compass. In other kinds of iron, the domains reorient themselves rapidly to conform to the direction of a changing external field, and soon take random directions if the field is removed. A ferromagnetic substance which retains much of its magnetism in the absence of an external field, is said to have high remanence or retentivity. The strength of a reverse field (one of opposite polarity) required to reduce the magnetism of a magnet to zero is called the coercivity or coercive force of the magnet. Hence, a compass magnet should have high remanence in order to be strong, and high coercivity so that stray fields will not materially affect it. For convenience, iron is called “hard” if it has high remanence, and “soft” if it has low remanence.

The direction of a magnetic field is usually represented by lines, called lines of force. Relative intensity in different parts of a magnetic field is indicated by the spacing of the lines of force, a strong field having the lines close together. If a piece of unmagnetized iron is placed in a magnetic field, the lines of force tend to crowd into the iron, following its long axis, and the field is stronger in the vicinity of the iron, somewhat as shown in Figure 903a. If the iron becomes permanently magnetized and is removed from this field, the lines of force around the iron follow paths about as shown in Figure 903b.

[Figure 903a in Bowditch, Pub. No. 9: Lines of force crowd into ferromagnetic material placed in a magnetic field.]

[Figure 903b in Bowditch, Pub. No. 9: Field of a permanent magnet.]

The region in which the lines of force enter the iron is called the south pole, and the region in which they leave the iron is called the north pole. Thus, the lines of force are directed from south to north within the magnet, and from north to south in the external field. Every magnet has a north pole and a south pole. If a magnet is cut into two

pieces, each becomes a magnet with a north pole and south pole. A single pole cannot exist independently. If two magnets are brought close together, unlike poles attract each other and like poles repel. Thus, a north pole attracts a south pole but repels another north pole. The Earth itself has a magnetic field (Section 906), with its magnetic poles being some distance from the geographical poles. If a permanent bar magnet is supported so that it can turn freely, both horizontally and vertically, it aligns itself with the magnetic field of the Earth, which at most places is in a general north-south direction and inclined to the horizontal. Since the north pole of the magnet points in a northerly direction, the Earth's magnetic pole in the Northern Hemisphere has south magnetism. Nevertheless, it is called the north magnetic pole because of its geographical location. For a similar reason, the pole in the Southern Hemisphere, although it has north magnetism, is called the south magnetic pole. To avoid confusion, north magnetism is usually called “red,” and south magnetism, “blue.” The red (north) pole of a magnet is usually painted red, and in some cases the south (blue) pole is painted blue. The north magnetic pole of the Earth is a blue pole, and the south magnetic pole is a red pole.

The magnetism of soft iron, in which remanence is low, depends upon the position of the iron with respect to an external field. It is strongest if the long axis is parallel to the lines of force, and decreases to a minimum if the material is rotated so that the long axis is perpendicular to the lines of force. Figure 905 shows a rod of soft iron which will acquire induced magnetism, meaning there will be a change in the strength and polarity as it is rotated within the Earth's magnetic field. In position 1, the blue pole is located at position x in the material, which is oriented along the long axis of the material, and the polarity is at its strongest. In position 3, the material is now perpendicular the Earth's field and the poles lie along the sides of the bar, the weakest configuration. At position 5, the bar is once again aligned with the Earth's field so the poles are once again at their strongest; however, the polarity has changed. Position “x”, initially a “blue” pole, is now a “red” pole. It should be noted the bar could be viewed as being either horizontal or vertical, the result is the same.

If a bar of soft iron is placed vertical in northern magnetic latitudes (as in any part of the United States), the north (red) end of a compass magnet brought near it will be attracted by the upper end of the bar, and repelled by the lower end. If the bar is inverted, so that its ends are interchanged, the upper end (which as the lower end previously repelled the compass needle) will attract the north end of the needle, and the lower end will repel it. Thus, the polarity of the rod is reversed, either end having blue magnetism if it is at the top. This changing polarity of soft iron in the Earth's field is a major factor affecting the magnetic compasses of a steel vessel.

[Figure 905 in Bowditch, Pub. No. 9: Field of a permanent magnet.]

The Earth itself can be considered to be a gigantic magnet. The horizontal component of this field is a valuable reference in navigation, for it provides the directive force for the magnetic compass, which indicates the ship's heading in relation to the horizontal component of this field.

The world-wide pattern of the Earth's magnetism is roughly like that which would result from a short, powerful, bar magnet near the Earth's center, as shown in Figure 906. The geographical poles are at the top and bottom, and the magnetic poles are offset somewhat from them. This representation, however, is greatly simplified. The actual field is more complex, and requires measurement of its strength and direction at many places before it can be defined accurately enough to be of practical use to the navigator. Not only are the magnetic poles offset from the geographical poles, but the magnetic poles themselves are not 180° apart and, in general, a magnetic compass aligned with the lines of force does not point toward either magnetic pole. In 2000, the north magnetic pole was located at latitude 80.972°N, longitude 109.640°W and the south magnetic pole was at latitude 64.661°S, longitude 138.303°E. The 2020 location of the north magnetic pole was 86.50°N and 164.04°E and the south magnetic pole was 64.07°S and 135.88°E. The entire magnetic field of the Earth, including the magnetic poles, undergoes a small daily or diurnal change, and a very slow, progressive secular change. In addition, temporary sporadic changes occur from time to time during magnetic storms. During a severe storm, variation may change as much as 5°, or more. However, such disturbances are never so rapid as to cause noticeable deflection of the compass card, and in most navigable waters the change is so little that it is not significant in practical navigation. Even when there is no temporary disturbance, the Earth's field is considerably more intricate than indicated by an isomagnetic chart. Natural magnetic irregularities occurring over relatively small areas are called

magnetic anomalies, but the navigator generally refers to these phenomena as local disturbances. Notes warning of such disturbances are shown on charts. In addition, artificial disturbances may be quite severe when a vessel is in close proximity to other vessels, piers, machinery, electric currents, etc.

[Figure 906 in Bowditch, Pub. No. 9: Terrestrial magnetism]

The elements of the Earth’s field are as follows: Total intensity (F) is the strength of the field at any point,

measured in a direction parallel to the field. Horizontal intensity (H) is the horizontal component of

the total intensity. At the magnetic equator, which cor-

responds roughly with the geographic equator, the field

is parallel to the surface of the Earth, and the horizontal

intensity is the same as total intensity. At the magnetic

poles of the Earth, the field is vertical and there is no

horizontal component. The direction of the horizontal

component at any place defines the magnetic meridian

at that place. This component provides the desired

directive force of a magnetic compass. Vertical intensity (Z) is the vertical component of the total

intensity. It is zero at the magnetic equator. At the mag-

netic poles it is the same as the total intensity. While

the vertical intensity has no direct effect upon the

direction indicated by a magnetic compass, it does

induce magnetic fields in vertical soft iron, and these

may affect the compass. Variation (V, Var.), (sometimes referred to as declination

in geophysics) is the angle between the geographic and

magnetic meridians at any place. The expression mag-

netic variation is used when it is necessary to distin-

guish this from other forms of variation. This element

is measured in angular units and named east or west to

indicate the side of true north on which the (magnetic)

northerly part of the magnetic meridian lies. For com-

putational purposes, easterly variation is sometimes

designated positive (+), and westerly variation nega-

tive (-). Magnetic dip (I), (sometimes referred to as inclination in

geophysics) is the vertical angle, expressed in angular

units, between the horizontal at any point and a line of

force through that point. The magnetic latitude of a

place is the angle having a tangent equal to half that of

the magnetic dip of the place.

The World Magnetic Model is a joint product of the United States' National Geospatial-Intelligence Agency (NGA) and the United Kingdom's Defence Geographic Centre (DGC). The WMM was developed jointly by the National Centers for Environmental Information (NCEI, Boulder, CO, USA) and the British Geological Survey (BGS, Edinburgh, Scotland).

[Figure 907a in Bowditch, Pub. No. 9: World Magnetic Model https://ngdc.noaa.gov/geomag/WMM/]

The World Magnetic Model is the standard model used by the U.S. Department of Defense, the U.K. Ministry of Defence, the North Atlantic Treaty Organization (NATO) and the International Hydrographic Organization (IHO), for navigation, attitude and heading referencing systems using the geomagnetic field. It is also used widely in civilian navigation and heading systems. The model, associated software, and documentation are distributed by NCEI on behalf of NGA. The model is produced at 5-year intervals, with the current model expiring on December 31, 2024. Figure 907b and Figure 907c show magnetic variation and annual change (2020 epoch) for the world. The lines connecting points of equal magnetic variation are called isogonic lines. These are not magnetic meridians (lines of force). The line connecting points of zero variation is called the agonic line. Red contours are positive or east, blue contours are negative or west and green is agonic or zero.

Directions relative to the northerly direction along a geographic meridian are true. In this case, true north is the reference direction. If a compass card is horizontal and oriented so that a straight line from its center to 000° points to true north, any direction measured by the card is a true direction and has no error (assuming there is no calibration or observational error). If the card remains horizontal but is rotated so that it points in any other direction, the amount of the rotation is the compass error. Stated differently, compass error is the angular difference between true north and compass north (the direction north as indicated by a magnetic compass). It is named east or west to indicate the side of true north on which compass north lies.

If a magnetic compass is influenced by no other magnetic field than that of the Earth, and there is no instrumental error, its magnets are aligned with the magnetic meridian at the compass, and 000° of the compass card coincides with magnetic north. All directions indicated by the card are magnetic. As stated in Section 906, the angle between geographic and magnetic meridians is called variation (V or Var.). Therefore, if a compass is aligned with the magnetic meridian, compass error and variation are the same.

When a compass is mounted in a vessel, it is generally subjected to various magnetic influences other than that of the Earth. These arise largely from induced magnetism in metal decks, bulkheads, masts, stacks, boat davits, etc., and from electromagnetic fields associated with direct current in electrical circuits. Some metal in the vicinity of the compass may have acquired permanent magnetism. The actual magnetic field at the compass is the vector sum, or resultant of all individual fields at that point. Since the direction of this resultant field is generally not the same as that of the Earth's field alone, the compass magnets do not lie in the magnetic meridian, but in a direction that makes an angle with it. This angle is called deviation (D or Dev.). Thus, deviation is the angular difference between magnetic north and compass north. It is expressed in angular units and named east or west to indicate the side of magnetic north on which compass north lies. Thus, deviation is the error of the compass in pointing to magnetic north, and all directions measured with compass north as the reference direction are compass directions. Since variation and deviation may each be either east or west, the effect of deviation may be to either increase or decrease the error due to variation alone. The algebraic sum of variation and deviation is the total compass error.

For computational purposes, deviation and compass error, like variation, may be designated positive (+) if east and negative (-) if west. Variation changes with location, and can be obtained from charts. Deviation depends upon the magnetic latitude and also upon the individual vessel, its trim and loading, whether it is pitching or rolling, the heading (orientation of the vessel with respect to the Earth's magnetic field), and the location of the compass within the vessel. Therefore, deviation is not published on charts.

In practice aboard ship, the deviation is reduced to a minimum, as explained later in this chapter. The remaining value, called residual deviation, is determined on various headings and recorded in some form of deviation table. Figure 910 shows the form used by the United States Navy. This table is entered with the magnetic heading, and the deviation on that heading is determined from the tabulation, separate columns being given for degaussing (now called magnetic silencing) (DG) off and on (section 927). If the deviation is not more than about 2° on any heading, satisfactory results may be obtained by entering the values at intervals of 45° only. If the deviation is small, no appreciable error is introduced by entering the table with either magnetic or compass heading. If the deviation on some headings is large, the desirable action is to reduce it, but if this is not practicable, a separate deviation table for compass heading entry may be useful. This may be made by applying the tabulated deviation to each entry value of magnetic heading, to find the corresponding compass heading, and then interpolating between these to find the value of deviation at each 15° compass heading. Another method is to plot the values on cross-section paper and select the desired values graphically.

An important point to remember regarding deviation is that it varies with the heading. Therefore, a deviation table is never entered with a bearing. The deviation table should be protected from damage due to handling or weather, and placed in a position where it will always be available when needed.

As indicated in Section 908, a single direction may have any of several numerical values depending upon the reference direction used. One should keep clearly in mind the relationship between the various expressions of a direction. Thus, true and magnetic directions differ by the variation, magnetic and compass directions differ by the deviation, and true and compass directions differ by the compass error.

If variation or deviation is easterly, the compass card is rotated in a clockwise direction. This brings smaller numbers opposite the lubber's line. Conversely, if either error is westerly, the rotation is counterclockwise and larger numbers are brought opposite the lubber's line. Thus, if the heading is 090° true (Figure 911, A) and variation is 6°E,

[Figure 907b in Bowditch, Pub. No. 9: Main Field Declination (WMM 2020 Epoch).]

[Figure 907c in Bowditch, Pub. No. 9: Annual Change Declination (WMM 2020 Epoch).]

[Figure 910 in Bowditch, Pub. No. 9: Deviation table.]

the magnetic heading is 090°- 6°= 084° (Figure 911, B). If the deviation on this heading is 2°W, the compass heading is 084°+ 2°= 086° (Figure 911, C). Also, compass error is 6°E-2°W= 4°E, and compass heading is 090°- 4°= 086°. If compass error is easterly, the compass reads too low (in comparison with true directions), and if it is westerly, the reading is too high. Many rules-of-thumb have been devised as an aid to the memory, and any which assist in applying compass errors in the right direction are of value. However, one may forget the rule or its method of application, or may wish to have an independent check. If they understand the explanation given above, they can determine the correct sign without further information. The same rules apply to the use of gyro error. Since variation and deviation are compass errors, the process of removing either from an indication of a direction (converting compass to magnetic or magnetic to true) is often called correcting. Conversion in the opposite direction (inserting errors) is then called uncorrecting. 910. Degaussing is off. The gyro error (GE) is 1° E. A lighthouse bears 306.5° by magnetic compass. Required:

[Figure 911 in Bowditch, Pub. No. 9: Effects of variation and deviation on the compass card. Example: A vessel is on course 215° true in an area where the variation is 7°W. The deviation is as shown in Figure]

(1) Magnetic heading (MH).

(2) Deviation.

(3) Compass heading (CH).

(4) Compass error.

(5) Gyro heading.

(6) Magnetic bearing of the lighthouse.

(7) True bearing of the lighthouse.

(8) Relative bearing of the lighthouse. Solution:

TH 215°

V 7° W

(1) MH 222°

(2) D 1.5°W

(3) CH 223.5° The deviation is taken from the deviation table (Figure 910) to the nearest half degree.

(4) Compass error is 7° W + 1.5° W = 8.5° W.

TH 215°

GE 1° E

(5) Hpgc 214°

CB 306.5°

D 1.5° W

(6) MB 305°

V 7°

(7) TB 298°

(8) RB=TB-TH=298°-215°= 083°. Note: Relative bearings are usually measured from 0° at the heading clockwise through 360°.

The materials of which a vessel is constructed are not, in general, selected for their magnetic properties. As a result, many degrees of permeability, remanence, and coercivity (Section 902) exist within its structure. Detailed analysis of the complex field existing at a magnetic compass is a specialized study not ordinarily required of the navigator. However, a general knowledge of the basic principles involved is of value to the navigator in helping him understand better the behavior of his magnetic compasses.

For most purposes, a vessel can be considered to be composed of two types of material: “hard iron” and “soft iron”. “Hard iron” is all material having some degree of permanent magnetism. This magnetism is acquired largely during construction of the vessel, when the rearrangement of the domains (Section 901) is facilitated by the bending, riveting, welding, and other violent mechanical processes. Since a vessel remains on a constant magnetic heading while it is on the building ways, a field of permanent magnetism becomes established, the positions of the poles being dependent largely upon the orientation of the hull with respect to the magnetic field of the Earth. Consider a case of a vessel constructed in an area where both the variation and the magnetic dip are 0° and it is comprised of only hard iron. Figure 912a shows that if the bow is pointed north during construction the bow will acquire red polarity and the stern will acquire blue polarity. These poles lie in the fore-and-aft axis of the vessel and are therefore the fore-and-aft component of the permanent magnetism.

[Figure 912a in Bowditch, Pub. No. 9: Permanent magnetic field in a vessel built at the magnetic equator; oriented N/S during construction.]

When this vessel is swung clockwise from 000° through 360°, the effect of these poles on the compass are shown in Figure 912b. On a heading of 000° magnetic the deviation 0° (Figure 912b - a). It then begins to increase in a westerly direction, reaching a maximum value on a heading of 090°, then slowly returns to 0° on a heading of 180° (Figure 912b a-e). The deviation then reverses sign, increasing to a maximum easterly value on a magnetic heading of 270° after which it returns to 0° when the vessel is once again headed 000° magnetic. Thus, the deviation is zero on headings of magnetic north and south and maximum of magnetic headings east and west. When the vessel is headed either north or south the permanent magnetism does not cause a deflection of the compass needle but only strengthens or weakens the directive force of the compass. On headings of 090° and 270° magnetic the vessel was perpendicular to the Earth's field and the deviation was greatest. This type of deviation is referred to as semicircular and for fore-and-aft permanent magnetism it behaves like a sine curve (Figure 912d). If the vessel had been constructed on a heading of magnetic east, the poles would have developed in the athwartship with the port side acquiring red polarity and the starboard side acquiring blue polarity. The effect of the athwartship permanent magnetism on the compass is shown in Figure 912c. It can be seen that on headings of magnetic east and west there is no deviation and the vessels pole only strengthen or weaken the directive force of the compass. Maximum deviation occurs on headings of magnetic north and south when the vessel's poles are perpendicular to the Earth's magnetic field. This type deviation is also referred to as semicircular and for athwartship permanent magnetism it behaves like a cosine curve (Figure 912d).

If a vessel is constructed on a heading of magnetic north, at a place where the magnetic dip is 70°N (the approximate value at the midpoint of the east coast of the United States), its field of permanent magnetism is about as shown at the left of Figure 912e. The upper and stern portions are magnetically blue, while the lower and forward portions are magnetically red. If the vessel is built on a heading of magnetic east, the starboard and upper portions are blue, and the port and lower portions are red, as shown by the stern view at the right of Figure 912e. If this same vessel were constructed in the southern hemisphere where the lines of force are directed upward at a 70° angle, the lower and stern portions would be magnetically blue and forward and upper portions are magnetically red.

In reality the orientation of the construction bay is geographically constrained and is arbitrary. If the heading of

[Figure 912b in Bowditch, Pub. No. 9: Deviation due to fore-and-aft permanent magnetism.]

[Figure 912c in Bowditch, Pub. No. 9: Deviation due to fore-and-aft permanent magnetism.]

[Figure 912d in Bowditch, Pub. No. 9: Semicircular deviation due to fore-and-aft and athwartship permanent magnetism.]

the vessel constructed at a place where the magnetic dip is 70° but the heading is magnetic northeast, the upper, starboard, and stern portions are blue, and the lower, port, and forward portions red (Figure 912e). The red and blue portions for any given vessel can be visualized by drawing a sketch similar to that of Figure 912e, with the correct orientation, and the three components of permanent magnetism can be as shown in Figure 912g.

The “permanent” magnetism thus acquired during construction is less permanent than that of a permanent magnet such as one of those used in a compass, and is modified somewhat after launching, particularly if the vessel remains on another heading for a considerable time during fitting out. The change is especially rapid during the first few days after launching, when the domains of the softer iron become reoriented. At this stage, deviation due to permanent magnetism may change several degrees. Further changes in the vessel's permanent magnetism may occur during long periods of being moored on a constant heading, or during a run of several days on nearly the same heading. This change is gradual and affects the strength, but usually not the polarity, of the magnetic field. The permanent field may be changed quickly, in polarity as well as in strength, if the vessel grounds, collides with another vessel, is struck by lightning, undergoes magnetic treatment, etc. The effect that the per-

[Figure 912e in Bowditch, Pub. No. 9: Permanent magnetism of a vessel built on heading magnetic north (left) and magnetic east (right) at a place where the magnetic dip is 70°N.]

[Figure 912f in Bowditch, Pub. No. 9: Permanent magnetism of a vessel built on heading magnetic northeast at a place where magnetic dip is 70°N.]

[Figure 912g in Bowditch, Pub. No. 9: Components of permanent magnetic field.]

manent magnetism of hard iron has upon a compass depends upon the position and strength of the poles relative to the compass. When the poles are in line with the north-south axis of the compass card, the only effect is to strengthen or weaken the directive force of the compass. When the compass heading is approximately 90° away, so that the poles are east and west of the compass, the deviating effect is maximum. The direction of the deviation is the same as that of the blue pole with respect to the compass.

“Soft iron” is all that material in which induced magnetism (Section 902) is present. With respect to its effect upon the magnetic compass, it is classed as either vertical or horizontal. Unlike hard iron, its magnetic field changes quickly as its orientation with respect to the Earth's field changes. It also changes as the strength of the Earth's field changes. For some purposes induced magnetism can be treated as if it were concentrated in two bars of soft iron, one vertical and the other horizontal. The polarity depends upon the position of the vessel relative to the Earth's magnetic field, and the strength depends upon the strength of the vertical and horizontal components of the Earth's field. This is illustrated in Figure 912e. In north magnetic latitude the bottom of the vertical rod has red magnetism and the top has blue magnetism. In south magnetic latitude these are reversed. In both north and south magnetic latitudes the magnetic north end of the horizontal bar has red magnetism, and the magnetic south end has blue magnetism. Thus, whatever the position of the rod, that part in the direction of magnetic north has red magnetism, and that part in the direction of magnetic south has blue magnetism. That is, each end has magnetism opposite to that of the magnetic pole indicated by the direction in which it is pointed.

The effect upon a magnetic compass of the induced magnetism in soft iron depends upon the strength and direction of the field relative to the compass. The cumulative effect of the induced magnetism in vertical soft iron is generally on the centerline of the vessel (if of conventional construction), and for a compass located forward, as on the bridge, is aft of the compass. In magnetic north latitude the effect is generally that of a blue pole at the level of the compass card. In magnetic south latitude the pole is red. On a heading of compass north or south the pole is in line with the magnets of a centerline compass and serves only to strengthen or weaken the directive force. On a heading of compass east or west the pole is perpendicular to the north-south axis of the compass card, and the deviating force is greatest.

For a compass located on the centerline of a vessel of conventional construction, the horizontal soft iron close enough to have appreciable effect upon the compass is arranged in a more-or-less symmetrical manner with respect to the compass. Thus, on any cardinal compass heading, the fore-and-aft and athwartship horizontal soft iron is either in line with the compass magnets or equally and similarly arranged on both sides. No error is introduced by such symmetrical horizontal soft iron because the iron north and south of the compass magnets serves only to strengthen or weaken the directive force, and that east and west of the compass sets up an equal and opposite field on each side. On intercardinal headings, the poles of the induced magnetism are offset and a maximum deviating force occurs. That part of horizontal soft iron which is not symmetrically arranged with respect to the compass, the asymmetrical soft iron, produces deviation which is maximum on the cardinal headings and zero on the intercardinal headings (by compass). This type of deviation is particularly great in a compass not mounted on the centerline of the vessel. It may also produce deviation which is constant on all headings.

As far as its effect upon the compass is concerned, the magnetic field at a centerline compass located forward on a vessel of conventional construction, and on an even keel, is essentially the same as that which would result from four sources: (1) the Earth's magnetism; (2) a single blue pole the location and strength of which depends upon the magnetic history of the vessel; (3) a single pole which is blue in north magnetic latitude and red in south magnetic latitude, is on the centerline aft of the compass, and increases in strength with higher magnetic latitude; and (4) a single blue pole on the starboard side for easterly headings and on the port side for westerly headings, being of zero strength on a heading of north or south and decreasing in strength with increased magnetic latitudes. The single pole concept assumes that the effect of one pole predominates. The locations of the poles depend partly upon the position of the compass to which they apply. The actual field surrounding any magnetic compass may be considerably more complex than indicated.

There are at least two possible solutions to the problem of compass error. The error can be permitted to remain, and the various directions interconverted by means of variation and deviation, or compass error, as explained in Section 911; or the error can be removed. In practice, a combination of both of these methods is used.

Variation depends upon location of the vessel, and the navigator has no control over it. Variation does not affect the operation of the compass itself, and so is not objectionable from this standpoint.

Deviation is undesirable because it is more troublesome to apply, and the magnetic field which causes it partly neutralizes the directive force acting upon the compass, causing it to be unsteady and sluggish. As the vessel rolls and pitches, or as it changes magnetic latitude, the magnetic field changes, producing a corresponding change in the deviation of an unadjusted compass. Deviation is eliminated, as nearly as practicable, by introducing at the compass a magnetic field that is equal in magnitude and opposite in polarity to that of the vessel. This process is called compass adjustment, or sometimes compass compensation,

although the latter designation is now more generally applied to the process of neutralizing the effect due to degaussing of the vessel (Section 927).

In general, the introduced field is of the same kind of magnetism as well as of the same intensity as those of the field causing deviation. That is, permanent magnets are used to neutralize permanent magnetism, and soft iron to neutralize induced magnetism, so that the adjustment remains effective with changes of heading and magnetic latitude. A relatively small mass of iron near the compass introduces a field equal to that of a much larger mass at a distance.

When a compass is properly adjusted, its remaining or residual deviation is small and practically constant at various magnetic latitudes, the directive force is as strong as is obtainable on all headings, and the compass returns quickly from deflections and is comparatively steady as the vessel rolls and pitches.

As indicated in Section 906, the magnetic field of the Earth is horizontal at the magnetic equator, and vertical at the magnetic poles, the change occurring gradually as a vessel proceeds away from the magnetic equator. At any place, the relative strength of the horizontal and vertical components depends upon the magnetic dip. The directive force of a magnetic compass, provided by the horizontal component of the Earth's magnetic field, is maximum on or near the magnetic equator and gradually decreases to zero at the magnetic poles. Within a certain area surrounding each magnetic pole the directive force is so weak that the compass is unreliable (Section 3421).

Deviation changes with a change of the relative strength of either the deviating force or the directive force. Thus, with either an increase in deviating force or a decrease in directive force, the deviation increases. However, if both the deviating and directive forces change by the same proportion, and with the same sign, there is no change in deviation. Also, if a deviating force is neutralized by an equal and opposite force of the same kind, there is no change of deviation with a change of magnetic latitude.

Permanent magnetism is the same at any latitude. If the permanent magnetism of the vessel is neutralized by properly placed permanent magnets of the correct strength, a change of magnetic latitude can be made without introduction of deviation. But if residual deviation due to permanent magnetism is present, it increases with a change to higher latitude. The deviating force remains unchanged while the directive force decreases, resulting in an increase in the relative strength of the deviating force.

As magnetic latitude increases, the vertical component of the Earth's magnetic field becomes stronger, increasing the amount of induced magnetism in vertical soft iron. At the same time the directive force of the compass decreases. Both effects result in increased deviation unless the deviating force is neutralized by induced magnetism in vertical soft iron.

As magnetic latitude increases, the induced magnetism in the horizontal soft iron decreases in the same proportion as the decrease in the directive force of the compass, since both are produced by the horizontal component of the Earth's magnetic field. Therefore, any deviation due to this cause is the same at any latitude.

Compass adjustment might be accomplished by locating the pole of each magnetic field, and establishing another pole of opposite polarity and equal intensity at the same place, or of less intensity and nearer to the compass; or a pole of opposite polarity and suitable intensity might be established at the correct distance on the opposite side of the compass. Thus, a blue pole east of a compass attracts the red northern ends of the compass magnets and repels the blue southern ends. Both effects cause rotation of the compass magnets and the attached compass card in a clockwise direction, producing easterly deviation. Either a red pole east of a compass, or a blue pole west of it, causes westerly deviation. If there are two fields of opposite polarity, one will tend to neutralize the other. If the intensities of the two fields are equal at the compass, one will cancel the other, and no deviation occurs.

Because of the complexities of the magnetic field of a vessel, and the fact that each individual field making up the total is present continuously, the process of isolating individual poles would be a difficult and time-consuming one. Fortunately, this is unnecessary. The vessel's field is resolved into certain specified components. Each of these components, regardless of its origin or the number of individual fields contributing to it, can be neutralized separately. Each component is called a parameter, and the various parameters are designated by letter, as follows: Permanent magnetism (Figure 915a).

[Figure 915a in Bowditch, Pub. No. 9: Permanent magnetism parameters.]

[Figure 915b in Bowditch, Pub. No. 9: Induced magnetism parameters.]

Parameter P is the fore-and-aft component. It is positive

(+) if it is the equivalent of a blue pole forward of the

compass, and negative (-) if red. Parameter Q is the athwartship component. It is positive if

it is the equivalent of a blue pole to starboard.

Induced magnetism has nine parameters, each the equivalent of that produced by a slender rod of soft iron. Each end of a rod is positive if it is forward, to starboard, or below the compass. Each rod is positive if both ends are positive or if both ends are negative, and negative if the two ends are of opposite sign. The rods are shown in Figure 915b.

Deviation which is easterly throughout approximately 180° of heading and westerly throughout the remainder is called semicircular deviation, indicating that its sign remains unchanged throughout a semicircle. Deviation caused by permanent magnetism and that caused by induced magnetism in vertical soft iron are semicircular. Deviation which changes sign in each quadrant, being easterly in two opposite quadrants and westerly in the other two, is called quadrantal deviation. It is caused by induced magnetism in horizontal soft iron. The types of deviation resulting from the various parameters are called coefficients. There are six, as follows: Coefficient A is constant on all headings. If its cause is

magnetic, as from an asymmetrical combination of

parameters, it is a “true” constant. If its cause is

mechanical, as from an incorrectly placed lubber's line,

or mathematical, as from an error in computation of

magnetic azimuth, it is an “apparent” constant. Coefficient B is semicircular deviation which is propor-

tional to the sine of the compass heading. It is maxi-

mum on compass headings east or west, and zero on

compass headings north or south. Coefficient B is

caused by permanent magnetism, and also by induced

magnetism in asymmetrical vertical soft iron. Coefficient C is semicircular deviation which is propor-

tional to the cosine of the compass heading. It is maxi-

mum on compass headings north or south, and zero on

compass headings east or west. Coefficient C is caused

by permanent magnetism or by induced magnetism in

asymmetrical vertical soft iron athwartship of the com-

pass. Coefficient D is quadrantal deviation which is proportional

to the sine of twice the compass heading. It is maxi-

mum on intercardinal compass headings, and zero on

cardinal compass headings. Coefficient D is caused by

induced magnetism in horizontal soft iron which is

symmetrical with respect to the compass. Coefficient E is quadrantal deviation which is proportional

to the cosine of twice the compass heading. It is maxi-

mum on cardinal compass headings, and zero on inter-

cardinal compass headings. Coefficient E is caused by

induced magnetism in horizontal soft iron which is

asymmetrical with respect to the compass. Coefficient J is the change of deviation for a heel of 1°

while the vessel is on compass heading 000°.

The determination and use of the approximate coefficients in the analysis of compass deviation are discussed in Section 924.

The location of a magnetic compass greatly influences the amount and type of deviation, as well as the adjustment. Thus, if a compass is on the centerline, forward, the effective pole of vertical soft iron is aft of it; but if the compass is on the afterpart of the vessel, the effective pole is forward. If the compass is not on the centerline, as the steering compass of an aircraft carrier, the magnetic field of the vessel is not symmetrical with respect to the compass. If a compass is located in a steel pilot house, the surrounding metal acts as a shield and reduces the strength of the magnetic field of the Earth. This is of particular significance in high magnetic latitudes, where the directive force is weak.

Many factors influence the selection of a position for the compass. The most important consideration is the use to be made of it. A steering compass is of little use unless it is located so that it can be seen by the steersman. A compass to be used for emergency steering should be at the emergency steering station. A compass to be used for observing bearings or azimuths, or a standard compass to be used for checking other compasses, should be located so as to have a clear view in most directions.

However, some choice is possible. A compass should not be placed off the centerline if it can be placed on the centerline and still serve its purpose. It should not be placed near iron or steel equipment that will frequently be moved, if this can be avoided. Thus, a location near a gun, boat davit, or boat crane is not desirable. The immediate vicinity should be kept free from sources of deviation - particularly those of a changing nature - if this can be done. That is, no source of magnetism, other than the structure of the vessel, should be permitted within a radius of several feet of the magnetic compass. Some sources which might be overlooked are electric wires carrying direct current; magnetic instruments, searchlights, wind shield wipers, electronic equipment, or motors; steel control rods, gears, or supports associated with the steering apparatus; fire extinguishers, gas detectors, etc.; and metal coat hangers, flashlights, keys, pocketknives, metal cap devices, or nylon clothing. The effect of some items such as an ammeter or electric windshield wiper varies considerably at different times. If direct current is used to light the compass, the wires should be twisted. A magnetic compass cannot be expected to give reliable service unless it is properly installed and protected from disturbing magnetic influences.

If a small magnet is pivoted at its center of gravity in such manner that it is free to turn and dip, it will tend to align itself with the magnetic field of the Earth (Section 906). It thus provides a directional reference and becomes a simple compass. However, such a compass would not be adequate for use aboard ship. For this purpose, a compass should have a stronger directive element than that provided by a single, pivoted magnet, should have provision for measuring various directions, should have some means of damping the oscillations of the directive element, should be approximately horizontal, and should have some means of neutralizing local magnetic influences.

In a mariner's compass, several magnets are mounted parallel to each other. To them is attached a compass card having a compass rose to indicate various directions. Both magnets and compass card are enclosed in a bowl having a glass top through which the card can be seen. The bowl is weighted at the bottom and is suspended in gimbals in such manner that it remains nearly horizontal as the vessel rolls and pitches. In nearly all modern compasses the bowl is filled with a liquid that supplies a buoyant force almost equal to the force of gravity acting upon the directive element and card. This reduces the friction on the pivot (a metal point in a jeweled bearing), and provides a means of damping the oscillations of the compass card. The card is mounted in such manner as to remain in an essentially horizontal position. A mark called a lubber's line is placed on the inner surface of the bowl, adjacent to the compass card, to indicate the forward direction parallel to the keel when the bowl is correctly installed. The gimbals used for mounting the compass bowl are attached to a stand called a binnacle, which in most installations is permanently and rigidly attached to the deck of the vessel, usually on its longitudinal center line. Most binnacles provide means for neutralization of local magnetic influences due to magnetism within the vessel. A cover or “hood” is provided to protect the compass from the elements, dust, etc.

After the compass has been selected and installed, proper adjustment and compensation are important, and future care of the instrument should not be neglected. It should be checked and overhauled at regular intervals, and any indication of malfunctioning or deterioration, however slight, should not be over looked. Discoloration of the liquid or the presence of a bubble, for instance, indicates a condition that should be investigated and corrected at once. If it becomes necessary to add liquid, one should be certain that he has the correct substance, and should attempt to

determine the source of the leak. Except as a temporary expedient, this is best done by a professional. Some compasses should be protected from prolonged exposure to sunlight, to prevent discoloration of the card and liquid.

The compass card is composed of light, nonmagnetic material. In nearly all modern compasses the card is graduated in 360°, increasing clockwise from north through east, south, and west. Some compass cards are graduated in “points”, usually in addition to the degree graduations. There are 32 points of the compass, 11-1/4° apart. The four cardinal points are north, east, south, and west. Midway between these are four intercardinal points at northeast, southeast, southwest, and northwest. These eight points are the only ones appearing on the cards of compasses used by the U.S. Navy. The eight points between cardinal and intercardinal points are named for the two directions between which they lie, the cardinal name being given first, as north northeast, east northeast, east southeast, etc. The remaining 16 points are named for the nearest cardinal or intercardinal point “by” the next cardinal point in the direction of measurement, as north by east, northeast by north, etc. Except for the cardinal and intercardinal points, and occasionally the two-point graduations, all of which are used to indicate directions generally (as “northwest winds”, meaning winds from a general northwesterly direction), the point system has become largely historical. Figure 918a shows a modern marine magnetic compass.

[Figure 918a in Bowditch, Pub. No. 9: A modern marine magnetic compass.]

Because of its essential simplicity, a magnetic compass does not easily become totally inoperative. Being independent of any power supply or other service, a magnetic compass may survive major damage to its ship without losing its utility. Despite its great reliability, however, a magnetic compass is subject to some limitations. Since it responds to any magnetic field, it is affected by any change in the local magnetic situation. Hence, the undetected presence or change of position of magnetic material near the compass may introduce an unknown error.

Larger compasses or repeaters are usually provided with a bearing circle or azimuth circle (Figure 918b). These devices take a variety of forms, but consist essentially of two parts: (1) a pair of sighting vanes attached to a ring which fits snugly over the compass, and (2) a mirror to reflect the compass graduation into the line of sight. The use of these devices is similar to that of the bearing bar and azimuth instrument. The azimuth circle has a pivoted reflecting surface attached to the far vane, to permit observation of celestial bodies. In most cases it also has a reflecting mirror and prism mounted on opposite sides of the ring, midway between the vanes. The prism is covered with opaque material except for a thin, vertical slot at its center. The surface of the mirror is curved so that reflection of sunlight falling upon it is in the form of a slender vertical line (at the distance of the prism) of about the same width as the slot. When the azimuth circle is adjusted so that this line of light falls upon the slot, a thin, bright line appears on the compass card graduations at the bearing of the sun. Most bearing and azimuth circles are provided with reverse compass rose graduations to permit reading of relative bearings or azimuths (by the vanes) at a mark on top of the compass bowl, in line with the lubber's line; bubbles for indicating the level position during observation; means for adjusting the snugness of the fit over the compass bowl; and handles for turning the device.

[Figure 918b in Bowditch, Pub. No. 9: Azimuth / Bearing circle.]

The compass is housed in a binnacle. Most binnacles provide means for housing or supporting the various objects used for compass adjustment, as well as the equipment for compensating for deviation caused by degaussing. Figure 918c shows a modern compass binnacle, with slots for holding the fore-and-aft and athwartship magnets the tube for the heeling magnet, the Finder's bar tube and the quadrantal spheres.

Permanent magnetism can be considered concentrated in a single pole, the position of which depends upon the

[Figure 918c in Bowditch, Pub. No. 9: Modern magnetic compass binnacle showing Flinder Bar tube, Quadrantal Spheres and the fore-and-aft /athwartships magnets.]

magnetic heading upon which the vessel was constructed, and the subsequent magnetic history of the vessel. Figure 919a indicates the condition if the permanent magnetism can be considered concentrated in a single blue pole which is directly south of the compass when the vessel is headed magnetic northeast. The only effect on this heading is to weaken the directive force. No deviation is produced because the pole is in line with the compass magnets. On heading magnetic southwest, the pole is also in line with the compass magnets and there is no deviation, but the directive force is strengthened. On any other heading, the pole is not in line with the compass magnets, and deviation occurs, being in the same direction as that of the blue pole from the compass, since the blue pole attracts the red northerly ends of the compass magnets and repels the blue southerly ends. The maximum effect occurs when the compass heading is approximately 90° from that of zero deviation. In Figure 919a the headings shown on the compass card are the magnetic headings of the vessel. Their offset from the lubber's line shows the direction and relative magnitude of deviation.

The usual method is to adjust for the fore-and-aft (parameter P) and athwartship (parameter Q) components separately. These are shown in Figure 919b. The vertical parameter R does not produce deviation while the vessel is on an even keel. Its effect when the vessel heels is discussed in Section 923. Thus, the effect of a single blue pole at the position shown in Figure 919a is the same as that which would be produced by two weaker poles as shown in Figure 919b. On heading east or west by the compass, parameter Q does not produce deviation directly. However, on easterly headings it does weaken the directive force due to the Earth's magnetic field and therefore the deviating force of parameter P (causing deviation coefficient B) is relatively stronger and has a greater deviating effect. On a westerly heading the directive force would be strengthened, with a corresponding decrease in the B coefficient of deviation. By weakening the directive force on easterly headings, parameter Q also makes the compass sluggish on these headings. In high latitudes, where the horizontal component of the Earth's magnetic field is weak, the compass may lose its directivity at a greater distance from the magnetic pole. Nearer the pole, it might point in the opposite direction.

Many binnacles provide a group of several small tubes or “trays” extending in a fore-and-aft direction below the compass. One or more permanent magnets can be inserted in these trays, and the whole assembly moved up or down to vary the effect upon the compass. Figure 919c shows the situation if a single magnet is placed with its red end aft. The field at the compass is in the opposite direction of that of parameter P, and if it is of equal strength, the effect of this parameter is eliminated.

If now the vessel is headed north or south by the compass, the only pole remaining is that due to parameter Q (causing deviation coefficient C), as shown in Figure 919d. A set of trays in an athwartship direction below the compass permits insertion of one or more permanent magnets to neutralize the remaining permanent magnetism. The effect of inserting a single magnet with red end to starboard is shown in Figure 919e. With both components removed, the field at the compass is completely neutralized.

Both the fore-and-aft (B) and athwartship (O trays are in pairs with an equal number of trays on each side of the vertical axis of the compass. In each set of trays it is gener-

[Figure 919a in Bowditch, Pub. No. 9: Deviation due to permanent magnetism if the resultant field is that of a blue pole on the starboard quarter of the vessel. Black lines passing through the compass represent the Earth’s magnetic lines of force.]

[Figure 919b in Bowditch, Pub. No. 9: The horizontal component of the permanent field of Figure 919a resolved into its components, parameters P and Q.]

ally desirable to use an even number of magnets equally distributed on each side, to produce a symmetrical field around the compass. However, under some conditions, maximum reduction of deviation occurs with an odd number of magnets, particularly when two magnets at maximum distance from the compass overcorrect. If there is a choice, a greater number of magnets at a distance is preferable to a lesser number close to the compass.

With each parameter, the trays to use are those which are approximately perpendicular to the compass magnets. The magnets are placed so that the red ends will be on that side of the compass corresponding to the deviation. Thus, if deviation is easterly, the magnets should be placed so that the red ends will be east of the compass (forward if the heading is east, and to starboard if the heading is north). However, if the wrong end is inserted in the trays, the fact will be immediately apparent because the compass card will

[Figure 919c in Bowditch, Pub. No. 9: The field of permanent magnet below the compass and opposing parameter P of Figure 919b.]

[Figure 919d in Bowditch, Pub. No. 9: The permanent field of Figure 919a after neutralization of parameter P.]

[Figure 919e in Bowditch, Pub. No. 9: The field of a permanent magnet below the compass and opposing parameter Q of Figure 919b.]

rotate in the wrong direction. If the binnacle is not constructed to receive appropriate corrector magnets, these might be secured to some supporting surface near the compass.

During adjustment, the unused magnets should be kept far enough from the compass so that they will not affect it.

Figure 920a shows the effect upon the compass of a single blue pole on the centerline of the vessel, aft of the compass. This is a typical situation for induced magnetism in vertical soft iron, for a centerline compass located in the forward part of a vessel in magnetic north latitude. On heading north by compass there is no deviating force, but the directive force is weakened. In high northern latitudes, where this pole becomes strong and the directive force becomes weak, magnetism of this type, if not neutralized, can cause the compass to be unreliable in a much larger area than if the force is neutralized. On a heading of south by compass there is no deviation, but the directive force is strengthened. On headings with an easterly component the deviation is westerly, and on headings with a westerly component the deviation is easterly. In each case the maximum occurs when the vessel is on compass heading approximately east or west. Thus, the deviation due to induced magnetism in vertical soft iron is semicircular, coefficient B. In Figure 920a, the headings shown on the compass card are the magnetic headings of the vessel. Their offset from the lubber's line shows the direction and relative magnitude of deviation.

[Figure 920a in Bowditch, Pub. No. 9: Deviation due to induced magnetism in vertical soft iron if the resultant field is that of a blue pole on the center line aft of the compass.]

The deviating force due to induced magnetism in vertical soft iron is neutralized by placing a bar of soft iron in a vertical position on the opposite side of the compass from the effective pole due to the field of the vessel. This piece of metal is called a Flinders bar, after Captain Matthew Flinders, RN (1774-1814), an English navigator and explorer who is generally given credit for discovering both the effect and method of adjustment.Today, most binnacles for large ships provide a tube for insertion of a Flinders bar. The bar consists of various lengths of soft iron placed end to end; with the remainder of the tube being filled with spacers of nonmagnetic material, usually wood, brass, or

aluminum. The standard Flinders bar is two inches in diameter and is divided into six sections, one each of 12, 6, 3, and 1-1/2 inches, and two of 3/4 inch. This permits use of any multiple of 3/4 inch to 24 inches. All the iron pieces should be above the spacers in the tube, without a gap between pieces, the largest piece being on top. The upper end is then about two inches above the level of the compass card. For short lengths, one or more spacers should be omitted so that about 1/12th of the length of the bar is above the level of the compass card. Figure 920b illustrates the effect of the proper amount of Flinders bar to offset the deviation caused by the blue pole aft of the compass. In the northern hemisphere the upper portion of the Flinders bar takes on blue polarity. If the vessel steams into the southern hemisphere, the pole aft of the compass becomes red but so does the upper portion of the Flinders bar.

The various pieces should be inserted in the tube carefully. If they are dropped, they may acquire some permanent magnetism. This reduces their effectiveness for the purpose intended. Each piece should be tested from time to time to determine whether or not it has acquired permanent magnetism. This can be done by holding it vertical with one end east or west of the compass and very near the compass magnets, noting the reading of the compass, and then inverting the piece so that the ends are interchanged. If the reading differs, permanent magnetism has been acquired by the iron rod. The temporary change of reading while the rod is being inverted should be ignored. In making the test, one should be careful to place the rod in the same position relative to the compass before and after inversion. On an easterly or westerly heading the Flinders bar holder can be used. A small amount of permanent magnetism can be removed by holding the rod approximately parallel to the lines of force of the Earth's field, with the blue pole of the rod toward the north, and tapping one end of the rod gently with a hammer. Several alternate tests and treatments may be needed to make the rod magnetically neutral. If this process is not effective in removing the permanent magnetism, the rod should be heated to a dull red and allowed to cool slowly.

The procedure for determining the proper length of the Flinders bar can be found the Handbook of Magnetic Compass Adjustment (Figure 920c). Once the correct amount of Flinders bar has been installed, no change should be needed unless there is a substantial change in the amount or location of vertical soft iron, or unless the compass is relocated. If the correct length and location of Flinders bar for another vessel of similar construction and compass location have been determined previously, the same length can be used for the compass being adjusted. If a large change in magnetic latitude can be made without appreciable change of deviation on headings east and west, the amount of Flinders bar is correct. If the deviation changes, readjustment is needed. By studying the structure of the vessel, an experienced compass adjuster may be able to make a reasonably accurate estimate of the length to use.

[Figure 920b in Bowditch, Pub. No. 9: Use of the Flinders Bar to reduce/eliminate deviation due to induced magnetism in vertical soft iron. Black lines passing through the compass represent the Earth’s magnetic lines of force.]

[Figure 920c in Bowditch, Pub. No. 9: Handbook of Magnetic Compass Adjustment. https://msi.nga.mil/api/publications/download?key=16920 950/SFH00000/HoMCA.pdf&type=view]

That part of horizontal soft iron which is symmetrically arranged with respect to the compass can be considered

[Figure 921a in Bowditch, Pub. No. 9: Deviation caused by induced magnetism in symmetrical horizontal soft iron. Black lines passing through the compass represent the Earth’s magnetic lines of force.]

equivalent to two rods extending through the compass, one in a fore-and-aft direction (-a rod) and the other in an athwartship direction (-e rod). The deviation caused by both of these rods is quadrantal, but of opposite sign. If both rods were equally effective in causing deviation, they would cancel each other and no deviation would result on any heading. In most vessels, however, the athwartships iron dominates, and deviation due to all horizontal soft iron can generally be considered to be that which would result from a single (-) e rod. In Figure 921a the deviation resulting from such a rod is shown for various magnetic headings in any latitude. There is no deviation on any cardinal heading, but the directive force is weakened on heading magnetic east or west. The maximum deviation occurs on intercardinal headings by compass, being easterly in the northeast and southwest quadrants, and westerly in the other two quadrants. This is coefficient D deviation. In Figure 921a the headings shown on the compass card are the magnetic headings of the vessel. Their offset from the lubber's line shows the direction and relative magnitude of deviation.

The field causing this deviation is neutralized by installing two masses of soft iron abeam of the compass, on opposite sides and equidistant from its center. Such iron is usually in the form of hollow spheres or cylinders, called quadrantal correctors. These can be moved in or out in an athwartship direction along brackets on the sides of the binnacle.

Quadrantal correctors act as (+) e parameters which neutralize the (-) e parameter of the athwartships iron. As shown in Figure 921b, the portion of the corrector adjacent to the compass is always of opposite polarity to the deflecting force. The amount of the correction can be adjusted by moving the correctors toward or away from the compass card. If the inboard limit of travel is reached without fully removing the deviation, larger correctors are needed. If overcorrection occurs at the outboard limit, smaller correctors are needed. A single corrector can be used, but this produces an unbalanced field which is less desirable than a balanced one. In general, large correctors at a greater distance are preferable to small correctors close up because there is less mutual induction between the correctors if they are widely separated. In the rare case when quadrantal deviation is westerly on heading northeast (coefficient D is negative, the fore-and-aft horizontal soft iron predominating), the quadrantal correctors should be mounted fore-and-aft on the binnacle.

Figure 921c shows the approximate amount of deviation correction to be expected from correctors of various sizes, shapes and distance from the center of a standard U. S. Navy 7 1/2-inch compass. The data apply to either the athwartships or fore-and-aft position.

Like the Flinders bar (Section 920), the quadrantal correctors should be handled carefully, and checked from time to time to see if they have acquired permanent magnetism.

[Figure 921b in Bowditch, Pub. No. 9: Adjustment for symmetrical horizontal soft iron. Black lines passing through the compass represent the Earth’s magnetic lines of force.]

[Figure 921c in Bowditch, Pub. No. 9: Effect of various quadrantal correctors.]

The test can be made by rotating each corrector through 180° without altering its distance from the center. If the compass heading changes, the correctors have acquired permanent magnetism which can be removed by tapping with a hammer when the blue pole is toward the north, or by removing the spheres, heating them to a dull red, and permitting them to cool slowly.

The following rule for improving the adjustment for coefficient B if no better method is available: Remove the deviation observed on magnetic east or west headings by means of fore-and-aft B magnets when the vessel has arrived at places of weaker vertical magnetic field, and by means of Flinders bar when it has arrived at places of stronger vertical magnetic field, whether in the Northern or Southern Hemisphere.

If the horizontal soft iron is not arranged symmetrically with respect to the compass, resulting in an effective pole which is on neither the fore and-aft nor athwartships axis through the compass, quadrantal deviation with its maximum values on cardinal headings (coefficient E) results. Constant deviation (coefficient A) may also be used by this arrangement. Either coefficient E or A is due to a combination of parameters.

For a centerline compass on a ship of conventional construction, any deviation due to induced magnetism in asymmetrical horizontal soft iron is small, and many installations make no provision for neutralizing the effect. However, some binnacles are provided with a pair of E-Links, which are bars that can be attached to the side brackets to permit the quadrantal correctors to be slewed somewhat with respect to the compass. When this has been done, the horizontal axis through the correctors and the compass makes an angle with the athwartship axis of the compass.

After a compass has been adjusted, any remaining constant deviation due to magnetic coefficient A is likely to be very small. If such deviation exists, its cause is likely to be chiefly mechanical. If a compass is used primarily for determining the heading (as a steering compass), all constant deviation can be removed by realignment of the binnacle so as to rotate the lubber's line by the required amount.

All of the effects discussed previously refer to a vessel on an even keel. When the vessel heels, conditions are altered. Deviation which now appears or the change of deviation from that when the vessel was on an even keel, is called heeling error. For a constant angle of heel and a steady heading, this error remains essentially unchanged. However, it tends to increase as the heel becomes greater, and to reverse sign as the heel changes from one side to another. Therefore, if a vessel is rolling or pitching, the compass tends to oscillate. This increases the difficulty of reading the compass.

The cause of heeling error is the displacement of the permanent and induced magnetic fields with respect to the compass. Figure 923 shows a vessel heeled to starboard on heading magnetic north or south, in north magnetic latitude. The vessel was constructed in north magnetic latitude. On an even keel the vertical parameter R of permanent magnetism for a centrally located compass is directly below the compass, with the blue pole nearer the compass. When the

[Figure 923 in Bowditch, Pub. No. 9: Effect of heel.]

vessel is heeled as shown at A, the blue pole is to port of the compass, causing deviation toward that side. A vertical rod of soft iron below the compass (parameter k) exerts a similar influence, as shown at B. An athwartship horizontal rod through the compass has no deviating effect while the vessel is on an even keel, but when it heels as shown in Figure 923, the vertical component of the Earth's field causes the port end to acquire a blue pole and the starboard end a red pole (parameter e), as shown at C. Each of the three causes results in a blue pole being established on the port or high side of the vessel. This causes the red north ends of the compass magnets to be attracted to this side. If the heading is magnetic north, the deviation is westerly, and if magnetic south, it is easterly. This effect is offset somewhat by the changed magnetic field surrounding the quadrantal correctors. On heading magnetic east or west, these components have no deviating effect, but the directive force of the compass is strengthened or weakened. When the vessel pitches, the effects described for north-south and east-west headings are reversed. On a heading other than a cardinal direction (magnetic) the effect is some combination of the two. The magnetic situation varies not only with the heading, but also with the magnetic latitude and the magnetic history of the vessel.

Although heeling error is due in part to permanent magnetism and in part to induced magnetism, the induced magnetism generally exerts the greater influence. The most effective method of neutralizing this effect would be to attack each parameter separately. This would require the placement of soft iron above the compass. Since this would not be a convenient arrangement, the condition is improved by placing a vertical permanent magnet, called a heeling magnet, centrally below the compass, and adjusting its height until the error is minimized. In north magnetic latitude, the red end is placed uppermost in most installations. As the vessel proceeds to lower magnetic latitudes, parameter R becomes less effective in producing deviation because of the stronger directive force due to the horizontal component of the Earth's magnetic field. Parameters k and e become weaker because of decreased intensity of the vertical component of the Earth's field, and the strengthening of the horizontal component also reduces their effect. Therefore, the heeling magnet requires readjustment as the magnetic latitude changes. As the vessel approaches the magnetic equator, the heeling magnet should be lowered. After the vessel crosses the magnetic equator, it may be necessary to invert the heeling magnets, so that the opposite end is uppermost. A change in the setting of the heeling magnet may introduce deviation on headings of compass east or west because of altered induction between the heeling magnet and the Flinders bar. This should be removed by means of the fore-and-aft (B) magnets in the trays below the compass.

If adjustment for heeling error is made when the vessel is tied up or at anchor, it is best done by listing the vessel on a northerly or southerly heading, and adjusting the heeling magnet until the reading of the compass is restored to what it was before the vessel heeled. If the adjustment is made at sea, the vessel should be placed on a heading of compass north or south. If there is little rolling, the vessel can be listed and the compass reading restored, as at dockside. If the vessel rolls moderately on this heading, the heeling magnet should be placed at that height at which oscillation of the compass card is minimum. If the setting for minimum oscillation is different on north and south headings, the mean position should be used. Any yawing of the vessel should be considered when reading the compass under rolling conditions.

The approximate position of the heeling magnet can be determined by means of an instrument known as a heeling adjuster or a vertical force instrument, a form of dip needle. This consists of a small magnet balanced about a horizontal axis by means of a small adjustable weight. A scale indicates the distance of the weight from the axis. The instrument is taken ashore and balanced at a place where the Earth's field is undisturbed, the magnet being in a magnetic north-south direction, approximately. The instrument is then taken aboard ship, the compass removed from its binnacle, and the heeling adjuster installed in its place. The heeling magnet is then moved up or down until the magnet

of the instrument is level. This should be approximately the correct setting. This method is used principally when the listing of a vessel is difficult or impractical.

An analysis consists of determining the approximate value of each of the six coefficients, and studying the results. The purpose of the analysis is to give the compass adjuster an understanding of the magnetic properties of the vessel. This provides the basis for the approximate placement of the various correctors, and suggests possibilities for further refinement in the adjustment. Without an analysis, compass adjustment is a more-or-less mechanical process. Fewer mistakes are likely to be made by the person who understands the nature of the magnetic field he seeks to neutralize.

The first step in an analysis is to record the deviation on each cardinal and intercardinal heading by the compass to be analyzed. For the purpose of analysis, easterly deviation is considered positive (+), and westerly deviation negative (-). Approximate values of the various coefficients are: Coefficient A - mean of deviation on all headings. Coefficient B - mean of deviation on headings 090° and

270°, with sign at 270° reversed. Coefficient C - mean of deviation on headings 000° and

180°, with sign at 180°reversed. Coefficient D - mean of deviation on intercardinal head-

ings, with signs at headings 135° and 315° reversed. Coefficient E - mean of deviation on cardinal headings,

with signs at 090° and 270° reversed. Coefficient J - change of deviation for a heel of 1° while

the vessel heads 000° by compass. It is considered pos-

itive if the north end of the compass card is drawn

toward the low side, and negative if toward the high

side. Example: A magnetic compass which has not been adjusted has deviation on cardinal and intercardinal compass headings as follows: Required: The approximate value of each coefficient. Solutions: A = (-1.5 + 34.0° + 31.0° + 13.5° + 8.0° - 1.5° - 29.0° -

36.0°) / 8 = + 2.3° B = (31.0° + 29.0°) / 2 = + 30.0° C = (-1.5° - 8.0°) / 2 = - 4.8° D = (34.0° - 13.5° - 1.5° + 36.0°) / 4 = + 13.8° E = (-1.5° – 31.0° + 8.0° + 29.0°) / 4 = + 1.1° J = (-13.5° + 1.5°) / 10 =- 1.2° Answers: A = +2.3°, B = +30.0°, C = -4.8°, D = + 13.8°, E = + 1.1°, J = -1.2°. On any compass heading (CH) the deviation (d) from each coefficient acting alone is: Coefficient A: constant at + 2.3° Coefficient B: +30.0° sin CH Coefficient C: -4.8° cos CH Coefficient D: +13.8° sin 2CH Coefficient E: +1.1°cos 2CH Coefficient J: -1.2° cos CH

For a vessel on an even keel, the total deviation on any compass heading is the algebraic sum of the deviation due to each of the first five coefficients. For the compass of the example given above, are shown in graphical form in Figure 924. Since the various coefficients are only approximated by the method given above, the curve of total deviation found in this way should not be expected to coincide exactly with a curve drawn from values found by measurement on the various headings.

The shapes of the curves of Figure 924 are typical of those of an unadjusted compass of a large steel ship. However, an analysis of the results indicates the following:

Coefficient A is normally negligible. The presence of more than 2° of constant error indicates an abnormal condition which should be discovered and corrected. If the vessel has been in service for some time without major structural change, and no misalignment of the lubber's line of the compass or the pelorus or gyrocompass used for measuring deviation has been noted previously, it is probable that a mistake has been made in determining the azimuth or bearing used for establishing deviation.

Coefficient E is normally negligible for a compass located on the centerline of the vessel. This vessel has an excessive amount, which should be corrected by slewing the quadrantal correctors, using an E-link.

Since deviation is east on heading 090° and west on 000°, it is probable that the blue pole of the vessel's permanent field is on the port bow. The compass being unadjusted, no Flinders bar is in place, and the large B deviation on heading 090° is a combination of deviation from induced magnetism in vertical soft iron and that due to the permanent magnetism of the vessel. Since the deviation on heading 270° is nearly the same as that on 090°, but of opposite sign, adjustment on one of these headings should result in nearly correct adjustment on the other. Since some B and C deviation occurs on intercardinal headings, while no D deviation occurs on cardinal headings, adjustment for B and C should be made before that for final D adjustment.

[Figure 924 in Bowditch, Pub. No. 9: Coefficients and total deviation of an unadjusted magnetic compass.]

There are several reasons for correcting the errors of a magnetic compass, even if it is not the primary directional reference: 1. It is easier to use a magnetic compass if the deviations

are small. 2. Even known and fully compensated deviation intro-

duces error because the compass operates sluggishly

and unsteadily when deviation is present. 3. Even though the deviations are compensated for, they

will be subject to appreciable change as a function of

heel and magnetic latitude.

Theoretically, it doesn’t matter what the compass error is as long as it is known. But a properly adjusted magnetic compass is more accurate in all sea conditions, easier to steer by, and less subject to transient deviations which could result in deviations from the ship’s chosen course. Therefore, if a magnetic compass is installed and meant to be relied upon, it behooves the navigator to attend carefully to its adjustment. Doing so is known as “swinging ship”.

While a professional compass adjuster will be able to obtain the smallest possible error curve in the shortest time, many ship’s navigators adjust the compass themselves with satisfactory results. Whether or not a “perfect” adjustment is necessary depends on the degree to which the magnetic compass will be relied upon in day-to-day navigation. If the magnetic compass is only used as a backup compass, removal of every last possible degree of error may not be worthwhile. If the magnetic compass is the only steering reference aboard, as is the case with many smaller commercial craft and fishing vessels, it should be adjusted as accurately as possible.

Prior to getting underway to swing ship, the navigator must ensure that the process will proceed as expeditiously as possible by preparing the vessel and compass. The following tests and adjustment can be done at dockside, assuming that the compass has been installed and maintained properly. Initial installation and adjustment should be done by a professional compass adjuster. 1. Check for bubbles in the compass bowl. Fluid may be

added through the filling plug if necessary. Large bub-

bles indicate serious leakage, indicating that the com-

pass should be taken to a professional compass repair

facility for new gaskets. It is important to note that not

all commercially available compass fluids are compat-

ible with all compasses, especial compasses that were

original alcohol-filled. Very early fluid-filled com-

passes from the late 1800’s were filled with a mixture

of alcohol and water. Compass oil became more com-

monly used after the 1940s. If unsure about the type of

fluid, it is advisable to contact a professional before

adding any. 2. Check that the compass is centered on the vertical axis

of the binnacle. If it is, and the vessel is on an even

keel, there is no change of reading as the heeling mag-

net is raised and lowered in its tube. An adjustment

should be made to the gimbal rings if the compass is

off center. There should be no play in the position of

the compass once it is centered. 3. The lubber's line, too, should be checked to be sure it is

in line with the longitudinal axis of the vessel. This

can be done by sighting on the jackstaff if the compass

is on the centerline. 4. Check for free movement of gimbals. Clean any dust or

dirt from gimbal bearings and lubricate them as recom-

mended by the maker. 5. Check for magnetization of the quadrantal spheres by

moving them close to the compass and rotating them.

If the compass needle moves more than 2 degrees, the

spheres must be annealed to remove their magnetism.

Annealing consists of heating the spheres to a dull red

color in a non-magnetic area and allowing them to cool

slowly to ambient temperature. 6. Check for magnetization of the Flinders bar by invert-

ing it, preferably with the ship on an E/W heading. If

the compass needle moves more than 2 degrees the

Flinders bar must be annealed. 7. Synchronize the gyro repeaters with the master gyro so

courses can be steered accurately.

8. Assemble past documentation relating to the compass

and its adjustment. Have the ship’s degaussing folder

ready. 9. Ensure that every possible metallic object is stowed for

sea. All guns, doors, booms, and other movable gear

should be in its normal seagoing position. All gear nor-

mally turned on such as radios, radars, loudspeakers,

etc. should be on while swinging ship. 10. Vessel trim should be normal, and the vessel free from

list, so that no heeling error is present. 11. Have the International Code flags Oscar-Quebec ready

to fly.

Once underway to swing ship, the following procedures will expedite the process. Choose the best helmsman aboard and instruct him to steer each course as steadily and precisely as possible. Each course should be steered steadily for at least two minutes before any adjustments are made to remove Gaussin error. Be sure the gyro is set for the mean speed and latitude of the ship. All adjustment headings should be magnetic. The variation is applied to the desired magnetic heading, to determine the equivalent true heading. Any gyro error is then applied to determine the equivalent gyro heading. This is the method commonly used by vessels equipped with a reliable gyrocompass. Example: It is desired to place a vessel on magnetic cardinal and intercardinal headings during a compass adjustment, using the gyrocompass. The variation in this area is 6°W, and the gyro error is 1°E. Required: Headings per gyrocompass (pgc). Solution: For magnetic north the equivalent true heading is 000° - 6° =354° and the gyro heading is 354° -1° =353°. The same procedure is done for all remaining headings. Answer: Pgc headings: 353°, 038°, 083°, 128°, 173°, 218°, 263°, 308°.

Figure 926a summarizes all the various magnetic conditions in a ship, the types of deviation curves they create, the correctors for each effect, and headings on which each corrector is adjusted. When adjusting the compass, always apply the correctors symmetrically and as far away from the compass as possible. This preserves the uniformity of magnetic fields about the compass needle. Figure 926b discuss the mechanics of magnetic compass adjustment.

Occasionally, the permanent magnetic effects at the location of the compass are so large that they overcome the Earth's directive force (H in Figure 906). This condition will not only create sluggish and unsteady sectors, but may even freeze the compass to one reading or to one quadrant, regardless of the heading of the ship. Should the compass become so frozen, the polarity of the magnetism which must be attracting the compass needles is indicated; hence, correction may be effected simply by the application of permanent magnet correctors to neutralize this magnetism. For

[Figure 926a in Bowditch, Pub. No. 9: Summary of compass errors and adjustments.]

[Figure 926b in Bowditch, Pub. No. 9: Mechanics of magnetic compass adjustment.]

example, a ship whose compass is frozen to a north reading would require fore-and-aft B corrector magnets with the positive ends forward in order to neutralize the existing negative pole which attracted the compass. If made on an east heading, such an adjustment would be evident when the compass card was freed to indicate an east heading. Whenever such adjustments are made, the ship should be steered on a heading such that the unfreezing of the compass needles will be immediately evident.

The navigator (or compass adjuster if one is employed) should have a pelorus and a table of azimuths prepared for checking the gyro, but the gyrocompass will be the primary steering reference. Normally the adjuster will request courses and move the magnets as he or she feels necessary, a process much more of an intuitive art than a science. If a professional adjuster is not available, use the following sequence: 1. If there is a sea running, steer course 000° and adjust

the heeling magnet to decrease oscillations to a mini-

mum. 2. Come to course 090°. When steady on course 090°, for

at least two minutes, and adjust the fore-and-aft per-

manent magnets until the compass heading coincides

with the magnetic heading, thus removing ALL coeffi-

cient B on this heading. Use magnets in pairs, from

the bottom up, with the trays at the lowest point of

travel. When overcorrection occurs, remove the two

highest magnets and raise the trays until all deviation

has been removed. If two magnets overcorrect, use a

single magnet. It is not necessary to determine in

advance which direction the red ends should occupy,

for a mistake will be immediately apparent by an

increase in the deviation. 3. Come to a heading of 180° (or 000°) and when steady

for at least 2 minutes, adjust the athwartship perma-

nent magnets until the compass heading coincides with

the magnetic heading, thus removing ALL coefficient

C on this heading. Use the same technique as in step

2. 4. Steady on magnetic heading 270° (090° if 270° was

used in step 2) and remove half the deviation with the

fore-and-aft magnets.

5. Steady on magnetic heading 000° (180° if 000° was

used in step 3) and remove half the deviation with the

athwartship magnets. 6. Steady on 045° (or any intercardinal magnetic head-

ing) and adjust the position of the quadrantal spheres

until the compass heading coincides with the magnetic

heading, thus removing ALL coefficient D on this

heading. Leave the quadrantal correctors at equal dis-

tances from the compass. 7. Steady on 135° (or any intercardinal heading 90° from

the previous course) either and remove half the devia-

tion by adjusting the positions of the quadrantal correc-

tors, leaving them at equal distances from the compass. 8. Secure all correctors in their final positions and record

their number, size, positions, and orientation, as

appropriate, on the bottom of the deviation table form

(if a standard form such as that shown in Figure 910 is

used). 9. Swing ship for residual deviation. That is, determine

the remaining deviation on a number of headings at

approximately equal intervals. Every 15° is preferable,

but if the maximum deviation is small, every 45° (car-

dinal and intercardinal headings) may suffice. 10. If the vessel has degaussing, energize the degaussing

coils and repeat the swing. 11. Make a deviation table (Section 909) for each condi-

tion (degaussing off and on), giving values for head-

ings at 15° intervals if the maximum deviation is large

(more than about 2°), or at 45° intervals if the maxi-

mum deviation is small. Record values to the nearest

half degree.

The deviation of all compasses aboard the vessel can be determined from a single swing if the heading by each compass is recorded at the moment the magnetic direction is noted. If deviation of one compass is determined by means of a magnetic bearing or azimuth, the readings of this compass can then be used to establish the magnetic headings for determining the deviation of each other compass (see Handbook of Magnetic Compass Adjustment).

Compass adjustment is best made when the sea is relatively smooth, so that steady headings can be steered, and heeling error is absent. The setting of the heeling magnet can be checked later, preferably at the next time that the vessel is on a north or south heading and rolling moderately.

An analysis of deviation can be made either before or after adjustment. If this reveals an excessive amount of A (constant) deviation, the source of the error should be found and corrected (Section 922), if mechanical or mathematical. If an appreciable amount of E deviation is present, £Minks should be used and the spheres slewed. This is particularly to be anticipated for compasses which are not on the centerline.

The procedure outlined above is for initial adjustment aboard a new or radically modified vessel. Deviation on the heading being used for navigation should be checked from time to time and any important differences from the values shown on the deviation table should be investigated. At sea, it is good practice to compare the magnetic and gyrocompasses at intervals not exceeding half an hour. The error of one or both of these compasses should be checked twice a day when means are available. In pilot waters deviation checks should be made as convenient opportunities present themselves.

Whenever there is reason to question the accuracy of the deviation table, the ship should be swung at the first opportunity and a new table made up if there are significant changes in the old one. Suitable occasions for swinging ship would be after a deviation check indicates a significant error or after any event that might result in changes in the magnetic field of the vessel (Section 912). Intervals of swing should not exceed three months even when there is no reason to question the accuracy of the deviation table. If a swing indicates the presence of large maximum deviation, the compass should be readjusted. Unless there is reason to change it, the Flinders bar length should remain the same. Other adjustments are altered as needed, none of the correctors being removed at the beginning of adjustment. Whenever the vessel crosses the magnetic equator, the opportunity should be used to check the deviation on magnetic headings east and west. Any adjustment needed should be made by means of the fore-and-aft CB) magnets. Upon crossing the magnetic equator, the heeling magnet should be inverted.

The Flinders bar and quadrantal correctors should be checked for permanent magnetism at intervals of about a year, or more often if such magnetism is suspected.

A steel vessel has a certain amount of permanent magnetism in its “hard” iron and induced magnetism in its “soft” iron. Whenever two or more magnetic fields occupy the same space, the total field is the vector sum of the individual fields. Thus, near the magnetic field of a vessel, the total field is the combined total of the Earth’s field and the vessel’s field. Not only does the Earth’s field affect the vessel’s, the vessel’s field affects the Earth’s field in its immediate vicinity.

Since certain types of explosive mines are triggered by

the magnetic influence of a vessel passing near them, a vessel may use a degaussing system to minimize its magnetic field. One method of doing this is to neutralize each component of the field with an opposite field produced by electrical cables coiled around the vessel. These cables, when energized, counteract the permanent magnetism of the vessel, rendering it magnetically neutral. This has severe effects on magnetic compasses.

A unit sometimes used for measuring the strength of a magnetic field is the gauss. Reducing of the strength of a magnetic field decreases the number of gauss in that field. Hence, the process is called degaussing.

The magnetic field of the vessel is completely altered when the degaussing coils are energized, introducing large deviations in the magnetic compass. This deviation can be removed by introducing an equal and opposite force with energized coils near the compass. This is called compass compensation. When there is a possibility of confusion with compass adjustment to neutralize the effects of the natural magnetism of the vessel, the expression degaussing compensation is used. Since compensation may not be perfect, a small amount of deviation due to degaussing may remain on certain headings. This is the reason for swinging the ship with degaussing off and again with it on, and why there are two separate columns in the deviation table.

A simplified diagram of the distortion of the Earth’s magnetic field in the vicinity of a steel vessel is shown in Figure 928a. The field strength is directly proportional to the line spacing density. If a vessel passes over a device for detecting and recording the strength of the magnetic field, a certain pattern is traced. Figure 928b shows this pattern. Since the magnetic field of each vessel is different, each produces a distinctive trace. This distinctive trace is referred to as the vessel’s magnetic signature.

Several degaussing stations have been established in major ports to determine magnetic signatures and recommend the current adjustments needed in the various degaussing coils to render the vessel magnetically neutral. Since a vessel’s induced magnetism varies with heading and magnetic latitude, the current settings of the coils may sometimes need to be changed. A degaussing folder is provided to the vessel to indicate these changes and to document other pertinent information.

A vessel’s permanent magnetism changes somewhat with time and the magnetic history of the vessel. Therefore, the data in the degaussing folder should be checked periodically at the magnetic station.

For degaussing purposes, the total field of the vessel is divided into three components: (1) vertical, (2) horizontal fore-and-aft, and (3) horizontal athwartships. The positive 928a. (+) directions are considered downward, forward, and to port, respectively. These are the normal directions for a vessel headed north or east in north latitude.

[Figure 928a in Bowditch, Pub. No. 9: Simplified diagram of distortion of Earth’s magnetic field in the vicinity of a steel vessel.]

[Figure 928b in Bowditch, Pub. No. 9: A simplified signature of a vessel of Figure]

Each component is opposed by a separate degaussing field just strong enough to neutralize it. Ideally, when this has been done, the Earth’s field passes through the vessel smoothly and without distortion. The opposing degaussing fields are produced by direct current flowing in coils of wire. Each of the degaussing coils is placed so that the field it produces is directed to oppose one component of the ship’s field.

The number of coils installed depends upon the magnetic characteristics of the vessel, and the degree of safety desired. The ship’s permanent and induced magnetism may be neutralized separately so that control of induced magnetism can be varied as heading and latitude change, without disturbing the fields opposing the vessel’s permanent field. The principal coils employed are the following:

Main (M) coil. The M coil is horizontal and completely encircles the vessel, usually at or near the waterline. Its function is to oppose the vertical component of the vessel’s combined permanent and induced fields. Generally

the induced field predominates. Current in the M-coil is varied or reversed according to the change of the induced component of the vertical field with latitude.

Forecastle (F) and quarterdeck (Q) coils. The F and Q coils are placed horizontally just below the forward and after thirds (or quarters), respectively, of the weather deck. These coils, in which current can be individually adjusted, remove much of the fore-and-aft component of the ship’s permanent and induced fields. More commonly, the combined F and Q coils consist of two parts; one part the FP and QP coils, to take care of the permanent fore-and-aft field, and the other part, the FI and QI coils, to neutralize the induced fore-and-aft field. Generally, the forward and after coils of each type are connected in series, forming a split-coil installation and designated FP-QP coils and FI-QI coils. Current in the FP-QP coils is generally constant, but in the FI-QI coils is varied according to the heading and magnetic latitude of the vessel. In split-coil installations, the coil designations are often called simply the P-coil and I-coil.

Longitudinal (L) coil. Better control of the fore-and-aft components, but at greater installation expense, is provided by placing a series of vertical, athwartship coils along the length of the ship. It is the field, not the coils, which is longitudinal. Current in an L coil is varied as with the FI-QI coils. It is maximum on north and south headings, and zero on east and west headings.

Athwartship (A) coil. The A coil is in a vertical fore-and-aft plane, thus producing a horizontal athwartship field which neutralizes the athwartship component of the vessel’s field. In most vessels, this component of the permanent field is small and can be ignored. Since the A-coil neutralizes the induced field, primarily, the current is changed with magnetic latitude and with heading, maximum on east or west headings, and zero on north or south headings.

The strength and direction of the current in each coil is indicated and adjusted at a control panel accessible to the navigator. Current may be controlled directly by rheostats at the control panel or remotely by push buttons which operate rheostats in the engine room.

Appropriate values of the current in each coil are determined at a degaussing station, where the various currents are adjusted until the vessel’s magnetic signature is made as flat as possible. Recommended current values and directions for all headings and magnetic latitudes are set forth in the vessel’s degaussing folder. This document is normally kept by the navigator, who must see that the recommended settings are maintained whenever the degaussing system is energized.

Unless the degaussing system is properly secured, residual magnetism may remain in the vessel. During degaussing compensation and at other times, as recommended in the degaussing folder, the “reversal” method is used. The steps in the reversal process are as follows:

1. Start with maximum degaussing current used since

the system was last energized.

2. Decrease current to zero and increase it in the oppo-

site direction to the same value as in step 1.

3. Decrease the current to zero and increase it to three-

fourths maximum value in the original direction.

4. Decrease the current to zero and increase it to one-

half maximum value in the opposite direction.

5. Decrease the current to zero and increase it to one-

fourth maximum value in the original direction.

6. Decrease the current to zero and increase it to one-

eighth maximum value in the opposite direction.

7. Decrease the current to zero and open switch.

In some instances, degaussing can be made more effective by changing the magnetic characteristics of the vessel by a process known as deperming. Heavy cables are wound around the vessel in an athwartship direction, forming vertical loops around the longitudinal axis of the vessel. The loops are run beneath the keel, up the sides, and over the top of the weather deck at closely spaced equal intervals along the entire length of the vessel. Predetermined values of direct current are then passed through the coils. When the desired magnetic characteristics have been acquired, the cables are removed.

A vessel which does not have degaussing coils, or which has a degaussing system that is inoperative, can be given some temporary protection by a process known as flashing. A horizontal coil is placed around the outside of the vessel and energized with large predetermined values of direct current. When the vessel has acquired a vertical field of permanent magnetism of the correct magnitude and polarity to reduce to a minimum the resultant field below the vessel for the particular magnetic latitude involved, the cable is removed. This type protection is not as satisfactory as that provided by degaussing coils because it is not adjustable for various headings and magnetic latitudes, and also because the vessel’s magnetism slowly readjusts following treatment.

During magnetic treatment all magnetic compasses and Flinders bars should be removed from the ship. Permanent adjusting magnets and quadrantal correctors are not materially affected, and need not be removed. If it is impractical to remove a compass, the cables used for magnetic treatment should be kept as far as practical from it.

The degaussing of ships for protection against magnetic influence mines creates additional effects upon magnetic compasses, which are somewhat different from the permanent and induced magnetic effects. The degaussing effects are electromagnetic, and depend on:

1. Number and type of degaussing coils installed.

2. Magnetic strength and polarity of the degaussing

coils.

3. Relative location of the different degaussing coils

with respect to the binnacle.

4. Presence of masses of steel, which would tend to

concentrate or distort magnetic fields in the vicinity

of the binnacle.

5. The fact that degaussing coils are operated inter-

mittently, with variable current values, and with

different polarities, as dictated by necessary

degaussing conditions.

The magnetic fields created by the degaussing coils would render the vessel’s magnetic compasses useless unless compensated. This is accomplished by subjecting the compass to compensating fields along three mutually perpendicular axes. These fields are provided by small compensating coils adjacent to the compass. In nearly all installations, one of these coils, the heeling coil, is horizontal and on the same plane as the compass card, providing a vertical compensating field. Current in the heeling coil is adjusted until the vertical component of the total degaussing field is neutralized. The other compensating coils provide horizontal fields perpendicular to each other. Current is varied in these coils until their resultant field is equal and opposite to the horizontal component of the degaussing field. In early installations, these horizontal fields were directed fore-and-aft and athwartships by placing the coils around the Flinders bar and the quadrantal spheres. Compactness and other advantages are gained by placing the coils on perpendicular axes extending 045°-225° and 315°- 135° relative to the heading. A frequently used compensating installation, called the type K, is shown in Figure 933. It consists of a heeling coil extending completely around the top of the binnacle, four intercardinal coils, and three control boxes. The intercardinal coils are named for their positions relative to the compass when the vessel is on a heading of north, and also for the compass headings on which the current in the coils is adjusted to the correct amount for compensation. The NE-SW coils operate together as one set, and the NW-SE coils operate as another. One control box is provided for each set, and one for the heeling coil.

The compass compensating coils are connected to the power supply of the degaussing coils, and the currents passing through the compensating coils are adjusted by series resistances so that the compensating field is equal to the degaussing field. Thus, a change in the degaussing currents is accompanied by a proportional change in the compensating currents. Each coil has a separate winding for each degaussing circuit it compensates.

Degaussing compensation is carried out while the vessel is moored at the shipyard where the degaussing coils are installed. This process is usually carried out by civilian professionals, using the following procedure:

[Figure 933 in Bowditch, Pub. No. 9: Type K degaussing compensation installation.]

Step 1. The compass is removed from its binnacle and a dip needle is installed in its place. The M coil and heeling coil are then energized, and the current in the heeling coil is adjusted until the dip needle indicates the correct value for the magnetic latitude of the vessel. The system is then secured by the reversing process.

Step 2. The compass is replaced in the binnacle. With auxiliary magnets, the compass card is deflected until the compass magnets are parallel to one of the compensating coils or set of coils used to produce a horizontal field. The compass magnets are then perpendicular to the field produced by that coil. One of the degaussing circuits producing a horizontal field, and its compensating winding, are then energized, and the current in the compensating winding is adjusted until the compass reading returns to the value it had before the degaussing circuit was energized. The system is then secured by the reversing process. The process is repeated with each additional circuit used to create a horizontal field. The auxiliary magnets are then removed.

Step 3. The auxiliary magnets are placed so that the compass magnets are parallel to the other compensating coils or set of coils used to produce a horizontal field. The

procedure of step 2 is then repeated for each circuit producing a horizontal field.

When the vessel gets under way, it proceeds to a suitable maneuvering area. The vessel is then steered so that the compass magnets are parallel first to one compensating coil or set of coils, and then the other. Any needed adjustment is made in the compensating circuits to reduce the error to a minimum. The vessel is then swung for residual deviation, first with degaussing off and then with degaussing on, and the correct current settings determined for each heading at the magnetic latitude of the vessel. From the values thus obtained, the “DG OFF” and “DG ON” columns of the deviation table are filled in. If the results indicate satisfactory compensation, a record is made of the degaussing coil settings and the resistance, voltages, and currents in the compensating coil circuits. The control boxes are then secured.

Under normal operating conditions, the settings do not need to be changed unless changes are made in the degaussing system, or unless an alteration is made in the length of the Flinders bar or the setting of the quadrantal spheres. However, it is possible for a ground to occur in the coils or control box if the circuits are not adequately protected from moisture. If this occurs, it should be reflected by a change in deviation with degaussing on, or by a decreased installation resistance. Under these conditions, compensation should be done again. If the compass will be used with degaussing on before the ship can be returned to a shipyard where the compensation can be made by experienced personnel, the compensation should be made at sea on the actual headings needed, rather than by deflection of the compass needles by magnets. More complete information related to this process is given in the degaussing folder.

If a vessel has been given magnetic treatment, its magnetic properties have changed, necessitating readjustment of each magnetic compass. This is best delayed for several days to permit the magnetic characteristics of the vessel to settle. If compensation cannot be delayed, the vessel should be swung again for residual deviation after a few days. Degaussing compensation should not be made until after compass adjustment has been completed.

A gyroscope consists of a spinning wheel or rotor contained within gimbals which permit movement about three mutually perpendicular axes, known as the horizontal axis, the vertical axis, and the spin axis. When spun rapidly, assuming that friction is not considered, the gyroscope develops gyroscopic inertia, tending to remain spinning in the same plane indefinitely. The amount of gyroscopic inertia depends on the angular velocity, mass, and radius of the wheel or rotor.

When a force is applied to change alignment of the spin axis of a gyroscope, the resultant motion is perpendicular to the direction of the force. This tendency is known as precession. A force applied to the center of gravity of the gyroscope will move the entire system in the direction of the force. Only a force that tends to change the axis of rotation produces precession.

If a gyroscope is placed at the equator with its spin axis pointing east-west, as the Earth turns on its axis, gyroscopic inertia will tend to keep the plane of rotation constant. To the observer, it is the gyroscope which is seen to rotate, not the Earth. This effect is called the horizontal Earth rate, and is maximum at the equator and zero at the poles. At points between, it is equal to the cosine of the latitude.

If the gyro is placed at a geographic pole with its spin axis horizontal, it will appear to rotate about its vertical axis. This is the vertical Earth rate. At all points between the equator and the poles, the gyro appears to turn partly about its horizontal and partly about its vertical axis, being affected by both horizontal and vertical Earth rates. In order to visualize these effects, remember that the gyro, at whatever latitude it is placed, is remaining aligned in space while the Earth moves beneath it.

The gyrocompass depends upon four natural phenomena: gyroscopic inertia, precession, Earth’s rotation, and gravity. To make a gyroscope into a gyrocompass, the wheel or rotor is mounted in a sphere, called the gyrosphere, and the sphere is then supported in a vertical ring. The whole is mounted on a base called the phantom. The gyroscope in a gyrocompass can be pendulous or non-pendulous, according to design. The rotor may weigh as little as half a kilogram to over 25 kg.

To make it seek and maintain true north, three things are necessary. First, the gyro must be made to stay on the plane of the meridian. Second, it must be made to remain horizontal. Third, it must stay in this position once it reaches horizontal regardless what the vessel on which it is mounted does or where it goes on the Earth. To make it seek the meridian, a weight is added to the bottom of the vertical ring, causing it to swing on its vertical axis, and thus seek to align itself horizontally. It will tend to oscillate, so a second weight is added to the side of the sphere in which the rotor is contained, which dampens the oscillations until the gyro stays on the meridian. With these two weights, the only possible position of equilibrium is on the meridian with its spin axis horizontal.

To make the gyro seek north, a system of reservoirs filled with mercury, known as mercury ballistics, is used to apply a force against the spin axis. The ballistics, usually

four in number, are placed so that their centers of gravity exactly coincide with the CG of the gyroscope. Precession then causes the spin axis to trace an ellipse, one ellipse taking about 84 minutes to complete. (This is the period of oscillation of a pendulum with an arm equal to the radius of the Earth.) To dampen this oscillation the force is applied, not in the vertical plane, but slightly to the east of the vertical plane. This causes the spin axis to trace a spiral instead of an ellipse and eventually settle on the meridian pointing north.

The total of the all the combined errors of the gyrocompass is called gyro error and is expressed in degrees E or W, just like variation and deviation. But gyro error, unlike magnetic compass error, and being independent of Earth’s magnetic field, will be constant in one direction; that is, an error of one degree east will apply to all bearings all around the compass.

The errors to which a gyrocompass is subject are speed error, latitude error, ballistic deflection error, ballistic damping error, quadrantal error, and gimballing error. Additional errors may be introduced by a malfunction or incorrect alignment with the centerline of the vessel.

Speed error is caused by the fact that a gyrocompass only moves directly east or west when it is stationary (on the rotating Earth) or placed on a vessel moving exactly east or west. Any movement to the north or south will cause the compass to trace a path which is actually a function of the speed of advance and the amount of northerly or southerly heading. This causes the compass to tend to settle a bit off true north. This error is westerly if the vessel’s course is northerly, and easterly if the course is southerly. Its magnitude depends on the vessel’s speed, course, and latitude. This error can be corrected internally by means of a cosine cam mounted on the underside of the azimuth gear, which removes most of the error. Any remaining error is minor in amount and can be disregarded.

Tangent latitude error is a property only of gyros with mercury ballistics, and is easterly in north latitudes and westerly in south latitudes. This error is also corrected internally, by offsetting the lubber’s line or with a small movable weight attached to the casing.

Ballistic deflection error occurs when there is a marked change in the north-south component of the speed. East-west accelerations have no effect. A change of course or speed also results in speed error in the opposite direction, and the two tend to cancel each other if the compass is properly designed. This aspect of design involves slightly offsetting the ballistics according to the operating latitude, upon which the correction is dependent. As latitude changes, the error becomes apparent, but can be minimized by adjusting the offset.

Ballistic damping error is a temporary oscillation introduced by changes in course or speed. During a change in course or speed, the mercury in the ballistic is subjected to centrifugal and acceleration/deceleration forces. This causes a torquing of the spin axis and subsequent error in the compass reading. Slow changes do not introduce enough error to be a problem, but rapid changes will. This error is counteracted by changing the position of the ballistics so that the true vertical axis is centered, thus not subject to error, but only when certain rates of turn or acceleration are exceeded.

Quadrantal error has two causes. The first occurs if the center of gravity of the gyro is not exactly centered in the phantom. This causes the gyro to tend to swing along its heavy axis as the vessel rolls in the sea. It is minimized by adding weight so that the mass is the same in all directions from the center. Without a long axis of weight, there is no tendency to swing in one particular direction. The second source of quadrantal error is more difficult to eliminate. As a vessel rolls in the sea, the apparent vertical axis is displaced, first to one side and then the other. The vertical axis of the gyro tends to align itself with the apparent vertical. On northerly or southerly courses, and on easterly or westerly courses, the compass precesses equally to both sides and the resulting error is zero. On intercardinal courses, the N-S and E-W precessions are additive, and a persistent error is introduced, which changes direction in different quadrants. This error is corrected by use of a second gyroscope called a floating ballistic, which stabilizes the mercury ballistic as the vessel rolls, eliminating the error. Another method is to use two gyros for the directive element, which tend to precess in opposite directions, neutralizing the error.

Gimballing error is caused by taking readings from the compass card when it is tilted from the horizontal plane. It applies to the compass itself and to all repeaters. To minimize this error, the outer ring of the gimbal of each repeater should be installed in alignment with the fore-and-aft line of the vessel. Of course, the lubber’s line must be exactly centered as well.

Since a gyrocompass is not influenced by magnetism, it is not subject to variation or deviation. Any error is constant and equal around the horizon, and can often be reduced to less than one degree, thus effectively eliminating it altogether. Unlike a magnetic compass, it can output a signal to repeaters spaced around the vessel at critical positions.

But it also requires a constant source of stable electrical power, and if power is lost, it requires several hours to settle on the meridian again before it can be used. This period can be reduced by aligning the compass with the meridian before turning on the power.

The directive force of a gyrocompass depends on the amount of precession to which it is subject, which in turn is dependent on latitude. Thus the directive force is maximum

at the equator and decreases to zero at the poles. Vessels operating in high latitudes must construct error curves based on latitudes because the errors at high latitudes eventually overcome the ability of the compass to correct them.

The gyrocompass is typically located below decks as close as possible to the center of roll, pitch and yaw of the ship, thus minimizing errors caused by the ship’s motion. Repeaters are located at convenient places throughout the ship, such as at the helm for steering, on the bridge wings for taking bearings, in after steering for emergency steering, and other places. The output can also be used to drive course recorders, autopilot systems, plotters, fire control systems, and stabilized radars. The repeaters should be checked regularly against the master to ensure they are all in alignment. The repeaters on the bridge wing used for taking bearings will likely be equipped with removable bearing circles, azimuth circles, and telescopic alidades, which allow one to sight a distant object and see its exact gyrocompass bearing.

The magnetic compass has considerable limitations, chiefly that of being unable to isolate the Earth’s magnetic field from all others close enough to influence it. It also indicates magnetic north, whereas the mariner is most interested in true north. Most of the work involved with compensating a traditional magnetic compass involves neutralizing magnetic influences other than the Earth’s, a complicated and inexact process often involving more art than science. Residual error is almost always present even after compensation. Degaussing complicates the situation immensely.

The electro-mechanical gyrocompass has been the standard steering and navigational compass since the early 20th century, and has provided several generations of mariners a stable and reliable heading and bearing reference. However, it too has limitations: It is a large, expensive, heavy, sensitive device that must be mounted according to rather strict limitations. It requires a stable and uninterrupted supply of electrical power; it is sensitive to shock, vibration, and environmental changes; and it needs several hours to settle after initialization.

Fortunately, several new technologies have been developed which promise to greatly reduce or eliminate the complications brought on by the limitations of both the mechanical gyroscope and traditional magnetic compasses. Sometimes referred to as “electronic compasses,” the digital flux gate magnetic compass and the ring laser gyrocompass are two such devices. They have the following advantages:

1. Solid state electronics, no moving parts

2. Operation at very low power

3. Easy backup power from independent sources

4. Standardized digital output

5. Zero friction, drift, or wear

6. Compact, lightweight, and inexpensive

7. Rapid start-up and self-alignment

8. Low sensitivity to vibration, shock, and tempera-

ture changes

9. Self-correcting

Both types are being installed in many vessels as the primary directional reference, enabling the decommissioning of the traditional magnetic compasses and the avoidance of periodic compensation and maintenance.

The most widely used sensor for digital compasses is the flux-gate magnetometer, developed around 1928. Initially it was used for detecting submarines, for geophysical prospecting, and airborne mapping of Earth’s magnetic fields.

The most common type, called the second harmonic device, incorporates two coils, a primary and a secondary, both wrapped around a single highly permeable ferromagnetic core. In the presence of an external magnetic field, the core’s magnetic induction changes. A signal applied to the primary winding causes the core to oscillate. The secondary winding emits a signal that is induced through the core from the primary winding. This induced signal is affected by changes in the permeability of the core and appears as an amplitude variation in the output of the sensing coil. The signal is then demodulated with a phase-sensitive detector and filtered to retrieve the magnetic field value. After being converted to a standardized digital format, the data can be output to numerous remote devices, including steering compasses, bearing compasses, emergency steering stations, and autopilots.

Since the influence of a ship’s inherent magnetism is inversely proportional to the square of the distance to the compass, it is logical that if the compass could be located at some distance from the ship, the influence of the ship’s magnetic field could be greatly reduced. One advantage of the flux gate compass is that the sensor can be located remotely from the readout device, allowing it to be placed at a position as far as possible from the hull and its contents, such as high up on a mast, the ideal place for most vessels.

A further advantage is that the digital signal can be processed mathematically, and algorithms written which can correct for observed deviation once the deviation table has been determined. Further, the “table,” in digital format, can be found by merely steering the vessel in a full circle. Algorithms then determine and apply corrections that effectively

flatten the usual sine wave pattern of deviation. The theoretical result is zero observed compass deviation.

Should there be an index error (which has the effect of skewing the entire sine wave below or above the zero degree axis of the deviation curve) this can be corrected with an index correction applied to all the readings. This problem is largely confined to asymmetric installations such as aircraft carriers. Similarly, a correction for variation can be applied, and with GPS input (so the system knows where it is with respect to the isogonic map) the variation correction can be applied automatically, thus rendering the output in true degrees, corrected for both deviation and variation.

It is important to remember that a flux gate compass is still a magnetic compass, and that it will be influenced by large changes to the ship’s magnetic field. Compensation should be accomplished after every such change. Fortunately, as noted, compensation involves merely steering the vessel in a circle in accordance with the manufacturer’s recommendations.

Flux-gate compasses from different manufacturers share some similar operational modes. Most of them will have the following:

SET COURSE MODE: A course can be set and “remembered” by the system, which then provides the helmsman a graphic steering aid, enabling him to see if the ship’s head is right or left of the set course, as if on a digital “highway.” Normal compass operation continues in the background.

DISPLAY RESPONSE DAMPING: In this mode, a switch is used to change the rate of damping and update of the display in response to changes in sea condition and vessel speed.

AUTO-COMPENSATION: This mode is used to determine the deviation curve for the vessel as it steams in a complete circle. The system will then automatically compute correction factors to apply around the entire compass, resulting in zero deviation at any given heading. This should be done after every significant change in the magnetic signature of the ship, and within 24 hours of entering restricted waters.

CONTINUOUS AUTO-COMPENSATION: This mode, which should normally be turned OFF in restricted waters and ON at sea, runs the compensation algorithm each time the ship completes a 360 degree turn in two minutes. A warning flashes on the display in the OFF mode.

PRE-SET VARIATION: In effect an index correction, pre-set variation allows the application of magnetic variation to the heading, resulting in a true output (assuming the unit has been properly compensated and aligned). Since variation changes according to one’s location on the Earth, it must be changed periodically to agree with the charted variation unless GPS input is provided. The GPS position input is used in an algorithm which computes the variation for the area and automatically corrects the readout.

U.S. Naval policy approves the use of flux gate compasses and the lay-up, but not the removal of the traditional binnacle mounted compass, which should be clearly marked as “Out of Commission” once an approved flux gate compass has been properly installed and tested.

Optical gyroscope use can be classified under two major types: ring laser gyroscope (RLG) and fiber optic gyroscope (FOG). Both of these sensors make use of French Physicist Georges Sagnac's observation of rotation relative to inertial space thus bearing the name, Sagnac Effect. This principle states that if two beams of light are sent in opposite directions around a “ring” or polyhedron and steered so as to meet and combine, a standing wave will form around the ring. If the wave is observed from any point, and that point is then moved along the perimeter of the ring, the wave form will change in direct relationship to the direction and velocity of movement. While Sagnac's work was in pursuit of identifying the “ether” that was postulated in the late 19th century as the medium that supported the propagation of light waves, the effect that he predicted and measured was found to be rooted in general relativity. Sagnac is given significant credit, because he was the first person to report the experimental observation for a polygonal interferometer mounted on a turn-table. The practical realization of a Sagnac interferometer as a rotation sensor came only after the invention of the laser and other optical components. The Sagnac interferometer can be implemented in a resonant cavity as in the case of the RLG or in a non-resonant interferometer configuration of which the commercially available FOG is an example. While it is true that a FOG can be configured as a resonant cavity, this type of device has not yet achieved commercial success and will not be described herein.

The ring laser had its beginnings in England, where in the 1890’s two scientists, Joseph Larmor and Sir Oliver Lodge (also one of the pioneers of radio), debated the possibility of measuring rotation by a ring interferometer. Following Sagnac's 1913 observation. It wasn’t until 1963 that D. T. M. Davis Jr. and W. Macek of Sperry-Rand Corporation tested and refined the concept into a useful research device. Initially, mirrors were used to direct light around a square or rectangular pattern. But such mirrors must be made and adjusted to exceptionally close tolerances to allow useful output, and must operate in a vacuum for best effect. Multilayer dielectric mirrors with a reflectivity of 99.9999 percent were developed. The invention of laser light sources and fiber-optics has enabled the production of small, light, and dependable ring laser gyros. Mirror-based devices continue to be used in physics research.

The ring laser gyrocompass (RLG) operates by measuring laser-generated light waves traveling around a fiber-optic ring. A beam splitter divides a beam of light into two counter-rotating waves, which then travel around the fiber-optic ring in opposite directions. The beams are then recombined and sent to an output detector. In the absence of rotation, the path lengths will be the same and the beams will recombine in phase. If the device has rotated, there will be a difference in the length of the paths of the two beams, resulting in a detectable phase difference in the combined signal. The signal will vary in amplitude depending on the amount of the phase shift. The amplitude is thus a measurement of the phase shift, and consequently, the rotation rate. This signal is processed into a digital readout in degrees. This readout, being digital, can then be sent to a variety of devices which need heading information, such as helm, autopilot, and electronic chart systems.

A single ring laser gyroscope can be used to provide a one-dimensional rotational reference, exactly what a compass needs. The usefulness of ring laser gyrocompasses is clear in that they share many of the same characteristics of flux gate compasses. They are compact, light, inexpensive, accurate, dependable, and robust. The ring laser device is also unaffected by magnetic influences that would certainly impact the traditional compass, and even such that might adversely affect a remotely mounted flux gate compass.

Ring laser gyroscopes can also serve as the stable elements in an inertial guidance system, using three gyros to represent the three degrees of freedom, thus providing both directional and position information. The principle of operation is the same as for mechanical inertial navigation devices, in that a single gyro can measure any rotation about its own axis. This implies that its orientation in space about its own axis will be known at all times. Three gyros arranged along three axes each at 90 degrees to the others can measure accelerations in three dimensional space, and thus track movement over time.

Inertial navigation systems based on ring lasers have been used in aircraft for a number of years, and are becoming increasingly common in maritime applications. Uses include navigation, radar and fire control systems, precise weapons stabilization, and stabilization of directional sensors such as satellite antennas.

A non-resonant Sagnac interferometer is used as the basis of what is referred to as the interferometric fiber optic gyro (IFOG) often shortened to simply FOG. Resonant fiber optic gyros have been developed but at this time have not become commercially practical.

The development of the FOG required its own enabling technology, namely low loss, single mode optical fibers that became available in the mid-1970s. Vali and Shorthill first proposed the fiber optic gyro in 1975. The FOG is composed of a light source, a coupler, a fiber coil and a detector. Light is launched from the source and coupled through a fiber optic coil in both the clockwise and counter-clockwise directions. Based on the Sagnac effect, the optical path seen by the two beams interfere and the intensity detected is a function of the phase difference and hence the angular rate of the gyro.

The interferometric architecture of the FOG has a poor sensitivity at low rates as due to cosine nature of the phase difference and near zero phase at the peak of the cosine function. To achieve better sensitivity, it is necessary to modulate the light which is accomplished in modern FOG configurations through the use of an electro-optic phase modulator. Light passing through the modulator is phase shifted in proportion to the applied voltage. Differential phase shifts between the clockwise and counter-clockwise beams are sustained for only one transit time of the light through the coil and thus the modulation must be applied every transit time.

Phase modulation of the light improves the sensitivity at low angular rates. However, the high rate non-linearity, light intensity variation, photo-detector sensitivity, preamp gain and background intensity all affect the open loop output of the FOG. For this reason, it is important for higher accuracy and greater dynamic range to operate the FOG in a closed loop fashion. The same device that accomplished the phase shifting of the light is typically used to close the loop in the FOG. Because the angular rate sensed by the FOG appears as an interferometer phase shift, it may be nulled out by applying a phase rebalance in additional to the phase shift with the modulator. A complication arises due to the fact that the modulator can produce a differential phase shift between clockwise and counter-clockwise light beams only during the transit time of the light through the fiber coil and a given angular rate produces a persistent phase shift between the light beams. To achieve phase nulling, it is necessary to increase the phase applied at every transit time. A periodic reset is required when the maximum voltage that is supplied to the modulator is reached. The magnitude of this rest must be exactly 2(pi) to avoid introducing a gyro error.

The sensitivity of the FOG is theoretically limited by the photon shot noise which emerges from the statistical distribution of energy of the photon impinging on the photo detector. While the Sagnac sensitivity increases with the length of the fiber, the photon energy decreases with fiber length due to attenuation of the light as it travels through the fiber. Thus a tradeoff must be done when choosing the size of the FOG for a given application. Errors in the FOG output arise through a number of sources. Rayleigh backscattering is the dominant error source in the FOG. This comes about when backscatter of one beam interferes with the other light beam. Low coherence light sources are used to reduce this effect. Two popular light sources for FOGs are the superluminescent diode (SLD) and the broadband fiber source (BFS). The change in the index of refraction of the fiber as a function of the intensity of the light induces an

error through the optical Kerr effect. This effect is also reduced through the use of low coherence light sources. The thermal gradient effect due to uneven heating of the fiber coil is typically the major challenge to achieving required performance in the FOG. The light beam will experience propagation delays due to temperature differences along the length of the fiber. These propagation delays are not the same for the two counter propagating beams which results in a gyro error. Sophisticated coil winding designs, such as quadrupole or octopole can help to minimize this effect. Finally, birefringence effects, from the fiber, can result in errors; good control of the light polarization if required.

The FOG has gained a wide acceptance and is found in a wide variety of applications from undersea to outer space. The performance of the FOG as a gyro is dependent primarily on the diameter of the fiber coil and the length of the fiber. Thus the size of the FOG can vary significantly from coil diameters of approximately an inch with less than 100 meters of fiber to diameters of several inches containing multiple kilometers of fiber depending on the application and performance requirement. The FOG has been shown to have better reliability than that of the RLG and further eliminates the need for any high voltages that are required to initiate and maintain the plasma in the RLG. For these reasons, the marketplace is moving from RLG to FOG. Also, while there are only a few manufacturers of RLG left around the globe, and it is estimated that there may be more than a dozen manufacturers of FOG based systems worldwide.

The Hemispherical Resonator Gyro (HRG) belongs to a class of gyros referred to as Coriolis Vibratory Gyros (CVG). The physics of the HRG is based on the forces arising from the Coriolis Effect which describes the motion of a body undergoing uniform motion in a rotating frame of reference. The HRG was conceived in 1890 when physicist G.H. Bryan struck a wineglass, making an interesting discovery of how the tone from a glass behaved when it was rotated about its stem. To understand the operation of an HRG, consider a thin hemispherical shell, although other suitable configurations can also be used, such as cylindrical, whereas the rim of the shell can be made to vibrate by applying appropriate force and technique. The lowest fundamental mode is characterized by four nodes and four antinodes of vibration. The rim of the shell will then have a radial velocity component at the antinodes and a tangential velocity component at the nodes.

When the shell is subject to an angular rate about its sensitive axis, which is perpendicular to the plane of the standing wave pattern, Coriolis forces are generated. These forces are proportional to the applied angular rate and are orthogonal to both the applied rate vector and the shell's velocity vectors. The result of these forces is standing wave whose nodes and antinodes are now shifted with respect to the original pattern. The superposition of the original wave and the new orthogonal wave result in a phenomenon in which the resultant wave rotates relative to its own casing and to inertial space through an angle that is proportional to the angular rotation of the gyro case. The resultant pattern precesses in the opposite sense. The angular gain factor is a function of the geometrical design and provides a very stable gyro scale factor. The electrical sensing of pattern is typically accomplished through capacitive elements that are implemented between the shell and another element separated from the shell by a suitable gap.

The HRG is attractive as a result of the very low noise figure, one or two order of magnitude better than what can be achieved with either an RLG or FOB of comparable design. Furthermore, due to the simplistic nature of the sending element, the HRG has realized extraordinary reliability with tens of millions of failure-free operations exhibited in space applications. The challenges with the HRG are also related to the simplicity of the sensing element since that results in complexity of the electronics required for operation, HRG electronic functions are broadly grouped into the following categories:

1. Reference phase generation and frequency control

2. Amplitude control

3. Pattern angle readout

4. Quadrature suppression

5. Force-to-rebalance mode of operation

6. Whole angle mode of operation

In the force-to-rebalance mode of operation, the nodes and antinodes are capacitively held in place. The capacitive force required to do this is a measure of the angular rate experienced by the HRG. In this mode of operation, the bias errors can be minimized; however, the gyro scale factor is a function of the electronics and temporal trends in scale factor are observed as the electronics age. The force-to-rebalance mode is limited by the available capacitive forcing. This limits the angular rate range typically to less than 100 deg/sec for practical devices. In the whole angle mode of operation, the pattern is allowed to precess and so the angular rate range is limited only by the processing electronics. As mentioned, the geometric scale factor is very stable and hence scale factor performance of the HRG is excellent in the whole angle mode; however, the bias performance tends not be as good as in the force-to-rebalance mode.

Ship’s heading is the angle, expressed in degrees clockwise from north, of the ship’s fore-and-aft line with respect to the true meridian or the magnetic meridian. When this angle is referred to the true meridian, it is called a true heading. When this angle is referred to the magnetic meridian, it is called a magnetic heading. Heading, as indicated on a particular compass, is termed the ship’s compass heading by that compass. It is essential to specify every heading as true (T), magnetic (M), or compass. Two abbreviations simplify recording of compass directions. The abbreviation PGC refers to “per gyro compass,” and PSC refers to “per steering compass.” The steering compass is the one being used by the helmsman or autopilot, regardless of type.

Variation is the angular measure between the magnetic meridian and the true meridian at a given location. If the northerly part of the magnetic meridian lies to the right of the true meridian, the variation is easterly. Conversely, if this part is to the left of the true meridian, the variation is westerly. The local variation and its small annual change are noted on the compass rose of all navigational charts. Thus the true and magnetic headings of a ship differ by the local variation.

As previously explained, a ship’s magnetic influence will generally cause the compass needle to deflect from the magnetic meridian. This angle of deflection is called deviation. If the north end of the needle points east of the magnetic meridian, the deviation is easterly; if it points west of the magnetic meridian, the deviation is westerly.

A summary of heading relationships follows:

1. Deviation is the difference between the compass

heading and the magnetic heading.

2. Variation is the difference between the magnetic

heading and the true heading.

3. The algebraic sum of deviation and variation is the

compass error.

The following simple rules will assist in correcting and uncorrecting the compass:

1. Compass least, error east; compass best, error west.

2. When correcting, add easterly errors, subtract west-

erly errors (Remember: “Correcting Add East”).

3. When uncorrecting, subtract easterly errors, add

westerly errors. Some typical correction operations follow: Compass Deviation Magnetic Variation True -> +E, -W

358° 5°E 003° 6°E 009°

120° 1°W 119° 3°E 122°

180° 6°E 186° 8°W 178°

240° 5°W 235° 7°W 228° +W, -E <-

[Figure 946 in Bowditch, Pub. No. 9: Examples of compass correcting.]

Use the memory aid “Can Dead Men Vote Twice, At Elections” to remember the conversion process (Compass, Deviation, Magnetic, Variation, True; Add East). When converting compass heading to true heading, add easterly deviations and variations and subtract westerly deviations and variations. “Truly Valiant Marines Don’t Cry at Weddings” is another phrase used to remember compass correction where Westerly error is added.

The same rules apply to correcting gyrocompass errors, although gyro errors always apply in the same direction. That is, they are E or W all around the compass.

Complete familiarity with the correcting of compasses is essential for navigation by magnetic or gyro compass. Professional navigators who deal with them continually can correct them in their heads quickly and accurately.

The question (Q171 #17). The chart indicates the variation was 3°45'E in 1988, and the annual change is increasing 6'. If you use the chart in 1991 how much variation should you apply?

The working.

1. The compass rose gives the variation at the chart's own date and how much it changes each year. Here: 3°45'E in 1988, changing 6' a year.
2. Count the years from the chart's date to the year you are using it: 1991 − 1988 = 3 years.
3. Multiply: 3 years × 6' = 18'.
4. 'Increasing' means increasing in the direction the variation already has. This one is easterly, so it grows easterly: 3°45'E + 18' = 4°03'E.

Answer: 4°03'E.

What the paper is testing. Two traps. 45' + 18' is 63', which is 1°03' and not 0°63' — carry the minutes over into degrees. And 'increasing' is not 'easterly': on a chart whose variation is westerly, an increasing change makes it MORE westerly. The paper's 3°27'E is what you get by subtracting the 18' instead of adding it, and 3°27'W by getting the hemisphere wrong as well.

The question (Q171 #42). Your vessel is steering course 149°psc, variation for the area is 13°E, and deviation is 4°E. The wind is from the northeast, producing a 4° leeway. What true course are you making good?

The working.

1. Correcting — going from the compass toward true — easterly errors are added. Start at the compass course: 149°psc.
2. Apply the deviation first, because deviation is the error between the compass and the magnetic meridian: 149 + 4 = 153° magnetic.
3. Then the variation, which is the error between magnetic and true: 153 + 13 = 166°T. That is the vessel's HEADING.
4. Leeway is not a compass error at all: it is the angle between the heading and the track the vessel actually makes over the ground, and it is always to leeward.
5. The wind is from the northeast, 045°T. On a heading of 166°T that is 239° relative — the port quarter. Wind on the port side pushes the vessel to starboard, so the track lies to the RIGHT of the heading.
6. 166 + 4 = 170°T made good.

Answer: 170°T.

What the paper is testing. The side the wind is on is the whole question. Applying the leeway without working it out gives 162°T here — which is one of the four options. Work out the wind's bearing relative to the heading every time: 0–180 relative is the starboard side and pushes you to port, 180–360 is the port side and pushes you to starboard.

The question (Q171 #43). Your vessel is steering course 299°psc, variation for the area is 7°W, and deviation is 4°W. The wind is from the southwest, producing a 3° leeway. What true course are you making good?

The working.

1. Correcting, westerly errors are subtracted — the mirror of the previous example. Start at 299°psc.
2. Deviation first: 299 − 4 = 295° magnetic.
3. Then variation: 295 − 7 = 288°T, the heading.
4. The wind is from the southwest, 225°T. On a heading of 288°T that is 297° relative — the port bow this time, not the quarter, but still the port side, so again the vessel is pushed to starboard.
5. 288 + 3 = 291°T made good.

Answer: 291°T.

What the paper is testing. 313°T is on the paper, and it is exactly what you get by adding the westerly errors instead of subtracting them and then adding the leeway: 299 + 7 + 4 + 3. 299°T is what you get by applying nothing at all. If the answer comes out as the course you started with, you have forgotten something.