A SOLUTION TO THE
CLASSICAL PROBLEM OF A
MAGNETIC COMPASS IN A STEEL SHIP
John B. Moore, Jr.
Naval Sea Combat Systems Engineering Station
Norfolk, Virginia
BIOGRAPHY
Mr. John B. Moore, Jr. is head of the Combat Support Systems Department of the Naval Sea Combat
Systems Engineering Station in Norfolk, Virginia. Among the responsibilities of his department are shipboard
navigation systems. He has over twenty-five years experience in naval ship navigation systems and has been a
member of ION since 1968. He has previously worked in the aerospace and shipbuilding industries. He has
earned BS and MS degrees in electrical engineering. His other interests include sailing and he has sailed
extensively the Chesapeake Bay and the East Coast of the United States.
ABSTRACT
The use of a magnetic compass in a steel ship is a classical problem of compensation which has
encumbered the U.S. Navy and the entire maritime world. The ability to compensate for deviation of magnetic
compasses in steel ships has been difficult and virtually impossible in many applications.
Certain type ships, such as aircraft carriers where large masses in
causing deviation to vary, have made the problem particularly perplexing.
Naval -Sea Systems Command and conducted by Naval Sea Combat
developed remote sensing compass technology, which originally emerged
steel ship applications.
the ship are not stationary, thereby
A recent program sponsored by the
Systems Engineering Station has
in the yachting community, to the
This paper will show the results of such applications that render the classical problem of a magnetic
compass in the steel ship non-existent.
INTRODUCTION
The Naval Sea Combat Systems Engineering Station in Norfolk, Virginia is responsible for developing
and Installing improvements for most navigation equipment-used in ships of the U.S. Navy. It has been known
for decades that magnetic compasses installed in the fleet were difficult to compensate and compensation was
good for only short periods of time. Rarely were these magnetic compasses used for steering a ship, and it is
known that a Captain Is extremely reluctant to leave porl without his gyrocompass functioning property.
Indeed, our command's involvement in gyrocompass repair attests to this. However, with this background, ft
has been the policy of the U.S. Navy to require that magnetic compasses be Installed In all ships. No ship is
complete without a magnetic compass.
Recent developments in electronic magnetic compass technology referred to as the fluxgate compass,
has had wide application in yachting. Compasses and auto pilot equipment of wide description and with an
international list of .-manufacturers have been produced. With the distinct advantage of a remote sensor, which
can be installed at significant distance from the display, ft became worthy of Investigation as a solution to our
problem. The basic question to answer was, could such a magnetic field sensor be installed sufficiently far away
from the hull of the ships to minimize Its influence and be reliably compensated.
Following a check of available fluxgate compasses on the market, ft was decided to purchase compasses
made by KVH Industries, Inc. of Middletown, RI. The objective, of course, was not to identify a source for this
equipment, but to verify the application of the fluxgate compass technology as a solution to the Navy's problem.
Furthermore, this approach is in line with policy to employ commercially
approach Is in line with policy to employ commercially available equipment when possible.
During the testing of this equipment in the ships of the U.S. Navy, not only did we verify acceptable
performances In Navy ships environment, but In cooperation with KVH, unique ship installation problems were
identified and solved.
DISCUSSION
The magnetic compass has been the mariner's source for heading and bearing information since ancient
times. What people should be given credit for first identifying the phenomenon displayed by the lodestone as ft
returned to the same orientation in the horizontal plane when suspended by a string or floated in oil is unknown
(References (1) and (2)). However, the mariner's reliance and faith in the magnetic compass is legendary.
Historian Samuel Elliot Morison comments on the exquisite skill in dead reckoning in the days of Columbus and
Magellan who could steer their wooden ships by the compass for many leagues to land falls with great accuracy.
To that degree, this skill is not extant today (See Reference (3)). The emergence of ships of iron and steel have
spelled dffficult times for this ancient and revered instrument. Concurrent with the growth of the steel ship has
been the development of the
gyrocompass.
Doubtless, the gyrocompass'
evolvement was driven by the difficulties in the use of the magnetic compass.
The effects of the steel ship on the magnetic compass are well documented and understood, but a
fundamental overview is required in order to understand the effects on the magnetic compass and the test results.
MAGNETIC COMPASS PERFORMANCE
The magnetic compass aligns to the vectorial sum of the earth's magnetic field and the magnetic field of
the ship. The metal mass of the ship (hull and contents) can create a magnetic field equal to or greater than earth's.
Orientation of the ship during construction, location of the magnetic compass in the ship, cargo and contents,
electronic equipment, and electrical cables are just a few of the areas that influence the magnetic field of the steel
ship. Furthermore, these may have long term effects (hard iron effects) or may be more transient in nature (soft iron
effects). The net result is that compensating the magnetic compass is a very complex process and is as much art as
science. Furthermore, once
compensation is achieved, ft Is reliable for an unknown period of time. The tome on the magnetic compass is
Reference (4).
Clearly, ff the magnetic field of the steel ship could be eliminated or significantly minimized, there would
remain only the magnetic field of the earth to influence the magnetic compass. The potential for the fluxgate
compass to achieve this is done by locating the sensor away from the major mass of the hull. It is known that the
magnetic field strength decreases rapidly with distance (inverse squared function). It was therefore believed that f
the fluxgate sensor was placed at the highest available location on the ship's mast, vying with other antenna
requirements, perhaps the desired results could be obtained.
SYSTEM DESCRIPTION
The early version of the system, as shown in Figure (1), consisted of the fluxgate sensor, the junction
control box, and a heading indicator box. Not shown was a commercially available power supply converting
ships 120 volt 60 Hertz to 24 volt DC for system power. The Helmholtz coils incorporated In the fluxgate sensor
provided a NorthSouth and East-West compensation for deviation. This system was installed for testing up
through the USS JOHN F. KENNEDY (CV 67), and a more advanced system with automatic compensation
capability was Installed In ships after KENNEDY. Compensation procedures similar to "swinging ship' were
performed In order to obtain adjustments to the Helmholtz coils. There was nothing complicated in this process
and references to this procedure abound. Compensation of the system after KENNEDY was automatic and reference
will be made to that process later in this paper.
PREUMINARY TESTING
To verity the Initial feasibility of the fluxgate compass system- In a steel ship, a 74' LCM C4ass landing
craft was employed and operations took place out of the Naval Amphibious Base at Norfolk, Virginia. The sensor
was bolted to a relatively short mast on top of the Pilot House, and a MK 27 gyrocompass was temporarily
Installed for the test as the LCM did not have one as standard equipment. In this craft we were able to get 24
volts directly off the ship's battery bank and no power converter was required. The craft had an old type Remote
Magnetic Heading System (RMHS) Compass. Experience with the RMHS Compass has shown that ft had been
highly influenced by ship's deviation and had a poor
reliability record. Initial gyrocompass checks were performed using surveyed shore ranges and the
fluxgate compass was compensated by using standard swing ship methods. These preliminary
results indicated clearly that the fluxgate compass was superior to the existing RMHS Compass
and had less than one degree error referenced to the gyrocompass on all headings.
The next ship in which the system was tested was the 135' LCU Class. The sensor was
mounted 15' up the mast, at approximately the center line of the ship. As the LCU already had a
MK 27 gyrocompass installed, we were able to use this as reference for testing. The ship's own
magnetic compass was a standard Navy 5' magnetic compass. To test the influence of cargo on
deviation, the LCU was loaded and offloaded with large trucks and bulldozers during the test and
no change in deviation was observed, indicating that the fluxgate sensor was sufficiently distant up
the mast to eliminate the influence of cargo. Gyrocompass performance was verified during exft
and return to Norfolk, Virginia, again observing surveyed range as before. A complete swinging of
the LCU in the Chesapeake Bay using gyrocompass reference provided a deviation table with
most readings at one degree error or less. The average deviation for the entire deviation table was
.83 degrees. As a result of these tests, the Navy has initiated the process of Installing this new
fluxgate compass on all LCUs and LCMS. These results were considered highly successful and we
felt confident to proceed to our most challenging test scheduled next in an aircraft carrier.
LARGE SHIP TESTING
The aircraft carrier poses the most challenging environment to the magnetic compass
because of the significant movement of aircraft, deck equipment, large antenna installations, mass
of the ship, a very strong degaussing system and the asymmetrical magnetic compass installation,
i.e. significantly off center installation of the compass.
The fluxgate sensor was installed approximately 100' up the ships mast with indicators on
the bridge and In the chart room. The first system installed and tested in KENNEDY had
insufficient compensation adjustment to the Helmholtz coils. Deviation experienced was fully 35
degrees and only about 30 degrees could be removed. We returned the system to the KVH
Corporation to modify the compensation circuitry to provide additional adjustment. Following
this modification, the system
was reinstalled and KENNEDY was taken to sea for a swing ship. With the modification, it was
found that we could compensate for this large deviation. Data was taken by referencing the ship's
more accurate MK 19 gyrocompass as a heading reference.
Figure (2) shows the test results following swing ship and compensation adjustments. As can
be seen, the graph has two error curves; one for ship's degaussing energized and one for deenergized.
While these curves indicate deviation correct up to a maximum of about 17 degrees, the errors were
found to be stable, that Is the ships deviation was constant and unaffected by movement of aircraft,
equipment and electromagnetic emission on the mast. This large deviation correction Is primarily
explained by the asymmetrical Installation which causes what are known as lntercardinal errors. As
the Helmholtz coils compensate for North-South and East-West errors only, these curves indicate
uncompensated intercardinal errors. As the normal operating mode of the ship is with degaussing
on, compensation was performed In this mode.
DEVIATION COMPENSATION
While R was determined In KENNEDY testing that the remotely installed fluxgate sensor
was at sufficient distance from the mass of the ship to virtually eliminate effects of aircraft
movements, the asymmetrical installation of the sensor still caused a large periodic deviation.
However, since the deviation was stable, a deviation table could be obtained and applied to compass
readings for satisfactory use.
Fortuitously at this point in our testing KVH came forward with an offer to use their nemy
developed automatic compensating fluxgate system. Their new system utilized solid state fluxgate
technology and a microprocessor controlled autocompensation algorithm to compensate for
deviation. As the algorithm is considered proprietary, ft will not be covered here. However, the
benefits and ease of calibration provided a significant improvement and was able to solve and
compensate for the asymmetrical- effects of the installation.
DIGITAL FLUXGATE COMPASS SYSTEM (DFGMC)
As the KENNEDY was no longer available for testing, follow-on testing was conducted in
the USS DWIGHT D. EISENHOWER (CVN 69). The installation arrangement was essentially as in
KENNEDY with the sensor up the ship's mast about 100'. Compensation of the new digital
fluxgate compass system only
required a 360 degree turn of the ship for the algorithm to compute the compensation constants. Also, during
normal ships turning operations, the system is continuously verifying compensation parameters and refining the
compensation as necessary.
Figure (3) shows the deviation of the DFGMC and the ship's Installed magnetic compass. The
EISENHOWER's magnetic compass had recently been compensated, thereby explaining errors of no more than 6.6
degrees. Following the automatic calibration of the DFGMC, this data was collected. The data shows a median
offset of two degrees West. This offset could have been entered into the system as a heading offset, thereby
correcting further the errors. With this offset uncorrected, a maximum error of 3 degrees Is observed. Had the
offset been entered, then all errors would have been one degree or less with the exception of 240 degrees which
would have been two degrees.
OTHER SHIP CLASSES
Evaluation of the DFGMC Is continuing In other ship classes. Systems have been installed In USS
THOMAS S. GATES (CG 51), an AEGIS Class Destroyer, the USS STUMP (DD 978), a SPRUANCE Class
Destroyer and the USS GUADALCANAL (LPH 7), an Amphibious Assault ship. As both the GATES' and the
STUMP's topside super structure was aluminum, ft was found that the DFGMC fluxgate sensor performed
satisfactorily when mounted in the chart room. It was not necessary to mount ft on the mast. Figures (4) through
(6) show DFGMC and magnetic compass deviation as measured against the ship's gyrocompasses. The DFGMC
performance clearly exceeds the standard magnetic compass performance except in the STUMP. The STUMP's
compass was a 7 1/2 inch model. This data was collected by the ship's personnel. As can be seen, further
reduction in the DFGMC deviation could have been obtained if heading indexing values had been entered. Also,
while the STUMP's magnetic compass performance was comparable to the DFGMC, the problem of the transient
nature of the deviation remained. Furthermore, the continuous calibration verification of the DFGMC clearly was
an advantage.
CONCLUSION
While the evaluation of this new technology Is still ongoing, ft seems a low risk to conclude that a
solution has been found to the classic problem of a magnetic compass in a steel ship. The combination
of the new fluxgate technology, coupled with and enhanced by microprocessor capability, allows for sensing the
magnetic field at a remote location where deviation is minimal and essentially stable, and compensation methods
reduced to an automatic hands-off process. Preliminarily R Is planned to employ the DFGMC as a back-up aircraft
Inertial navigation system alignment capability in the next generation of aircraft carriers, and ft Is fully expected
that this technology will restore the mariner's confidence in the magnetic compass as a reliable source of accurate
heading.
REFERENCES
1.
130
Dutton's Navigation and Piloting, Chapter 9, Page
2.
1,
American Practical Navigation (Bowdftch), Volume
Section 111, Page 9
3.
Morrison, S. E., The European Discovery of
America, the Southern Voyager, Page 176
4.
See Reference (2), Chapter 3, Page 201