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Boat Tech Guide: Compass Basics

Heading Sensors - Electronic Compasses

-updated February 2009
by Chuck Husick

The discovery of the earth's magnetic field and its use in determining direction without relation to landmarks or celestial features such as the sun, moon or stars was one of the most significant events in the history of human exploration of the planet. Early directional devices, a piece of natural magnetic material, lodestone, or later, a magnetized needle, were extraordinarily valuable navigation assets.

While a mariner from 200 years ago would be perfectly at home with any of today's magnetic compasses, he would be amazed by today's electronic heading systems and particularly by the way in which the information they supply is interwoven in our navigation systems.

Regardless of what other magnetic or true north heading system is on a boat a conventional magnetic compass must always be on board. The magnetic compass is unique in that it operates with no need for external power other than its torque reaction to the earth's magnetic field. No batteries required! Unless mechanically damaged it will always work and will be reasonably accurate provided it was properly installed and compensated for the presence of nearby magnetic materials.

While the magnetic compass is sufficient for navigation except in the polar regions it cannot readily provide the heading information required by autopilots, chart plotters, some types of radar displays and satellite tracking antennas. The source of magnetic heading information in electronic form on most recreational vessels is supplied by a sensor system called a magnetic flux gate. The typical flux gate is comprised of a core of easily magnetized metal on which four coils of fine wire have been wound. An AC voltage is applied to one of the coils. The relative magnitude of the voltage induced in each of the other coils will depend on the angle of each coil to the earth's magnetic field. The magnetic heading information supplied will be accurate as long as the coil assembly is parallel to the earth's surface. For this reason, the coil assembly is usually suspended in a housing, which may, like many compasses, be filled with light oil to dampen its movements as the boat proceeds through the water.

Unlike conventional compasses which require manual adjustment to compensate for local magnetic fields, most flux gate systems can be automatically compensated often by pressing a button and performing a 540 to 720 degree turn in one direction in a period of from two to three minutes. The compensation procedure can be repeated whenever the user suspects that a change may have occurred in the local magnetic environment.

These flux gate or flux detector compasses are quite accurate, however they do suffer from the same errors that occur with conventional compassses. In the northern hemisphere they are least accurate on headings close to north, most accurate on headings close to south (with the reverse true in the southern hemisphere). They can provide momentarily misleading heading information on east or west headings when subject to rapid acceleration or deceleration, such as might happen when the boat is surfing down a wave. Rapid turns can create time lag errors, it takes a moment for the system to catch up with the boat. Many of today's electronic compass systems, especially those associated with autopilots incorporate rate of turn gyros that compensate for these short term errors, materially improving the autopilot's performance, especially in following sea conditions.

Large yachts may install a north seeking gyrocompass, similar to those used on large ships. Once too large, power hungry and costly for use on yachts, quite small but still quite costly (more than $10,000) units are now offered by a number of manufacturers. These gyros typically require 1-3 hours after application of power to align themselves with the earth's gravitational field before they are ready for use. They provide electrical signals to remote indicators and for use with autopilots and all other on board devices that need a source of heading information. They reference to true north, not magnetic north.

The sensors used to control the movements of automatic satellite tracking antennas can be used to provide very precise magnetic heading information. The KVH GyroTrac, a part of their satellite antenna products is available as a stand-alone heading system and can provide exceptional performance at a fraction of the cost of a conventional ship's gyro. Similar systems are available from other manufacturers, often as part of a comprehensive instrumentation system. Heading sense systems that rely upon information derived from GPS are available and can provide information referenced to true north, eliminating the need to adjust for local magnetic variation. Such systems typically provide fast response to changes in vessel heading, making them an ideal source for both autopilots and for chartplotters, especially when radar information or satellite or photo images are overlaid on the chart.

Your choice of magnetic heading sensors, which should always include a standard steering compass will depend on the value you place on ease of compensation (automatic with most flux detector based systems) and your need to provide electronic heading information for other systems. Regardless of which systems you choose it is always well to remember that it is very unlikely that any two sensors will provide identical information all of the time. Choose one you trust, use it for navigation and refer to the others only if the primary source of data fails.





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