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.
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
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.