Fundamentals · 6 minute read
Autopilot heading sensors
How autopilot heading sensors work, from fluxgate compasses to nine-axis solid state and satellite compasses, with mounting, calibration and Australian notes.
The heading sensor is the autopilot's eyes. Everything the course computer does starts with the question of which way the boat is pointing, and the quality and stability of that answer sets the ceiling on how well the pilot can steer. A perfect drive fed by a poor compass produces a boat that wanders.
Three technologies matter on cruising boats: the , the solid-state nine-axis sensor that has largely replaced it, and the that sidesteps magnetism altogether. Each has a place, and the choice interacts with the boat's construction and where you can physically mount the thing.
Key facts
- Fluxgate
- Gimballed magnetic sensor, the Autohelm-era standard
- Nine-axis
- Magnetometers, accelerometers and gyros in one small unit
- Satellite
- Heading from multiple GNSS antennas, immune to magnetism
- Mounting
- Low, central, away from engines, speakers and cabling
- Calibration
- A slow circle or two in flat water, plus a heading offset
- Steel boats
- Usually the case for a satellite compass
Fluxgate autopilot heading sensors
A senses the horizontal component of the earth's magnetic field with a coil arrangement and reports a heading. In autopilot use it was typically hung in gimbals so it stayed roughly level as the boat heeled and pitched. Autohelm and early Raymarine systems, and many first-generation Simrad and Navico pilots, all steered on fluxgates and steered well in moderate conditions.
Their weaknesses appear at the edges. They are slow to settle after a fast turn, they can be upset by heel, and because they are magnetic they respond to anything ferrous or electrically noisy that gets near them. Many are still in service on Australian boats and working fine, but a fluxgate is the first thing to look at when an older pilot steers untidily in a seaway.
Solid-state nine-axis sensors
Modern sensors combine three-axis magnetometers, three-axis accelerometers and three-axis gyros in a sealed unit with no moving parts. The Raymarine EV-1 sensor core, the B&G Precision-9 and Garmin's attitude and heading reference modules all work this way. The gyros and accelerometers let the unit compensate for heel, pitch and rate of turn, so the heading stays useful while the boat is moving properly rather than only when it is level.
That is why these sensors changed autopilot behaviour so noticeably. Knowing rate of turn as well as heading lets the course computer anticipate rather than react, which is the heart of the adaptive steering in current systems. Many also self-learn their compensation over the first hours of use rather than demanding a formal swing.
They are still magnetic devices. A nine-axis sensor bolted next to an alternator or a speaker will produce rubbish just as a fluxgate would, and the fact that it self-calibrates can mask a bad location until something changes.
Satellite compasses
A derives heading from the relative position of two or more GNSS antennas in a single housing. Because it is not magnetic, it is unaffected by the boat's steel, by the engine, or by deviation in any form, and it provides heading that is true rather than magnetic along with rate of turn and attitude data. Furuno's SC series is the best known family.
They cost more and they need a clear sky view, which means a mounting position clear of the rig, radar and other obstructions. On a sailing yacht with an aluminium mast and rigging that is a real constraint worth discussing with the installer.
For steel boats, and for commercial and larger power vessels where magnetic compensation is a persistent nuisance, a is often the sensible answer. It also suits boats with large magnetic loads that change, such as those carrying significant steel cargo or heavy machinery.
Where to mount a magnetic sensor
Location is the single biggest determinant of how well a magnetic heading sensor performs. Aim for low in the boat to minimise the effect of pitch and roll, close to the centreline and near the boat's centre of motion, and as far as practical from anything with a magnet or a heavy current in it.
The usual offenders on cruising boats are engines and alternators, battery banks and their cables, bow thrusters, anchor windlasses, autopilot drive motors, loudspeakers, and the steel in a keel-stepped mast or an engine bed. Hand-held VHF radios, tool bags and tinned food stowed in the wrong locker have all caused mysterious heading faults.
Before you drill, hold the sensor in the intended spot with the engine running and the usual loads on, and compare the heading against a hand bearing compass or the boat's steering compass on several headings. Ten minutes of checking beats a season of odd behaviour.
- At least the manufacturer's stated distance from engines, alternators and battery cables
- Well clear of loudspeakers, which are a very common and very overlooked cause
- Low and near the centreline, away from the ends of the boat
- Not on a bulkhead shared with the windlass, thruster or drive motor
- Aligned to the fore and aft line, or the offset entered at calibration
- Accessible enough to inspect the connector once a year
Record what is near the sensor
Deviation, calibration and the Australian magnetic field
Deviation is the error caused by the boat's own magnetism. Calibration measures it, and every magnetic heading sensor needs it done. The procedure is usually slow circles in flat water with minimal traffic, at low speed, taking a few minutes, followed by setting a heading offset so the sensor agrees with a known transit or with the ship's compass.
Australian waters add a wrinkle worth knowing about. The earth's magnetic field is steeply inclined at Australian latitudes, and the dip angle increases as you go south towards Tasmania. A steeply inclined field is harder on simple gimballed fluxgates when the boat is heeled or rolling, because heel tilts the sensing element out of the horizontal plane. Nine-axis sensors compensate for tilt directly, which is one reason they behave better on a heeled yacht. The detail is more complex than this summary and worth reading in your manufacturer's documentation.
Magnetic variation is a separate matter and changes across the continent and over time. Modern systems apply variation automatically from a model or from GNSS data, but check that the pilot, the plotter and your paper charts are consistent about whether they are showing true or magnetic.
Heading is not course over ground
Heading is where the bow points. Course over ground is where the boat actually goes, which is heading plus the effects of current and leeway. A pilot in compass mode holds heading, so in a current the boat tracks somewhere else, which is exactly why exists.
On the east coast, the East Australian Current can set a boat several degrees off its heading for hours. In the tidal streams north of Mackay or through the Kimberley the difference can be far larger. If you are wondering why the boat is not arriving where the bow is pointing, nothing is broken.
Practically, that means using heading mode when you want the sails happy and when you want to arrive at a mark. A good instructor will make you steer both on the same leg so you feel the difference rather than reading about it.
Questions
Frequently asked
Do I need to replace my old fluxgate compass?
Not if the pilot steers well and the fluxgate is in good order, since many still give years of service. Consider replacing it if you are upgrading the course computer anyway, or if the pilot wanders in a seaway and you have ruled out drive and tuning problems. Modern nine-axis sensors handle heel and rate of turn much better.
Where should the heading sensor be mounted on a yacht?
Low, close to the centreline, near the boat's centre of motion, and at least the manufacturer's stated distance from engines, battery cables, speakers, thrusters and the autopilot drive. Test the location with a hand bearing compass before drilling holes. Accessibility matters too, because the connector needs an annual look.
Is a satellite compass worth it on a cruising yacht?
On a steel boat, usually yes, because magnetic compensation is a constant battle. On a fibreglass yacht with a reasonable mounting location, a modern nine-axis sensor does the job for much less money. The satellite compass also needs a clear sky view, which is harder to find on a sailing boat.
How often should I calibrate the compass?
At least annually, and whenever anything magnetic or high-current is added, moved or removed near the sensor. Recalibrate too if the pilot starts holding course well on one tack and poorly on the other. Modern sensors that self-learn still benefit from a formal swing after a refit.
Why does the pilot's heading differ from my plotter's course?
Because heading and course over ground are different quantities. Heading is where the bow points, course over ground includes current and leeway, and in the East Australian Current or a northern tidal stream the difference can be several degrees or more. Use track mode when the destination matters and heading mode when the sails matter.
Practise autopilot operation with an instructor
Habits are learned at the helm with someone watching. The training section explains what a good instructor covers and where to find one in your state.