The essentials in 30 seconds
- 3 technologies coexist: fluxgate compass (entry-level, 80% of recreational fleet), fiber-optic gyro (pure racing, IMOCA), 9-axis attitude sensor (modern 2020+, current standard).
- Navico Precision 9 = 2026 standard. Magnetometer + MEMS gyro + 3-axis accelerometer combo. Heading accuracy ±2° static, ±5° in rough seas. Natively compatible with B&G/Simrad/Lowrance.
- Raymarine Evolution: Raymarine T909 fluxgate compass compatible with Axiom + EV-100/200/400.
- Offshore racing: Madintec MAD Sensor Motion (racing attitude sensor) or KVH fiber-optic gyro (premium).
- Workshop verdict: Precision 9 for 80% of sailboat refits 30-50 ft. Raymarine T909 if Raymarine-only ecosystem. Madintec only for serious IRC racing.
The electronic compass is the sailboat's orientation sensor. It feeds the autopilot with heading, the displays with heading, and the radar with course. No good compass, no good autopilot — it's the first thing checked when an autopilot oscillates. This article distinguishes the 3 available technologies and the brands sold at Skysat.
For autopilots using the compass, see B&G vs Raymarine autopilot for 40-50 ft sailboats.
Compass role in the autopilot chain
The compass continuously sends (10-20 Hz) three critical data points:
- Magnetic heading: direction the bow points. Feeds the autopilot in "Compass" mode and the radar for orientation.
- Heel: lateral tilt angle. Useful for autopilot compensation in rough seas.
- Pitch: longitudinal tilt angle. Compensates autopilot corrections in swell pumping.
Fluxgate compass — technical detail
Electromagnetic sensor measuring Earth's magnetic field. Historical technology since the 1970s.
- Accuracy: ±2° in calm water, drift to ±5-8° in rough sea (ferromagnetic effects + acceleration).
- Frequency: 5-10 Hz.
- Limitations: sensitive to nearby metal masses (engine, stainless steel rigging, batteries). Install at least 1 m away from these masses.
- Models: Raymarine T909, Garmin GHC50, B&G H5000 RC42.
Fiber-optic gyro — technical details
Fiber-optic gyro measures rotations via the Sagnac effect. Technology derived from military inertial navigation.
- Accuracy: ±0.1° immune to magnetic interference.
- Frequency: 50-100 Hz.
- Limitations: slow drift (1-3°/h) without external reference (GPS heading).
- Models: KVH (USA), Furuno (Japan).
- For racing/commercial use only.
9-axis attitude sensor — modern solution
Combined 3-axis magnetometer + 3-axis MEMS gyro + 3-axis accelerometer in a single housing. Software fusion for heading + attitude + acceleration.
- Accuracy: ±2° static, ±3-5° in rough sea (acceleration compensation).
- Frequency: 10-50 Hz depending on model.
- Advantage: compensates magnetic interference via gyro + accelerometer fusion. More stable in rough sea than fluxgate alone.
- Models: Navico Precision 9 = standard for cruising 2026. Madintec MAD Sensor Motion = standard for French racing.
Technical comparison chart
| Model | Technology | Accuracy | Frequency |
|---|---|---|---|
| Raymarine T909 | Fluxgate | ±2° static, ±8° rough sea | 10 Hz |
| Navico Precision 9 | 9-axis sensor | ±2° static, ±5° rough sea | 10 Hz |
| B&G H5000 RC42 | Fluxgate | ±2° static, ±8° rough sea | 10 Hz |
| Madintec MAD Sensor Motion | 9-axis racing sensor | ±1° static, ±3° rough sea | 50 Hz |
| KVH fiber gyro | Fiber gyro | ±0.1° | 100 Hz |
Workshop verdict by program
Coastal cruising 30-40 ft — basic fluxgate
If existing Raymarine Evolution autopilot: T909 is sufficient. If full refit: Precision 9 from the start for long-term stability.
Offshore cruising 40-50 ft — Precision 9
Skysat workshop standard. Native integration with B&G + Lowrance + Simrad + Raymarine via NMEA 2000. Stable in rough sea.
Offshore racing IRC / RORC — Precision 9 or Madintec
Precision 9 sufficient for club-level IRC. Madintec MAD Sensor Motion for serious IRC and offshore with fine polar diagrams.
IMOCA / America’s Cup — KVH fiber gyro
Pure high-level racing. Unmatched accuracy.
Installation and calibration
Workshop rules — compass installation
- Position: at the center of the boat, mid-height, at least 1 m away from any metal mass (engine, batteries, rigging).
- Orientation: respect the front marking (sensor arrow pointing toward bow).
- Calibration: mandatory after installation. Make 2-3 full turns of the boat in calm water, at 2-3 knots, without current. The compass learns the boat’s magnetic signature.
- Variation adjustment: adjust local variation (Brest area = -1°, Caribbean area = -15°) via software.
- Re-calibration: every 2-3 years or after significant modifications onboard (adding batteries, antenna, metal solar panels).
FAQ — Electronic compass
Is a cockpit magnetic compass sufficient?
No, it is complementary. The cockpit magnetic compass is mandatory (Division 240) for visual safety. The electronic compass powers the autopilot and chartplotter. Both are required.
Does GPS heading replace the compass?
No. GPS provides COG (Course Over Ground = ground track), not HDG (heading). Critical difference when current is present. The compass is mandatory in addition to GPS.
Why does my compass drift in rough seas?
A fluxgate alone is sensitive to accelerations and moving metal masses. Compensate with a 9-axis IMU (Precision 9) that filters accelerations via integrated gyro and accelerometer.
How often should calibration be performed?
Initial calibration is mandatory. Recalibrate every 2–3 years or after major modifications aboard. The boat’s magnetic signature slowly evolves due to re-caulking (corrosion), paint (metallic paint), or added ferrous equipment.
Is an attitude sensor useful for the autopilot?
Yes. This is the key difference between fluxgate and 9-axis systems. The autopilot receives heading + heel + pitch, enabling intelligent compensation for oscillations in rough seas. Documented reduction of unnecessary corrections by 30–50%.
Skysat distributes B&G/Navico, Raymarine, Garmin, and Madintec.
