The essentials in 30 seconds
- 3 technologies coexist: magnetic fluxgate (entry level, 80% of recreational fleet), fiber-optic gyro (pure racing, IMOCA), 9-axis attitude sensor (modernity 2020+, current standard).
- Navico Precision 9 = 2026 standard. Magnetometer + MEMS gyro + 3-axis accelerometer combo for ~ $533 USD ex-works. Heading accuracy ±2° static, ±5° in rough seas. Native B&G/Simrad/Lowrance compatible.
- Raymarine Evolution: Raymarine T909 fluxgate compass (~ $183 USD ex-works) compatible with Axiom + EV-100/200/400.
- Offshore racing: Madintec MAD Sensor Motion (racing attitude sensor ~ $850 USD ex-works) or KVH fiber gyro (high-end, $3,000–$5,000 USD).
- Workshop verdict: Precision 9 for 80% of 30–50 ft sailboat refits. Raymarine T909 if pure Raymarine ecosystem. Madintec only for serious IRC racing.
The electronic compass is the sailboat’s directional sense. It feeds the autopilot with heading, the displays with heading, and the radar with course over ground. No good compass means no good autopilot — it’s the first thing we check when an autopilot oscillates. This article distinguishes the 3 available technologies and the brands stocked by Skysat.
For autopilots using the compass, see B&G vs Raymarine autopilot.
The role of the compass 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 (heel angle): lateral tilt angle. Useful for autopilot compensation in rough seas.
- Pitch (pitch angle): longitudinal tilt angle. Compensates autopilot corrections in swell pumping.
Magnetic fluxgate — technical detail
Electromagnetic sensor measuring Earth’s magnetic field. Historic technology since the 1970s.
- Accuracy: ±2° in calm water, drift to ±5–8° in rough seas (ferromagnetic effects + acceleration).
- Frequency: 5–10 Hz.
- Limitation: sensitive to nearby metallic masses (engine, stainless rigging, batteries). Install ≥ 1 m away from these masses.
- Brands: Raymarine T909 (~ $183 USD ex-works), Garmin GHC50, B&G H5000 RC42.
Fiber-optic gyro — technical detail
Fiber-optic gyro measures rotation via the Sagnac effect. Technology inherited from military inertial navigation.
- Accuracy: ±0.1° immune to magnetic disturbances.
- Frequency: 50–100 Hz.
- Limitation: slow drift (1–3°/h) without external reference (GPS heading).
- Brands: KVH (USA), Furuno (Japan).
- Price: $3,000–$15,000 USD ex-works. For racing/commercial only.
9-axis attitude sensor — the modern solution
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 seas (acceleration compensation).
- Frequency: 10–50 Hz depending on model.
- Advantage: compensates magnetic disturbances via gyro + accelerometer fusion. More stable in rough seas than fluxgate alone.
- Brands: Navico Precision 9 (~ $533 USD ex-works) = 2026 recreational standard. Madintec MAD Sensor Motion (~ $850 USD ex-works) = French racing standard.
Technical comparison chart
| Model | Technology | Accuracy | Frequency | Price USD ex-works |
|---|---|---|---|---|
| Raymarine T909 | Fluxgate | ±2° static, ±8° rough | 10 Hz | ~ $183 |
| Navico Precision 9 | 9-axis sensor | ±2° static, ±5° rough | 10 Hz | ~ $533 |
| B&G H5000 RC42 | Fluxgate | ±2° static, ±8° rough | 10 Hz | ~ $850 |
| Madintec MAD Sensor Motion | 9-axis racing sensor | ±1° static, ±3° rough | 50 Hz | ~ $850 |
| KVH fiber gyro | Fiber gyro | ±0.1° | 100 Hz | ~ $5,000 |
Workshop verdict by program
Coastal cruising 30–40 ft — basic fluxgate
If existing Raymarine Evolution autopilot: T909 (~ $183 USD ex-works) is enough. For full refit: Precision 9 from the start for long-term stability.
Offshore cruising 40–50 ft — Precision 9
Skysat workshop standard. ~ $533 USD ex-works, native B&G + Lowrance + Simrad + Raymarine integration via NMEA 2000. Stable in rough seas.
Offshore racing IRC / RORC — Precision 9 or Madintec
Precision 9 sufficient up to club-level IRC. Madintec MAD Sensor Motion for serious IRC and offshore with fine polar data.
IMOCA / America’s Cup — KVH fiber gyro
Pure high-level racing. Investment $5,000+ USD ex-works but unmatched accuracy.
Installation and calibration
Workshop rules — compass installation
- Position: center of the boat, mid-height, ≥ 1 m from any metallic mass (engine, batteries, rigging).
- Orientation: respect the front marking (sensor arrow toward bow).
- Calibration: mandatory post-installation. Make 2–3 full boat circles in calm water at 2–3 knots, no current. The compass learns the boat’s magnetic signature.
- Magnetic variation adjustment: set local variation (Brest zone = -1°, Caribbean zone = -15°) via software.
- Re-calibration: every 2–3 years or after significant modifications aboard (added batteries, antenna, metal solar panels).
FAQ — Electronic compass
Is a cockpit magnetic compass sufficient?
No, it’s complementary. A cockpit magnetic compass is mandatory (Division 240) for visual safety. The electronic compass feeds the autopilot and chartplotter. Both are required.
Does GPS heading replace the compass?
No. GPS gives COG (Course Over Ground = ground track) not HDG (heading). Critical difference in the presence of current. The compass is mandatory in addition to GPS.
Why does my compass drift in rough seas?
Fluxgate alone = sensitivity to accelerations + moving metallic masses. Compensate with a 9-axis sensor (Precision 9) that filters accelerations via integrated gyro and accelerometer.
How often to calibrate?
Initial calibration mandatory. Recalibrate every 2–3 years or after major modifications aboard. The boat’s magnetic signature slowly evolves with re-caulking (corrosion), paint (metallic paint), added ferrous gear.
Is an attitude sensor useful for the autopilot?
Yes. It’s the #1 difference between fluxgate and 9-axis. The autopilot receives heading + heel + pitch, enabling intelligent compensation of oscillations in rough seas. Documented reduction of unnecessary corrections by 30–50%.
Skysat distributes B&G/Navico, Raymarine, Garmin and Madintec. 2026 USD ex-works prices are indicative distributor pricing.
