Two different measurement principles
| Criterion | Satellite compass | Magnetic compass |
|---|---|---|
| Reference | Geometry between two GNSS antennas | Earth's magnetic field |
| Local interference | Little sensitivity to onboard metal | Sensitive to motors, cables and metal |
| Heading at rest | Yes while both antennas track satellites | Yes after suitable compensation |
| Installation | Clear sky view and mechanical alignment | Magnetically clean location and calibration |
| Additional data | Position and attitude depending on the system | Heading, sometimes IMU attitude |
Why magnetic heading can change
A magnetic sensor observes a weak field. High-current cables, loudspeakers, motors, steel structures and later equipment changes can alter the local field. Calibration corrects a given configuration but cannot remove a disturbance that changes with electrical load.
When satellite heading is especially useful
Radar overlay
When stable orientation is needed to align independent data layers.
Autopilot
When heading and rate of turn must remain coherent at low speed.
Bathymetry
When position and attitude must accompany every depth sample.
Difficult magnetic environment
When a clean magnetic installation location is unavailable.
Complementary redundancy
A vessel may retain an independent magnetic compass as a backup while using satellite heading as the primary network source. Redundancy works best when source selection and failure behaviour are documented.