GNSS Compass for Vessels: Dual-Antenna Heading vs Gyro
A GNSS compass gives a vessel true heading from two GNSS antennas mounted a fixed distance apart — no spinning mass, no magnetic deviation, and typically a fraction of the cost of a marine gyrocompass. Heading accuracy scales with the antenna baseline: as a rule of thumb, about 0.1–0.2° per metre of separation. What actually decides whether it works on your bridge is antenna placement, multipath off the superstructure, and how the system behaves when GNSS is disturbed.
Specifying heading for a vessel or USV? Send your bridge layout and accuracy requirement and we will size the baseline, receiver and antenna kit. Request a quote — our engineers reply with a configuration, not a catalogue.
How a dual-antenna GNSS compass measures heading
A GNSS compass is one receiver with two antenna inputs running moving-base RTK. It measures the carrier-phase difference of the same satellite signal arriving at both antennas and solves the baseline vector between them to millimetre-level precision. Heading comes from that vector; a third antenna also yields pitch and roll. Two consequences matter when buying:
- Heading is a relative measurement. It needs no external correction service. A base station or NTRIP feed is required for absolute position, not for heading.
- Satellite availability drives time-to-heading. Multi-frequency, multi-constellation tracking (GPS, GLONASS, Galileo, BeiDou) means a faster fixed solution after power-up or an outage.
GNSS compass vs gyrocompass vs magnetic compass
| System | Accuracy | Needs GNSS? | Moving parts | Best fit |
|---|---|---|---|---|
| Dual-antenna GNSS compass | ≈0.1–0.2° per metre of baseline | Yes | None | Workboats, USVs, DP-capable craft |
| Gyrocompass (fibre-optic / ring laser) | IMO standard: ≤0.75° × secant(latitude) | No, self-contained | Optical assembly, periodic service | SOLAS vessels needing type-approved heading |
| Magnetic / fluxgate compass | Degrades with deviation and heeling | No | None | Back-up heading, small craft |
The trade is straightforward: a gyrocompass is independent of satellites but costs the most and needs servicing; a magnetic compass is cheapest and least reliable on steel; a GNSS compass sits between them — accurate, no moving parts, but dependent on a usable sky view and protection from interference.
What heading accuracy can you expect from a given baseline?
| Antenna baseline | Rule-of-thumb accuracy | Where it fits |
|---|---|---|
| 0.5 m | ≈0.3–0.4° | Small craft, space-constrained builds |
| 1 m | ≈0.15–0.2° | General workboat and USV heading |
| 2 m | ≈0.08–0.1° | Survey and hydrographic vessels |
| 4 m or more | ≈0.05° and better | Vessels with long fore-aft mounting runs |
These figures are geometric and assume ideal conditions; real installations add baseline flex, multipath and antenna phase-centre error, so treat them as a sizing guide rather than a specification. Doubling the baseline roughly halves the heading error — it is usually cheaper to gain accuracy by extending the baseline than by buying a higher-grade receiver.
Antenna placement: the decisions that set real accuracy
- Mount rigidly. Baseline flex converts directly into heading error; deck flex, mast whip and thermal expansion all count.
- Take the longest practical baseline along the fore-aft axis, and never mount both antennas on structures that can move independently.
- Respect radiating sources. VHF/AIS antennas, radar and satcom are the usual causes of degraded on-board GNSS reception. Keep clear of them and of reflecting steel.
- Plan for sky view and multipath. A GNSS compass cannot resolve heading without satellites, and flat metal decks and superstructure are effective reflectors. Check mask angle and reflections at the actual mounting positions, not on the drawing.
Interference, jamming and signal loss at sea
Heading is a GNSS-dependent output, so anything degrading GNSS degrades heading. In harbours and near high-power transmitters, in-band interference raises the noise floor and slows or blocks the ambiguity fix. Receiver-level mitigation such as AIM+ interference and spoofing mitigation, built into Septentrio-based hardware, suppresses narrowband and wideband interference before it reaches the tracking loops, which is what keeps the baseline solution alive in a busy RF environment. Where heading must survive an outage, pair the compass with an inertial or magnetic reference and define the switching logic up front.
When a GNSS compass is the right choice
- Yes, if: the vessel is not required to carry a type-approved gyrocompass, you need sub-degree heading, and you have a rigid fore-aft mounting run of about 1 m or more.
- Yes, with care, if: you operate in a high-interference port — specify interference mitigation as a baseline requirement, not an option.
- Not as the sole source, if: the vessel is SOLAS-class or the operation must continue through extended GNSS denial without a second sensor.
Next step: sizing a heading system
Bring us the constraints that matter — vessel type, mounting space, required heading accuracy and the RF environment — and we will specify a receiver, antenna pair and baseline layout that fits. Dual-antenna heading is available on the rugged industrial GNSS receiver box and on OEM hardware for series builds. Contact our engineering team for a vessel heading configuration, or read more on GNSS for marine and offshore applications.
Frequently Asked Questions
Can a GNSS compass replace a gyrocompass?
On non-SOLAS craft, workboats, USVs and many commercial vessels, yes. On SOLAS-regulated vessels a type-approved gyrocompass normally remains the primary heading source, with the GNSS compass used as an accurate secondary or for specific operations such as DP support.
How many antennas does a GNSS compass need?
Two antennas minimum for heading, mounted with a rigid baseline. A third antenna adds pitch and roll.
What heading accuracy should I specify?
Work from the baseline you can physically install: roughly 0.2° at 1 m and about 0.1° at 2 m in ideal conditions. Specify the baseline first, then the receiver accuracy class that supports it.
Does a GNSS compass need RTK corrections?
Not for heading. The two antennas are processed as a moving-base pair, so the relative baseline solution is computed on board. Corrections are needed for absolute position accuracy only.
What happens to heading if GNSS is jammed?
Heading degrades or drops out with the GNSS solution. Interference mitigation such as AIM+ maintains tracking through significant jamming, but for guaranteed continuity pair the compass with an inertial or magnetic heading reference.
