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Navigation & Communication
GPS and GNSS Receivers
A GNSS receiver gives position, speed over ground and time to the bridge, and on a DP unit it is usually the primary position reference. The sub-Category covers single- and multi-constellation receivers, differential (DGNSS) receivers, beacon and L-band correction receivers, dual-antenna GNSS compasses, and their antennas, for drilling units and offshore vessels.
How a marine GNSS receiver fixes position
The receiver measures the range to each satellite from the signal's travel time and solves for position and clock error. A single-frequency receiver has to model the ionospheric delay. A dual- or multi-frequency receiver measures it directly, which is the main reason DP-grade receivers track L1 with L2 or L5. Corrections then remove most of the remaining error.
- SBAS (WAAS, EGNOS and similar) broadcasts corrections from geostationary satellites.
- IALA beacon DGNSS sends corrections on 283.5 to 325 kHz from coastal stations, with limited range offshore.
- L-band services deliver differential or precise point positioning (PPP) corrections by satellite on a paid subscription, and reach decimetre-level accuracy.
A GNSS compass uses two antennas a fixed distance apart to measure heading from carrier phase. It is approved as a transmitting heading device under ISO 22090-3, not as a gyrocompass.
Bridge receivers and DP references
A bridge receiver feeds ECDIS, AIS, the radars, GMDSS radios and the voyage data recorder. A DP receiver feeds the DP controller and must deliver integrity data, so the controller can weight or reject it. DP class 2 and 3 units carry at least three position reference systems. IMCA guidance warns against relying on GNSS for all of them, because ionospheric scintillation, jamming or a correction outage hits every receiver at once. Two DGNSS receivers on different correction services, plus a hydroacoustic or laser reference, is a common arrangement.
Key specifications
| Parameter | Why it matters | Typical classes or units |
|---|---|---|
| Constellations and frequencies | Satellite count near the derrick, ionospheric correction | GPS, GLONASS, Galileo, BeiDou; L1, L2, L5 |
| Correction inputs | Accuracy and redundancy | SBAS, beacon, L-band PPP, RTCM SC-104 |
| Accuracy | Fitness as a DP reference | Metres standalone, decimetres with PPP |
| Output rate | DP control loop | 1, 5, 10 or 20 Hz |
| Interfaces | Fit to receiving systems | IEC 61162-1, -2, -450; 1PPS timing |
Standards and what a used receiver must show
IEC 61108 sets the performance tests, one part per system. Part 1 covers GPS, part 2 GLONASS and part 4 shipborne beacon DGNSS receivers. A bridge receiver also needs IEC 60945 environmental testing and the flag's approval route, usually the wheel mark or class type approval. A DP receiver is accepted through the class DP rules, the FMEA and annual DP trials, which IMCA M 190 sets out. Firmware counts. The certificate and the DP system's interface list both refer to it.
Failure modes and inspection
- Antennas take the weather. Look for cracked radomes, water in the connector and failed low-noise amplifiers.
- Coaxial cable loss grows with length and corrosion, so ask for the installed cable type and run.
- Receivers built before the GPS week-number rollover of April 2019 needed firmware updates. Any receiver still on old firmware may report wrong dates.
- Older receivers have no interference or spoofing monitoring.
On test, log satellite count, carrier-to-noise ratio and position scatter over several hours against a known point. Read the options list from the receiver, because extra constellations, heading and high-accuracy modes are often software keys.
Documentation
- Type-approval certificate matching model and firmware.
- Options and licence list read from the unit.
- Antenna model and cable details from the original installation.
- The last DP trials report where the receiver served as a reference.
Integration with your unit
Enter the antenna lever arms from the vessel's reference point, and feed the receiver roll and pitch from an MRU so the DP controller can remove motion from the antenna position. Check that the DP system reads the sentences the receiver sends, such as GGA and GST. Correction subscriptions belong to the account holder and do not transfer with the hardware, so budget for your own.
Used, surplus or new
A used DP receiver of the model already fitted is the cleanest swap, because the DP vendor has already integrated its messages. Unused surplus multi-constellation receivers are worth taking. A GPS-only bridge receiver costs little but limits redundancy. Related equipment is in motion reference units and DP position reference systems. Naventr passes your questions on firmware and options straight to the Seller.
FAQ
Frequently asked questions
Can a used DGNSS receiver be moved to another vessel?
Yes, the hardware moves freely, but the correction subscription usually stays with the previous account holder. Set up your own correction service, enter the new antenna lever arms, and prove the receiver as a position reference during DP trials before you rely on it.
What standard should a marine GNSS receiver be approved to?
A bridge GNSS receiver is approved to the IEC 61108 part for its system, with IEC 60945 for environmental testing. IEC 61108-1 covers GPS, part 2 GLONASS and part 4 beacon DGNSS. DP use is accepted through class DP rules rather than a separate IEC standard.
Is a GNSS compass a substitute for a gyrocompass?
Not by default, because a GNSS compass is approved as a transmitting heading device under ISO 22090-3, not as a gyrocompass under ISO 8728. It can supply heading to radar or DP where the rules allow. SOLAS V/19 asks ships of 500 GT and up for a gyrocompass or other non-magnetic heading means, so get your flag state to accept a GNSS compass in that role before you rely on it.
Why do DP units use two different correction services?
Two correction services remove a common-mode failure, because one service outage or bad correction would otherwise degrade both receivers at once. Most DP class 2 and 3 units run separate receivers on separate L-band or beacon sources and add a non-GNSS position reference.
Are surplus GNSS antennas interchangeable?
Only when the antenna covers the same frequencies and constellations as the receiver and meets its gain and power requirements. A single-frequency antenna on a multi-frequency receiver throws away the ionospheric correction. Use the antenna models the receiver's manual lists.
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