Offshore survey, construction and subsea work all depend on a position the crew can trust. When GNSS signals are jammed or spoofed, that trust is lost. Survey data may no longer tie to the seabed, a DP vessel may lose a key position reference, and diving or construction activities may have to be suspended.
GNSS interference used to be treated as a navigation problem. IMCA’s April 2026 guidance on GNSS jamming and spoofing now treats it as an operational risk for DP and offshore survey.
This article explains the main types of interference and how each one affects survey and subsea work. It also covers the GNSS hardware and backup navigation technology that help keep operations running when satellite positioning cannot be relied on.
Key Takeaways
- Jamming drowns out satellite signals, so position quality degrades or the fix is lost. It is often easier to recognise than spoofing.
- Spoofing can be particularly difficult to detect, because the receiver may continue to output apparently valid but incorrect position or time data.
- Meaconing (replaying genuine signals with a delay) and time spoofing can bias position or time without an obvious alarm.
- A stable position output does not prove position integrity. Equipment can be working normally while the data it receives is wrong.
What is GNSS interference?
GNSS (Global Navigation Satellite System) is the collective term for satellite positioning constellations: GPS, Galileo, GLONASS and BeiDou. A receiver calculates its position and time from the signals it receives from several satellites.
Tracking more usable satellites, with good geometry and signal quality, generally improves availability and positioning robustness.
The weakness is that these signals are very faint by the time they reach the Earth’s surface. That makes them vulnerable to being drowned out or imitated. GNSS interference is any signal, deliberate or accidental, that prevents a receiver from calculating a correct position or time.
Jamming vs Spoofing: What is the difference?
Jamming transmits noise on GNSS frequencies so the receiver can no longer track the satellites. Position quality degrades, the solution becomes unstable, or the fix drops out entirely. Jamming is often easier to recognise than spoofing, because the loss of signal is usually visible to the operator.
Spoofing transmits counterfeit GNSS signals. If the receiver accepts them, it may continue to output a position or time that looks valid but is incorrect. That is why spoofing can be particularly difficult to detect.
In short, jamming tends to leave a vessel uncertain of its position. Spoofing can leave it confident and wrong.
Other forms of GNSS Interference
Meaconing
Meaconing captures genuine GNSS signals and re-broadcasts them, normally with a delay introduced. Because the underlying signals are genuine, some checks on the navigation message may still pass. The receiver’s range measurements, and so its position or time, can be biased without an obvious warning.
Time spoofing
GNSS is a timing source as well as a positioning source. Time spoofing manipulates signal timing so the receiver’s reported time becomes biased or shifted away from true time, while the reported position may still look reasonable.
This matters for survey work because timestamps tie the data together. Multibeam echosounder pings, INS and gyro outputs, subsea acoustic positioning and RTK corrections are all aligned by time. A timing offset can blur that alignment. The survey may look acceptable on deck and only show problems during processing.
Asynchronous spoofing
Here the counterfeit signals are not synchronised with the genuine ones in timing or strength. This type is generally easier to detect, because it may produce abrupt position or time discontinuities, or loss of lock. It can still disrupt operations for long enough to matter.
Related effect: AIS position anomalies
AIS is a separate VHF-based system, but the position it transmits commonly comes from GNSS. AIS messages are not cryptographically authenticated, so false positions can arise in two ways:
- Deliberately, where false AIS messages are transmitted
- Indirectly, where a vessel’s genuine AIS equipment broadcasts a wrong position because its GNSS source has been spoofed
Either way, a false AIS track can place a phantom vessel in a work area, hide a real one, or show a vessel in the wrong place on a client’s monitoring screen. That degrades traffic awareness and simultaneous operations (SIMOPS) planning, even when a vessel’s own navigation is sound.
Why GNSS interference is a growing concern for offshore operators
IMCA’s April 2026 guidance notes that interference, jamming and spoofing are now being observed across multiple operating areas, including the Baltic, Black Sea, Eastern Mediterranean, Red Sea and Arabian Gulf. At the same time, DP, navigation, survey and subsea activities depend heavily on GNSS.
The vessel types most exposed are those where positioning drives the work:
- DP vessels
- Diving support vessels
- Survey vessels
- Construction support vessels
How GNSS Interference Affects Offshore Operations
| Operation | Potential impact |
|---|---|
| Hydrographic and geophysical survey | Position jumps off the planned line, growing position error, height changes, and loss of satellite tracking. Data without a trusted position may not tie to the seabed, so lines may need to be re-run. |
| DP and construction support | Degraded DGNSS quality, repeated DP position reference warnings, and reduced confidence in position references. |
| Diving operations | Where station keeping relies on GNSS, interference raises diver safety and asset integrity concerns. This needs GNSS-loss contingencies and clear stop-work criteria. |
| ROV and inspection, repair and maintenance (IRM) | Where the ROV’s absolute position depends on an affected surface GNSS reference, the georeferenced ROV position or recorded survey data may shift relative to the asset. The vehicle’s local INS/DVL navigation may remain coherent. |
| Metocean buoys and monitoring platforms | GNSS-equipped buoys and platforms may report erroneous position or timing data, which can trigger false movement or geofence alarms. |
| Jack-up rigs and drilling vessels | Position jumps during approach, positioning or DP-supported drilling operations, triggering alarms and a switch to other position references. |
| Drones and aerial survey | Flight path interference and compromised LiDAR and photographic data. |
The commercial impact follows directly. When a trusted position is lost, operations may have to stop while the vessel’s day rate continues. Lines may need to be run again, and missed weather windows put pressure on client schedules.
How to recognise GNSS interference on board
IMCA stresses that a stable position output does not prove position integrity. Equipment can be working normally while the positioning data it receives is unreliable. Warning signs include:
- A sudden drop in DGNSS or RTK solution quality
- Position jumps that do not match vessel motion
- Fewer satellites tracked than expected
- Unexplained changes in height
- Repeated DP position reference alarms
- Disagreement between GNSS and independent references such as INS, acoustic position references, radar or visual fixes
- GNSS time disagreeing with an onboard holdover clock, where one is fitted
How Offshore Operators are reducing the risk
No single measure removes the risk. Operators are building layered protection across three areas.
1. Hardware
- Interference-mitigating antennas, from low elevation angle nulling designs that suppress ground-based jamming near the horizon to controlled radiation pattern antennas (CRPA) for more demanding environments
- Careful antenna siting for a clear view of the sky
- Modern multi-constellation, multi-frequency receivers with built-in interference monitoring and integrity checks
- Signal authentication. Where the receiver supports it, Galileo’s OSNMA service authenticates navigation message data and helps detect some forms of spoofing. It does not protect against jamming or prevent all spoofing, so it complements other measures rather than replacing them.
2. Procedures
- A GNSS interference risk assessment at the project planning stage
- Activity Specific Operating Guidelines (ASOG) with defined thresholds, DP response procedures and clear stop-work criteria
- Position checks against radar, visual and survey references
- Conservative operational limits in known interference areas
- GNSS-loss contingencies built into diving project plans
- Suspending critical activities when position integrity cannot be verified
3. People
Better hardware only pays off through better decisions. Survey, DP and bridge teams need to understand what causes interference, how to recognise anomalies, and how to follow the approved response procedures. Regular drills keep that knowledge current.
GNSS-independent Navigation: The Main Options
The most effective safeguard for navigation continuity is a reference that does not depend on satellites. There are three main approaches.
| Approach | How it works | Strengths | Limitations |
|---|---|---|---|
| Inertial navigation system (INS) | Gyros and accelerometers track motion from the last known good position | Self-contained after initialisation; bridges GNSS outages without continuous external position updates | Position error grows over time without external aiding |
| INS + Doppler velocity log (DVL) | The DVL measures velocity over the seabed and constrains INS drift | Does not require deployed external beacons; well suited to ROVs and AUVs operating within bottom-lock range | For a surface vessel, bottom-lock range limits use to relatively shallow water. For a subsea vehicle, what matters is altitude above the seabed, not total water depth. Some drift remains over distance and time |
| INS + seabed acoustic aiding (LBL or sparse LBL) | Ranges to calibrated seabed transponders constrain INS drift | Can provide high-quality GNSS-independent positioning, including in deep water | Requires transponder deployment and calibration. Performance depends on geometry, environment and configuration |
Where USBL fits. USBL measures range and bearing between a vessel-mounted transceiver and a subsea transponder. For globally georeferenced subsea positioning, its output also depends on the vessel’s position, heading and attitude, so degraded GNSS can affect the resulting coordinates.
However, when a fixed seabed transponder is used as an acoustic position reference for DP, USBL can provide an independent relative reference for station keeping. Whether USBL is independent of GNSS therefore depends on how the system is configured and used.
The right choice depends on water depth or vehicle altitude, how long an outage must be tolerated, and the accuracy the task requires. Accuracy figures for any specific system should come from the manufacturer’s datasheet or verified field trials.
A note on timing. Inertial and acoustic aiding can maintain navigation continuity and provide independent position and motion checks when GNSS is degraded. Where GNSS is also used as the vessel or survey timing source, separate resilient timing or holdover capability may be required.
GNSS and inertial navigation systems from Unique Group
Unique Group supplies GNSS receivers, interference-mitigating antennas and inertial navigation systems for survey, ROV and subsea construction projects worldwide. Equipment is available for sale or rental depending on the system.
GNSS receivers and antennas for interference-affected areas
- C-NavX3 GNSS receiver: a multi-frequency GPS, GLONASS, Galileo and BeiDou dual-antenna receiver that provides position and heading. It supports C-Nav LEO PPP corrections delivered over Iridium. A 72-hour rolling data log supports incident investigation, and its QC messages follow IMCA S015 recommendations.
- C-Nav977XF+ LEANA antenna: a low elevation angle nulling antenna that typically suppresses GNSS-band signals arriving between -5° and around 15° elevation by 20 dB. This helps mitigate jamming from nearby ground-based transmitters. For more sophisticated jamming, the manufacturer recommends a CRPA antenna.
- Septentrio AsteRx-U3 Marine: a full-constellation, triple-frequency dual-antenna receiver for precise position and heading. Its AIM+ system, with a built-in spectrum analyser, monitors, flags and mitigates narrowband and wideband interference, including jamming and spoofing. RAIM+ adds receiver autonomous integrity monitoring.
Our regional survey team supports clients using these receivers in areas affected by GNSS spoofing.
Better receivers and antennas reduce the impact of interference, but strong jamming can still overwhelm any GNSS receiver. That is why an independent inertial reference remains essential.
Inertial and Hybrid Navigation Systems
- Applanix POS MV: GNSS-aided inertial navigation for survey vessels, providing position, heading and motion data.
- Sonardyne SPRINT-Nav 500: combines a SPRINT INS, a Syrinx DVL and a high-accuracy pressure sensor in a single housing, for AUVs, ROVs, towfish and ROTVs.
- Sonardyne SPRINT-Nav Mini: a compact hybrid navigator for smaller vehicles.
- Exail Rovins and Exail Rovins Nano: subsea inertial navigation systems that can be aided by DVL and acoustic positioning.
- Exail Phins 6000: a subsea INS for demanding survey and positioning tasks.
These sit alongside Unique Group’s wider subsea positioning portfolio, including USBL and LBL acoustic positioning.
Conclusion
GNSS interference is now a normal planning consideration for offshore work, particularly in regions such as the Arabian Gulf. Jamming often produces more obvious signal degradation or loss. Spoofing, meaconing and time spoofing can be harder to detect, and may compromise position or timing without an obvious alarm.
The operators best placed to keep working combine interference-mitigating antennas and receivers, trained crews following approved procedures, resilient timing, and independent inertial and acoustic position references.
Speak with our survey specialists to discuss your project requirements.
Frequently Asked Questions
What is the difference between GNSS jamming and spoofing?
Jamming blocks satellite signals with noise, so the receiver loses or degrades its position. Spoofing sends counterfeit signals, so the receiver may output a position or time that looks valid but is wrong.
Can GNSS spoofing affect survey data even if the position looks correct?
Yes. Time spoofing and meaconing can bias the receiver’s time without an obvious position error. Because survey sensors are aligned by timestamp, problems may only appear during processing.
Can a GNSS receiver protect against jamming and spoofing on its own?
It can help. Interference-mitigating antennas and receivers with interference monitoring reduce the impact and make anomalies easier to spot. However, strong jamming can still overwhelm any receiver, so an independent reference such as an INS remains essential.
How can a vessel keep navigating during GNSS interference?
By using independent position references appropriate to the operation. INS can bridge GNSS outages, while DVL aiding constrains inertial drift when bottom lock is available. LBL or sparse LBL provides seabed-referenced acoustic aiding for subsea navigation. For DP station keeping, a USBL system tracking a fixed seabed transponder can also provide an acoustic reference independent of GNSS. Independent radar, visual and other available references should be used alongside approved response procedures.
Does INS + DVL work in deep water?
For ROVs and AUVs, what matters is altitude above the seabed, so DVL aiding works at depth as long as the vehicle stays within bottom-lock range. For surface vessels in deep water, bottom-tracking DVL may no longer be practical. Seabed-referenced acoustic positioning, such as USBL or LBL, may be used instead, depending on the operation and system configuration.
Does an INS protect against time spoofing?
Not on its own. An INS maintains navigation continuity, but it does not provide a trusted time reference. Timing resilience needs a holdover clock or an alternative trusted time source.
