Vignette
It is 0230. You are transiting the Strait of Messina at 12 knots, bound north. Your OOW calls you to the bridge: the primary GPS has started jumping — position fixes shifting 0.2 nm laterally then snapping back, repeating every few minutes. Chart plotter track looks erratic. ECDIS is drawing the vessel through the shallows on the Sicilian side. Radar overlay is drifting off the coastline. The OOW asks what to do. This is your decision as Master.
The command layer
Your first act is not to diagnose the GPS — it is to confirm the vessel's actual position by independent means immediately: parallel indexing on the radar against known coastal returns, visual bearings to fixed charted objects, depth sounder against the charted profile. If those confirm safety, you buy time to investigate. If they do not, you slow down or heave to. The ECDIS display is not the sea; the sea is the sea.
GPS: what can go wrong
- Satellite geometry (PDOP/HDOP): poor dilution of precision in confined waters with mountains masking satellites produces degraded accuracy without necessarily alarming. Check the receiver's quality indicator.
- Multipath: reflections off canyon walls, tall superstructure, or near buildings in port. Classic symptom is the jumping fix you are seeing.
- Ionospheric and tropospheric delay: bends signal paths; worst at low elevation angles. Position bias rather than gross jumps.
- Selective availability / interference / jamming: GPS is an unencrypted civil signal. Spoofing and jamming incidents are documented in the Black Sea, eastern Mediterranean and Red Sea. Spoofing produces a plausible but false position — the receiver does not alarm.
- Receiver fault: antenna, cable, or processor degradation.
ECDIS-specific layering of error
ECDIS position error = GPS error + chart datum shift + compilation accuracy of the ENC. A GPS fix landing precisely on the receiver's display can still be wrong relative to the seabed if the ENC is compiled to a local datum not perfectly reconciled with WGS-84. The safety contour and safety depth are only as reliable as that chain.
Corrective hierarchy as Master
- Reduce speed — the single most effective safety margin whilst investigating.
- Establish position by independent means: radar ARPA ranges/bearings to fixed objects, visual clearing bearings, depth.
- Switch to secondary GPS or cross-check against a second independent receiver if fitted.
- If ECDIS is the primary means, revert to paper chart or the ECDIS back-up in accordance with your SMS procedures.
- Consider anchor or heave-to in deep water until the anomaly is resolved if confirmation of safe position cannot be obtained.
- Notify the company / flag state if a navigation incident has occurred or is imminent.
Cross-track monitoring
Radar parallel indexing requires no GPS input. In a GPS-degraded environment it is your most reliable real-time lateral positioning tool. You should be using it as a matter of routine in confined waters regardless of GPS status — that discipline is what the examiner wants to hear.
LOG entry
Record the GPS anomaly, the independent position-checks carried out, decisions made and the time GPS was restored or permanently failed. This protects you legally and satisfies SMS requirements.