Position Fix Degradation on a Fjord Approach
M/V Nordkapp Trader, an 18,000 GT general cargo ship, is approaching the pilot station at Ålesund through Breisundet at 0300 in visibility 2 nm, wind SW 20 kn. Two GPS receivers display positions 0.35 nm apart. The ECDIS radar overlay is offset about 0.2 nm to starboard of the ENC chart features. A visual bearing of the Kleivanes light agrees with the ECDIS bearing, but the radar ARPA bearing to the same light differs by 4°. The last confirmed visual fix was 22 minutes ago. One GPS antenna was replaced yesterday; the GNSS status page shows six satellites, HDOP 2.8, and no RAIM. The radar has no chart overlay. Speed is 12 kn, and the pilot boards at 0400. The bridge team consists of the OOW, a helmsman, and a cadet.
1. What is your assessment of the vessel's position accuracy, and what immediate actions should the OOW take before the pilot boards?
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Assessment. The position is not reliable enough to continue at 12 kn. Three independent inconsistencies exist: the two GNSS positions differ by 0.35 nm, the radar overlay is offset by 0.2 nm, and the radar bearing differs from the visual and ECDIS bearing by 4°. Because the visual bearing agrees with the ECDIS bearing, the gyro is probably correct and the radar bearing alignment or heading input is suspect. The GNSS status is poor: six satellites, HDOP 2.8, no RAIM, and a recently replaced antenna raise the possibility of a bad installation, multipath, or interference.
Immediate actions. Inform the master. Reduce to a safe speed for 2 nm visibility and the available UKC. Increase the fix frequency to every 3–5 minutes. Station a lookout forward and use radar ranges to charted points plus the echo sounder as an independent check. Prepare the anchor and brief the bridge team. Call VTS and the pilot station and state that the position is uncertain. Do not rely on a single ECDIS position.
2. How would you determine whether the error lies with the GPS, the gyro, or the radar, and what fix methods would you use?
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Isolating the fault. Compare visual compass bearings of two fixed objects with the gyro repeater and with the radar electronic bearing line. If visual and gyro agree, the gyro is good and the radar bearing scale or heading input must be checked. If radar ranges agree with the chart but the radar bearing is offset, the radar bearing alignment is misaligned. For GNSS, check the antenna cable, connector, and position offset; compare GPS 1 and GPS 2 against a terrestrial fix. Look for jamming or spoofing indicators such as an abrupt position jump, impossible speed, or lost RAIM.
Fix methods. Use a three-bearing visual fix, radar ranges to two identifiable points, radar range and bearing, echo-sounder contour matching, and parallel indexing. Record each fix, the method, and the time in the log. If no fix can be confirmed, anchor or stand off clear of the channel rather than proceed on a doubtful ECDIS position.
3. What bridge team briefing and external reporting should be made?
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Brief the bridge team on the degraded GNSS and the suspect radar bearing. Assign fix intervals, assign a dedicated radar/visual cross-check, and set a trigger: if no confirmed fix is obtained within 5 minutes, reduce speed further or anchor. Tell the pilot on arrival that GNSS is degraded and that the radar bearing alignment is suspect, so the pilot does not build a plan on a false position.
Report to VTS and, if spoofing or jamming is suspected, to the coast guard or national authority. Log all actions, positions, and times. Consider switching to a second ECDIS or GNSS receiver if available. Do not make an ECDIS-only approach in a narrow fjord approach with a 0.35 nm position split.