Position Discrepancy Approaching a Traffic Separation Scheme
At 0340 on a clear night with visibility 5 nm, your 18,000 GT bulk carrier is inbound at 12 knots, 2.5 nm south of the westbound lane of a traffic separation scheme. The ECDIS, fed by GPS 1, places the vessel 0.4 nm east of the planned track. A visual fix taken two minutes ago from a lighthouse bearing 315° (compass) and a lit buoy bearing 082° (compass) plots 0.7 nm west of the ECDIS position. The radar, on the 6 nm range, shows the same buoy 0.2 nm farther away than the charted range. The gyro error is recorded as 1.5°W but was last checked four hours ago. An overtaking vessel is 1.2 nm astern on your starboard quarter, making 16 knots. The master is not on the bridge. Entry into the lane is due in 15 minutes.
1. How would you resolve the position discrepancy, and which position would you use for the lane entry? Explain the checks you would make before changing course.
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Treat the ECDIS position as suspect until the discrepancy is explained. The visual fix and the radar range are independent of GPS, but both depend on correct identification and on gyro error. First check the gyro error by converting the lighthouse's charted true bearing to a compass bearing using the logged 1.5°W error and comparing it with the observed 315°(C). If the difference is not about 1.5°, the visual fix is displaced. Check the buoy on the chart and on radar: a 0.2 nm range difference at 6 nm is within plotting error, but together with the 0.7 nm westerly visual offset it supports a real westerly error in the ECDIS. Then check the ECDIS datum (WGS84), the GPS antenna offset, any manual position offset, and compare GPS 1 with GPS 2, the echo sounder and dead reckoning. If the visual and radar evidence agree within about 0.2 nm, use the visual/radar fix for the lane entry. Plot fixes every few minutes and use a parallel index. If the discrepancy remains unexplained, reduce speed, call the master and do not enter the TSS on an uncertain position.
2. What bridge team actions and reports would you make before entering the TSS, given the overtaking vessel and the master's absence?
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Call the master immediately: the vessel is near a TSS, there is a position discrepancy and an overtaking vessel, and the master's standing orders will require it. Muster the bridge team, confirm the lookout is closed up, put the engine on stand-by and switch to hand steering. Establish VHF contact with the overtaking vessel if possible, but do not rely on VHF; under COLREGs the overtaking vessel must keep clear, so hold your course and speed unless collision risk develops. Check the TSS rule: if you are joining the westbound lane, join at a small angle; if crossing, cross on a heading as near as practicable at right angles to the traffic flow. Do not cut across the overtaking vessel's bow. Confirm the next waypoint, safety depth and under-keel clearance, and check that the ECDIS route is on the correct datum. Record the fixes and actions in the log, inform VTS or the port if required, and report to the master before entry.