Twelve Degrees of Heading Error
A 45-foot workboat has a 4 kW open-array X-band radar, a new GPS, and a gyrocompass. After a technician replaces the heading interface cable, the radar is set to north-up and true bearing. The gyro repeater at the helm reads 045° true. The radar’s heading readout shows 033° and every radar echo and AIS target is about 12° to port of its visual bearing. Own-ship position on the radar is correct. Sea clutter is symmetrical, and the heading marker rotates properly with the antenna. The technician connected a NMEA heading sentence from the gyro but did not check the sentence type. Magnetic variation in the area is 12°E. The captain notices the error only when comparing a buoy’s radar bearing with a hand bearing compass. The radar is used for pilotage in a narrow channel.
1. Diagnose the likely interface error and explain the 12° offset.
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The likely error is that the technician connected a magnetic heading sentence, such as HDM, while the radar is set to display true bearings. The gyro repeater reads 045° true. With 12°E variation, magnetic heading is 045° − 12° = 033°M. If the radar receives 033°M but treats it as true, the heading marker and all bearing data are shifted 12° counterclockwise. That matches the symptom: echoes and AIS targets appear about 12° to port of their visual bearings.
The GPS position is correct because position is independent of heading. The symmetrical sea clutter and proper rotation show the antenna and scanner are working. The fault is in heading input interpretation, not in the antenna or receiver. GPS course over ground is not a substitute for heading, especially at low speed or when the vessel is crabbing in a channel.
2. How should the technician verify the fault and correct the interface?
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The technician should compare the radar heading readout with the gyro repeater and with a known visual bearing. If the radar shows 033° while the gyro shows 045° true, the 12° difference is the clue. The installation menu should be checked for the selected heading source and sentence type. The radar should receive HDT from the gyro for true heading; if only HDM is available, the radar must be configured to apply the correct variation or to display magnetic bearings consistently.
The technician should not correct the problem by rotating the antenna or by entering a false bearing offset unless the manufacturer’s procedure requires an alignment offset and the heading source is otherwise correct. After correcting the sentence type, verify the heading marker against the gyro, check a buoy or fixed object with a hand bearing compass, and confirm that AIS target bearings and ARPA vectors agree. The correction should be documented in the installation record.
3. Why is this error especially serious for ARPA and AIS in a narrow channel?
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A heading error rotates the entire true-bearing reference frame. ARPA computes target course, speed, CPA, and TCPA from relative bearing, own-ship heading, and own-ship speed. If heading is 12° low, every target’s true bearing is shifted, and the resulting vectors can point in the wrong direction. A target that is actually clear may appear to be opening or closing incorrectly, and a crossing target may be misjudged.
AIS adds another layer because AIS target positions and courses are referenced to true north. If the radar’s heading input is wrong, the AIS target symbols can appear offset from the radar echoes even though both are reporting the same vessel. In a narrow channel, a 12° bearing error at short range can translate into a significant lateral error and can lead the operator to steer the wrong side of a buoy or an oncoming vessel.
The captain should treat the display as unreliable until the heading source is corrected and cross-checked. Visual bearings and a hand bearing compass remain the independent check.