Radar and AIS Overlap on a Night Approach
A 24 m workboat approaches a harbour entrance at night in light rain. The X-band radar is set to 3 NM range with gain at 55 percent and sea clutter at 40 percent. The operator reports that the harbour breakwater echo appears intermittently and that a small unlit skiff was seen visually at about 0.4 NM before it showed on the radar. A Class B AIS transponder is fitted, and the chart plotter overlays AIS targets on the radar picture. The radar antenna is mounted 4.2 m above the waterline on a mast, and the scanner is tilted 4 degrees up from horizontal to reduce deck spray returns. Wind is Force 4 with a short chop.
1. Explain why the skiff was detected visually before radar, and identify the radar settings and installation factors that most likely contributed.
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The skiff was probably small, low freeboard, and non-reflective, so its radar cross-section was tiny. At 0.4 NM it subtended a very small vertical angle from a 4.2 m antenna, and in a short chop the skiff was intermittently masked by sea clutter. A 4-degree upward tilt raises the antenna beam axis, reducing illumination of close, low targets and worsening close-range detection; that tilt helps with spray but trades away near-field performance. High sea clutter (40 percent) suppresses small echoes near the vessel, and 55 percent gain may be too low for weak targets. The intermittent breakwater echo suggests marginal tuning or shadowing rather than a hard fault. Practical judgement: reduce sea clutter in the entrance, raise gain cautiously until speckle appears, use a shorter range with offset, and treat AIS as complementary, not a substitute, because Class B AIS may not be fitted to a small skiff and its update rate and transmit power are limited. Visual lookout remains primary in rain and clutter.
2. The operator proposes relying on the AIS overlay to track all traffic in the entrance. Give a reasoned assessment and a coverage-check procedure using the installed equipment.
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AIS is not a detection system in the radar sense: it only shows cooperative targets that are transmitting valid, correctly configured data. Small craft, unlit skiffs, and vessels with faulty or disabled transponders will be invisible. Class B transmissions are lower power and may be missed at range or in congested areas. The overlay can also mislead if the AIS target positions are stale or if heading and course vectors are confused. A coverage check should compare radar echoes with AIS targets over several minutes: note any radar echo without an AIS corollary, and any AIS target with no echo. Confirm the vessel's own AIS data (MMSI, dimensions, position) is correct, because errors propagate to other vessels. Verify antenna height and cable condition, and check that the AIS receiver is not desensitised by the VHF radio transmitting nearby. Do not reduce visual or radar watchkeeping because the overlay looks complete.
3. State whether the 4-degree upward tilt should be changed and describe how you would verify the radar's close-range performance after any adjustment.
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The tilt should be reviewed, not blindly removed. A small upward tilt can reduce spray and deck returns, but 4 degrees is significant at close range and can lift the beam off low targets. If the antenna is mounted where spray is heavy, a modest tilt or a radome with better close-range beam shape may be preferable. Verification should be done in calm conditions first, then repeated in the chop: use a known small target such as a buoy or a measured skiff at 0.2, 0.4, and 1 NM, record whether it paints consistently, and compare gain and sea clutter settings. Check for shadow sectors caused by the mast or superstructure by observing a known target while turning slowly. Confirm the antenna is level athwartships and that the mounting has not loosened. Document the baseline so future performance changes can be detected.