Question 1

You are on watch and notice a large vessel on AIS target bearing 010° at 8nm showing CPA 0.1nm in 30 minutes. However, there is no corresponding radar return at that position. What do you do?

Accepted Answer

Treat the discrepancy as significant. AIS and radar provide independent information — if AIS shows a target but radar shows nothing at that position: (1) check radar range scale and ensure the target's range is within effective radar range; (2) adjust gain and clutter control to check for a suppressed return; (3) consider that the AIS position may be erroneous (GPS error on the target vessel, AIS data entry error, or AIS transmission from a vessel at a different position than shown — "spoofed" or corrupt AIS); (4) look out visually in the direction of the AIS target; (5) attempt VHF Ch 16 or working channel contact; (6) do NOT use the AIS CPA/TCPA data as the basis for collision avoidance decisions — COLREGs require using all available means (radar is primary for collision avoidance); (7) treat the situation as a potential close-quarters situation with an unknown target. AIS limitations are specifically examined at OOW level because of the industry-wide concern about over-reliance on AIS for collision avoidance. The IMO Res. A.1106(29) on AIS Operational Guidance and MSC/Circ.1252 specifically warn that AIS must not be used as the primary collision avoidance tool — COLREGs Rule 7 requires proper radar watchkeeping and Rule 5 requires visual lookout. Known AIS failure modes: (a) GPS position error on the target — vessel is not where AIS says it is; (b) manual position entry — some vessels enter position manually rather than GPS-derived; (c) dynamic data lag — AIS updates course/speed/heading at intervals; the displayed course may not reflect a recent manoeuvre; (d) spoofing/jamming in certain geographic areas (Gulf, Strait of Hormuz areas have documented AIS anomalies); (e) the vessel may have a faulty AIS transponder transmitting wrong data. The correct response: treat the AIS information as advisory and investigate the discrepancy actively.

Question 2

You are OOW in restricted visibility. A contact appears on your radar at 8nm bearing 020°. You acquire it on ARPA. After 3 minutes, ARPA shows CPA 0.2nm, TCPA 22 minutes, true course 185°, true speed 14 knots. Your examiner asks what you must know about the reliability of this ARPA data.

Accepted Answer

ARPA data must be treated with caution: (1) Acquisition time — ARPA requires a minimum track period (typically 1-3 minutes for manual acquisition, longer for auto-acquisition) before vector data is reliable; at 3 minutes the data is just stabilising; (2) course and speed accuracy depends on own-ship course and speed input — gyro and log errors directly transfer into ARPA target data; (3) target swap — if targets are close together, ARPA may swap tracking from one target to another without warning, producing false vector data; (4) the true vector shown assumes the target maintains its current course and speed — any manoeuvre by the target will require time for ARPA to recalculate; (5) CPA/TCPA is a mathematical projection, not a guarantee. Maintain visual watch and sound signals under Rule 19. SOLAS V/19 (carriage requirements) mandates ARPA on vessels 10,000 GT+, but carriage does not mean the OOW can defer to ARPA without understanding its limitations. The IMO Performance Standards for ARPA (Res. MSC.64(67) Annex 4) specify that tracking accuracy after a stated initialisation period must be within defined limits — but these are performance minimums, not guarantees. The practical concern at sea: target swap is a documented hazard in areas with multiple vessels close together (anchorages, traffic separation schemes) — ARPA can transfer its tracking from one vessel to another, instantly producing a false vector. The OOW must cross-check ARPA data against visual bearings, true motion plot, and the developing situation. Rule 19(d) COLREGs requires the OOW in restricted visibility to take action in time to avoid collision, using sufficient sea room — relying solely on ARPA without confirming by radar plotting or other means is not seamanship.

Question 3

You take over the bridge watch at night. Your examiner asks you to describe how you would check that the radar is correctly tuned and performing satisfactorily before relying on it for collision avoidance.

Accepted Answer

Radar performance checks: (1) Performance monitor — use the built-in performance monitor (plume test) to verify the radar transmitter and receiver are working to spec; (2) Tune the radar using Auto-tune or Manual-tune until target returns are sharp and well-defined; (3) Check gain — set gain to just below the noise level, then increase until light speckle appears across the screen, then reduce slightly; (4) Sea clutter (STC/GAIN) — adjust to suppress sea return in the immediate area without suppressing real targets; (5) Rain clutter (FTC) — apply only in precipitation; (6) Check range scales — verify the scale matches the situation (approach into confined waters: 3-6nm; open ocean: 12-24nm); (7) Verify heading marker alignment with gyro; (8) Note radar blind sectors on this vessel (from funnel/mast positions — listed on the bridge radar display). Radar operational competence is a core OOW requirement under STCW A-II/1. The performance monitor (also called the echo box or plume test) checks that the radar transmitter is putting out adequate power and that the receiver is sensitive enough — it produces a characteristic plume on the screen whose length can be compared against a baseline recorded when the radar was serviced. A degraded plume indicates a failing magnetron or receiver fault. Blind sectors: every vessel has radar blind sectors caused by the mast, funnel, and superstructure — these must be documented and posted at the radar. The OOW must know the blind sector positions on each vessel they serve on and ensure targets in those arcs are monitored by other means. Gain and clutter controls: over-suppression of sea clutter can mask real targets at short range — the aim is to suppress the bulk of sea return while retaining real target returns. The correct technique is to observe the effect on a known target while adjusting, not to set by approximation.