Question 1

Your vessel is entering a port approach channel at night in clear weather. Your examiner asks how radar should be used in this situation and what limitations apply.

Accepted Answer

Radar use in pilotage waters: use radar as one of multiple position-fixing aids — parallel indexing is the primary technique: select a prominent radar-conspicuous feature (jetty end, headland, offshore buoy) and monitor the vessel's position relative to the index line drawn parallel to the intended track. Set range scale to 3nm or less for good definition of channel aids. Limitations: (1) chart to radar correlation — buoys and channel marks may not appear at their charted positions due to WGS84/local datum differences, swinging, or being off-station; (2) minimum range blind spot (typically 25-50m from the radar antenna) — parallel indexing cannot detect objects too close to the ship; (3) radar resolution — closely spaced objects may merge into single returns at short range; (4) pilot has responsibility for the approach — provide radar information to the pilot but maintain an independent check. Parallel indexing is one of the most important practical radar techniques for OOWs. It was developed in the 1960s (Captain Blee) and remains a core competency. The technique: draw a line on the radar display parallel to the intended track, offset by the perpendicular distance to a chosen radar-conspicuous object. As long as the object's echo remains on the parallel index line, the vessel is tracking correctly. Any deviation from the line is immediately apparent. The limitation re chart correlation is important: in port approaches, chart datum and radar-observed positions may differ by tens of metres due to survey data age or GPS/chart datum mismatch — the OOW must verify that radar returns correlate with visual observations before relying on them for close-quarters navigation. Master-Pilot relationship: the pilot is an adviser with specialist local knowledge; the master (and by delegation the OOW) retains navigational responsibility. ISM Reg 5 and the Master-Pilot Exchange checklist (MSC/Circ.1428) formalise this.

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.