Question 1

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.

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

In restricted visibility, you have two radar contacts. Contact A: relative motion vector shows movement from 350° relative, right-to-left across your bow. Contact B: relative motion vector is stationary on the screen bearing 270° relative at 4nm. What does each situation indicate and what action do you take?

Accepted Answer

Contact A — relative motion moving from 350° right-to-left: Contact A is on a converging course (crossing from starboard to port — own vessel is crossing behind it, or it is crossing ahead; vector analysis required). Action: plot the relative motion line (OAW/PAD plot) to determine CPA; if CPA is not safe, alter course and/or speed early and substantially. Contact B — stationary relative vector: Contact B bears constant on 270° and the relative vector is zero — it is either at anchor or a shore feature, or it is a vessel on an exactly parallel course at the same speed. Action: monitor for any change in the stationary plot; if the range is decreasing and the bearing is constant, it indicates a collision course — take action promptly. Radar plotting analysis is the fundamental OOW competency and is always examined. Relative motion analysis: in relative motion mode, a stationary relative vector on the display means the contact's position relative to own vessel is not changing — if range is also constant, the vessels are in formation or the contact is stationary; if range is decreasing while bearing is constant, this is the classic collision course indicator (steady bearing, decreasing range — Rule 7(d)). True motion vs relative motion: true motion shows own vessel's movement across the chart, which is useful for geographic situational awareness; relative motion is better for collision avoidance plotting because the relative motion line directly shows the projected closest point of approach. COLREGs Rule 19: in restricted visibility, avoiding action must avoid (a) alteration to port for a vessel forward of the beam (except when overtaking) and (b) alteration towards a vessel abeam or abaft the beam. Rule 19(e): if a close-quarters situation develops despite action, reduce to minimum steerage or stop and navigate with extreme caution.