You are called to the bridge at 0200. Your OOW reports a target at 6 miles, closing on a steady bearing, CPA 0.1 nm in 18 minutes. Visibility is 4 miles, you are transiting a TSS, and the ARPA is tracking 14 targets. Before you act, you need to know whether you can trust what the screen is showing you — and why.
What the ARPA is actually doing
ARPA acquires targets and computes vectors based on successive position fixes over an observation period. The accuracy of every derived CPA, TCPA, true course and speed is entirely dependent on accurate own-ship inputs: log speed and gyro heading. If either is wrong, every vector on screen is wrong. As Master you must know what sensor feeds the ARPA and verify those inputs before trusting the output — particularly after manoeuvring, in shallow water (log errors), or in high current (course made good ≠ heading).
Stabilisation modes — a command decision
- Sea-stabilised (relative vectors): vectors show target motion relative to own ship. Used for collision avoidance; shows what the target appears to be doing from your perspective. Does not account for current — a buoy will appear to move if there is set.
- Ground-stabilised (true vectors): own-ship input is from GPS or ground track. True vectors show targets' actual courses and speeds over ground. Essential for parallel indexing in pilotage; correctly represents stationary objects. If GPS is unreliable, ground stabilisation will corrupt all vectors.
Switch mode consciously and brief the watch. An ARPA showing sea-stabilised vectors in a cross-tide pilotage passage will misrepresent fixed hazards.
Parallel indexing
Parallel indexing (PI) is the pre-planned use of a radar range/bearing relationship between a fixed echo and your intended track. You draw an index line parallel to the planned track, offset by the chosen distance from a conspicuous radar-prominent object. During passage, you maintain that line on the fixed echo — any deviation is immediately visible as cross-track error. It is an active monitoring technique, not passive plotting.
For PI to be reliable: the radar must be on a fixed range scale that gives adequate resolution; the reference object must be genuinely radar-prominent (cliffs or hard rock, not a flat sandy headland); and the radar must be ground-stabilised or the PI recalculated for sea stabilisation. Pre-plan PI lines on the chart and transfer to the radar overlay or maintain a PI notebook with range and bearing offsets for each leg.
Principal limitations and errors
- Sea clutter: return from wave crests, suppressed by STC — can mask real targets close-in, especially low freeboard vessels or liferafts
- Rain clutter: FTC suppresses but can also remove weak targets
- Blind sectors and shadow sectors: caused by masts, funnels and other structures; mark these on the radar screen reminder card and never rely on a single bearing sweep through a known blind sector
- Radar horizon: proportional to aerial height — low-lying dangers (shoals, growlers) may not be detected until dangerously close
- False echoes: side-lobe returns, multiple reflections between two large targets, indirect echoes from nearby structures
- ARPA lag: vectors take time to settle after a manoeuvre — an OOW who alters course and immediately reads the new ARPA CPA will see erroneous data. The observation period means ARPA lags reality; state this clearly if asked in the oral
- Target swap: ARPA can lose track of one target and lock onto a nearby different target, particularly in a crossing situation or overtaking scenario in a busy seaway
What you do as Master in the vignette
Verify gyro and log inputs. Check stabilisation mode is appropriate. Identify whether the 0.1 nm CPA has been allowed sufficient ARPA observation time. Cross-check visually and by manual plotting if in any doubt. Apply COLREGs — in a TSS you are not relieved of your obligations under Rule 8, 16 and 17. If the target is the give-way vessel and not acting, you must take early and substantial action as the stand-on vessel once risk of collision is confirmed.