ARPA
An Automatic Radar Plotting Aid acquires and tracks targets and continuously computes each one's CPA, TCPA, true course and speed, displaying vectors and raising warnings when a target breaches set limits. Its accuracy is only ever as good as the radar and the sensor inputs feeding it: targets can be swapped or lost in clutter or when they pass close, there is a processing delay of up to a few minutes after any manoeuvre before the vectors settle, and the trial-manoeuvre function is a planning aid, not a promise. ARPA supports the officer's judgement under the IRPCS; it does not make the decision.
AIS
| Data class | Examples | Note |
|---|---|---|
| Static | Name, call sign, MMSI, type, dimensions | Entered once; can be wrong |
| Dynamic | Position, COG, SOG, heading, rate of turn | From the ship's sensors; updated frequently |
| Voyage | Draught, destination, ETA, cargo | Manually entered; often stale or wrong |
| AIS transmits three classes of data. |
AIS gives identity and intent that radar cannot, and it sees around headlands that block radar, but it shows only what another vessel actually transmits — which may be delayed, wrong, or entirely absent, since many small craft carry no AIS. Target-swap between radar and AIS, and reliance on another ship's manually entered data, are known traps. The OOW uses AIS to complement radar and the visual watch, never as the sole basis for a collision-avoidance decision.
WARNING — Aids assist; the rules decide ARPA and AIS inform the picture, but Rule 7 warns against making assumptions on scanty information, and no rule lets you hand the decision to a box. Confirm with radar and eyes, then act under the IRPCS.
NOTE — Not every target transmits Fishing boats, yachts and some warships may show on radar but not on AIS, or on AIS but weakly on radar. Treat the two as complementary, and hunt for the target the other one is missing.