CPA / ARPA — closest point of approach and radar plotting

Closest Point of Approach (CPA) and Time to Closest Point of Approach (TCPA) are the two core numbers a watchkeeper derives from radar plotting to assess whether another vessel poses a risk of collision. CPA is the minimum distance that will exist between own ship and a target if both vessels maintain their present course and speed; TCPA is how long until that minimum distance occurs. Historically these were derived by manual plotting; today Automatic Radar Plotting Aids (ARPA) compute and continuously update them for every tracked target.

Relative motion vs true motion

A radar display can be presented in relative-motion mode, where own ship stays fixed at the centre and targets move according to their motion relative to own ship, or true-motion mode, where own ship and all targets move across the display according to their actual courses and speeds over the ground or through the water. Relative motion is the classical basis for manual CPA plotting — a steady target on a steady relative bearing that is closing in range is on a collision course, which is itself the practical statement behind COLREGs Rule 7 on risk of collision.

The manual plotting triangle

Before ARPA, and still taught as the underlying principle every ARPA solution is built on, the officer of the watch (OOW) plots successive radar ranges and bearings of a target on a reflection plotter or manoeuvring board. Connecting the plotted positions gives the target’s relative-motion line. Combining that relative-motion vector with own ship’s known course and speed vector — the classic “vector triangle” — yields the target’s true course and speed, and the point where the relative-motion line passes closest to own ship’s centre gives CPA, with the distance along that line to that point giving TCPA.

ARPA — automating the plot

An Automatic Radar Plotting Aid automatically acquires and tracks targets, continuously computing and displaying each target’s CPA, TCPA, true course, and true speed, and can display predicted vectors and alarm the watchkeeper if a target’s CPA/TCPA breaches operator-set limits. SOLAS Chapter V, Regulation 19 sets carriage requirements for radar and, above defined gross tonnage thresholds, ARPA (in modern fits this function is commonly delivered through an integrated bridge/ECDIS radar overlay rather than a standalone ARPA unit, but the underlying carriage requirement and functional standard remain the SOLAS V/19 basis). STCW sets the underlying competence standard for radar observation and ARPA use that deck officers must demonstrate.

Vector modes — relative vs true

Mode	What the vector shows	Best for
Relative vector	Target’s motion relative to own ship	Directly reading CPA/TCPA and collision risk
True vector	Target’s actual course and speed over ground/water	Understanding what the target is actually doing, aspect, and traffic separation compliance

Trial manoeuvre

Before committing to an avoiding action, ARPA’s trial manoeuvre function lets the OOW simulate a proposed course and/or speed change and see the resulting predicted CPA/TCPA against all tracked targets, without actually altering the ship’s heading or engine order. This lets the watchkeeper test whether a manoeuvre under COLREGs Rule 8 (action to avoid collision — to be positive, made in ample time, and readily apparent) will actually open CPA to a safe distance against the target being considered, and check it does not close CPA dangerously with any other tracked target, before executing it.

Limitations — what ARPA does not solve

Target swap. In crowded traffic, the tracking algorithm can lose a target and lock onto an adjacent one, producing a discontinuous and misleading vector.
Sea and rain clutter. Heavy clutter can suppress genuine target returns or create false ones, degrading acquisition and tracking accuracy.
Small or non-metallic targets. Small craft, some fishing vessels, and targets with poor radar cross-section may not be reliably acquired at all.
Over-reliance. ARPA is a decision-support tool, not a substitute for the proper lookout by sight and hearing required under COLREGs Rule 5, nor for independent visual and plotted assessment of risk of collision under Rule 7.

Primary sources

· Convention on the International Regulations for Preventing Collisions at Sea (COLREGs), Rules 5, 6, 7, 8, and 19 — International Maritime Organization
· SOLAS Chapter V, Regulation 19 (Carriage requirements for shipborne navigational systems and equipment) — International Maritime Organization
· STCW Convention and Code — radar and ARPA competence standards — International Maritime Organization