CPA / TCPA Calculator

Two vessels on intersecting tracks: how close will they pass, and when? Closest Point of Approach and Time to CPA are the two numbers every watchkeeper checks against an incoming radar contact. This calculator gives you the geometry, an animated view, and a configurable danger ring.

What it does

• Enter your own course and speed, then the target's initial bearing and range plus its course and speed. The calculator shows:

• A live own-centred plot of both vessels, the view stays on you as time scrubs forward.

• Each vessel's velocity arrow (drawn for half an hour of run, for scale) and its projected track as a dashed line.

• A pink danger ring around own ship at a configurable CPA threshold (default 0.5 NM).

• The CPA positions of both vessels with a dashed line linking them and the CPA value labelled on top.

• A live range line from own ship to the target's current position.

• The read-out updates as you scrub time: live range, closing rate (in knots, positive when the range is shrinking, zero at the moment of CPA), and the target's current bearing off your bow.

Animation controls

• Play runs the scrub at 1 simulated hour per 3 wall seconds.

• Reset to t=0 snaps back to the starting geometry.

• Jump to TCPA snaps to the moment of closest approach.

When to use it

Underway watchkeeping, manually plot incoming targets as a check on AIS/ARPA.

Pre-passage planning around busy waypoints and traffic separation schemes.

Training & exams, explain why two slow boats can have a much shorter TCPA than they look like they should.

Caveats

Pure geometry. No allowance is made for tide set, for radar/AIS errors, or for the target manoeuvring. Use it as a first-look check and back it up with your usual watch-keeping practice and the COLREGs.