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CPA and TCPA Calculator
Closest point of approach and time to CPA from relative bearing, range and both vessels' courses and speeds. For training and planning arithmetic.
Closest approach is below 1 nm. Apply COLREGS, your watch-keeping procedures and radar plotting. Training and planning arithmetic only — not a collision-avoidance tool; it does not replace radar plotting, ARPA or the master's judgement.
Method
Straight-line relative motion.
The target's velocity relative to your ship sets the closest approach. Real tracks change, so the result is a snapshot, not a forecast.
Relative motion
Relative velocity is the target's velocity minus yours. TCPA is found from the dot product of the relative position and velocity.
Limits
Assumes both vessels hold course and speed. Radar plotting, ARPA and the COLREGS remain the basis for any decision.
Opening targets
If the vessels are already separating, the closest approach was in the past and the current range is reported.
Get in touch
Plan the passage around the close-quarters picture.
Course to steer and UKC calculations sit alongside this one on the deck.