Coastal Skipper and Yachtmaster Quiz · Electronic navigation
What do the CPA and TCPA figures for this AIS target tell you?
A chart plotter showing your own vessel and one AIS target, a triangle up and to the right with a dashed line pointing where it is going. A data panel for the target, NORDIC STAR, reads: range 3.2 M, SOG 16.0 knots, COG 255°, CPA 0.2 M, TCPA 12 minutes.
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If neither vessel alters course or speed, she will come within 0.2 miles of you, 12 minutes from now. CPA is the closest point of approach and TCPA the time until it happens, both worked out from present courses and speeds. They do not say which side she will pass or whether she will cross ahead or astern. The figures rely on the data both vessels are transmitting, and many small craft do not transmit AIS at all, so they never replace a lookout by sight, hearing and radar.
