Closing speed

Closing speed is the rate at which range to a target decreases, the radial component of relative velocity along the line of sight between own ship and the contact. On a radar plot it equals the relative-motion distance run over the plotting interval, so a six-mile relative run in six minutes gives a closing speed of 60 knots. It drives time to CPA: TCPA is current range divided by closing speed. A high closing speed compresses the time available to act under COLREGs Rule 8, so officers read it directly from ARPA or derive it from the OAW plot.