TCPA (Time to CPA)

Time to CPA is the interval until a target reaches its closest point of approach, computed by ARPA from the closing speed and current range along the relative-motion line. It tells the watchkeeper how long is available before the encounter peaks and therefore how soon a Rule 8 maneuver must be made for it to be in ample time. A short TCPA at close range leaves no room for a succession of small alterations, which Rule 8(c) discourages in favor of one substantial change.