A short review of the three vessel motion vectors will demonstrate the advantage of using them to picture how course or speed changes will affect a target’s CPA.
The echo trail "draws" the Relative Motion
(R - M) vector for you.
Relative Motion displayed by the echo trail is NOT the target's true heading. It combines the course and speed of both the target and your own ship.
For a more thorough explanation, review the Relative vs True Motion lesson.