Question 1

Two timed echoes of a contact give you…

Accepted Answer

Her relative track — and from it CPA and TCPA — The line through the marks, extended, is the relative track; the perpendicular from own ship to it is the CPA, and the mark spacing times it.

Question 2

A contact’s relative track extends straight through your own ship at the screen centre. That means…

Accepted Answer

Collision course — CPA zero, the radar version of a steady bearing — Relative track through the centre IS the steady-bearing-closing-range fact, drawn. Act early and boldly.

Question 3

Why must your own course and speed stay steady between the two plotting marks?

Accepted Answer

Your manoeuvre bends the RELATIVE track and invalidates the plot — The plot is drawn in the relative frame your own motion defines. Alter mid-plot and you must start the interval again.

From blips to CPA

This lesson is part of a paid course