Collision Assessment with Radar

Radar can show whether a contact is closing and whether the bearing is steady. A constant bearing with decreasing range indicates collision risk. MARPA or ARPA may calculate CPA and TCPA, but the navigator must understand the assumptions behind the numbers.

Relative motion is often clearer for collision avoidance because it shows how targets move relative to your vessel. True motion can be useful for overall traffic awareness, but it depends on accurate heading, speed, and position inputs. If those inputs are wrong, the displayed vectors can mislead.