When may the closest point of approach (CPA) of a contact on a relative motion RADAR be determined?

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Multiple Choice

When may the closest point of approach (CPA) of a contact on a relative motion RADAR be determined?

To determine the closest point of approach, you must establish the contact’s motion relative to your own vessel. That requires measuring how the contact moves over time so you can determine its relative speed and direction. Two position marks, spaced by at least a minute, give enough information to define the relative velocity and produce a reliable trajectory. With that trajectory you can project where the distance between you will be smallest and when that point will occur—the CPA. Relying on a single detection doesn’t provide a velocity vector, so the future path is uncertain. Changing course alters the relative motion and would change the projection, and visibility conditions don’t by themselves enable CPA determination without a track. So, you determine the CPA only after you’ve marked the contact twice over a minimum interval to establish its motion.

To determine the closest point of approach, you must establish the contact’s motion relative to your own vessel. That requires measuring how the contact moves over time so you can determine its relative speed and direction. Two position marks, spaced by at least a minute, give enough information to define the relative velocity and produce a reliable trajectory. With that trajectory you can project where the distance between you will be smallest and when that point will occur—the CPA.

Relying on a single detection doesn’t provide a velocity vector, so the future path is uncertain. Changing course alters the relative motion and would change the projection, and visibility conditions don’t by themselves enable CPA determination without a track. So, you determine the CPA only after you’ve marked the contact twice over a minimum interval to establish its motion.

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