For those that do not understand MARPA is a Radar feature of alot of modern Radar. Its stands for Mini-automatic radar plotting aid which is alot like a radar target pretending to be an AIS signal.
When you put your cursor on a radar target and select “Track Target” (every radar manufacturer is slightly different) it will circle the Target and start calculating facts for it from its track, like speed, heading, CPA etc. Just like is an AIS target.
You can then click on this and see this information and it will let you treat boats and even weather systems like they are an AIS target and know how close they are going to get, when they are going to be that close etc.