> it does not judge whether the distance is closing or widening. If a ship passes perpendicular to our course but less than the defined radius behind us, we get

> it does not judge whether the distance is closing or widening. If a ship passes perpendicular to our course but less than the defined radius behind us, we get an alarm--exactly the same alarm as if it were directly in front of us and moving toward us. I've used civilian AIS systems and I can assure you they can do exactly as you describe. It's called closest point of approach (CPA and TCPA) > AIS has a feature where you can send messages as a broadcast to all nearby ships. Consumer AIS does not do this. You are referring to DSC. A consumer marine DSC/AIS VHF radio can send and receive DSC, receive AIS, and, if they have enough screen/CPU, can show CPA/TCPA tracks. For ocean passages we set our TCPA for 30 minutes and in port 2 minutes.