What is the difference IN ARPA RADAR & AIS navigation system?

AIS can't be used for navigation support, for reference only.

Radar ARPA used for navigation & collision avoidance.

AIS can detect target ships with wider area coverage and provide more voyage information than ARPA radar.

In radar we get the actual echos of targets out in the sea.

In AIS, we receive only the data sent by the other ship's AIS transmitter. These data, are from the ships sensors