AIS does not replace RADAR; instead, it augments it, telling you not only what a ship has done, but what it is doing right now. And it allows you to "see" ships that are not visible to RADAR due to terrain or even distance, as long as they are in VHF range.
And beyond this, NOAA has begun deploying both AIS-equipped aids to navigation, as well as "virtual" aids to navigation. The latter are shore-based transmitters that broadcast a location and type as an aid to navigation in the proper place. If you go there, you'll find nothing at all, but it will show on your AIS display.
So you know that Eolian had to have an AIS receiver. I managed to get one in a trade for an old cell phone:
|It's a tiny little thing|
|Temporary hookup; the "antenna" is just a length of coax|
Eolian is the boat at the bottom of the display surrounded by the red circle - the guard zone I have set. If a boat enters the guard zone, or will enter it in the next three minutes, the chartplotter issues an alarm.
So now I need to climb the mizzen mast and attach the coax to the abandoned VHF antenna up there - I knew it would prove useful some day...