And if you really want to pass radar targets via AIS, you could make RAVDM messages and use type 27 position reports with mmsi equal to your arpa target number. Someone could even propose a change to ITU-1371 for type 1 or 27 to specifically designate radar targets.
Some figures from IALA Recommendation on the Inter-VTS Exchange Format (IVEF) Service, IALA V-145:
In NMEA 0183 4.0 (I bought this damn doc):
- Sentence: RSD – Radar System Data
- Talker: Radar and/or Radar Plotting RA
My notes for people (not in the United States) setting up their own VTS and/or WhaleAlert and/or Met/Hydro systems:
You can transmit standard IMO 289 AIS msg 8-1-22 using a simple computer (e.g. RaspberryPi + Linux), ssh via keys (just use passwordless), using any of a Class-A, ATON, or Basestation transceiver. You just have to send it serial BBM messages. I originally used a class-A transceiver which left a "ship" at the location of the transceiver on short and then switched to a blue force Class A transceiver (which really is just a bios patch to the unit) to transmit only the area notices without sending the 1-2-3-5-27 messages a normal class b will send.
https://github.com/schwehr/ais-area-notice will send IMO 289 standard 8-1-22 messages.
You could even do this with a software defined radio and have the entire hardware system cost < $300 USD (raspberrypi, gps board, sdr board, cell modem, power amplifier, cables, rf filters and antennas [ build your own J-Pole antennas ]) and the software would then be free after you get the basic glue scripts written. SDR AIS transmission is specifically not allowed in the US, but would be totally awesome if allowed by the "competent regional authority" [that's the official term] in your area.
A word of warning:
As I tell everyone I talk to, I strongly encourage you to make sure you are always passthrough the original AIS NMEA through the system. You will likely need it at a later date as your needs expand. And IVEF does not give you very much of what comes through on the AIS channels and there are some super important missing things in the specification.