AIS stands for “Automatic Identification System” and is the marine counterpart of aviation's transponder. The IMO (International Maritime Organization) requires that all ships travelling internationally with 300 gross register tons or more are required to have a Class A AIS system. Some countries require that small vessels carry Class B receivers and some even require transmitters and there are ongoing discussions about expanding AIS use in the EU.
AIS uses GPS and other information on a vessel and transmits this via VHF. The signals are digital and can be read by anyone with an appropriate AIS receiver (available for under US$300 these days). This information can be sent to chart plotter or ECDIS systems and displayed. Part of the signal that is sent is the course/heading/bearing/speed/GPS position so systems can compute if there is a chance of collision or crossing and this, in turn, triggers off alarms. Class A vessels transmit a lot of information including their destination, vessel type and rate of course change while Class B vessels (such as that on Zanshin) transmit much less data.
I love playing with the AIS system. Targets are displayed as triangles and I can click on them and get their AIS data which also includes their MMSI (unique ID given to boats with communications equipment) which allows contacting them via DSC so that nobody else catches on. Plus it is better to hail “ULT Behemoth” than to send “Big Ship About To Run Me Down” on the radio! In addition to receiving this information, my AIS unit also broadcasts my boat's information and thus warns the big ships if there is a chance of collision or if the CPA (closest point of approach) is too near for their liking. That will let me sleep better, since AIS coupled with the radar alarm system I've got two independent systems preventing things from going “bump” in the night.
A really cool site is at Marine Traffic, one can track and see AIS vessels worldwide in near-real-time!
 After spending some time getting the AIS system installed and networked, I turned on the chartplotter and, as one can see, the AIS display is working.
[18°4'2.2"N 63°5'14.72"W ] | | After spending some time getting the AIS system installed and networked, I turned on the chartplotter and, as one can see, the AIS display is working. | |
 Seadream I on the AIS
[18°26'36.35"N 64°45'6.47"W ] | | Seadream I on the AIS | |
 Seadream 1 AIS data
| | Seadream 1 AIS data | |
 I love playing with the AIS system. In this case, after asking myself how long the Wind Spirit actually is.
| | AIS on the chartplotter | |
 Normally only very large commercials vessels and megayachts have AIS systems. On this chart while I was heading for the drawbridge opening you can all those grey triangles denoting an AIS target.
| | AIS Targets galore | |
