AIS – Automatic Identification System – is a system whereby ships (boats, yachts) broadcast information about their location, speed, course, and much more on marine VHF channel 87 and 88 as digital data. (Tune your VHF there if you want to hear what digital data sounds like.)
There are two classes of AIS devices, Class A and Class B. Class A devices are intended for commercial vessels, and transmit with more power and more frequently than the Class B devices, which are primarily intended for recreational craft.
For either class, there are two kinds of devices, transceivers(transmitter/receivers, also incorrectly called ‘transponders’ – a term meaning transmitter/responder, a device that provides data when interrogated by an incoming signal), and receivers. As you might suppose, transceivers both broadcast their ship’s data and also receive data from other transceivers. The other type of device, the receiver, collects data but does not broadcast it. In practical terms, vessels with transceivers can see each other, but tho vessels with receivers can see those with transceivers, they are invisible to other vessels.
Read the full post on Windborne in Puget Sound.