Since it was made available in the U.S. about 18 months ago, AIS has revolutionized marine traffic communications. AIS, a vessel's automatic identification system, is the maritime equivalent of a radar transponder in the air-traffic control system. The AIS transponder (a combination of a transmitter and receiver) broadcasts a signal that tells every AIS receiver in range the vessel's MMSI (or maritime address), position and a lot more. The signal is transmitted on VHF channel 87B or 88B and can be received and decoded with a relatively inexpensive AIS receiver. Connect the AIS receiver's output to a compatible chart plotter or radar display and an icon of the reporting vessel pops up on the screen, changing what was an anonymous blip to a named vessel. A key click will disclose the vessel's course, heading, speed and type, plus its length, beam, draft and destination. The value of this information, especially in congested waters, is obvious. Need to communicate with the vessel? If you already know its MMSI, enter it in your VHF/DSC radio, choose a working channel, press the radio's enter button and you will be in contact. If you navigate busy commercial waters an AIS receiver should be high on your priority list.