AIS
Used extensively as an aid to maritime navigation and safety by ships, aircraft and fixed or floating structures, an AIS transponder continually broadcasts the host vessel’s positional and status information.
This enables other similarly equipped vessels in the vicinity to receive, decode and display that information, along with information from other navigational systems (radar, GPS, depth recorders), to provide a comprehensive picture of the maritime traffic in the local area. Some installations employ just a receive-only version for monitoring and recording.

Each AIS system consists of one VHF transmitter, two VHF TDMA receivers and one VHF DSC receiver (optional in class B). Using the omni-directional broadcast of formatted wireless-data in the VHF maritime band in 25 or 12.5 kHz channels, an AIS system employs two simultaneous Rx channels and a single, frequency-switched, Tx channel.

The AIS system will normally operate in an autonomous continuous mode. Using Self-Organising Time Division Multiple Access (SOTDMA) technology, AIS messages are packed into over-air time slots that are accurately synchronized using either GNSS or local timing information. An AIS system can handle well over 4,500 reports per minute using individual 26.6ms time slots on both Rx channels. Each system determines its own transmission schedule based upon data-link traffic history and a knowledge of future actions by other stations.Although only one radio channel is necessary, each station transmits and receives over two radio channels to avoid interference problems, and to allow channels to be shifted without communications loss.