AIS also known as (Universal) Automatic Identification System (UAIS) is a worldwide radio system for ship collision avoidance and navigational advice. AIS-equipped ships continuously transmit short messages containing information like position, course over ground, speed over ground, and so on. All information may be sent from ship to ship, as well as well as from ship to shore. This is very useful for Vessel Traffic Systems (VTS) in congested areas such as harbors, rivers and archipelagos. Tranmissions may be in broadcast mode or individually addressed. The messages may contain a fixed content in binary format, free text or binary information defined by international, regional or national authorities (see table below).

Transmission is 9600 Baud GMSK modulation over 25 or 12.5 kHz channels using the HDLC packet protocol. Each station transmits and receives over two radio channels to avoid interference problems. Self-Organizing Time Division Multiple Access (SOTDMA) is used. One Time Division Multiple Access (TDMA) frame corresponds to one minute and contains 2250 slots.

Each frame contains an 8-bit ramp up and a 24-bit synchronization sequence. At the beginning and end of each frame, a HDLC flag (01111110) is sent. After the 8-bit preamble, 168 bits of data and a 16-bit CRC checksum are sent in a default packet (one slot). Long transmission packets may occupy two to five continuous slots. Bit stuffing and NRZI encoding is used.

Traffic can be monitored on the AIS1 Channel 87B 161.975 MHz and AIS2 Channel 88B 162.025 MHz

From Options | Display Mode, All frames or Error free frames may be selected.

AIS Binary Messages