AIS Automatic Identification System for Ships is a traffic system that uses VHF transponders on vessels to periodically send out location information. It was developed by Swedish inventor Hakan Lans and is similar in concept to the ADS-B transponder system used on aircraft for position reporting.
AIS uses two VHF marine channels 87B (161.975 MHz) and 88B (162.025 MHz). Transmission is by 9.6Kbps GMSK modulation using HDLC packet protocol. VHF channel access is by SOTDMA Self Organized Time Division Multiplexing. AIS uses navigational information from external on board GPS, Inertial Navigation and Ship Control systems. AIS has several different classes of equipment such as Class A, Class B, Base Station and Aids to Navigation. Depending on the Class, different types of information are transmitted when the ship is in port, or underway.
AIS can be decoded using very simple equipment. An RTL-SDR receiver as described in an earlier post can be used together with a simple vertical whip antenna installed outdoors with line of sight to the shipping of interest. Packet decoding can be done by programs like Multipsk, a popular amateur radio program developed by Patrick Lindecker F6CTE.
“Automatic Identification System”, Wikipedia
“All About AIS”, Created by IMO/IEC Experts
“RTL-SDR Receiver”, NooElec-NESDR
“MULTIPSK” Digital Mode Decoder by Patrick Lindecker F6CTE