Automatic Identification System (AIS)

An automatic Identification System (AIS) is an automated tracking system that displaysother vessels in  the vicinity. The broadcast transponder system operates in the VHF mobile maritime band. Your ship also shows on the screens of other vessels in the vicinity, provided your vessel is fitted with AIS. If AIS is not fitted or not switched on, there is no exchange of information on ships via AIS. The AIS onboard must be
switched on at all times unless the Master deems that it must be turned off for security reasons or anything else. The working mode of AIS is continuous and autonomous. It is fitted on ships for the identification of ships and navigational marks. However, it is only an aid to navigation and should not be used for collision avoidance. Vessel Traffic Services (VTS) ashore use AIS to identify, locate and monitor vessels.

1. Class A: Mandated for all vessels 300 GT and above engaged on international voyages as well as all passenger ships

Marine comunication

2. Class B: Provides limited functionality and is intended for non-SOLAS vessels. Primarily used for vessels such as pleasure crafts

AIS operate principally on two dedicated frequencies or VHF channels:

  • AIS 1: Works on 161.975 MHz- Channel 87B (Simplex, for the ship to ship)
  • AIS 2: 162.025 MHz- Channel 88B (Duplex for the ship to shore)

Originally, AIS was used terrestrially, meaning the signal was sent from the boat to land and had a range of roughly 20 miles (also considering the earth’s curvature). As ships began sailing further away from land, they started sending the signal to low orbit satellites, relaying information back to land. This meant ships could sail as far as they liked, and we’d always have peace of mind knowing exactly where they were and how they were doing.

The AIS system consists of one VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and a standard marine electronic communications link to shipboard display and sensor systems. Position and timing information is normally derived from an integral or external GPS receiver. Other information broadcast by the AIS is electronically obtained from shipboard equipment through standard marine data connections.

Although only one channel is necessary, each station transmits and receives over two radio channels to avoid interference and communication loss from ships. A position report from one AIS station fits into one of 2250 time slots established every 60 seconds. AIS stations continuously synchronize themselves to avoid overlap of slot transmissions.