Also known as: AIS
AIS (Automatic Identification System) is a maritime safety system in which ships and shore stations broadcast their identity, position, course, and speed on VHF marine frequencies. It uses GMSK modulation in a self-organising TDMA scheme so many vessels share two channels without a central controller.1
Overview
Each station transmits position reports keyed to its MMSI identifier on 161.975 MHz (AIS 1) and 162.025 MHz (AIS 2). Receiving AIS gives a live map of nearby vessel traffic, the maritime counterpart to ADS-B.
Technical characteristics
| Property | Value |
|---|---|
| Band | VHF marine, ~162 MHz |
| Access | SOTDMA |
| Modulation | GMSK, 9600 bps |
| Framing | HDLC-style with CRC-16 |
History
Standardised by the ITU and mandated by the IMO for SOLAS vessels from the early 2000s to improve collision avoidance and traffic monitoring.1
Deployment
Commercial shipping, port authorities, vessel-traffic services, and hobbyist tracking.
Decoding it with GopherTrunk
GopherTrunk demodulates the GMSK bursts, frames them, and decodes position reports. See the AIS decoder page.
Sources
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Automatic identification system — Wikipedia, for the maritime VHF AIS system, its GMSK/SOTDMA air interface, message content, and ITU/IMO standardisation. ↩ ↩2