Also known as: AIS, CSTDMA
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 that many vessels share just two channels without a central controller. AIS is to shipping what ADS-B is to aviation — a continuous, receivable, unencrypted position broadcast that underpins collision avoidance and traffic monitoring.1
Overview
Each AIS station transmits position reports keyed to its MMSI (Maritime Mobile
Service Identity) on 161.975 MHz (AIS 1 / channel A) and 162.025 MHz (AIS 2 /
channel B), alternating between the two for diversity. Reports are carried in
HDLC-style packets with bit-stuffing, a training preamble, a start/stop flag, and a
CRC-16; a shore receiver frames the GMSK bit
stream and emits the familiar !AIVDM NMEA sentences. There are 27 defined message
types: dynamic position reports (types 1–3), the base-station report (type 4), the
static/voyage report giving name, dimensions, and destination (type 5), safety and
binary messages, and the Class B reports used by smaller craft. The result is a live
map of nearby vessel traffic.
Two equipment classes exist. Class A, mandatory on larger SOLAS vessels, uses SOTDMA (Self-Organising TDMA): each transmitter listens, learns which of the 2250 slots per minute per channel are free, and reserves future slots, so the network schedules itself with no master. Reporting rate scales with speed and manoeuvre — every 2–10 s for a moving ship, every 3 minutes at anchor. Class B, for leisure and small commercial craft, transmits at lower power and less often, typically using CSTDMA (carrier-sense TDMA) that simply checks a slot is clear before using it.
Technical characteristics
| Property | Value |
|---|---|
| Band | VHF marine, 161.975 / 162.025 MHz |
| Channel spacing | 25 kHz |
| Access | SOTDMA (Class A) / CSTDMA (Class B) |
| Modulation | GMSK, BT 0.4 |
| Bit rate | 9600 bps |
| Framing | HDLC with bit-stuffing, CRC-16 |
| Slots | 2250 per minute per channel |
| Identity | 9-digit MMSI |
History
AIS was developed in the 1990s and standardised by the ITU in Recommendation ITU-R M.1371, with IEC test standards and IMO carriage requirements. The IMO SOLAS convention mandated Class A AIS on most vessels over 300 gross tons on international voyages and all passenger ships, phased in from 2002 to 2004, to reduce collisions and give coastal authorities a real-time traffic picture. Class B was added later to bring smaller vessels into the picture affordably.12
Deployment
AIS is now ubiquitous in commercial shipping, port operations, vessel-traffic services (VTS), and search-and-rescue. Satellite AIS receivers extend coverage over open ocean where shore stations cannot reach, feeding global ship-tracking services. On the safety side it works alongside DSC distress calling and EPIRB 406 beacons, and AIS-SART transmitters broadcast a man-overboard or lifeboat position using the same air interface. As with ADS-B, the open unencrypted format created a large hobbyist reception community.
Decoding it with GopherTrunk
GopherTrunk tunes the two AIS channels, demodulates the 9600 bps GMSK bursts, performs HDLC de-framing (flag detection and bit de-stuffing), validates the CRC-16, and decodes the message payload into position reports and vessel data. Because AIS is unencrypted and the modulation is straightforward, it is one of the marine protocols GopherTrunk handles end to end, alongside its marine VHF and DSC support. See the AIS decoder page for the live plot.
Sources
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Automatic identification system — Wikipedia, for the maritime VHF AIS system, its GMSK/SOTDMA air interface, Class A/B equipment, message types, and ITU/IMO standardisation. ↩ ↩2
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Recommendation ITU-R M.1371 — International Telecommunication Union, the primary technical standard defining the AIS physical layer (GMSK, 9600 bps), SOTDMA slot structure, and message formats. ↩