AIS — Automatic Identification System

AIS is a self-organising VHF transponder system that broadcasts a vessel's identity, position, course, speed, and other data at regular intervals on two dedicated VHF channels. It was introduced under SOLAS Chapter V Regulation 19 and made mandatory in stages between 2002 and 2008 for international voyages of vessels ≥ 300 GT, all passenger ships, and cargo vessels ≥ 500 GT on domestic voyages. AIS is now the foundational layer for situational awareness on the bridge, for shore-based traffic monitoring (VTS), and for global vessel tracking via satellite.

AIS does not replace radar; it complements it. Radar shows you what is out there regardless of whether the target wishes to be seen; AIS shows you what each cooperating target wants you to know about it. Both pictures should agree — when they don't, treat the AIS picture with suspicion.

Class A vs Class B

Class A — mandatory on SOLAS-class vessels. SOTDMA (Self-Organised Time Division Multiple Access) over two channels (AIS 1 — 161.975 MHz, AIS 2 — 162.025 MHz). 12.5 W transmit power. Minimum reporting interval 2 seconds. Carries all 27 standard message types and integrates with ECDIS and the bridge alarm system.
Class B — fitted to non-SOLAS vessels (yachts, fishing, small commercial). CSTDMA (Carrier Sense TDMA) — listens before transmitting. 2 W transmit power. Reporting interval typically 30 seconds (5 seconds for SO/CS-TDMA "Class B+ / B-SO"). Limited message set.

Reporting intervals (Class A)

Vessel state / speed	Interval
At anchor / moored, < 3 kt	3 minutes
At anchor / moored, > 3 kt	10 seconds
0–14 kt	10 seconds
0–14 kt + changing course	3⅓ seconds
14–23 kt	6 seconds
14–23 kt + changing course	2 seconds
> 23 kt	2 seconds

Static, dynamic, and voyage data

· Static (every 6 minutes / on request) — MMSI, IMO number, callsign, name, ship type, dimensions, GPS antenna position.
· Dynamic (per the table above) — position, COG, SOG, heading, rate of turn, navigational status, UTC time.
· Voyage (every 6 minutes / on change) — draft, hazardous cargo flag, destination, ETA, route plan.
· Safety — short safety-related text messages (broadcast or addressed to a specific MMSI).

Navigational status codes

Set manually on the AIS controller; changing this is part of bridge handover routine. Code 14 is reserved for AIS-SART, MOB-AIS, and EPIRB-AIS — never set this manually.

0Under way using engine
1At anchor
2Not under command (NUC)
3Restricted manoeuvrability (RAM)
4Constrained by her draft
5Moored
6Aground
7Engaged in fishing
8Under way sailing
9Reserved (HSC / WIG)
10Reserved (HSC / WIG)
11Power-driven vessel towing astern (regional)
12Power-driven vessel pushing ahead or towing alongside (regional)
13Reserved
14AIS-SART (active), MOB-AIS, EPIRB-AIS
15Undefined / default

MMSI

The 9-digit Maritime Mobile Service Identity is allocated by the seafarer's flag administration. The first three digits are the Maritime Identification Digits (MID — country code). For ships, the last six are unique to the vessel. Specialised MMSIs use the form 974 xxx xxx (AIS-SART, MOB-AIS, EPIRB-AIS) or 970 xxx xxx (SAR aircraft). Coast stations use 00 + MID + 4 digits. Group MMSIs use 0 + MID + 5 digits.

AIS-SART, MOB, and EPIRB-AIS

· AIS-SART — broadcasts an AIS Class A position report every minute when activated. MMSI begins 970. Replaces or supplements the X-band radar SART.
· MOB-AIS (man overboard) — personal beacon; transmits position when activated. MMSI begins 972.
· EPIRB-AIS — combined 406 MHz EPIRB and AIS transmitter. MMSI begins 974. AIS transmission supports on-scene location.

Spoofing, jamming, and the "dark fleet"

AIS data is unauthenticated — anything that can transmit on the AIS channels can claim any identity and any position. Documented misuses: false "ghost" tracks (sanctions-evasion tankers transmitting positions in one ocean while physically operating in another); position offsets to obscure visits to sanctioned ports; spoofed identities in conflict zones. Treat AIS data as an aid, never as authoritative truth — cross-check with radar, ECDIS, visual bearings, and shore-side intelligence.

Satellite AIS & long-range identification

Satellite-based AIS receivers (operated by exactEarth, ORBCOMM, Spire, and others) capture VHF AIS broadcasts globally and provide near-real-time worldwide ship tracking. SOLAS-mandated LRIT (Long-Range Identification and Tracking) is a separate, encrypted system reporting position to flag-state and authorised SAR/security users every 6 hours, regardless of AIS. AIS is open and ubiquitous; LRIT is closed and authoritative.