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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.
| 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 |
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.
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 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-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.