AIS (Automatic Identification System) is a VHF radio system ships use to broadcast their identity, position, course, speed and navigational status automatically to other ships, shore stations and aircraft. The International Maritime Organization requires it on ships of 300 gross tonnage and upwards on international voyages, on cargo ships of 500 gross tonnage and upwards on other voyages, and on all passenger ships regardless of size.
§01What is AIS?
AIS was adopted under SOLAS Chapter V, regulation 19. The carriage requirement became effective for all covered ships by 31 December 2004.1Source 1 · International Maritime OrganizationAIS transpondersimo.org The IMO says AIS shall automatically provide the ship’s identity, type, position, course, speed, navigational status and other safety information to appropriately equipped shore stations, other ships and aircraft.1Source 1 · International Maritime OrganizationAIS transpondersimo.org The technical standard is ITU-R M.1371, which defines a VHF system using time-division multiple access.2Source 2 · International Telecommunication UnionRecommendation ITU-R M.1371: Technical characteristics for VHF automatic identification system using TDMA in the maritime mobile serviceitu.int
§02How does AIS work?
Each shipborne unit has one VHF transmitter, two TDMA receivers and a DSC receiver, connected to the ship’s navigation sensors. Stations synchronize with one another and share time slots. A position report fits into one of 2,250 slots established every 60 seconds, and stations self-organize to avoid overlapping transmissions.3Source 3 · U.S. Coast Guard Navigation CenterHow AIS Worksnavcen.uscg.gov
| AIS | ADS-B | |
|---|---|---|
| Domain | Ships (and aids to navigation) | Aircraft and airport vehicles |
| Radio | Maritime VHF, TDMA | 1090 MHz or 978 MHz |
| Stable identifier | MMSI (and IMO number in static data) | 24-bit ICAO address |
| Position source | Ship’s own navigation (usually GNSS) | Aircraft’s own navigation (usually GNSS) |
| Who requires it | IMO SOLAS carriage rules | National and regional airspace rules |
§03Example: reading a vessel’s day
A cargo ship’s dynamic reports (position, speed, course, status) arrive frequently, while its static and voyage data (name, dimensions, destination, ETA) arrive less often and are typed in by the crew. In analysis, join the two on MMSI, then check that destination and ETA agree with the track. Manually entered fields are the ones most often stale.
§04Common misconceptions
- “Every boat has AIS.” Carriage rules cover specific ships. Many smaller vessels carry Class B or nothing.1Source 1 · International Maritime OrganizationAIS transpondersimo.org4Source 4 · U.S. Coast Guard Navigation CenterTypes of Automatic Identification Systemsnavcen.uscg.gov
- “A gap means the ship switched it off.” Gaps also come from receiver coverage, slot congestion and equipment faults. The IMO also allows AIS to be off where international rules provide for protecting navigational information.1Source 1 · International Maritime OrganizationAIS transpondersimo.org
- “AIS positions are verified.” Like ADS-B, AIS is self-reported, and fields such as destination are manual entries.
§05How Kimo uses AIS
The AIS connector loads position and static reports into a vessel tracks model keyed by MMSI, with receiver coverage stored next to each gap so analysts can tell “no signal” from “no receiver”. Kimo Defense Intelligence can fuse AIS with ADS-B and OSINT for maritime and coastal awareness. The AIS gaps post walks through gap scoring, and anonymized case studies show it in use.
§06Related terms
- ADS-B: the aviation equivalent.
- Data fusion: combining AIS with other tracking sources.
- OSINT: turning open AIS data into verified findings.
Frequently asked questions
What is an MMSI?
What is the difference between AIS Class A and Class B?
Can AIS be received from space?
Sources
4 references- AIS transponders (opens in a new tab)International Maritime Organizationimo.org
SOLAS V/19 carriage requirements; effective date; information AIS shall provide; operation at all times with exceptions.
- Recommendation ITU-R M.1371: Technical characteristics for VHF automatic identification system using TDMA in the maritime mobile service (opens in a new tab)International Telecommunication Unionitu.int
Technical standard for AIS (current version M.1371-6).
- How AIS Works (opens in a new tab)U.S. Coast Guard Navigation Centernavcen.uscg.gov
Transceiver architecture; 2,250 slots per 60 seconds; self-organizing slot selection.
- Types of Automatic Identification Systems (opens in a new tab)U.S. Coast Guard Navigation Centernavcen.uscg.gov
Class A vs Class B equipment.
External sources were accessed at the time of writing. Kimo product details, customers and figures in examples are illustrative unless a source is cited.
