AIS overview

AIS (Automatic Identification System) is a self-organizing radio network that ships use to broadcast their position, course, speed, and identifying information to other ships and to shore stations. Every commercial vessel over 300 gross tons and all passenger ships are required to transmit AIS continuously.

AIS operates on two VHF channels:

Channel 87B (161.975 MHz)
Channel 88B (162.025 MHz)

Transmissions are self-organized using TDMA (time-division multiple access). Each ship listens, finds an unused time slot, and transmits in it. The network reconfigures itself as ships enter and leave a coverage area.

MMSI: the vessel's identity

Every AIS-equipped vessel has an MMSI (Maritime Mobile Service Identity), a 9-digit number assigned by the vessel's flag state. The MMSI is the primary key for joining AIS data to vessel registration databases. In the US, the FCC issues MMSIs as part of the Ship Radio Service license; our /ships-aircraft vessel search uses the MMSI to join FCC registration data with live AIS position.

The leading digits identify the issuing country: 366-369 are US-issued, 232-235 are UK, 440-441 are South Korea, and so on (per ITU MID list).

Class A vs Class B

The standard distinguishes two transmitter classes:

Class A: required for commercial vessels over 300 GT and all

passengers. Higher transmit power (12.5 W), reports every 2 to 10 seconds when moving, every 3 minutes when stationary. Carries the full static-data set (vessel name, callsign, type, dimensions, destination, ETA, draught).

Class B: voluntary, for recreational and smaller commercial.

Lower power (2 W), reports every 30 seconds when moving. Slimmer static-data set. Many sailboats and yachts carry Class B.

Vessel type codes

AIS encodes vessel type as a single integer 0-99:

20-29: Wing-in-ground craft (rare).
30: Fishing vessel.
31-32: Tug.
33: Dredger.
35: Military.
36: Sailing.
37: Pleasure craft.
40-49: High-speed craft.
50: Pilot vessel.
51: Search and rescue.
52: Tug (service).
55: Law enforcement.
60-69: Passenger (61-64 are hazmat categories A-D).
70-79: Cargo (71-74 hazmat A-D).
80-89: Tanker (81-84 hazmat A-D, with 81 being the most

dangerous category).

90-99: Other.

What every field on the live ship panel means

Position: lat / lng, updated every few seconds for movers.
Course (COG): course over ground in degrees true. The actual

direction the vessel is moving, accounting for wind and current.

Speed (SOG): speed over ground in knots.
Heading: where the bow is pointing (may differ from COG

if currents are pushing the vessel sideways). 511 means not available.

Destination: free-text field. Often a real port (USNYC =

New York) but sometimes "TO SEA" or operator-set local jargon.

ETA: estimated arrival time, set manually by the bridge.
Nav status: under way / at anchor / moored / aground /

engaged in fishing, etc.

Length and beam: dimensions in meters (from antenna A+B

and C+D positional offsets).

Draught: current loaded depth in meters. Higher draught means

laden; lower means unladen.

IMO number: the permanent vessel identifier (different from

MMSI, which can change with reflagging). Use this to look up cargo manifest history on MarineTraffic or VesselFinder.

Where the data comes from

Our /ships-aircraft map and vessel search pull live AIS from AISStream.io, a community-fed network of land-based AIS receivers. Coverage is best near coastlines and ports; open-ocean and remote-area coverage is patchier.