A nine-digit Maritime Mobile Service Identity uniquely identifying a vessel's radio station. The first three digits are the Maritime Identification Digits of the flag state — 232 for the UK, 366 for the USA.

Why are AIS payloads six-bit encoded?

To pack binary data into printable ASCII that survives NMEA 0183 transport. Each payload character carries six bits, so the real fields must be read as bit offsets rather than bytes.

What is the difference between Class A and Class B AIS?

Class A is mandatory for larger commercial vessels, transmits at higher power and reports more often. Class B is the lower-power voluntary standard for leisure and small craft, using message type 18.

AIS Vessel Decoder

Decode AIS NMEA 0183 vessel-tracking payloads into readable fields.

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The ais-decoder decoder is on the roadmap. The heavier binary and cryptographic parsers ship after the core text and token decoders — all of which are already live and working.

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What is the AIS Vessel Decoder?

AIS (Automatic Identification System) broadcasts vessel identity, position, course and speed over VHF, packaged in NMEA 0183 sentences whose six-bit-ASCII payload an AIS decoder unpacks into structured fields.

An AIS sentence looks like !AIVDM,1,1,,A,13aG?P0P00PD;88MD5MTDww@2D7k,0*53. The fields give the fragment count and index (long messages split across sentences), the radio channel, the armoured payload and a checksum. The payload uses six-bit ASCII: each character carries six bits, and the message's real fields are bit ranges within that stream, not byte-aligned values.

Message type 1, 2 and 3 are position reports carrying MMSI, navigation status, rate of turn, speed over ground, position, course and heading. Type 5 is the static and voyage report — vessel name, call sign, IMO number, dimensions, draught and destination. Type 18 covers the lighter Class B transponders fitted to smaller craft.

MMSI is the key identifier, and its first three digits are the Maritime Identification Digits of the flag state. Like ADS-B, AIS is unencrypted, which is why public vessel-tracking sites exist and why spoofed AIS tracks are a documented problem in maritime security.