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AIS Vessel Decoder

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

Hardware, Radio & TelecomFree
INPUT
OUTPUT

The result appears here as you type.

Decoding runs in your browser. What you paste is not sent to a server.

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.

Example

Input
!AIVDM,1,1,,B,177KQJ5000G?tO`K>RA1wUbN0TKH,0*5C
Output
Message type        1
MMSI                477553000
Navigation status   5 — Moored
Speed over ground   0.0 kn
Longitude           -122.345833
Latitude            47.582833
Course over ground  51.0°
True heading        181°
Timestamp (second)  15
Messages: 1

What is the AIS Vessel Decoder?

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.

How to use the AIS Vessel Decoder

  1. Paste the full !AIVDM sentence, including all fragments of a multi-part message.
  2. The six-bit payload is unpacked and the message type identified.
  3. Read the MMSI, position, speed and any static vessel data.

When to use it

  • Debugging an AIS receiver or a marine data feed.
  • Decoding archived vessel traffic for research or incident reconstruction.
  • Learning the six-bit AIS payload format for a decoder project.

AIS Vessel Decoder at a glance

Sentence!AIVDM / !AIVDO, NMEA 0183 with checksum
PayloadSix-bit ASCII armouring — not byte-aligned
Key types1/2/3 position · 5 static & voyage · 18 Class B
MMSI9 digits; first 3 identify the flag state

Frequently asked questions

What is an MMSI?

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.

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