AIS message type
AIS message type 18: Standard Class B equipment position report
The Class B equivalent of type 1: position, speed, course and heading. What type 18 carries, who sends it, how often, and how to read it.
What type 18 is
Type 18 carries a Class B transponder's MMSI, speed, position, course, heading and timestamp in a single slot. It has no navigational status and no rate-of-turn field, which is why Class B vessels show no status on a map. It also has flags for whether the unit has a display, DSC, band-switching and message 22 capability.
How to read it
Class B is the transponder fitted to most pleasure craft, small fishing boats and workboats. Its transmit power is lower (2 W for CS, 5 W for SO against 12.5 W for Class A) and Class A traffic has priority, so Class B reports can be thinner at the edge of coverage.
Field layout
Bit offsets are counted from zero at the first bit of the decoded payload.
| Bits | Field | Notes |
|---|---|---|
| 0–5 | Message type | |
| 6–7 | Repeat indicator | 0 = original, 1 to 3 = repeated by a repeater; 3 means do not repeat |
| 8–37 | MMSI | |
| 46–55 | Speed over ground | 1023 = not available |
| 56 | Position accuracy | |
| 57–84 | Longitude | |
| 85–111 | Latitude | |
| 112–123 | Course over ground | |
| 124–132 | True heading | |
| 133–138 | UTC second |
Paste a real sentence into the AIS message decoder to see these fields filled in.
All message types
| Type | Name |
|---|---|
| 1 | Position report, scheduled (Class A) |
| 2 | Position report, assigned schedule (Class A) |
| 3 | Position report, response to interrogation (Class A) |
| 4 | Base station report |
| 5 | Static and voyage related data (Class A) |
| 6 | Binary addressed message |
| 7 | Binary acknowledge |
| 8 | Binary broadcast message |
| 9 | Standard SAR aircraft position report |
| 10 | UTC and date inquiry |
| 11 | UTC and date response |
| 12 | Addressed safety related message |
| 13 | Safety related acknowledgement |
| 14 | Safety related broadcast message |
| 15 | Interrogation |
| 16 | Assignment mode command |
| 17 | DGNSS broadcast binary message |
| 18 | Standard Class B equipment position report |
| 19 | Extended Class B equipment position report |
| 20 | Data link management message |
| 21 | Aid-to-navigation report |
| 22 | Channel management |
| 23 | Group assignment command |
| 24 | Static data report (Class B) |
| 25 | Single slot binary message |
| 26 | Multiple slot binary message with communications state |
| 27 | Long-range AIS broadcast message |