AIS message type
AIS message type 1: Position report, scheduled (Class A)
The standard Class A position report: where the ship is, how fast, which way. What type 1 carries, who sends it, how often, and how to read it.
What type 1 is
Type 1 is the message behind most dots on any AIS map. It carries the MMSI, navigational status, rate of turn, speed over ground, position, course over ground, true heading and a seconds-of-UTC timestamp. A Class A transponder sends it on a schedule it chooses for itself (self-organised time division, SOTDMA), faster when the ship is moving quickly or turning.
How to read it
Everything in a type 1 except the navigational status comes from sensors: the status is typed in by the crew. A speed of 102.3 knots, a course of 360.0 degrees, a heading of 511 and a position of 181 degrees longitude are the "not available" values, not measurements.
Field layout
Bit offsets are counted from zero at the first bit of the decoded payload. Types 1 and 2 and 3 share this layout.
| 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 | |
| 38–41 | Navigational status | |
| 42–49 | Rate of turn | -128 is not available |
| 50–59 | Speed over ground | 1023 = not available |
| 60 | Position accuracy | |
| 61–88 | Longitude | 1/10,000 minute; 181° = not available |
| 89–115 | Latitude | 1/10,000 minute; 91° = not available |
| 116–127 | Course over ground | 3600 = not available |
| 128–136 | True heading | 511 = not available |
| 137–142 | UTC second | 60 = not available, 61 to 63 = special modes |
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 |