AIS message type
AIS message type 3: Position report, response to interrogation (Class A)
Same layout as type 1, sent when a station asks the ship to report. What type 3 carries, who sends it, how often, and how to read it.
What type 3 is
Type 3 is a type 1 layout sent because another station polled the ship with an interrogation (type 15). It is the only one of the three Class A position messages that is a reply rather than a broadcast.
How to read it
Position decoders treat 1, 2 and 3 identically. A rare type 3 usually means a coast station or another ship asked for a fix.
Field layout
Bit offsets are counted from zero at the first bit of the decoded payload. Type 3 uses the same layout as type 1.
| 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 |