AIS message type
AIS message type 4: Base station report
A shore station announcing itself, its position and the UTC date and time. What type 4 carries, who sends it, how often, and how to read it.
What type 4 is
Base stations broadcast type 4 so that ships in range can synchronise their slot timing to UTC and know a shore station is present. It carries the full UTC year, month, day, hour, minute and second plus the station's own position.
How to read it
A type 4 is not a vessel. Its MMSI begins 00 followed by a country code, and its position is a fixed antenna site.
Field layout
Bit offsets are counted from zero at the first bit of the decoded payload. Types 4 and 11 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 | |
| 38–51 | UTC year | 0 = not available |
| 52–55 | UTC month | |
| 56–60 | UTC day | |
| 61–65 | UTC hour | 24 = not available |
| 66–71 | UTC minute | 60 = not available |
| 72–77 | UTC second | 60 = not available |
| 78 | Position accuracy | |
| 79–106 | Longitude | |
| 107–133 | Latitude | |
| 134–137 | Position-fixing device |
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 |