AIS message type
AIS message type 21: Aid-to-navigation report
A buoy, beacon, light or virtual mark reporting its identity and position. What type 21 carries, who sends it, how often, and how to read it.
What type 21 is
Type 21 identifies an aid to navigation by type (a 5-bit code covering lighthouses, beacons, cardinal and lateral marks, safe-water marks and others), gives its 20-character name, position, dimensions and whether it is off its charted position. A virtual flag marks a mark that exists only as a broadcast, with no physical structure at that spot.
How to read it
The sender's MMSI begins 99. A buoy that drags its anchor sets the off-position flag. A virtual aid to navigation is a transmission only; there is nothing physical at that position.
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 | |
| 38–42 | Aid-to-navigation type | |
| 43–162 | Name | 20 six-bit characters; an extension can follow the main message |
| 163 | Position accuracy | |
| 164–191 | Longitude | |
| 192–218 | Latitude | |
| 219–227 | Dimension A | |
| 228–236 | Dimension B | |
| 237–242 | Dimension C | |
| 243–248 | Dimension D | |
| 249–252 | Position-fixing device | |
| 253–258 | UTC second | |
| 259 | Off position | set when the mark is away from its charted position (valid only when the UTC second is 0 to 59) |
| 269 | Virtual aid | 1 = virtual: broadcast only, no physical mark |
| 270 | Assigned mode |
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 |