What Do AIS Vessel Data Fields Mean?
Click on any ship on a tracking map and you are shown a panel full of numbers and codes: MMSI, COG, SOG, heading, nav status, draught. This is AIS vessel data, and once you know what each field means, it tells you a great deal about who a ship is, what it is doing and where it is heading.
This guide explains every common AIS data field, grouped the way the system itself groups them.
The three types of AIS information
AIS information falls into three groups, each updated in a different way:
- Static data describes the ship itself. It is programmed into the transponder at installation and rarely changes.
- Dynamic data describes the ship's movement. It comes automatically from the ship's sensors and is broadcast every few seconds.
- Voyage data describes the current trip. The crew enters it manually, so it is only as accurate as the last update.
Static AIS data fields
| Field | What it means |
|---|---|
| MMSI | Nine-digit radio identity. The first three digits show the flag state. |
| IMO number | Seven-digit permanent hull identifier. Broadcast by Class A transponders only. |
| Vessel name | Up to 20 characters. Can change over the ship's life. |
| Call sign | The ship's international radio call sign. |
| Ship type | A two-digit code. For example, 70–79 are cargo ships, 80–89 tankers, 60–69 passenger ships, 30 fishing and 52 tugs. |
| Dimensions (A, B, C, D) | Distances from the GPS antenna to the bow, stern, port side and starboard side. Added together they give length and beam. |
| Position fixing device | The source of the position, usually GPS or another GNSS. |
Dynamic AIS data fields
| Field | What it means |
|---|---|
| Latitude and longitude | The ship's position, usually from its GPS receiver. |
| Position accuracy | High (better than 10 meters) or low (worse than 10 meters). |
| Timestamp | The UTC second at which the position was taken. |
| SOG (speed over ground) | Speed in knots relative to the seabed, not the water. A value of 102.3 means "not available". |
| COG (course over ground) | The actual direction of travel over the ground, in degrees. |
| True heading | The direction the bow is pointing, from the gyrocompass. A value of 511 means "not available". |
| ROT (rate of turn) | How fast the ship is turning, in degrees per minute. |
| Navigational status | What the ship says it is doing (see below). |
Note the difference between COG and heading. Heading is where the bow points; COG is where the ship is actually going. In strong currents or crosswinds, the two can differ by several degrees, and that gap is useful information in itself.
Navigational status codes
The crew sets the navigational status on the transponder. The most common values are:
| Code | Status |
|---|---|
| 0 | Under way using engine |
| 1 | At anchor |
| 2 | Not under command |
| 3 | Restricted maneuverability |
| 4 | Constrained by her draught |
| 5 | Moored |
| 6 | Aground |
| 7 | Engaged in fishing |
| 8 | Under way sailing |
| 15 | Not defined (default) |
Because it is set by hand, the status is sometimes left unchanged. A ship showing "moored" while travelling at 12 knots has simply not been updated, so always read status alongside speed.
Voyage AIS data fields
| Field | What it means |
|---|---|
| Destination | Free text of up to 20 characters, often a port name or UN/LOCODE. |
| ETA | Estimated time of arrival at the destination, in UTC. |
| Draught | Current maximum draught in meters. A deeper draught than usual suggests the ship is loaded. |
| Hazardous cargo | Category of dangerous goods on board, if any. |
Voyage data is the most valuable for commercial users and the least reliable, because it depends on the crew. A destination like "FOR ORDERS" is common for ships waiting for charterer instructions.
How to read AIS tracking data with confidence
Raw vessel AIS data is useful, but a few habits make it far more reliable:
- Cross-check fields against each other. Speed, status and position should tell the same story.
- Look at the timestamp. An old position is not the ship's current position, especially in open ocean where satellite updates are less frequent.
- Watch the draught. Comparing draught on arrival and departure is a quick way to infer whether a ship loaded or discharged cargo.
- Treat destination and ETA as indications. Confirm with the agent or operator when the decision matters.
- Notice the gaps. A long silence in a ship's track can mean poor coverage, a switched-off transponder or deliberate concealment.
See AIS vessel information in context with Navtrac
Navtrac brings this AIS vessel information together on an interactive map, alongside the vessel's track, port calls and the maritime traffic around it. Real-time AIS data and predictive analytics help you turn individual fields into decisions about routes, arrivals and fleet performance.
Explore AIS tracking data on Navtrac and see what every field is telling you.