Electronics
What Is AIS on a Boat? Class A vs Class B Explained
Published · Last checked October 2026
AIS (Automatic Identification System) is a VHF-based system that lets vessels broadcast their identity, position, course and speed to each other automatically. On a plotter you see nearby ships as targets with name, MMSI and closest point of approach. Ships over 300 gross tonnes on international voyages must carry Class A transponders. Yachts typically fit Class B, which transmits less often and at lower power, or a receive-only unit that shows others but does not show you.
How AIS works
Each AIS unit takes its position from GPS and transmits short digital messages on two dedicated VHF frequencies (161.975 and 162.025 MHz). Every receiver in range decodes them and plots the targets. Because it is VHF, range is roughly line of sight: often 10 to 20 miles between ships with high antennas, less for a yacht with a low antenna.
Class A, Class B and receivers
| Type | Who uses it | Transmits | Notes |
|---|---|---|---|
| Class A | Commercial ships (mandatory over 300 GT on international voyages) | Every few seconds when under way, 12.5 W | Includes destination, ETA, navigational status |
| Class B (SOTDMA) | Yachts and small craft | Up to every 5 s at speed, 5 W | Better slot access than CSTDMA |
| Class B (CSTDMA) | Yachts and small craft | Typically every 30 s, 2 W | Cheaper; may be filtered in busy areas |
| Receive only | Yachts | Nothing | You see ships; they do not see you |
A transponder needs an MMSI number and must be listed on your Ofcom Ship Radio Licence.
Using AIS for collision avoidance
- Set a CPA (closest point of approach) and TCPA (time to CPA) alarm, for example 1 mile and 12 minutes.
- Use the vessel name to call a ship directly on VHF or by DSC.
- Check targets against what you see and against radar.
- Remember the COLREGs still apply: AIS data does not change who gives way.
Limits of AIS
Not every vessel carries it: small fishing boats, many yachts and some warships do not transmit. Positions depend on the other vessel's GPS and set-up, and Class B targets update less often. AIS is an aid to keeping a lookout, not a replacement for one. Learn more in the radar and AIS lesson and electronic navigation lesson.