Module 15 - Restricted Visibility
AIS in Fog
AIS (Automatic Identification System) is a powerful complement to radar in restricted visibility. AIS-equipped vessels broadcast their identity (MMSI number and vessel name), position, Course Over Ground (COG), Speed Over Ground (SOG), and navigational status. Your AIS receiver or chart plotter can display these targets and compute CPA (Closest Point of Approach) and TCPA (Time to CPA), giving you an early warning of collision risk.
AIS has significant limitations. Not all vessels carry AIS — small fishing boats, dinghies, kayaks, and many leisure craft under 15 metres are not required to fit it. Class B transponders (common on yachts) transmit at lower power and less frequently than Class A units (mandatory on commercial vessels over 300 GT and all passenger ships). AIS data depends on the other vessel having their unit switched on and correctly configured.
Never rely on AIS alone. A vessel not appearing on AIS may still be present. Use AIS as one layer in your overall situational awareness: combine it with radar, visual lookout, listening for fog signals, and VHF monitoring. Treat AIS as a complement to — never a substitute for — keeping a proper lookout by all available means (Rule 5).
Key points
- AIS shows vessel identity, position, COG, SOG, and status
- CPA and TCPA can be computed from AIS data
- Not all vessels carry AIS — especially small craft
- Class A: commercial vessels (higher power, more frequent updates)
- Class B: leisure craft (lower power, less frequent updates)
- Never rely on AIS alone — always combine with radar and visual lookout
- AIS complements but does not replace a proper lookout (Rule 5)
Continue studying Restricted Visibility
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