Module 15 - Restricted Visibility
AIS in Fog
What you will learn
Use AIS for identity and situational awareness without mistaking transmitted ground-referenced data, target association or VHF contact for a complete collision-avoidance solution.
AIS can reveal useful information that radar cannot, but it reports what another system transmits. Targets can be absent, delayed, manually wrong or associated with the wrong echo.
AIS may provide MMSI, name, position, course over ground, speed over ground, heading when available, navigational status, dimensions and vessel type. Some data is sensor-generated and some is entered manually. Navigation status, dimensions and voyage data can be stale or wrong, and the AIS position may not coincide exactly with the radar echo. Treat the symbol as transmitted information rather than the physical vessel itself.
Not every vessel or floating hazard carries AIS. Class B equipment commonly used by recreational and fishing craft has different reporting performance from Class A, and received updates may be intermittent or delayed. A virtual or synthetic aid-to-navigation symbol may have no physical object at its plotted position, while a real unlit object or small craft may have no AIS symbol at all.
MCA guidance says collision-avoidance decisions should be based primarily on systematic visual and/or radar observations and comply with the COLREG. AIS course and speed are over ground, whereas collision decisions may need water-referenced motion and direct relative observations. Calling an AIS name by VHF does not guarantee that the correct target has been contacted; attempted passing agreements can waste time, create misunderstanding and distract from lawful observable action.
| AIS field | Possible value | Important limitation |
|---|---|---|
| Identity | MMSI, name and vessel type | Manual or configuration errors; wrong target association |
| Position and motion | Position, COG, SOG and sometimes heading | Sensor error, latency and ground-versus-water reference |
| Voyage data | Status, draught or destination where supplied | Manual entries may be stale or wrong |
| CPA and TCPA | Prediction from received and own-ship data | Not a COLREG rule, universal safe threshold or substitute for radar |
Separate a radar echo from an AIS symbol
Radar shows a closing echo fine on the starboard bow. An AIS target with a similar bearing is labelled as a fishing vessel, but its reported position is offset and its navigation status has not changed for several hours.
1. Keep two hypotheses
Do not associate the symbol and echo until repeated radar and AIS motion support the match.
2. Use direct risk evidence
Assess collision risk from the systematic radar observations and the complete Rule 19 picture.
3. Use AIS cautiously
Treat identity and transmitted data as supporting awareness; mark stale or conflicting fields as uncertain.
4. Avoid a negotiated shortcut
Do not replace COLREG-compliant action with a VHF passing agreement based on uncertain identification.
Sense check: An AIS name makes a target feel certain, but a confident label attached to the wrong echo is more dangerous than an honestly unidentified contact.
Common mistake or limitation
- Assuming every vessel, buoy and obstruction transmits AIS or that every displayed symbol represents a physical object.
- Treating manually entered status, dimensions or destination as current verified facts.
- Letting an AIS overlay replace the radar track even when association or reference data disagrees.
- Using an AIS name to negotiate a non-COLREG passing plan by VHF in a multi-target or restricted-visibility situation.
Recap
- AIS is transmitted data for awareness, not a complete physical traffic picture.
- Association, latency, manual fields and ground-referenced motion can mislead the decision.
- Use systematic radar or visual evidence where available and keep collision avoidance inside the COLREG.
Optional quick check
Section 7 of 8
An AIS symbol and radar echo are close but do not align. Which response is strongest?
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
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