Module 19 - Electronic Navigation
Collision Assessment with Radar
What you will learn
Use repeated radar observations and tracker outputs to assess collision risk without treating one vector, CPA or bearing trend as a promise.
Collision assessment is a time series. Radar, visual bearings, trails and tracker outputs support the judgement, but they depend on own motion, target manoeuvres, sensor inputs, acquisition time and the applicable COLREG situation.
A target whose bearing does not appreciably change while range decreases is a strong collision-risk warning. Risk can still exist when the bearing changes, particularly at close range or with a large vessel, tow or uncertain data. On relative-bearing displays, own heading must remain steady or the measurements must be converted and interpreted correctly before comparing them.
CPA and TCPA are projections from the motion data used by the tracker. They can be unstable during acquisition or after either vessel manoeuvres, and serious errors can follow incorrect heading or speed input, target swap, clutter or loss. Use systematic observations, appropriate ranges and a cautious error allowance rather than one snapshot.
Before selecting action, establish whether vessels are in sight and which COLREG rules apply. Take early, substantial and readily apparent action when required, then re-observe until finally past and clear. Old CPA/TCPA values do not verify the new situation after a manoeuvre.
Turn a vector into a monitored decision
A newly acquired target shows 0.2 M CPA, but the vector is still settling and the yacht's heading input has just changed.
1. Stabilise evidence
Continue repeated range and bearing observations and allow the tracker to settle where time and safety permit.
2. Check inputs
Verify heading, speed, motion mode, target identity and any own manoeuvre.
3. Apply the rules
Determine visibility and encounter geometry before choosing a Rule-compliant action.
4. Verify the result
Replot or reassess the new relative track and monitor until finally past and clear.
Sense check: A changing CPA immediately after acquisition or manoeuvre is a reason for more observation and margin, not false precision.
Common mistake or limitation
- One automatic vector or CPA value is not a complete collision assessment.
- A changing bearing does not always remove risk, especially at close range or with large targets.
- Relative bearing comparisons are invalid if own heading changes without being accounted for.
- AIS identity does not convert a radar-only target into a vessel in sight.
Recap
- Use repeated observations and allow for tracker acquisition and sensor error.
- Treat CPA/TCPA as projections, not promises.
- Apply the COLREG rule set that matches the actual visibility and encounter.
- Verify the effect of action until the vessels are finally past and clear.
Optional quick check
Section 8 of 18
After an avoiding alteration, why should the navigator not quote the old CPA as proof that the manoeuvre worked?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
- RYA Yachtmaster Offshore examination scope
- RYA Coastal Skipper/Yachtmaster Offshore Theory course
- RYA Digital First: the rise of eNavigation
- MCA MGN 610 Amendment 1 — navigation and voyage planning
- MCA MGN 379 Amendment 1 — electronic navigational aids
- IMO Convention on the International Regulations for Preventing Collisions at Sea
Continue studying Electronic Navigation
This public lesson is part of Revision Module 19. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.