Module 19 - Electronic Navigation
Radar Fixing and Parallel Indexing
What you will learn
Use verified radar ranges, bearings and parallel indexes as bounded observations, and explain what each can and cannot establish about position or track.
Radar can supply valuable position and track evidence, but reliability depends on set-up, target identity, geometry, timing, charting and the distinction between a line, a fix and a monitoring boundary.
Radar range is generally treated as more accurate than radar bearing on a correctly set-up small-craft set, but neither measurement is error-free. One range gives a circle of position; a bearing gives a line subject to beam width and heading alignment. A fix from several observations is only as defensible as the target identification, geometry, timing, charted feature and plotting.
Parallel indexing is a monitoring technique, not a position fix or an automatic safe-water guarantee. Set an index line parallel to the planned track with a checked offset from an identified feature or danger. If the echo changes relative to the line, investigate the vessel's achieved track, tide, steering and observation quality while the protected margin remains.
A radar overlay can help compare radar echoes with charted features, but an overlay may share GNSS and heading inputs with the chart display. Misalignment can reveal a problem; apparent alignment does not independently validate every source behind the overlay.
Use an index as an early boundary
In reduced visibility, a correctly identified coastline echo begins moving across the planned parallel index towards the danger side.
1. Verify
Confirm range scale, orientation, heading input, feature identity and the charted offset used.
2. Interpret
Treat the movement as evidence that the achieved track or observation differs from plan, not as an automatic helm order.
3. Protect
Use the pre-briefed speed, sea-room or turn-away action before the safe offset is consumed.
4. Cross-check
Compare with depth, other radar ranges, compass, log and position evidence with different failure paths.
Sense check: An index line can show movement relative to the planned boundary; it does not identify the cause or remove the need for a safe response.
Common mistake or limitation
- Several radar ranges do not guarantee a reliable fix if the echoes are misidentified, poorly separated or not simultaneous enough for the situation.
- An EBL bearing inherits heading-alignment and beam-width limitations.
- Parallel indexing monitors an offset; it is not a two-dimensional position fix.
- Radar overlay agreement can repeat the same GNSS or heading error used by the chart display.
Recap
- Treat range, bearing, fixes and index lines as different kinds of evidence.
- Verify target identity, geometry, timing and set-up before using a radar observation.
- Use parallel indexes as early monitored boundaries with stated safe sides.
- Cross-check radar with evidence that has a different failure path.
Optional quick check
Section 7 of 18
A coastline echo crosses the parallel index towards danger. What does that establish first?
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.