Module 6 - Position, Course & Speed
Radar and AIS for Position Confirmation
What you will learn
Use radar ranges, bearings, overlay and AIS as bounded evidence, identify their common inputs and decide what to do when the sources disagree.
Radar can provide independent range and coastline evidence; AIS provides transmitted situational data. Neither becomes reliable merely because it is overlaid on the chart.
Radar range derives from pulse travel time, while bearing accuracy depends on heading input, heading-marker alignment, beam width and target response. Range is often the stronger small-craft fixing observation, but clutter, target identity, range-marker calibration, performance, coastline shape, tide and scale still limit it. Two ranges intersect geometrically; they do not become a reliable fix until the objects and measurements are trustworthy and suitably separated.
Radar image overlay can expose a GNSS or alignment disagreement when fixed echoes fail to match charted features. It can also hide a common-input error: overlay position, orientation and scale may depend on GNSS, heading and chart data. Check the raw radar picture and inputs instead of accepting a tidy combined display.
AIS broadcasts information supplied by other vessels and shore systems. Not every vessel carries or transmits it, manually entered data can be wrong or stale, positions are GNSS-derived and antenna offsets matter. AIS supports situational awareness and identification; it does not replace lookout, radar observation or COLREG assessment.
Position confirmation is a source-management task. Name each observation, its time, physical origin and shared dependencies. When evidence conflicts, preserve sea room and investigate the common input before averaging outputs that may not measure the same thing.
Worked example
At 2210 two radar ranges intersect near the planned track, but the second object is a broad sloping headland and the heading marker has not been checked since equipment work.
- 1Keep both ranges and their times visible; do not label the intersection as certain.
- 2Confirm each echo against shape, adjacent features and expected range; a broad headland may not have one unambiguous chart point.
- 3Check radar performance, range marker and heading-marker alignment. Heading error affects overlay and bearings even if ranges remain plausible.
- 4Add a third independent observation such as a distinct radar range, visual line when available or depth profile, and use the danger-side uncertainty until the evidence converges.
Sense check: A sharp intersection can still be sharply wrong when both arcs were attached to misidentified or poorly defined targets.
| Source | Strength | Critical limitation |
|---|---|---|
| Radar range | Independent distance to a fixed echo | Target identity, calibration, clutter and coastline definition |
| Radar bearing | Direction to an echo | Beam width and heading-marker/input error |
| Radar overlay | Fast chart-picture comparison | Shared GNSS, heading, chart and set-up errors |
| AIS | Identity and transmitted movement context | Missing, stale, manual or GNSS-derived data |
Diagnose a displaced overlay
The raw radar coastline is stable, but the overlay sits 0.15 M east of the chart and nearby AIS targets appear similarly shifted.
1. Recognise common dependence
GNSS position, datum or antenna offset can displace own-ship overlay and GNSS-derived AIS presentation together.
2. Check radar separately
Use raw ranges to identifiable fixed objects and verify heading-marker alignment, scale and performance.
3. Protect and resolve
Increase margin or pause the approach, then check GNSS status, chart datum, sensor offsets and independent observations before restoring reliance on overlay.
Sense check: Overlay agreement is meaningful only after its position, heading, chart and radar inputs have been challenged.
Common mistake or limitation
- Calling two radar ranges automatically reliable without checking target identity, geometry and equipment performance.
- Using radar overlay as an independent check when it shares the suspect GNSS or heading input.
- Assuming absence from AIS means absence from the water, or using AIS alone for collision avoidance.
Recap
- Radar measurements need identified objects and checked performance.
- A neat intersection is not automatic reliability.
- Overlay inherits sensor and chart inputs.
- AIS supplements rather than replaces lookout and radar evidence.
Optional quick check
Section 8 of 8
Radar overlay and AIS targets shift together while raw radar ranges disagree. What should be investigated first?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
- RYA Coastal Skipper/Yachtmaster Offshore Theory Course
- MCA MGN 610 Amendment 1 — navigation and SOLAS Chapter V
- MCA MGN 379 Amendment 1 — electronic navigational aids
- IHO S-4 chart specifications, edition 4.10.0
- BIPM SI Brochure, ninth edition
- USCG Boat Crew Handbook — Navigation and Piloting
- GPS Standard Positioning Service Performance Standard, fifth edition
Continue studying Position, Course & Speed
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