Module 19 - Electronic Navigation
GNSS Integrity and Datum
What you will learn
Interpret GNSS geometry, integrity and datum information without converting a precise display or receiver status into a guaranteed position bound.
GNSS quality depends on signal environment, satellite geometry, receiver behaviour, antenna installation, datum handling and independent checks. Precision on the screen is not a measure of real-world certainty.
GNSS positions are usually referenced to WGS84, but charts may have a different or imperfect datum. Modern UKHO charts are normally compatible, yet older charts, harbour plans, and some foreign charts may carry datum warnings. A position can look precise on the plotter while still being offset from the charted world.
Integrity matters as much as accuracy. HDOP and other DOP values describe satellite geometry. RAIM can warn of inconsistent satellite range measurements where supported. Multipath, antenna masking, jamming, spoofing, and poor installation can all corrupt a position. Treat any sudden jump, impossible speed, or position that conflicts with depth or visual evidence as suspect.
Respond to a plausible but suspect fix
Near a steep harbour wall, the GNSS position shifts while DOP rises and the plotted track conflicts with a visual transit and depth trend.
1. Check status
Read DOP, integrity messages, alarms, antenna and constellation status without treating one indicator as a guarantee.
2. Check datum
Confirm the receiver output datum, chart note and any offset; avoid applying a correction twice.
3. Check independently
Use verified visual, radar, compass and depth evidence with different failure paths.
4. Protect margin
Slow or move to safe water before the uncertainty reaches the danger-side allowance.
Sense check: A low DOP describes favourable geometry; it does not prove that multipath, interference, spoofing, datum or chart error is absent.
Common mistake or limitation
- DOP is not a universal metre error or a guarantee of accuracy.
- RAIM or another integrity message operates within receiver-specific thresholds and does not validate chart data or every external error.
- A WGS84 label does not prove that the displayed chart and receiver apply compatible datum handling.
- More decimal places increase displayed precision, not necessarily accuracy.
Recap
- Lower DOP generally means better satellite geometry, not a complete accuracy statement.
- Read integrity and correction indicators according to the fitted receiver.
- Check datum notes and prevent missing or double-applied offsets.
- Challenge a suspect fix with evidence that does not share the same GNSS failure path.
Optional quick check
Section 2 of 18
A receiver shows low DOP and many decimal places, but the position conflicts with a verified transit. What is the defensible conclusion?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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.