Module 19 - Electronic Navigation
GNSS Integrity and Datum
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.
Key points
- Most GNSS positions use WGS84
- Check chart datum warnings
- High DOP means weaker satellite geometry
- Multipath is common near cliffs, bridges, harbours, and large vessels
- Jamming and spoofing can affect GNSS
Tip: A position shown to several decimal places is not automatically trustworthy. Precision on screen is not the same as real-world accuracy.
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