GNSS — how it works and how it fails
Global Navigation Satellite Systems (GPS and its equivalents) fix position by measuring the ranges to several satellites. Accuracy depends on the geometry of the satellites in view, expressed as the dilution of precision (DOP) — a tight, well-spread constellation gives a good fix, a poor spread a weaker one. The signal is degraded by ionospheric and tropospheric delay and by multipath (reflections off the ship and structures), and it can be lost, jammed or even spoofed. Differential and satellite-based augmentation systems (DGNSS, SBAS) improve accuracy to a few metres. Positions are referenced to the WGS84 datum and may need a datum shift before plotting on an older chart.
Depth and speed
| Instrument | Measures | Watch out for |
|---|---|---|
| Echo sounder | Depth below the transducer | Transducer offset below the waterline; weak returns over soft mud; second-trace echoes |
| Speed log (Doppler/EM/impeller) | Speed through the water or over the ground | Water vs ground reference; fouling; calibration error |
| Depth and speed sensors and their common errors. |
The echo sounder is a primary safety instrument: set correctly, with the transducer offset understood, it independently warns of shoaling water and confirms or contradicts the charted depth beneath a fix. The speed log's reference matters — a Doppler log may read speed over the ground in shallow water but speed through the water in deep water, which changes how you interpret it against the GNSS speed made good.
WARNING — Treat every reading as a measurement with an error GNSS is accurate until it is not. Keep a running EP, and cross-check the electronic position against a visual bearing, a radar range or the echo sounder before you rely on it near any danger. Never navigate on one box alone.
NOTE — Know your datum A position on WGS84 laid straight onto an older local-datum chart can sit tens or even hundreds of metres out. Check the chart's datum note and apply the shift, or use charts referred to WGS84.