Position fixing and monitoring
GNSS (GPS and its equivalents) gives a continuous, accurate position, but it can fail, be jammed, or read subtly wrong, and it is only as good as the datum and the chart it is plotted on. Treat it as one input among several: cross-check it against visual bearings, transits, the echo sounder and a running estimate. A chartplotter shows the GNSS position on an electronic chart with routes and alarms, but its chart may be less detailed or less up to date than the paper it replaces, so the skipper checks the source and the scale.
- Radar shows other vessels and the coastline in poor visibility and gives ranges and bearings for both fixing and collision avoidance.
- AIS shows the identity, course and speed of equipped vessels and the closest point of approach, but only for craft that transmit.
- Set and monitor sensible alarms — cross-track error, waypoint arrival, depth and anchor — without drowning in them.
- Always keep an independent means of navigating if the electronics or the power fail.
WARNING — Cross-check; do not worship the plotter GNSS is accurate until it is not, and the boat symbol on the plotter looks equally confident whether the position is right or wrong. Confirm it against a transit, a light or a sounding before you trust it near danger.
NOTE — Radar and AIS are complementary Radar sees everything that reflects, including craft with no AIS; AIS names and gives the intentions of vessels that transmit but is blind to those that do not. Used together they build a fuller picture than either alone.