Why does good navigational practice call for regularly cross-checking your primary GPS-derived position against an independent method, even when the GPS appears to be working normally?

A. It is not necessary in practice — a functioning GPS fix can always be trusted alone

B. A GPS receiver can fail silently (no fault alarm) or be affected by interference/spoofing without any obvious indication to the user, so periodic cross-checking against radar, visual bearings, or a second independent system is what actually catches an undetected error before it leads to a grounding or collision risk, rather than only reacting after something visibly goes wrong

C. Cross-checking is only required in restricted visibility

D. Cross-checking is the sole responsibility of the ECDIS system, not the OOW

A. The tidal height at the time of transit to confirm sufficient under-keel clearance

B. The yacht's fuel consumption for that leg

C. The colour of the seabed shown on the chart

D. Only the distance to the next waypoint

A. Switch the ECDIS to standby mode and navigate by GPS position alone

B. Ignore it — ECDIS alarms are frequently false and can be left running

C. Assess the situation against the planned route and charted depths, take action to maintain safe water, and inform the Master if there is any doubt

D. Acknowledge and silence the alarm immediately so it does not distract the helmsman