What is a Differential GPS (DGPS) position reference system, and why is differential correction used rather than standard GPS?

A. DGPS is identical to standard GPS, just using a different satellite constellation

B. DGPS eliminates the need for any other position reference systems on a DP vessel

C. Differential correction is only used at night when satellite visibility is reduced

D. DGPS applies correction signals from a reference station at a known fixed location to a vessel's GPS-derived position, removing common-mode errors (e.g. atmospheric delay, satellite clock/orbit errors) that affect both the reference station and the vessel similarly — this significantly improves position accuracy compared to standard GPS, which is necessary for the precision required to hold a DP vessel's position within a small operating envelope

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