Sensor Independence and Common-Mode Failure

Two displays are not necessarily two independent sources. A chart plotter, AIS overlay, DSC radio, tablet, and autopilot may all receive position from the same GNSS antenna or network. If that input is wrong, every screen can agree with the same error. A second plotter using the same antenna confirms only that the data have been distributed consistently; it does not confirm the vessel's position.

Trace the source behind each important value. Course over ground and speed over ground usually come from GNSS; heading should come from a compass or heading sensor; speed through the water comes from the log; depth comes from the echo sounder; AIS target positions are reported by the other vessels and may share their GNSS errors. A radar range to an identified headland, a visual transit, or a depth contour can be genuinely independent of own-ship GNSS, provided the feature has been identified correctly.

At command level, name the failure domain rather than counting instruments. Ask: what powers this sensor, where does its antenna or transducer sit, what network distributes it, and which displays depend on it? Build the passage plan so one failed breaker, damaged antenna cable, network fault, corrupted datum, or erroneous heading source cannot remove every safe method at the same time.