Module 19 - Electronic Navigation
Sensor Independence and Common-Mode Failure
What you will learn
Draw the dependency chain behind position, heading, speed, depth, chart and route information and select checks with genuinely different failure modes.
Independence is a property of the whole data path. Different brands, batteries or screens can still repeat one sensor, antenna, network, chart or route error.
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.
| Quantity | Typical source | Independent challenge |
|---|---|---|
| Own-ship position / SOG / COG | GNSS receiver and antenna | Verified radar range, transit, depth trend, compass-and-log DR |
| Heading | Magnetic, fluxgate or gyro sensor | Steering compass and known-object alignment check |
| Speed through water | Log transducer | Timed distance or reasoned comparison with SOG and stream |
| AIS target motion | Target sensors and transmission | Radar and visual observation |
Build a failure-domain map
The main plotter, tablet, DSC radio and autopilot agree, but all four receive position from one GNSS antenna and route data from one exported file.
1. List outputs
Write down each displayed or automated quantity: position, heading, SOG, STW, depth, target motion, chart and route.
2. Trace dependencies
For each quantity, identify sensor, antenna, transducer, power, network, software and user-entered data.
3. Group failures
Mark which outputs disappear or become wrong together if one dependency fails.
4. Choose checks
Select observations and backups whose critical paths differ from the failure being tested.
Sense check: Independent battery power protects against one power fault; it does not make shared GNSS, chart or route data independent.
Common mistake or limitation
- Counting screens rather than data paths overstates redundancy.
- An AIS overlay and own-ship plotter can share GNSS, heading and chart dependencies.
- A second receiver is not independent if it shares the same antenna, power or interference environment relevant to the failure.
- A backup route copied from the primary can repeat the same waypoint or planning error.
Recap
- Trace each value from sensor or user input to display and automation.
- Group common power, antenna, network, chart and route failures.
- Select checks that differ in the failure domain that matters.
- Test independence aboard; do not infer it from labels or device count.
Optional quick check
Section 11 of 18
A tablet has its own battery but uses the plotter's GNSS position and copied route. Which failure is it clearly independent of?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Electronic Navigation
This public lesson is part of Revision Module 19. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.