Cross-checking and carrying on when the plotter fails
1The lesson in plain words
Check the electronic position against something independent, at set times. When the plotter fails, carry on from your last logged position without drama.
One source is not enough
A cross-check compares your electronic position with observations independent of GNSS. Agreement increases confidence, but is no guarantee. If sources disagree, keep clear of danger while you recheck observations and instrument settings. One bearing, transit or depth reading is usually not a complete fix. Make independent checks at set times, not only when something looks wrong.
Quick checks
Depth: convert the sounder reading to depth below the waterline, allowing for its reference and offset. Subtract height of tide to compare with charted depth. Bearings: a compass bearing, corrected to true for variation and deviation, should pass through your plotted position. Transits: two identified charted objects in line give a position line. Radar range: distance to an identified charted object gives another constraint. The Position fixing unit explains suitable combinations and plotting.
The DR check
Compare GNSS with your independently kept DR (dead reckoning) or EP (estimated position) at least hourly, more often near danger. A discrepancy is a reason to investigate, not a diagnosis. From the same starting fix, a tidal stream you have not accounted for normally produces a growing offset; a constant gap may reflect an earlier fixing or datum error. A jump or unexplained trend needs independent checks. The Dead reckoning unit explains DR and EP.
A real backup
Up-to-date paper charts at a useful scale, with your route marked and a current log, let you continue independently while you establish your position. A separately powered phone or tablet with offline charts can cover a plotter or power fault. Its GNSS position is vulnerable to the same interference, but the chart can still support navigation using independent observations.
2Step by step
When the plotter fails
- 1
Make the boat safe: steer a safe compass course and slow down if needed.
Keep clear of danger and gain sea room before starting chartwork or fault-finding.
- 2
Record the current time and log; mark the last verified fix with its own time and log, then advance it to now by DR or EP.
The last fix belongs to its original time. Your present DR or EP remains uncertain; it is not a new fix.
- 3
Obtain an independent fix using suitable combinations of observations as soon as it is safe; use depth as a cross-check.
One observation usually only constrains position. An independent fix limits the uncertainty in your DR or EP.
- 4
Carry on by DR or EP between fixes, logging each one.
A written trail lets anyone take over the navigation.
- 5
Then look for the fault: power, fuse, antenna cable, settings.
Many failures are simple once the boat is safe.
- 6
Compare the restarted plotter with an independent fix before using it again, and keep checking.
Agreement increases confidence, but does not prove the system is free of faults or interference.
3Key data
- Depth check
- Waterline depth − height of tide ≈ charted depth
- Routine
- Compare GNSS with DR or EP every hour
- Transit
- Identified marks in line give one position line, not a full fix
- Phone or tablet
- Backs up the display, not the satellite signal
4Key points
- 1
Cross-check with methods that do not use GNSS: depth, bearings, transits, radar ranges.
- 2
Compare GNSS with your DR or EP every hour and investigate any jump.
- 3
If sources disagree, increase your safety margin and investigate; do not simply choose one.
- 4
A phone's GNSS shares the interference risk; offline charts can still support independent navigation.
- 5
After a failure, make the boat safe first, then fix your position, then find the fault.
5Frequent mistakes
Wrong: Cross-check only when the plotter looks wrong.
Right: Spoofing and chart errors can look normal. Check at set times, every hour and before each hazard.
Wrong: A tablet running a navigation app is a full backup.
Right: A separately powered tablet with offline charts can cover a plotter or power fault. Its GNSS position shares the interference risk, but the chart supports navigation using independent observations.
Wrong: When the plotter dies, start fault-finding at once.
Right: First steer a safe compass course and slow if needed. Keep clear of danger before recording positions, obtaining an independent fix or fault-finding.
6Practice case & exam keys
On a coastal passage at 6 kn, the plotter goes blank. Your last log entry, 40 minutes ago, gave your position and course. A headland with a charted rock off it lies 3 M ahead, and a church tower and a beacon are both charted and visible.
Show the answer
Steer clear of the rock on a safe compass course and slow if needed. Record current time and log; mark the last reliable fix with its own time and log, then advance it to now by DR or EP. Your present position is uncertain. Obtain an independent fix using the visible marks; cross-check with depth, allowing for offset and tide. Continue on paper between fixes, then fault-find. Compare the plotter with an independent fix before using it again, and keep checking.
Exam keys
- Independent checks: depth, bearings, transits, radar ranges, DR/EP.
- Compare GNSS with DR/EP at least hourly; investigate steady gaps and jumps with independent checks.
- Plotter failure: steer safe and slow if needed; record current time/log and the last fix's own time/log; update DR/EP, fix independently, then fault-find.
- Paper chart and log are the backup that works against jamming and spoofing.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
My notes for this unit
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