GPS failure: What would you do?

GPS failure: What would you do?

How would you react to GPS failure while sailing? James Stevens answers your Questions of Seamanship

GPS failure: a man looking at a GPS on a boat
If the position of your yacht suddenly changed on your chartplotter, how would you react? Credit: Lester McCarthy

GPS failure: What would you do?

Question:

Ian runs a highly successful software company. He has recently purchased Free Electron, a custom-made 14m sailing yacht.

He insisted on the very latest electronic systems so the navigation area is more sophisticated than most merchant ships. Ian is very adept at handling this equipment.

He has not taken any navigation courses because he reckons that his instructor will know less than him about the use of modern electronics.

Unsurprisingly, there are no ‘old-fashioned’ paper charts on board.

Ian has also taught himself how to sail and if the boat handling becomes a little difficult in the marina, he arranges for the launch to help with berthing.

A chart of Flamborough Head

Credit: Maxine Heath

He has successfully visited most of the harbours on the South Coast and is now sailing up the East Coast on a circumnavigation of the British Isles.

He is approaching Flamborough Head. The wind is southwesterly Force 5, the visibility about 2 miles and the autopilot is holding a steady course about 6 miles from the land in 22m of water. Suddenly four alarms go off at once.

The position of the yacht on the chartplotter jumps 20 miles on to the land giving a speed of 100 knots.

Alarms are coming from the plotter, the DSC radio, AIS and the autopilot, the latter is now steering straight for the land.

No problem, Ian turns off the instruments, hand steers and opens the navigation app on his phone. It is also giving a position on land.

What has happened and what does he do?

Continues below…

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Answer:

This is an unusual event. The GPS signal has been jammed, probably from the land. GPS signals come from satellites 20,000km from the earth and are very weak.

The technology provides us with a remarkably reliable and accurate system but it is susceptible to interference, sometimes from sunspots.

James Stevens

James Stevens, author of the Yachtmaster Handbook, spent 10 of his 23 years at the RYA as Training Manager and Yachtmaster Chief Examiner

Jamming is the malicious and usually criminal interference of the GPS signal to confuse the receiver or, more ominously, to give it a false position.

It is at this moment that Ian realises that a navigation course and a paper chart are not such a bad idea after all. Alternatively, a chart on a screen on which visual fixes and estimated positions can be drawn electronically would allow the voyage to continue.

This, of course, assumes some basic navigational knowledge and kit such as a hand-bearing compass.

His best bet is to anchor and wait for the visibility to clear or for the GPS to return to normal.

GPS is more likely to fail through faulty systems on board. Competent skippers therefore keep a record of their position and confirm it periodically with another source.

This can be visually, by echo sounder or a thumbnail EP by using fingers as dividers to verify the screen is accurate.

Used intelligently, electronic navigation is a huge asset but there has to be a plan if it fails.

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