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GPS - get position support

The incessant beeping of an alarm is a normal occurrence in today’s boating environment.

If it’s not a DSC safety message, it’s a phone ringing or another piece of equipment telling you it has gone wrong or is not getting the information it requires.

A few years back a beeping from down below would have got an immediate response but now it could be a multitude of minor irritants or signify a real emergency.

Today’s alarm was one that I had not encountered for a while – no GPS signal entering the chartplotter.  Poking my head out of the companionway I looked skyward for signs of satellites falling from the sky or signs that the Americans had declared war. But all was calm, so it must be something wrong with the chartplotter system itself.

We were in safe water, good visibility and I knew the area well, so today we were safe. However, I was not keeping a record of my position, a cardinal sin, had I been offshore and reliant on GPS it may have been a different story as I’d really have no idea where we were.

Who dunnit?

The problem on board was eventually traced to a crush in the antenna cable as it passed through a compartment onboard. After assembling all the clues, we confirmed it was the cabin boy, in the lazerette with the boathook that did it.

I had a similar problem a few years back when we lost GPS signal for a couple of hours a few times every day and we had to resort to trad-nav techniques. It took two days to deduce that the signal was lost each time that a particular watch was on deck and that one of the crew kept sitting on the back rail - directly on top of the small GPS patch antennae and blocking the signal.

We nicknamed the crewmember I-SAT for the rest of the trip as he’d been blocking the satellites.

It was on that trip we also came up with the GPS acronym of Get Position Support as we needed the positional support recorded in the log and an Estimated Position to monitor our position and stop it going horribly wrong.

Back yourself up with the basics

GPS and plotters are great and the system doesn’t break down often, but things do go wrong every now and then so be ready for it and do not leave yourself vulnerable.

GPS signals can also be jammed using equipment available on the internet for less than £100 that can stop or confuse signals. The signals are so weak that spurious UHF TV signals in the USA have shut out GPS coverage for a whole harbour and some malfunctioning marine electronics can have the same affect.

In fact the UK is researching land based electronic navigation such as eLoran, similar to the old Decca system, as a suitable positioning backup to GPS. So whilst GPS is great - remember to;

  • Keep a log of your position when you need to by either a regular plot on the chart or an entry in the log book.
  • The faster you travel or the more essential your exact position, the more it should be logged.
  • Ensure you have the ability and equipment onboard to work out your position and shape your next course.
  • Get Position Support – verify the GPS data by another means such as looking out of the window, a fix or a depth contour.

Simon Jinks, RYA Instructor, Examiner and yachting journalist

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