Benbow
Well-Known Member
An example of a GPS \'failure\'
I just looked at the passage planning thread which refers to the risk of GPS failure. I thought you may be interested in an experience of mine some years ago.
In a 60’ sailing vessel, coming up the Menai straits with 4 kn of tide plus a f4 up the chuff. Poor vis, getting dark. The boat draws 8 feet, if you know the area, you will know that at half tide you need to be rather careful.
I had previously ‘checked’ the GPS by plotting the position from it onto a big scale chart. It was absolutely spot on – it showed us tied up to a fish dock with perfect agreement with reality.
The GPS had been the primary means of nav on the passage across the Irish Sea and there was no evidence of an issue. However, as the pressure came on while creaming up the straits, it soon became clear that the GPS position was seriously adrift from reality – by around 1 mile. Because we had a formal passage plan and because we had been following it and were also using the radar for position fixes, we knew where we were and without hesitation abandoned the GPS as a source of information. In a more casual setting, I might well have come seriously unstuck, because the GPS was sitting there giving me a position, telling me that it had a good fix on a bunch of satellites, and showing me a SOG and COG which were perfectly credible.
We arrived with no drama, tied up at Menai Bridge, put the kettle on and sat down to look at the GPS. Plotting its position on the chart showed it absolutely spot on. Skipper gives me a look which clearly means ‘you bozo, there is nothing wrong with the GPS, what was all that about?’
That was a learning experience for me and it underlined the need for careful planning. A prize for the first person with an explanation.
I just looked at the passage planning thread which refers to the risk of GPS failure. I thought you may be interested in an experience of mine some years ago.
In a 60’ sailing vessel, coming up the Menai straits with 4 kn of tide plus a f4 up the chuff. Poor vis, getting dark. The boat draws 8 feet, if you know the area, you will know that at half tide you need to be rather careful.
I had previously ‘checked’ the GPS by plotting the position from it onto a big scale chart. It was absolutely spot on – it showed us tied up to a fish dock with perfect agreement with reality.
The GPS had been the primary means of nav on the passage across the Irish Sea and there was no evidence of an issue. However, as the pressure came on while creaming up the straits, it soon became clear that the GPS position was seriously adrift from reality – by around 1 mile. Because we had a formal passage plan and because we had been following it and were also using the radar for position fixes, we knew where we were and without hesitation abandoned the GPS as a source of information. In a more casual setting, I might well have come seriously unstuck, because the GPS was sitting there giving me a position, telling me that it had a good fix on a bunch of satellites, and showing me a SOG and COG which were perfectly credible.
We arrived with no drama, tied up at Menai Bridge, put the kettle on and sat down to look at the GPS. Plotting its position on the chart showed it absolutely spot on. Skipper gives me a look which clearly means ‘you bozo, there is nothing wrong with the GPS, what was all that about?’
That was a learning experience for me and it underlined the need for careful planning. A prize for the first person with an explanation.
