Another strange thing.

[cynthia]

Thursday 9th August. Approx. 11.30a.m. Sailing in f6, well reefed, boat speed approx. 7 kts. Enjoying the penultimate long sail before crossing the George's Channel and home at the end of a superb holiday. Went down to check GPS/plotter. Panic - the little boat on the screen was at the wrong side of Arklow Bank! Of course the GPS must be right, mustn't it? But how could we have crossed the bank (2m of water and us with 1.85 keel) without noticing? Well it had been a bit choppy, maybe it had been very shallow? Even so, we've never been so wrong before!

A quick check with the paper chart and vis. bearing confirmed that we were actually where we originally thought - the reported GPS position was completely wrong by 5 miles!! Now you expect GPS accuracy of 10m, otherwise for it not to work at all. This could have caused serious problems if we hadn't a paper chart back-up plot, or weren't in sight of land.

As we watched the screen the little black ship suddenly jumped sideways into the correct position.

Interrogation of the instruments later claimed we had done 27.4 kts of groundspeed in 60 kts of wind. As if...!?

Anyone else have a problem at this time?
Have I a bug in the software?
Any other suggestions?

Cynthia

 

[Peegee]

Can't comment on this particular case Cynthia , but ,last Tuesday we left Nieuwport, in not very nice conditions, and discovered that our GPS had gone down. Carried on using the good old fashioned way and trying to work out how much holiday money we had left to buy a replacement. On arrival in Ramsgate we chat to a fellow traveller who says " And it did'nt help that GPS was down all morning". Of course , we had not bothered trying it after initial problems so double checked, and sure enough it was all up and running again. I have only just got back on line , so I don't know if anybody had any info on the breakdown/switch off. Perhaps this is going to be the shape of things to come when the system goes private. or am I paranoid ?

 

[Guest]

mmm....60 kts of wind... Does your GPS have an anenometer?

 

[cynthia]

All the instrumentation is Raytheon and the separate bits 'talk' to each other, so GPS/plotter uses boat speed to calculate the true boat speed. So I'm told!

 

[Guest]

Hi cynthia,
I had the identical problem as Peegee.Called my GPS all the names under the sun, to find it working A1 friday morning!!.

 

[Guest]

Two interesting facts for you...

1 - GPS As this is controlled by the US Government, they can get it shut down at a whim, and week before last they were dropping bombs on Saddam again. It was only last year that Clinton authorised the removal of the degredation feature that increased the accuracy to +or- 15m and this was due to the reductiuon of strategic threat to the US and Europe from little beggars like Saddam. (ie he doesn't want them to find their way around in the desert)

2 - Also the police can request the shutdown of mobile 'phone "cells" so that anarchists or other naughty people cant get organised (like anti-capitalist rallys)

Cheers,
Rich.

 

[duncan]

it could have been the relationship to your chart software in this case could it not? Did you note the actual position it was giving you at and compare it or base everything on the picture?
Either way nasty......

Do GPS outages fget reported formally anywhere?

by the way did peegee see paulineb on this crossing - wasn't she crossing niewport - ramsgate at this time when she brole a nail (amongst other mishaps......)?

 

[cynthia]

Plotted screen position on paper chart - result the GPS position was replicated, however, the fact that we should have been about to bump into a cardinal marker which simply was nowhere in sight, was the clue that the GPS was wrong. Fortunately we were within sight of land, so a couple of quick fixes proved that we were actually where we thought we were, not where the GPS put us.

A bit worrying really, as our first thoughts were that the electronics must be right and it was tempting to simply put the wheel over and head for deeper water - in reality this would have put us on the banks!!

Can only think its a glitch in the software, so have contacted the Raytheon dealer who fitted the system. We've always assumed the reported position would be acccurate, or simply not working, not that it could ever give a false position.

Off to read Paul(ine)'s contribution - seems I've missed something!!!

 

[Guest]

A bit late to put my two cents in, but here goes.

GPS outages were more common back in the early 90s, indeed there was one week in 1992 when the GPS was down more than it was up. Our Magellan had a feature where you could check the schedule of satellite availability and so we knew when the GPS was "up" or "down". Friends with a Garman apparently didn't have that feature on their GPS. 9 years ago, technology has all changed so don't know if this advice is still given. Goes to show you shouldn't rely on ANY single aid to navigation, and that you should always maintain a DR course.

Although US and BA charts are pretty accurate for their respective countries, many places of the world have not been surveyed since GPS went in, and positions are often 1/2 mile to 1 mile different from what is given on GPS. Check your "datum" information on the charts, which can help. Sometimes nothing can - we once had three charts of a specific section of the Pacific, and wanted to plot our course to avoid a little lump of an island. Problem was that all three charts reported the island in a different place! Granted, total difference was about 2 miles, but in situations like that it's hard not to feel that all three charts are probably wrong and who knows where the silly lump of land is going to pop up!

Fair winds,