GPS warning

[lustyd]

So you don't accept first hand experience of a GPS unit stating a location many miles from where you are? I've experienced it with a Garmin 152 GPS - the plotted course blipped and put us at the top of Portsdown hill ... a little tricky in a 30'er with a 6' keel ...

Blimey, how much did the repairs cost? Or did you just stop driving the boat when you saw the green/brown/grey bit approaching? I don't doubt that GPS can go wrong, and I don't doubt that people can and will get lost because of it. What I do strongly doubt, however, is whether many boats will actually hit the land as a result. Even with absolutely no nav equipment on board it's possible to safely get the boat somewhere to stop by just following the coast until you see a harbour and then following the buoys or another boat, or absolute worst case by going slow as heck and using a lead line to find a channel. You don't need to know which harbour it is, what ocean you're on or even which way is north.
It's also worth highlighting to some on here to avoid unnecessary worry (and counteract the panic inducing posts about the end of the world following tech failure) that the vast majority of boats actually just know where they are and where they are going, with a tiny percentage venturing outside home waters. Those going outside of home waters will likely plan for GPS failure in one way or another, even if that just means carrying charts and logging GPS positions in a notebook without doing traditional nav. For most of us, this just simply isn't dangerous enough to worry about, and I include in that number myself on a trip to the Isle of Man in a couple of weeks from Scotland - I'd be more than happy to lose GPS and still feel I could get there safely.

All that said though, it's nice to know it's going on and I suspect that quite a few on here may plan a little more as a result.

 

[yesod]

So you don't accept first hand experience of a GPS unit stating a location many miles from where you are? I've experienced it with a Garmin 152 GPS - the plotted course blipped and put us at the top of Portsdown hill ... a little tricky in a 30'er with a 6' keel ...

almost certainly software issues in the receiver rather than solar related.

this solar cycle is much lower than the previous one, and no-one harps on about the great gps outage of september 2001, do they? most of the 'sky is falling in'-style reports stem from nasa's original estimation for cycle 24 which put it as being very large. turns out they were very very wrong in that respect, but we still get the same **** regurgitated by lazy journalists (or disaster recovery wonks)

 

[maby]

No. Anecdotal evidence doesn't mean anything to me. There are any number of reasons why a handheld GPS could provide an incorrect position, not least of which is operator error. How many times have we read of two devices giving different positions, only to find they were set to different datums?

It's not so uncommon for them to give incorrect readings for a variety of reasons. Jamming, both official and unofficial is one path - the military freely admit to carying out jamming exercises but they do give warning and generally do it in remote places where the number of people impacted will be small. There's plenty of unofficial ones around these days - driving along the M1 a few months ago, my satnav suddenly decided I was several miles off-course - I slowed down, let the nearby vehicles get a few hundred yards ahead of me, and it all snapped back to the right spot.

Other problems such as duff satellites and ionospheric conditions can also affect it, but they tend to be short lived. The system is carefully monitored and any satellite sending out bum data will quickly be rebooted or taken out of service. If it's ionospheric, corrections will soon be posted over WAAS or EGNOS - provided your receiver can handle them.

 

[ionbard]

The CYCA report into the Flinders Islet accident in 2009 makes interesting reading, partly relevant to this thread.

http://www.cyca.com.au/sysfile/downloads/CYCA_Flinders_Islet_Internal_Inquiry_Report.pdf

p26 onwards covers reliability and accuracy of GPS systems, and the role these played in the grounding.

while it doesn't refer to solar flares, it does make the point about over-reliance on these systems.

 

[Daydream believer]

Solar flates!!
Are they hot?
If they warm the place up a bit then bring them on i say
About time we got some global warming

( oh ! Hang on better not mention global warming we will start webcraft off again!!!)

 

[Cap'n Creosote]

I normally sail with 3 gps sets onboard. And I plot my position on paper. (Belt & Braces!)

But a couple of years ago, as I left Conwy, my hand held Garmin GPS60 reported my track and position as 2,300knots over the Mediterranean! Then it decided I was a couple of hundred miles off the west coast of Ireland.

The other Gps sets seemed unaffected.

I've wondered ever since what could have caused such misreporting.

I've put the Garmin track here, if anyone is curious enough to want to see it.
http://www.tanygraig.f9.co.uk/John/odds/Gps_fault.gdb
It's a Garmin Mapsource file (.gdp) I think you'd need mapsource on your computer to view it.
But it's very odd and worrying that the gps reported nonsense, instead of saying it couldn't get a fix.

 

[Andrew G]

Just to emphasise what IronBard referred to in post #24 above (see his link). In the "Flinders Islet Incident" two of Australia's most experienced, respected and competant sailors (skipper and navigator) died when a top-notch ocean racer drove, at full speed, up onto an islet being used as a rounding mark in a race (the remainder of the crew were lucky to survive). They were relying heavily on GPS and at that very time a survey vessel had to suspend survey work because of upto 100m errors/uncertanity in GPS positions.

GPS' can and do go "wrong" and the consequences can be very bad - any advance warning about the potential risk of this is well worth making and heeding. Andrew

 

[Jegs]

One cheap GPS handheld gave me an altitude reading of minus 100+ metres - I wasn't in a sub & I doubt the river Somme is that deep anyway.

John G

 

[ukmctc]

I'll just use the stargate.

 

[maby]

One cheap GPS handheld gave me an altitude reading of minus 100+ metres - I wasn't in a sub & I doubt the river Somme is that deep anyway.

John G

GPS altitude is very unreliable - the geometry does not work well.

 

[AntarcticPilot]

GPS altitude is very unreliable - the geometry does not work well.

The vertical error is greater than the horizontal error, but not by much - about 50% greater. However, what often confuses things is that the height reported is not height above sea-level, it is height above the ellipsoid. This varies from height above sea-level (crudely speaking) by up to 100m (near Hawaii and in the Ross Sea in Antarctica). it's usually between plus and minus 20 metres or so. This difference can easily be corrected for, but usually isn't in consumer GPS systems, as the required data and computation is a bit heavy for a power and cost limited bit of equipment. In a particular region it tends to be pretty much constant, so once you know what it is where are, you can allow for it. This (http://en.wikipedia.org/wiki/Geoid) explains the difference for those who want to take it further.

We notice these things because we perceive vertical distances as greater than horizontal ones. A horizontal error of 10 metres SEEMS much less than the same vertical error. After all, 10 metres horizontally is a few steps; 10 metres vertically is 3 flights of stairs. Over the years, I've heard many more map users complaining about a map showing the wrong surface elevation than people complaining something is in the wrong place by the same amount.

Of course, it isn't a factor in marine GPS. I don't know if the manufacturers do this in practice, but if a marine GPS used the assumption it was at (or near) sea-level, it would potentially improve the accuracy of the 2D fix somewhat. It's only likley to be useful in marginal conditions, though.