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Old 21 May 2009
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Location: Derbyshire, Britain; ± N52°58', W1°30'
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Exclamation GPS Reduced Accuracy / Outages 2010-2016

I saw a news report titled "Worldwide GPS may die in 2010, say US gov" here:

Worldwide GPS may die in 2010, say US gov • The Register

It's based on a US government report, which can be found here:

http://www.gao.gov/new.items/d09670t.pdf

Having read the report, the title and content of the 'news' report is alarmist. The facts presented in the US gov report show that there is some risk that a full set of 24 GPS satellites will not be able to be maintained from 2010-2016, due to late delivery of replacement satellites.

To be able to understand what will happen if there are only 23 satellites, we need to think about what kind of a gap this creates. Each satellite takes 11h58m to orbit, following the same path day in, day out. It has three other satellites following it on the same orbit. So there are 4 satellites equidistantly spaced on the same orbit (or sometimes more than 4 if there are more than 24 GPS satellites in operation). This is then duplicated to give 6 sets of these 4 satellites orbiting on 6 paths around the world.

So if one of the 24 fails, and is approaching your location, there will be a 2hr59.5m delay until the following active GPS satellite on the same orbit appears (albeit this is actually a little more complex due to the rotation of the earth). Whilst you're waiting you are will experience a comparative drop in GPS accuracy. This will happen twice a day and may result in difficulty in obtaining an initial fix when you 1st turn on, or dropping down to a 2D fix, (ISTR location accuracy being about 1.5km in this case). However, unless you haven't got a good view of the sky- I.E. you're in a city (urban canyon) or hilly terrain (genuine canyon), you're unlikely to notice these degredations.

As usual, out in the wilds you should have maps and compass anyway. This is a good reminder to pack them. Bear in mind it's still failure of your own GPS receiver that is the biggest risk to your ability to use GPS.

So, in summary, GPS accuracy may be reduced if enough satellites fail, which could result in traffic jams in cities, because all of a sudden sat-navs are lost and car drivers don't carry maps any more. It could also be a little more tricky finding that cached jerry can of fuel in featureless terrain.....
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Old 21 May 2009
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And the user's on-the-ground experience will be.....what? Accuracy will drop to "within 100 meters?" Units will take longer to get a fix? Or they'll be rendered entirely unusable for hours at a time? And...these things will be true once in a while, here and there, regularly or irregularly....?

The thought of tens of thousands of taxi drivers newly arrived in large cities suddenly unable to find their way around bothers me far more than the thought that I might not be led directly to the correct entrance of my chosen hotel anyplace on the face of the planet.

Feel free to tell me to read the articles you linked so I can answer my own questions, if you don't want to bother.

Thanks.

Mark
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Old 21 May 2009
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The US does from time to time turn the network off by switching to encrypted mode. Do not forget its primary use is military. By allowing free use of it, it discourages anyone else bearing the cost of setting up a similar system. This monoply is due to end during the next ten years (ish) as both China and europe are co-operating in launching their own systems.
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Old 22 May 2009
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Quote:
Originally Posted by markharf View Post
And the user's on-the-ground experience will be.....what? Accuracy will drop to "within 100 meters?" Units will take longer to get a fix? Or they'll be rendered entirely unusable for hours at a time? And...these things will be true once in a while, here and there, regularly or irregularly....?

My understanding is that there is at least an 80% chance that there will not be a problem.

That if there is a problem accuracy will drop, resulting in significant military tactical issues, but for civilians (unless you consider the threat of wayward ICBMs etc) a few flights may be cancelled or re-routed and GPS receivers could lose accuracy, particularly in cities and the mountains.

As I said above, I believe this could take accuracy down to 1.5km temporarily. I f it's a problem- park up for about 3 hours, then everything should be back to normal for a further 9 hours or so.
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Old 22 May 2009
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Thanks, Ian. I see that you did indeed answer my questions, although I didn't understand what you had written well enough to notice---a convincing demonstration of my electronic ineptitude.

I will add errant ICBM strikes to my list of anticipated hazards on my upcoming trip.

enjoy,

Mark
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