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how it works.
Now I may be wrong on a few details. But, i used to work with GPS for moving oil rigs and such.
Anyways, the Crapping up of signals you used to get was called selective availability. It's been turned off. But on occasions the signals do get messed up, when the US started to hit Iraq this time, the positions went all funny.
There can be problems when theres solar flares too and also down by brazil and area you can get something called scintilation which messes up accuracy.
You can get subscriptions to GPS services, but they're Differentially corrected signals, sometimes refered to as DGPS. they bring down the error on the postion, Just before I was made redundant, they (my employers) made a DGPS system that was good to within cm's as opposed to 20mtrs or so on the standard.
You also get Differnt kinds of GPS, Normal GPS and dual frequency GPS which is more accurate.
Unless you need pinsharp accuracy I'd not bother with the DGPS stuff myself.
If theres any questions give me a shout.
MiniG
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