The US DoD occasionally used to activate 'Selective Availabilty' which would degrade the accuracy of GPS for non-US military (or non-authorised) users randomly. Bill Clinton put a stop to this in (I think) May 2000. The trouble is that the GPS signal is so weak that it's relatively easy to attenuate and jam the signal. I was told that the signal strength is comparable to trying to see a 20 watt lightbulb from 10,000 miles away!
The US are working on methods of checking that GPS signals are accurate but I'm not sure what checking systems are available for it here in the UK.
I'm sure that most users are aware of the potential inaccuracy of GPS and would never rely on it as a primary navigation aid.
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