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The Economist[1] had a story about the London Stock Exchange getting jammed GPS-signal 10 minutes every day. I believe they are using something similar.

[1]: http://www.economist.com/news/international/21582288-satelli...

Gps satellites all send slightly incorrect information... A separate radio signal correction is transmitted to correct it. That way, during wartime (or in high value target areas) the US can basically mess up enemy gps enabled weapons by broadcasting misleading satellite and correction data.

That doesn't answer the question of how they could make GPs inaccurate only in a specific area.

Low power GPS jammer at the Whitehouse, perhaps?

A terrestrial transmitter spoofs or overpowers the GPS signal.

GPS satellites have a signal strength on par with a night light's, so it's easy to overpower in a small area using a low power transmission.

I don't know how they do it, but if you drive by the Pentagon with a GPS unit you'll see it go crazy.