In the past I've seen concerns about wide-scale GPS jamming (not just localized tampering), and also that the government might turn off the signal. I didn't pay that much attention since I'd never heard anything about it actually happening over a large area. Granted the military could reinstate selective availablity (SA), the 100 meter +/- error. And while the military has its own encryped super-accurate GPS signal (accessible only to special government receivers) they still needed the civilian signal in order to jump on to the secure signal; no way they would obliterate the civilian signal. So I blew off the whole GPS jamming/blackout 'threat'.

Well, a few things have changed.

1) Lyle Brotherton is the author of The Ultimate Navigation Manual. In the book he cites specific events where the GPS signal was purposely jammed by the government in Britain. They aren't always secretive about it either; he says the UK government posts warnings well ahead of time that the GPS signal will be weak or lost in certain geographic areas while they test their jamming protocols; here's an example. Lyle's book and website are excellent, by the way.

2) The US military has developed a modernized GPS capability in which the encrypted GPS signals are available directly to the government-only receivers, with no need for the civilian signal. Thus a significant barrier to blacking out the civilian signal is removed.

3) Wired Magazine has a report describing new GPS satellites whose signal is harder to jam, but it makes no mention of the military's government-only signal capability. At least the article has a really cool photo of a GPS satellite. At first I thought it was a candy machine from Willy Wonka's factory...