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...

Europe is working on Galileo and Russia has GLONASS. I imagine that future nav devices will be able to use all three. However, it's hard to imagine that any combination of those systems would be especially difficult to jam or otherwise disable for a modern military.

GPS III satellites no longer have the Selective Availability feature.

No longer have the capability, or merely have the SA turned off?

I'm not privy to any inside information but I understand from what I read that the feature is not present on the latest satellites. According to an official US government statement at gps.gov

Localized jamming and spoofing by US, foreign and non-state actors is always possible, as has been recently reported in the online technical media.

There is a website which for us users might be useful: it's designed for pilots to check forecast gps disruptions prior to flight, but I imagine it might be of interest to ground-based gps users too.

http://www.raimprediction.net/

I expect that when the UK military plan gps jamming activities they coordinate with the CAA and relevant notams are issued to pilots well in advance. I would be surprised if was anything other than announced months ahead,

Jamming is super easy if you can physically locate a powerful radio transmitter where you need to jam. Just pump out lots of radio noise at the right frequency and no one can make out the relatively faint GPS signals above the clutter...

Come to think of it, spoofing the GPS signal shouldn't bee too hard, either. You know where the satellites will be at any given time, and you can calculate what the true signal will look like. Blocking the "true" signal with your spoofed version is a question of getting the right antenna and transmitter close enough to the area of interest.

I don't know any details (and anything interesting would probably be classified anyway), but I do know this is one area where the military has active research going on. I would also hazard a guess that systems less vulnerable to spoofing would be pretty high on the military Christmas wish list.

I seem to remember Dealextreme used to sell an L1 (1560-1580 Mhz)and L2 (1217-1237 Mhz) GPS Jammer i.e. for both the Military encrypted and Civilian Channels for about $30. Expanding the size of the jamming bubble just depends on the RF microwave power of the jamming antenna.

I have a Model:G30 Cell Phone and GPS Jammer which blocks 850-970Mhz CDMA/GSM 1560-1580Mhz L1 GPS and 1805-1930 Mhz DCS/PHS with 3 handy SMA microwave connectors which can be fed to tuned microwave Amplifiers.

It does a nice job at blocking GPS and older GSM 2 and 2.5 cellular phones . Once folks realise how easy it is to block GPS signals they begin to appreciate old school navigation.

L1 Spoofing is relativity easy as well as the GPS signal format is available on line.

http://www.navcen.uscg.gov/pubs/gps/sigspec/gpssps1.pdf

^ Agree it's not hard (or expensive!) to unselectively jam GPS. Note I don't address legality. It especially helps that the actual useful signal is below the noise floor to start with.

Interesting paper - not classified - on gps spoofing:
http://www.syssec.ethz.ch/research/ccs139-tippenhauer.pdf