Rainboe, the problem is terrorists. A primitive gps guided cruise missile can be put together for very little money using off the shelf components. It's not that difficult either.
Gps jamming is one way to stop it.
Some GPS rumors and non-rumors from SLF:
Rumor 1:
I think I've read somewhere that commercial GPS chip sets like
http://www.sirf.com/products/GSC3LPProductInsert.pdf have inbuilt max speed and max altitude just for that reason (60Kft, 1000knts in the link). It may not stop from using it on short range missiles, but longer range wont work.
Rumor 2:
One of the reasons Selective Availability (SA) was switched off is that during some 'recent' US deployment (Desert Storm? Balkans?), troops on the ground were using (on their own) commercial devices as navigation aid, which obviously where affected by the SA. Switching off SA made life of the troops on the ground easier.
Rumor 3:
UK is kind of against the militarization of Gallileo (using it to guide weapons), France and Germany want it. Could be because of better relationships with the USA, or maybe that the UK has suffered losses from European built guided weapons (Falklands, Exocet ASM), that with a military Gallileo signal will just get better.
Non rumor:
Gallileo when finished (2014+) wont be better in accuracy than the latest block of GPS satellites (Block III?). GPS will be superior by then. Gallileo is better than the current GPS satellites, but not the GPS of 2014+. The military signal of GPS may be even better by then (beam focusing etc).
Its the strategic independence in navigation that makes EC fund it fully after the failure of the PPP (Private-Public Partnership).
China is also chipping in some 300-400 million euros, they have their own local positioning system, and looking for a global one in the future.