See this
GPS jamming: No jam tomorrow | The Economist
I said a technical explanation, not a load of inaccurate journalistic hype. It is easy to knock out or desensitise a GPS receiver within a few feet of you by simply making the active antenna switch off. Jamming the "system" and more distant receivers is a totally different issue, which is why the military keep trying! The "weak" GPS transmitters are actually 16 times more powerful than the average satellite TV transmitter.