radeng:
It's not that hard to generate a jamming signal of adequate strength on the ground, the hard part is upsetting an aircraft which isn't as sensitive to GPS signals from below due to antenna position. I've also done some GPS stuff in the past, including using a neat simulator that would happily replay a synthesised signal set for checking receiver performance. It wouldn't be hard to increase the power output and presumably confuse receivers over a wider range..
I've heard it described that the power of a GPS signal at the receiver is equivalent to being able to see a candle in New York from London (if they were line-of-sight), although it's probably better up in the clear where you get a decent number of satellites in view.