My son is an engineer and is involved in contract work with ratheon out at the nellis test site. He tells me a bunch of the testing is involving ground based station deliberately jamming gps sigs to provide training for fighter aircraft in dealing with airborne gps interference.
As many of you engineer types might know, the gps system utilizes the 1.5ghz band. This high of freq provides for a very narrow bandwidth thus it propogates very well with very low power. However, due to the narrow bandwidth it is very easy to jam the signal.