Call me old fashioned, if you will, but the first defence to jamming is to change frequency. It would not surprise me if the GPS network had a frequency agility mode.
Second defence is to not use fixed frequencies at all, and to frequency hop. GSM phones have the capability to do this at 217Hz. Thus the effect of a jammed/interfered-with channel is averaged over the other time slices, and is negligible. Forward error correction is also applied, in order to be able to reconstruct the lost data.
Better still is to use a CDMA system, where the transmit bandwidth is so wide, and the amplitude so low, that the signal appears as noise to the adjacent channels.
Failing that, MLRS does a good job of anti-jamming.