GPS systems rely upon receiving signals from 3, or 4 different satallites, each satallite contains an atomic clock and each it transmitting a time signal. The GPS receiver measures the time difference between each satallite (each signal travels at the speed of light), so the relative time delays will enable the GPS receiver to work out the position.
Three satallites will only enable the position at sea level to be determined, but four will enable the altitude to be determined as well. I'm not sure how jamming this system makes much sense, given that long range missiles and aircraft are also using inertial guidance systems as well and that jamming a signal coming from a satallite presets some quite serious technical difficulties.
This may be a problem caused by something else, other than jamming. It does seem a bit strange though!