Anything can be jammed, you just need enough power. Having a secret spread spectrum code helps. It's the same principle but easier to understand in the freque

Anything can be jammed, you just need enough power. Having a secret spread spectrum code helps. It's the same principle but easier to understand in the frequency hopping case. In that case, suppose you want as much jamming power as signal power, you have to spread it out across the whole band because you don't know where the next frequency hop is going to be. So if say the system uses 1/100 of the available bandwidth at any one time, now the jammer needs to be 100x as powerful to jam the whole band with the same power as the jammed system. Still, the satellite puts out a low power signal from thousands of miles away, so the jammer starts with an advantage. Where the new block iii satellites will improve this is with a spot beam that can be directed at a particular area on earth where the jamming is expected, and locally get about 100x the power. I suspect that you'll see more directional antennas and phased arrays applied to GPS receivers as well, since the angular direction to the satellite is known, and it's difficult to put the jammer right in the path of between the receiver and the satellite.