In reply to 2E0KPO:
Well in theory there aren’t too many military receivers in civilian hands which means that people who need to have their GPS jammed by the MoD are very much more likely to use civilian GPS. The people with military GPS receivers are probably “our friends”. Given the sensitive nature of what is being tested you can never be sure if they are doing what they say they are doing. And of course if you jam one and not the other system then anyone with one of each receiver will know something is happening. Lots of bluff, double bluff and tripple bluffs take place!
Ground based jamming using brute force needs some quite big brute force! Most of the GPS receivers are looking up for their signal and of course the system is using spread spectrum so any ground based system has to put out a considerable signal strength over a wide bandwidth to jam the real signal. The effect has quite a limited range but in some scenarios would be sufficient. Drone aircraft which have a long endurance would make a good platform to carry a airborne jamming signal but they may not be able to carry a big enough transmitter and power supply for brute force jamming. You’d probably want to carry out spoofing though from one of these. You need to transmit a signal that appears to be from the satellites but is giving the wrong data so the ground based receiver calculates the wrong position.
If you just jam it so it doesn’t work at all then ground based users immediately know something is wrong. By jiggering about sufficiently with the spoof signal you can destroy the user confidence in their GPS as it appears to be working but it takes them sometime to realise it can’t be trusted.
Of course it’s much easier to jam ground based receivers than airborne ones. In the air you can have an antenna on top of the wing shielded from the ground and one on the bottom of the wing shield from the sky. A signal that comes up and differs from one coming down is a jamming signal and can be ignored.
Given the limited range it’s not surprising you’ve not really been aware.