Most frequency hopping equipment uses GPS's atomic clock to get their timing to enable them to all be on the right frequency at the right time, Whilst this is in its self very hard to intercept it is remarkably easy to jam a GPS signal.

Also as anyone can find what time it is on the atomic clock, if the can find out what frequencys you are using and when you hop (usually about 3 hops per second) it would be a simple matter to have a peice of r/x ing equipment hop at the same time as your network.

A point to note, America owns every single GPS satilite in orbit and unless they sell their copyright no body else can build one, if America decided to turn off their satilites or if someone "disabled" them GPS and everything that relies on GPS would be rendered useless.