Rampisham HIRTA (BBC World Service) knocks out our Garmin 496 GPS regulaly if flying too close. It freezes and the only way to get it working again is to remove it from the panel, prize the battery out, inset battery and start again. Bit of a PITA when it is bumpy !
I worked this out because our GPS used to regularly fail on the way to Dunkerswell low level, exactly when passing Rampisham and no where else. At higher levels it was never a problem.