FRA are based out of Teeside and they carry all sorts of nasty kit on them.
They fly around in Falcons with what look like missiles under there wings but actually they are ECM pods for playing with the military. Part of there job some times is to jam GPS, but as i know of 3 local lads with GPS jammers which they bought for 30 quid. I don't put much faith in steady signal for GPS.
They also have the job of calibrating all the instrument approaches in the UK.
You will hear them use the call sign calibrator.
MJ