After a few posts with IO540 about roof mounting and the rest.
I have asked one of a avionics techs andhe assures me that all our units have to be up to class 1 radio fit for BRNAV. which i presumes means arials in the best place checked every year etc etc.
Ours still go tits up on a regular baises. Usually only giving a RAIM error for 5-10 mins then clearing. GPS isn't as good as some make out.
MJ