If a GPS just receives signals, how could it interfere with the Aircraft systems? Just the same as my little cd player doesnt affect anything!
Without getting into the how immune avionics should be from interference I think I might be able to throw light on why a GPS reciever is seen as a potential problem. (Anyone with more up to date tech knowledge correct me from here on in!)
Although the GPS "just receives signals" it is the way in which it does it that is seen to be the problem. Most (all?) GPS units use a
superhet circuit to make the reciever more sensitive. This circuit generates a signal that is added to the incomming GPS signal, basically to boost it to a useful level. It is this signal that is seen as having the potential to interfere with systems.
The same applies to most modern radio recievers (Superhet or PLL) hence why we are not allowed to use them on board.
The link has a pretty good explanation of the resons and workings of the circuit.