How many of you pilots have used hand held GPS on the flight deck? (I know nobody will answer that)
GPS is a receiver only - it doesn't transmit.
As one poster pointed out, all the sensative electronics are tested and shielded for EMI.
EMC is not absolute.
Filtering components are no more reliable than any other electronics. They can and do fail.
Shielding can be damaged, left off or left unconnected after maintenance.