I can't see how you could appeal provided the camera is being used in the direction it's intended to be used. TRUVELO is bi-directional. GATSO works from behind so if you are photographed whilst approaching the NIP would never be anyway.
There is one interesting situation which AFAIK) has never been tested in court, and that is multiple GATSOs suspended side by side on overhead gantrys. It has been suggested that the devices have not been tested and approved to work in this configuration ie. that radar scatter from adjacent GATSOs would yield unreliable results. Perhaps we could argue a case if a GATSO derived NIP was received on account of a GATSO being too close to aircraft radar (M4/A408) Heathrow slipway, for example?
There is a rock solid and 100% guaranteed way to avoid speeding fines. Don't exceed the speed limit.