Ummm, 411A I thought you knew about aviation?
How do you shield a VOR against RF? Or and ADF, ILS or AWR for that matter? All rely on receiving RF to function, therefore fundamentally cannot be shielded. GPS has to be even more sensitive, as the signal it is receiving is less than the background noise. The systems used to recover signal fromt he noise clutter do protect it from a certain level of random intereference (not a huge amount though) but to someone deliberately interfering they could be used to increase the vulnerability of the system.