If a home-made GPS guided UAV was ever detected heading towards a US city I don't think it would take very many milliseconds before the whole GPS network was shutdown. Mr President probably has a button for that in his briefcase.
US Presidential Policy on GPS is for active local denial of GPS signals for Hostile forces within the battle zone, without disrupting civilian and commercial uses.
GPS.gov: United States Policy
It was also Presidential mandate that Selectivity Availability will never again be implemented into the GPS system and the new Block IIF satellites now launched come without the equipment required to implement S.A.
The US realises how dependent the world is on GPS. Don't forget the precipitating event making GPS available for civilian uses was the KAL007 shoot down.