Anybody can use commercial off the shelf aviation and marine GPS navigation gear to steer a plane or cruise missile to a target. This equipment is standard for boats and planes, it links the autopilot to the GPS. You just put in the exact coordinates, and tell the boat or plane to go to that "waypoint." And it will. Even if the plane is a Learjet loaded with explosives, and the "waypoint" is the Whitehouse.
It's not nearly that simple. At the very least the autopilot must be linked to an altitude control system in order to cause the AC to come down in the right place. Most such systems cannot be pre-programmed. They must be enabled by the pilot at the beginning of the descent. And they don't control *where* the AC hits a certian altitude, unless they're flying a pre-programmed GPS-ILS to a runway.
And it's not nice to give people ideas like this. I'm not talking about the terrorists, they've already thought about this. I'm talking about the thousands of bureaucrats in DC who don't have a technological clue. They are the one's liable to start threatening GPS shutdowns, which will be very damaging to the economy. You have no idea how many billions of dollars depend on this technology.
Local area jammers will protect DC. Leave the rest of the system intact.