If I were the DOD, I would set the standard GPS default to selective availability. After all, with SA on military receivers can still get fine positioning data. If someone could come up with a good reason to turn SA off, great, but I wouldn't leave it on otherwise.

The scenarios concerning GPS-piloted Cessnas full of nasty stuff come to mind, especially vis-a-vis the North Koreans. They probably don't have accurate IRBMs but they certainly could cobble together a Learjet-based delivery system.

SA is effectively cancelled out by using differential GPS (Using a correction transmitter to send out differences between GPS as-reported position and actual position). However, any 'home made cruise missile' would need to be fairly slow moving and manoevring. One of the features of p-Code is support for 'high-dynamic' vehicles. I've been away from the navigation field for some years, so others may know more. Does the Russian Glonass system have SA? -- Richard Parratt