[ghshephard]

Back in 1995 I wrote the mapping software for a "Tundra Tracker", this was the navigation software for a supply vehicle that could navigate in the arctic in whiteout conditions. The idea was to have someone drive a path while conditions were good, and we would then lock in all the waypoints. Rather than store all the waypoints for the path, we were able to eliminate about 99% of them with various splining algorithms.

One of our big concerns back then, though, was that US Military would degrade the SA (Selective Availability) signal - it had happened in the Gulf War, and our tracks, which were normally accurate to within 3-4 meters, went wacky and were almost 50 meters off at times.

The solution was to mount a radio antenna, have it calculate it's "True" location by averaging a couple days worth of GPS signals, and then, continually transmit the difference between a current GPS signal and it's known location. This Differential GPS let us calculate paths to within 1m of accuracy.

When I asked what prevented a military opponent from doing the same thing, I was told that the first thing in the battlefield that gets hit as a command center would be anything transmitting RF, particularly if it was believed to be for D-GPS. Also, D-GPS isn't that effective for missile tracking, as you need to get differential signals in areas outside of your zone of control.