I follow the GPS universe fairly closely. The term for the intentional degrading of accuracy for "unauthorized" (civilian) users is "Selective Availability" (SA). The story gets even better. Although the DoD hasn't described how SA works, it's now apparent that they add cryptographically generated "phase noise" to the timebase on each satellite, probably with a direct digital synthesizer. In other words, the satellite looks like it has a noiser atomic clock than it really does. "Authorized" users with the right keys can regenerate the same dither stream and subtract it out of their observations. During the Gulf War, the DoD quietly turned SA *off* because they could only meet their immediate need for vast quantities of receivers by tapping the commercial ("unauthorized") market. Of course, this did not go unnoted by the civilan GPS market; there were probably some red faces in the Pentagon. I do know there were a lot of broad grins in the civilian world. It's almost as if they had decided to use PGP on PC clones due to a lack of NSA-approved military crypto gear. Unfortunately, after the war, DoD turned SA back on again. However, the civies had a neat trick up their sleeves: "differential GPS". This involves placing a GPS receiver in a fixed spot and having it broadcast the difference between its known location and its current position as determined by GPS. Because most of the errors in GPS are strongly correlated between nearby receivers, this subtracts out almost all the errors in the mobile user's position. Not just SA, but ionospheric dispersion, orbital element inaccuracies, etc. The result is an accuracy of 1-3 meters, with or without SA. The really fun part is this. Guess who's leading the effort to deploy differential GPS beacons? The US Coast Guard! That's right, while one side of the military intentionally sabotages the signal, another military service (albeit under the Department of Transportation rather than the DoD) works to un-sabotage it! Your tax dollars at work. Actually, the Coast Guard says it would be doing differential GPS even if SA were turned off. With SA on, accuracy is typically on the order of 100m; without it, accuracy improves only to about 25m, and this is insufficient for many harbor approaches. And here's yet another delightful irony in the GPS saga. The DoD maintains a network of ground tracking stations that determine the orbit of each GPS satellite. Every few hours they uplink these orbital elements to the satellites so they can be broadcast to the users. There was talk for a time of encrypting the low order bits of the orbital elements as part of SA, but this apparently hasn't happened. Nevertheless, GPS is so useful to the international scientific community that they've set up their own network of tracking stations to produce and disseminate their own GPS orbital elements. And while the DoD-generated elements are good only to about 10-15m, the civilians, having many more stations and better techniques, generate sets good to less than 1m! Phil