I speculated that the government may in fact have designed the satellite systems so that they could be told to do several things in case of national emergency: * Shut down (probably possible; they may have actually mentioned this on the show). Problem is that lots of friendlies may grow to rely on the data for life-critical things, like guiding commercial airliners. It's worse; the military relies on GPS. During the Gulf War large numbers of combat vehicles could rendezvous with high accuracy and no radio contact in the dark of night; staggeringly important in limiting fratricide as well (though not as well as one might have liked...) * Shut down normal transmission and begin strongly encrypted transmission. No mention of this; apparently, the satellites were originally designed with some sort of weak system that made the data difficult to use for high-accuracy purposes, but that's been defeated (by the FAA or someone contracted thereto). * Enter into a bogus-cleartext with encrypted subchannel mode, where the plaintext is slyly made to be wrong, but using some subchannel encrypted "good stuff" is still available. Well, this sounds like the "selective availability" (SA) capability, which still exists and has in fact been turned on. It is defeatable, but only under certain circumstances (see below). GPS is a clock-based system; by talking to a constellation of satellites (I think the minimum is 4, with 5 being desired and 6 being best) and munging the timestamps received from them, GPS systems can compute their location to a degree of precision related to the precision of the clocks. SA essentially truncates a few bits off the low-end of the time stamps and makes them available in an encrypted form. Without the key to decrypt the low-order bits, you get location information to within a couple hundred meters or so; with the low-order bits, within about six feet. During the Gulf War, the US military was unable to get enough military GPS receivers (i.e. ones that could decrypt the selectively-available data). Instead, they bought commercial over-the-counter GPS receivers and turned SA off. As soon as they got enough SA-enabled receivers, they turned it on. SA is defeated by something called Differential GPS. Basically, a ground station at a fixed location constantly computes its position via GPS, computes the difference between the GPS location and its known correct location, and broadcasts the correction factor. A differential-capable GPS receiver computes its location via GPS and then applies the correction fac- tor from the nearest differential station. These corrections are obviously of decreasing accuracy the farther you get from the fixed-position station, but you can correct for that to some degree once you have an estimate of the direction and distance from your location to that of the differential broadcaster; apply corrections iteratively until they converge. The requirement, of course, is that you be someplace near a differential GPS station. These stations are maintained by the US Coast Guard and obviously exist only where there is navigable water (and certainly not everywhere, since they take time and money to build). I'm curious if anyone has attempted to crack the encryption used in selective availability; although Differential GPS solves my most important issues (coastal navigation) it's be nice to be able to use a handheld GPS receiver in the middle of Rock Mountain National Park or some such and still locate yourself on the bloody maps with some precision. Jason