This is a bit off list topics. Press D now if you don't like it.... In message Tue, 28 Dec 93 14:01:15 CST, m5@vail.tivoli.com (Mike McNally) writes:
* 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).
The GPS system was designed to have two modes, one highly acurate for military use, and a low accuracy version for commerce. Since initially no one had receivers, the whole system used military grade for the first few years. Before the Gulf war started, the military needed zillions of GPS units, so they bought commercial marine navigrtion units. After the war, there was talk of turning on the division, but that was made impractical with 'differential' calibration. To get military accuracy, all you have to do is take the commercial GPS to known places. Such as the surveying marks at the US Naval Acadamy, or "ground zero" in the Pentagon. Get your "imprecise" reading, calculate the difference between it at the known value, and use it for subsequent calculations. I expect that triangulation with a few known sites is better than a single one. Last I hear from the Sailing trade press was that the Gov still talked about going to the split setup, but there was little belief that they would. BTW, two satellites is enuff for a general location fix. Three identifies a place on the globe, four adds altitude (handy for planes, bombs, and missles) Pat Pat Farrell Grad Student pfarrell@netcom.com Department of Computer Science George Mason University, Fairfax, VA Public key availble via finger #include <standard.disclaimer>