Gps Essays (3291 words) - Global Positioning System, Geography

Gps The new Avionics Modernization Program (AMP) systems installed in the F-111E and EF-111A have raised their share of questions, so I have decided to continue my series of Everything You Always Wanted To Know handouts to you pilots, navigators, and maintenance technicians on how the cotton-picken' thing works. This informational pamphlet is an overview of the GPS system as a whole, NOT the system- specific hardware that you find in your respective aircraft. I'll cover the basic theory of operation here, and if there proves to be sufficient interest in platform-specific installation, that will be covered in a later supplement. THE BASICS GPS works by triangulation, the process of finding where you are by the angle to fixed known points. In the old method of DME position determination, you would tune one DME channel and draw a circle on your chart around the DME transmitter, the radius of which was your DME reading in nautical miles. Then you'd tune in a second DME station and repeat the process. On your chart at this point there would be two circles whose lines intersected at two points. Even a vague guess of your whereabouts would be enough to discard the bogus point, and you'd be left with a pretty good idea of your position. Better yet, take a cut from a third DME transmitter and draw a third circle on your chart. Now you'd have three intersecting circles and your position would be inside the little triangle formed by the intersection of the three circles. Got the picture? This is basically how GPS triangulates, except that instead of circles, we're dealing with intersecting spheres. TIMING IS EVERYTHING Think of GPS satellites as floating DME stations. They move along in orbit and that complicates things but forget about that for the moment. How can we measure distance? The satellites in the GPS are some 10,900 miles up, but they're not geostationary (they'd have to be much higher and thus would require more power to reach earthbound GPS receivers) and they travel along at a ground speed of about five miles a second. Like DME, GPS measures the time that it takes the signal to reach the receiver. However, unlike DME, the receiver doesn't have the benefit of a returning pulse from an interrogation to act as a baseline. It relies purely on one-way timing. You can see right away how it begins to get complicated. The speed of microwave communication is roughly the speed of light, and from 10,900 miles up, any pulse from the GPS takes about 1/17 (0.059) of a second to reach us. The math is simple enough. All we need to know is exactly when the signal left the satellite. And I do mean exactly. An error of a mere .001 of a second would trash the fix by a factor of 180 miles or so. Obviously, very accurate clocks are required. DO YOU HAVE THE EXACT TIME? Each satellite carries four atomic clocks internally, each of which uses the oscillation of cesium and rubidium atoms to keep extremely accurate time, accurate to within one second over more than 30,000 years. (For you graduates of the USAF Academy, that's one part in 1013, or one part in 10,000,000,000,000). All satellites in the system are synchronized at exactly the same time and they are kept within 176 nanoseconds of the Universal Time Code (UTC), plus accumulated jump seconds to account for things like solar time. Navigation messages from the satellites announce the difference between GPS time and UTC, providing self-recalibration of the clocks. Okay, we have accurate clocks in the satellites. Now all we need are accurate clocks in our GPS receivers, synch 'em up and we're in business. Of course, if your el cheapo K-Mart GPS receiver had a cesium clock, it'd cost about $200,000 and be about the size of a desktop computer. The way around that was to develop internal receiver clocks that are consistently accurate over relatively short periods of time, as long as they're reset often enough to keep them synched. Here's how the receiver clocks are reset: Remember how we explained that DME business, with three intersecting circles? Well, GPS does the same thing, only it uses three intersecting spheres to determine position. Let's for a moment assume that the receiver clock and satellite clock are exactly in synch. The receiver times the signal, figures the distance from three satellites and where the three spheres intersect...voila...that's our position. But, the receiver