Global Positioning Systems

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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 ...

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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 ...

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