The Global Positioning System (GPS) is a framework that allows anyone, with the aid of a small device, to identify exactly where they are on the earth’s surface. The GPS is made up of several satellites in accurate, well-known orbits around the earth. Each of these satellites contains a very accurate clock and a microwave transmitter. Each time the clock “ticks” (millions of times every second), it sends out a microwave pulse which is ‘stamped’ with the time and the ID of that specific satellite.
A hand-held GPS locator then receives these pulses. Since it is closer to some satellites than to others, the pulses it receives take less time to reach it from some satellites than from the others. The result is that the pulses it receives at a given instant are not all stamped with the same time. The locator then uses the differences in these timestamps to figure out which satellites it is closest to, and by how much. Because it knows the orbits of the satellites very accurately, this informs the device exactly where it is located (i.e. the device itself). This technology permits the device to pinpoint its location on the earth’s surface to within a one meter circle. To accomplish this accuracy, the clocks in the satellites must be accurate and the time stamps must be very precise. Specifically, they must be much more accurate than one part in ten billion.
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