We all appreciate the progress of technology, and it has become quite indispensable even for the most simple activities. Unlike the old days, when people used to determine their locations by the position of the stars, or, later, a compass, nowadays things are much easier for us: we activate the GPS when we travel, and the GPS receiver updates our location continually. Still, how do GPS receivers work?

First of all, GPS stands for global positioning system. GPS systems use satellites as reference points, and in fact are quite simple devices. The satellites orbiting Earth send out radio signals, detected by the receivers on the ground (or in ships/ airplanes). These signals include information about the satellite ’s location and the exact time, and the receiver uses a clock to measure the amount of time needed for the radio signal, traveling at 186,000 miles/second, to reach it. A second signal from another satellite narrows the possible locations to a circle, and then a third signal narrows it to only two possible locations for the receiver. The forth sphere is the surface of the Earth, narrowing the possibilities to one location. As the system has at least 24 operating satellites at all times, there are always at least three of them within a line of sight to the receiver.

Still, the process is more complicated in practice. An accurate calculation requires a time measurement with a precision of a few hundred nanoseconds (a nanosecond is one billionth of a second), and this can be done only with an atomic clock. However, the cost of an atomic clock reaches tens of thousands of dollars, and since most purchasers of GPS receivers would not afford that, some mathematical tricks are required.

Each satellite does carry an atomic clock, and at least one backup, so the times are synchronized on the satellites. The receiver calculates the real time from the satellite signals, and then calculates the distance. The result? Most receivers can determine your location to within 30 feet or so, and your positioning will improve up to 10 or 15 feet of your actual location, with the more satellites you can pick up. Also, in some places an additional location is provided by ground-based transmitters. In addition to the satellites, these signals allow the receiver to calculate a position within a few feet, as close as to tell the exact corner of the intersection you ‘re on.

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