## The U.S.-developed-GPS is a space-based satellite navigation system, which provides location and time information.

Have you ever ridden your bicycle without a destination in mind, lost yourself completely and then rummaged your bags to find your location using the maps on your mobile? Or do you use your smartphone to check-in to any place you visit in order to let friends nearby know that you are around?

If you’d answered yes to either of the above questions, you were probably using the built-in GPS receiver on your handset. Now wait, does this mean you only have a receiver? Then what is GPS supposed to be?

The Global Positioning System or GPS is actually a space-based satellite navigation system which provides location and time information. The GPS constellation includes 24 Earth-orbiting satellites distributed equally among six circular orbital planes. Developed by the United States of America and initially deployed as a military navigation system, the technology is now available for anyone with a receiver to use, even though it is maintained by the U.S. government.

So how is it that the receiver determines the location using these satellites? There are two things that a receiver needs to know in order to find out a location: the position in which at least four satellites are located and the distance between the receiver and each of these four satellites.

Once the receiver is able to locate four or more of these solar-powered satellites and the distance to each, this information is pieced together using a principle called trilateration. Trilateration is a mathematical process of determining absolute or relative locations by measuring distances, using the geometry of circles, spheres and triangles.

How does it work?

How is this principle employed in finding our location once we have the position and distance from four or more satellites? According to trilateration, in a two-dimensional plane, while two circles intersect in two points, three or more circles can intersect only in one point. Extrapolating the same principle to a three dimensional surface like our Earth we can say that four or more spheres can intersect only at one point.

The position of the satellite that the receiver has would be the centre of a sphere and the corresponding distance would be the radius. Having acquired data from four or more satellites, the receiver would be able to pinpoint the location in which these spheres intersect. This point is in fact the location of the GPS receiver.

However, in order to make this measurement, the clocks in the receiver and the satellite would have to be synchronised to the nanosecond. While satellites do have an atomic clock, it is not economically viable to build receivers that have one. This error is easily rectified by using a simple correction. The ordinary quartz clock in the receiver finds out its inaccuracy from incoming signals and resets itself to the current time that all the satellites correspond to.

Ever since U.S. opened it up to public usage, the GPS technology has been put to use in numerous ways (scientific research, geotagging, navigation, robotics, etc.) apart from the military surveillance for which it was initially built for. The Russian GLObal NAvigation Satellite System (GLONASS) is a similar technology used by the Russian intelligence, made available to civilians from 2007.

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