Q12Principle of Electronic Communication

GPS - Components and Working

GPS (Global Positioning System) consists of three segments: the space segment (a constellation of at least 24 satellites in medium Earth orbit, arranged so at least 4 are visible from any point on Earth at any time), the control segment (ground stations that track satellite orbits and clock accuracy, uploading correction data), and the user segment (the GPS receiver used by the end user).

Each satellite continuously broadcasts its precise orbital position (ephemeris data) and a precisely timed signal from its onboard atomic clock. The receiver measures the time delay between when each signal was transmitted and when it was received, and multiplying this delay by the speed of light gives the distance (pseudorange) to that satellite. Since the receiver's own clock is not as accurate as the satellites' atomic clocks, at least four satellite measurements are required: three to solve for the receiver's 3D position (latitude, longitude, altitude), and a fourth to solve for and correct the receiver's own clock bias error. Solving this system of four simultaneous equations (trilateration with clock-bias correction) gives the receiver's precise position and time.

A key practical refinement of the basic four-satellite trilateration principle is differential correction: because ionospheric and tropospheric delay, satellite clock drift, and orbital errors all introduce small but significant timing inaccuracies, civilian GPS receivers historically achieved only about 10-15 meters of accuracy, whereas augmentation systems (such as WAAS in the US or GAGAN in India) broadcast additional correction signals from ground reference stations of known precise location, improving achievable accuracy to a few meters for suitably equipped receivers.