How does Global Positioning System work?

The U.S. military developed and implemented GPS satellite network as a military radio-navigation system to figure out the exact position on the earth.
It’s made up of three parts: satellites, ground stations, and receivers.
GPS is a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails). Each satellite is built to last about 10 years. Each one transmits information about its position and the current time at regular intervals.These satellites make two complete rotations around the globe everyday such that at any point of time atleast four satellites are visible to the receiver.
The basis of GPS is "trilateration".Trilateration is a method of determining the relative positions of objects using the geometry of triangles.In order to make this simple calculation, the GPS receiver has to know the location of at least three satellites and the distance between our receiver and each of those satellites.
The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are further away than others.Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite.
To make the measurement we assume that both the satellite and our receiver are generating the same pseudo-random codes at exactly the same time.By comparing how late the satellite's pseudo-random code appears compared to our receiver's code, we determine how long it took to reach us.
Multiply that travel time by the speed of light and we've got distance.When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions.

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