This following applet is [b]How GPS works[/b]
Created by prof Hwang Modified by Ahmed
Original project [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=465.0]How GPS works[/url]

The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails). The U.S. military developed and implemented this satellite network as a military navigation system. Each of these solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky.
Each GPS satellite has atomic clocks on board. Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver.
The GPS receiver is located at sphere surfaces centered at each satellite.
So the location is at the intersection for those three sphere surface.

The intersection for two sphere surface is a circle.
And the intersection for the above circle and the other sphere surface can be at two intersect points. However, only one of the point is on the earth surface (the other one will be on the outer space, not on the earth surface). You unique location can be determined.

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