for Global Positioning System, a worldwide MEO satellite navigational system formed by 24 satellites orbiting
the earth and their corresponding receivers on the earth.
The satellites orbit the earth at approximately 12,000 miles above the surface
and make two complete orbits every 24 hours.
GPS satellites continuously transmit digital radio signals that contain data
o the satellites location and the exact
time to the earth-bound receivers. The satellites are equipped with atomic
clocks that are precise to within a billionth of a second. Based on this
information the receivers know how long it takes for the signal to reach the receiver on earth.
As each signal travels at the speed of light, the longer it takes the receiver
to get the signal, the farther away the satellite is. By knowing how faraway a satellite is, the receiver knows that it is located somewhere
on the surface of an imaginary sphere centered at the satellite. By using three satellites, GPS can calculate the longitude
and latitude of the receiver based on where the three spheres intersect. By using four satellites, GPS can also determine
GPS was developed and is operated by the U.S. Department of Defense. It was
originally called NAVSTAR (Navigation System with Timing and Ranging). Before its
civilian applications, GPS was used to provide all-weather round the-clock
navigation capabilities for military ground, sea, and air forces.