Page 3triangulation
How Does GPS Work?
The Simple Physics

The bases of GPS is using triangulation to locate a point.  Triangulating your position on Earth requires using distance measurements taken by three different satellites to your point.  To find our unknown point's location we need to know where we are measuring from.  The GPS satellites are precisely placed in a simple orbit well above the Earth's atmosphere.  They are carefully monitored by their ground stations and checked with radar.  The satellite's onboard computer knows where it is in its orbit at all times.

Measuring Distance                                                                                                                                                                                                                        Trimble Website
The GPS satellites use radio waves and precision timing to measure distance.  Knowing the velocity the radio wave travels and the amount of time the radio wave took to reach its destination, we can find the distance with a simple calculation.

Velocity X Time = Distance

Timing is Everything
Radio waves travel at the speed of light, which is a velocity of roughly 186,000 miles per second.  If the timing is off by just a thousandth of a second that translates to 186 miles of error.  Satellites have onboard clocks called atomic clocks that uses the oscillations of atoms as the clock's metronome and is the best way ever to measure time.  The receiver on earth does not have an atomic clock (they are very expensive), but uses an extra measurement from a fourth satellite to adjust for the timing error.  The receiver uses this fourth satellite measurement to cross-check the other three, and find a correction factor that intersects all the measurements at your point.

Radio Waves
GPS satellites transmit radio signals on two carrier frequencies known as L1 and L2.  The L1 carrier is at a frequency of 1575.42 MHz and carries both a status message and a pseudo-random code used for timing.  The L2 carrier is at a frequency of 1227.60 MHz and is used by the military.  The pseudo-random codes are unique to each satellite and the military's pseudo-random codes can be encrypted.

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