Global Positioning System satellites transmit signals to equipment on the ground.
GPS receivers passively receive satellite signals.
Accurate time is the secret
GPS operations depend on a very accurate time reference, which is provided by atomic clocks.
Each GPS satellite has atomic clocks on board.
- GPS satellites
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There are at least 24 operational GPS satellites at all times. The satellites, operated by the U.S. Air Force, orbit with a period of 12 hours. Ground stations are used to precisely track each satellite's orbit.
Determining Position
A GPS receiver "knows" the location of the satellites, because that information is included in satellite transmissions. By estimating how far away a satellite is, the receiver also "knows" it is located somewhere on the surface of an imaginary sphere centered at the satellite. It then determines the sizes of several spheres, one for each satellite. The receiver is located where these spheres intersect.
The GPS calculates using the "triangulation" method
knowing distances from two satellites places you somewhere along a circle that's between the two satellites (defined by the intersection of their "distance spheres") distances from three satellites usually intersect at two points, and if you're not flying around, one of these points will be on Earth's surface distances from four or more GPS satellites will intersect at just one point
This process works by finding the intersection of your distances from three or more satellites. Thus, describing it as "trilateration" is actually better than "triangulation", but neither term seems precisely correct from a technical standpoint.
[distance] = [speed] * [travel time]
We know [speed] because GPS signals are electromagnetic waves, which travel at light speed (approximately 300,000,000 meters per second).
We can figure [travel time] by comparing [time sent] and [time received].
Each GPS satellite has an on-board clock and includes [time sent] in the the signal it broadcasts.
A GPS receiver also keeps track of time, which gives [time received]
[Travel time] is the difference between these times.
GPS Accuracy
The accuracy of a position determined with GPS depends on the type of receiver. Most hand-held GPS units have about 10-20 meter accuracy. Other types of receivers use a method called Differential GPS (DGPS) to obtain much higher accuracy. DGPS requires an additional receiver fixed at a known location nearby. Observations made by the stationary receiver are used to correct positions recorded by the roving units, producing an accuracy greater than 1 meter.
