How GPS Works
GPS uses a network of 24 satellites as reference points to determine position. By accurately measuring the distance from three satellites the GPS can “triangulate” a position anywhere on earth. But how do you measure the distance to something floating in space. Basically, by timing how long it takes for a signal sent from a satellite to arrive at a receiver here on earth using a Pseudo Random Code (PRC).
A fundamental part of the GPS process, the PRC is a complicated digital code, or in other words, a complex sequence of “on” and “off” pulses. The code’s complexity helps makes sure that the receiver doesn’t accidentally “sync up” to some other signal. The complexity also makes it possible to use “information theory” to “amplify” the GPS signal, making GPS economical.
In addition, since each of the 24 satellites has its own unique PRC, the complexity guarantees the receiver won’t accidentally pick up another satellite’s signal. So all the satellites can use the same frequency without jamming each other, making it more difficult for a hostile force to jam the system. In fact, the PRC gives the DoD a way to control access to the system.
GPS comprises three segments. The Space Segment consists of satellites orbiting the earth once every 12 hours. More than 24 operational satellites are often in service as new ones are launched to replace older ones. The satellites repeat almost the same ground track (as the earth turns beneath them) once each day. They pass over any point on the earth once every 24 hours (4 minutes earlier each day). Four satellites are needed to compute the dimensions of X, Y, Z (position), and Time.
The Control Segment consists of tracking stations located around the world. The Master Control facility is headquartered at Schriever Air Force Base in Colorado. These stations measure signals from the SVs. The signals are then incorporated into orbital models for each satellite. The models compute precise orbital data (ephemeris) and SV clock corrections for each satellite. The Master Control Station then uploads ephemeris and clock data to the SVs, which send a subset of the orbital ephemeris data to GPS receivers using radio signals.
The User Segment consists of people employing GPS receivers. These receivers convert satellite vehicle signals into position, velocity, and time estimates. They make navigation the system’s primary function. Additional functions are mapping, tracking, locating, and timing. Receivers have been miniaturized to just a few integrated circuits and are becoming economical, making GPS technology more and more accessible to everyone.