The GPS (Global Positioning System) consists of a system of at least 24 satellites that continuously broadcast information so that a passive receiver can locate its position on the surface of the earth.
Satellites are in half sidereal-day orbits that are roughly circular (altitude is approximately 20,200 km) with four satellites in each of six orbital planes inclined at 55°.
The satellites are distributed so that generally a sufficient number are well above the horizon at any one time.
They broadcast on two carrier frequencies, 1575.42 MHz (≈ 19 cm) and 1227.60 MHz (≈ 24 cm).
Each satellite contains an "atomic clock" and broadcasts an encoded digital message that includes data such as a satellite identification, ephemeris of the satellite orbit, satellite clock error correction, approximate data for all other satellites, and a pseudo-random sequence ranging code.
The code used by non-military receiver is modulated at 1.023 MHz rate with a period of 1 ms. The use of a spread spectrum decreases problems of interference.
Data for each satellite is computed by central ground stations and uploaded periodically to keep the system accurate.
The passive nature of the system ensures that many users can simultaneously use the system, and no user need betray his or her position with an active signal.
Big Brother cannot track the individual GPS user unless a transmitter (such as a cell phone) is incorporated with a GPS receiver.