Global Positioning System (GPS)
GPS operates by accurately measuring the propagation time of signals transmitted from the satellites to the user's receiver. A nominal constellation of 24 satellites together with some active spares, in six orbital planes at about 20000 km altitude, provides a minimum of four satellites in view 24 hours a day at every point on the globe. Four satellites must be received simultaneously to determine both the receiver position (x, y, z) and the receiver clock's offset from the GPS system time. All satellites are monitored by ground control stations which determine the exact orbit parameters and the clock offset of the satellites' on-board atomic clocks. These parameters are uploaded to the satellites and become part of a navigation message which is retransmitted to the user's receiver by the satellites.
The high precision orbit parameters of the satellites are called ephemeris parameters, and a reduced precision subset of the ephemeris parameters is called a satellite's almanac. While ephemeris parameters must be evaluated to compute the receiver's position and clock offset, almanac parameters are used to check which satellites are in view from a given receiver position at a given time. Each satellite transmits its own set of ephemeris parameters, and almanac parameters of all existing satellites.
GPS system time differs from the universal time scale (UTC) by the number of leap seconds that have been inserted into the UTC time scale since GPS was initiated in 1980. The current number of leap seconds is part of the navigation message supplied by the satellites, so a receiver's internal real time can be based on UTC. Conversion to local time and handling of Daylight Savings Time each year is done by the receiver's microprocessor once these parameters have been programmed by the user.