Global Positioning System (GPS) satellites broadcast microwave

signals to enable GPS receivers on or near the Earth s surface to

determine location and synchronized time. The GPS system itself is

operated by the U.S. Department of Defense (DoD) for use by both the

military and the general public.

GPS signals include ranging signals, used to measure the distance to

the satellite, and navigation messages. The navigation messages

include ephemeris data, used to calculate the position of each satellite

in orbit, and information about the time and status of the entire satellite

constellation, called the almanac