The GPS system includes three parts: space GPS satellite constellation;

Ground control part - ground monitoring system;

User equipment Part GPS signal receiver.

The GPS satellite constellation

GPS satellite and constellation consists of 21 satellites and 3 spare satellites in orbitGPS satellite constellation, denoted by (21+3) GPS. 24 satellites evenly distributed in the 6 orbit plane, the orbital inclination of 55 degrees, between the orbital plane 60 degrees apart, that orbit of the ascension of the difference between the 60 degrees.Between each satellite within the orbital plane of the lift angle is 90 degrees, an orbitalplane 30 degrees ahead than the corresponding satellite satel west adjacent tracks.

In the twenty thousand km altitude of the GPS satellites, for the stars when the earthrotates, they are orbiting the earth for two weeks, which is around the earth one week to 12 stars. So, on the ground that observer, every 4 minutes ahead of schedule tomeet with a GPS satellite. In a satellite number above the horizon, with the time and place is different, at least can see 4 stars, could see up to 11. In the navigation and positioning using GPS signals, in order to clearing three-dimensional coordinates of the station, must be observed 4 GPS satellites, called positioning constellation.

The ground monitoring system

For navigation, GPS satellite is a dynamic known points. Star position is based onsatellite ephemeris parameters calculated, describe the motion of satellites and orbit.Each. Each of the GPS satellites broadcast ephemeris, is provided by the ground monitoring system. All the equipment on the satellite is working, and whether the satellite has been run along the orbit, to monitor and control the ground equipment.Another important role of ground monitoring system is to keep the satellite at the same time standard time system GPS. This requires the satellite ground station to monitor the time, the clock error. Then from the ground into the station to satellite,satellite navigation message is sent then the user equipment. Ground control system of GPS satellite includes a master control station, three into the station and five stations.

The GPS signal receiver

The GPS signal receiver's mission is: to capture a certain satellite elevation cut-offsignal measured satellite angle selected, and track the satellite operation, GPS on the received signal conversion, amplification and processing, in order to measure the GPS signal from the satellite to the propagation time receiver antenna, interpretationthe navigation message s