The satellites orbit the earth with a speed of 3.9 km per second and have a circulation time of 12 h sidereal time, corresponding to 11 h 58 min earth time. This means that the same satellite reaches a certain position about 4 minutes earlier each day. The mean distance from the middle of the earth is 26560 km. With a mean earth radius of 6360 km, the height of the orbits is then about 20200 km. Orbits in this height are referred to as MEO – medium earth orbit. In comparison, geostationary satellites like ASTRA or Meteosat – satellites orbit the earth at 42300 km, which is about twice the distance of GPS satellites and GPS tracking devices.The satellites are arranged on 6 planes, each of them containing at least 4 slots where satellites can be arranged equidistantly. Today, typically more than 24 satellites orbit the earth, improving the availability of the system. The inclination angle of the planes towards the equator is 55°, the planes are rotated in the equatorial plane by 60° against each other. This means that the orbits range from 55° north to 55° degrees south. Block I satellites had an inclination of 63° against the equator.
By this arrangement of the orbits it is avoided that too many satellites are to often over the north and south pole (like it was the case in the TRANSIT system, where the satellites ran on polar orbits).
However the orbits run far enough to the north and south to guarantee GPS availability in polar regions. Furthermore this arrangement leads to a rather stable constellation, as orbit disturbing factors like solar winds and gravitation fields have about the same influence on all of the satellites.
The number and constellation of satellites guarantees that the signals of at least four satellites can be received at any time all over the world.
The closer you get to the poles, the lower over the horizon the satellites are located. They can still be received very well, but in no case they are directly above. This may lead to a – typically insignificant – loss of the precision of the position determination. This effect, caused by the geometry of the satellite arrangement, happens from time to time on any spot of the earth surface and can be forecasted.
The above picture shows the ground track of satellite BIIR-07 (PRN 18) from 2001-10-18, 00:00 ’o clock to 2001-10-19, 00:00 ’o clock. The yellow arrow marks the 00:00 ’o clock time point. It can be seen that the orbit time is slightly shifted (about 4 minutes) in 24 h.
The yellow dot marks the satellite position at 09:30 pm. The satellite is positioned over Ethiopia. The correlating “zone of sight”, within which the satellite signal can be received, is mark in light blue. The graph was compiled with the FreeWare software WinOrbit (and slightly modified).
For a deeper insight into orbits of GPS satellites and other satellites, the following link can be recommended: J-Track 3D. A Java applet on this page can illustrate orbits and information of more than 500 satellites.