Satellite Gps

Today the Global Positioning System (GPS) is well established for military, commercial and private use. The full operational system consists of a constellation of 24 operational satellites with a few more in orbit as spares in case of the failure of one. The satellites are in one of six orbits. These are in planes that are inclined at approximately 55 degrees to the equatorial plane and there are four satellites in each orbit. This arrangement provides the earth user with a view of between five and eight satellites at any time from any point on the Earth. When four satellites are visible, sufficient information is available to be able to calculate the position on Earth.
The GPS satellites need to be monitored and controlled from the ground and it is necessary to be in contact with each satellite for most of the time to be able to maintain the level of performance required. To achieve this there is a master station located at Falcon Air Force Base, Colorado Springs, USA. However there are other remote stations are located on Hawaii, Ascension Island, Diego Garcia and at Kwajalein. Using all these stations the satellites can be tracked and monitored for 92% of the time. This results in two 1.5 hour periods each day when the satellite is out of contact with the ground stations.