Introduction to Global Satellite Positioning System (GPS)

7.2 Principle Of Global Positioning System

The GPS is a satellite-based positioning and navigation system. GPS provides continuous positioning and timing information anywhere in the world under any weather conditions. GPS is also a passively one-way ranging system and it can serve unlimited number of users. That is, users only take the GPS receivers to receive enough GPS signals without bidirectional communications among them. GPS receives will be able to calculate out the user positions they locate now.

GPS system generally consists of a constellation of 24 operational satellites (Leick, 2004). In order to ensure continuous worldwide converge for GPS positioning service, six orbital planes of satellites are organized and each four satellites are placed in an orbital plan. There are four to ten GPS satellites will be visible anywhere in the world under this constellation geometry. The sketch map of GPS constellation is shown in Figure 1. A GPS satellite routes around the earth in a nearby circular orbit, an elliptical shape, with an inclination of 55 degrees to the equatorial plane. The maximal radius of GPS orbit is about 26,560 kilometers measured from the earth center. The orbit period of GPS satellite is approximately 12 sidereal hours, which is about 11 hours and 58 minutes. Thus, GPS satellite will run around the earth twice per day. In order to ensure the availability of GPS positioning service, the number of satellites in the GPS constellation has always been more than 24 operational satellites.