Principles of Satellite Positioning
1. The Global Positioning System

The Global Positioning System GPS was developed by the U.S. military in the application of military positioning and navigational objectives. After the Defense Dept. took over the project it was soon expanded and is now being used in civil positioning and surveying applications.

The whole system consists of 24 satellites evenly distributed on 6 different orbits. The orbit is a near perfect circle at about 20000km altitude above the earth surface and each revolution takes about 12 hours. The arrangement of satellites is planned in such a way that at least four satellites are simultaneously risible above the horizon anywhere on the earth, 24 hours a day which is then capable of providing applications of navigation and precise positioning.

2. The Principle of Satellite Positioning Surveying

At the same time when a GPS Satellite is revolving around the earth it will transmit continuously satellite signal to the earth. Users on the earth will use various kinds of satellite receiver for the reception of these satellite signals. From the various characteristics and features of the signal received the distance between the satellite and the receiver may be derived as well as the baseline vector from each individual receiver scattered on the surface of the earth. A computation of data received with geometrical principle will find out the position of these receivers and likewise the applications in navigation, positioning and other survey are made possible.

3. The Formation of The GPS Satellite Signal , Its Application and Precision Level

The signal content of GPS satellite are mainly formed of code and carrier:

  1. Code is used for real-time positioning and applied for navigation on moving objects such as planes and ships. The precision level by this application was only good to around 50 to 100 meters. The recent and fast development of the Differential GPS can now pushed the precision level up to within 1 meter.
  2. Carrier signals are for computation after data collection and its major application is for high precision positioning. The resulting positioning will achieve centimeter accuracy.
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