The Global Navigation Satellite Systems (GNSS) include of the U.S.A. Global Positioning System (GPS), the Russian Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS), the upcoming European Galileo and the Chinese COMPASS system. The fundamental of satellite navigation is the simultaneous observation of at least four satellites with known coordinates. The measurements are carried out through correlation of the received satellite signal with an identical signal generated in the ground receiver. The difference between the received and the generated signal is directly proportional to the travel time of a signal from satellite to the receiver. As the satellite and receiver have different clocks, in addition to the three unknown coordinates of the receiver, an additional unknown appears in the observation equation. Therefore, only the simultaneous observation to at least four satellites enables the equation to be solved for all positioning parameters, i.e. for longitude, latitude, height, and clock correction.
