Document Type : Research Paper

Author

• Lotfollah Emad Ali (Translator)

Master of Geodesy (Surveying Organization of Khuzestan)

Abstract

By using coordinates of satellites and determining the receiver's distance from the satellite, the position of the GPS receiver is calculated and the distance is determined by measuring the time of the radio signal’s travel from satellite to receiver. But how are satellite coordinates determined and how is the time is measured? This is the topic of determining the satellite position and we know what it is and does, but we do not know how it works.
 In general, the determination of satellite position is a matter of triangulation. The receiver coordinates can be calculated by knowing the position of three satellites and measuring the distance between these satellites and the receiver. The distance between receiver and satellite is obtained by multiplying the radio signal’s travel time by the speed of waves (speed of light). But how we monitor the satellite's position permanently? The orbits of satellites can not be fully and accurately determined, and change by the effects of the forces imposed by celestial bodies. Since radio signals move at the speed of 300,000 km / s, inaccuracy in time measuring of the magnitude of one nanosecond leads to an error of 30 cm in distance measurement, while by using geometric receivers, precision  of the order of magnitude of one centimeter can be achieved.