GNSS, or more precisely GNSS-2, is an abbreviation for Glob al Navigation Satellite Systems – Second generation, and serves as a generic name for the class of modern global sa tellite based radio navigation systems. GNSS-2 consists mainly of the four major Global Navigation Satellite Systems known as: GPS (U.S.), GLONASS (Russia), Galileo (EU) and Bei-Dou-2 (China). All these global radio navigation systems are based on the same navigation principle, i.e. utilizing ultra- stable clocks in satellites to determine the user position by independent measurements of the transit time of electromagnetic signals transmitted from satellites in orbit, so-called Radio Navigation Satellite Services (RNSS). The typical performance of these global radio navigation systems is to provide absolute positioning to an observer on the surface of the Earth within the precision of 5-10 meter. However, this precision can be improved utilizing state of the art processing techniques such as Precise Point Positioning (PPP), currently demonstrating absolute positioning of 5-10 centimeters utilizing only one receiver. To achieve this astonishing precision in terms of absolute position, the rate of time as measured on the clock in the satellite must be known to better than a few nanoseconds. Since the satellites are constantly moving with respect to the observer and are also located at highly different gravitational potentials, effects predicted by both the Special- and General theories of Relativity must be considered in order to achieve the desired accuracy in the observed transit times. These systems are in fact one of the very few man made systems, outside of particle accelerators, that experience significant relativistic effects.