GNSS principles and comparison

Over the past years, there has been an increase in number of Global Navigation Satellite Systems (GNSSs). Some of these GNSS are already in service, but some of them still under development. As a result of developing these systems by different countries spread through different continents, users will soon be overwhelmed by lists of concepts, features, and idioms, which are announced by different competing systems. The purpose of this paper is to review main principle of any GNSS then to compare different GNSSs including GPS, GLONASS, Galileo, and Compass. However, in this paper, we focus more on GPS system as it is the only GNSS that is presently at a full operational capability and widely used in the last ten years.