This chapter first gives a brief overview of the GPS system from the macro perspective, then gives a similar overview of China’s independently planned, designed, and implemented BeiDou Satellite Navigation System. Emphasis is placed on the comprehensive description of the satellite constellation, signal system, carrier frequency band, performance index, and application scope of the GPS and BeiDou systems from the perspective of top-level design. At the same time, some historical development events relating to the satellite navigation system are described and remarked upon. This will provide readers who are new to the field with a preliminary understanding of the system’s composition as well as the historical evolution of the field, so as to grasp the characteristics of the existing system and the direction of future development. As a major event in satellite navigation in the twenty-first century, the modernization of the GPS system will be described in detail in this chapter, and the impact of this event on future GPS receivers will be analyzed. At the same time, the performance evolution trend of future GNSS receivers will be analyzed in combination with the deployment and implementation of China’s BeiDou Satellite Navigation System.