This chapter provides an overview of Global Navigation Satellite System. It begins with an introduction to the major satellite navigation systems, focusing on global navigation systems such as Global Positioning System, BeiDou Navigation Satellite System, Galileo, and Global Navigation Satellite System, as well as regional or augmented navigation systems like Quasi-Zenith Satellite System, Indian Regional Navigation Satellite System, and Satellite-Based Augmentation Systems. Each system has its own unique structure, functions, and signal specifications, contributing to the global satellite navigation landscape. Next, the chapter traces the evolution of positioning technology, from single point positioning to precise point positioning (PPP) and real-time kinematic positioning (RTK), leading to their derivative technologies, such as network RTK and PPP-RTK. Their respective operating principles are also discussed. Finally, the chapter highlights the unique advantages of RTK and provides a comparative analysis of the strengths and weaknesses of mainstream high-precision positioning methods.