This chapter describes the Global Navigation Satellite Systems (GNSS) used for positioning and navigation applications. Although a satellite navigation system is very complex, it is common to speak of a “satellite navigation system sensor” when referring to the tools that an end user needs in order to exploit the functionalities of a satellite navigation system, and this chapter will give a general survey of this topic. Firstly, the positioning problem within the framework of a positioning satellite system will be described. Then, general GNSS descriptions and prospects will be presented, outlining the main features of the American GPS – GNSS “gold standard”; the Russian GLONASS (revived and modernized a few years ago); the European Galileo and the Chinese Bei Dou (formerly called Compass), both successfully developing; along with the Japanese QZSS and the Indian IRNSS regional navigation satellite systems. Following these will be sections that include sources of errors and receiver structures common to all GNSS. Modernization and augmentation (GBAS and SBAS) of satellite systems are treated in a specific paragraph. Finally, some brief outlines of the aerospace applications of satellite systems will conclude the chapter. Major attention has been paid to the principles of satellite positioning, to descriptions of the measurements that can be obtained from satellite signals, the way they are processed, the effects that affect the measurements and how they propagate. These choices were motivated by the fact that their principles are common to all the GNSS and can provide the reader with the tools to approach the following subject matter. Many references are also given in the bibliography for the study of particular topics in depth, but the most recent novelties are quickly outlined because the relevant research is still ongoing and new applications are emerging.