This chapter begins with a brief overview of four global satellite systems for navigation and positioning, GPS, GLONASS, Beidou and Galileo (under development), and point out the differences between them. We have a cursory look at two regional navigation systems, the Indian NAVIC, and the Japanese QZSS. Most important applications of positioning systems are presented. There are navigation for cars, ships, airplanes and even ordinary pedestrians in the city or in the countryside, dissemination of time scale and search and rescue services. For high accurate metrology crucial is dissemination (distribution) of a precise time scale. All global navigation systems offer the function of dissemination of time scale. Important function of GNSS.