Knowing about Global Navigation Satellite System (GNSS)is imperative for engineers, scientists as well as civilians becauseof its wide range of applications in various fields, includingpersonal and vehicle navigation, aviation, defense,transportation, science, security, telecommunication, and survey.Global availability of signal and continuous service hasmade GNSS technology popular with a large number of users.This article covers various aspects of GNSS/GPS like architecture,working principle, signal structure and augmentation(GAGAN). The article also covers the Indian RegionalNavigation Satellite System (IRNSS) and its potentials.