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Global Navigation Satellite Systems (GNSS) is the generic term for the satellite-based radio positioning, navigation, and timing systems that provide autonomous geospatial positioning and time references on a global or regional basis ^[1]. A GNSS constellation comprises a space segment of satellites in precisely modeled orbits that continuously transmit ranging signals carrying navigation data formatted according to standardized interface specifications ^[2], a control segment of ground stations that tracks the satellites, uploads ephemerides and clock corrections, and maintains the constellation, and a user segment of receivers that measure pseudorange and carrier-phase observables to derive position, velocity, and time ^[1]. Position is determined by trilateration: a receiver measures the travel time of signals from at least four satellites, converting geometric range into three-dimensional coordinates and a common clock offset through solution of the ranging equations ^[1]. GNSS includes fully global systems such as the United States GPS, Russian GLONASS, European Galileo, and Chinese BeiDou, as well as regional augmentation systems, and is distinguished from terrestrial navigation aids by its space-borne transmitters and global coverage ^[3].