A Global Navigation Satellite System (GNSS) is a generic term denoting a satellite navigation system that provides continuous positioning over the globe. A GNSS involves a constellation of satellites orbiting Earth, continuously transmitting signals that enable users to determine their three-dimensional (3D) position with global coverage. The design and contents of GNSS concepts and techniques are often focused on their instrumental use in navigation. GNSS systems include GPS (Global Positioning System), Glonass (GLObal NAvigation Satellite System), Galileo, and Beidou. While the US GPS was historically the only fully operational GNSS for many years, the Russian Glonass was restored to full operation in December 2011, and the Chinese BeiDou and European Galileo systems are under development. Operational Principle The core principle of positioning is solving an elemental geometric problem by finding the distances (ranges) of a user to a set of GNSS satellites with known coordinates. The satellites’ coordinates are calculated from navigation data transmitted by the satellites. The basic observable in a GNSS is the time required for a signal (an electromagnetic wave) to travel from the satellite (transmitter) to the receiver. This travel time, when multiplied by the speed of light, provides a measure of the apparent distance, referred to as the pseudorange. To solve for the user's position, at least four satellites are needed to compute the three receiver coordinates and clock offset simultaneously. Using resulting signals and navigation data, user coordinates can be initially computed to an accuracy of several metres, although centimetre-level positioning can be achieved using more advanced techniques. GNSS Architecture A GNSS is organized into three primary segments: 1. Space Segment: This segment consists of satellite constellations designed with enough satellites to ensure users can view a minimum of four satellites at any time from any point on Earth’s surface. Its main functions are to generate and transmit code and carrier phase signals, and to store and broadcast the navigation message. The transmissions are regulated by highly stable atomic clocks placed onboard the satellites. 2. Control Segment (Ground Segment): This segment is responsible for the overall proper operation of the GNSS. Its duties include controlling and maintaining the status and configuration of the satellite constellation, predicting ephemeris and satellite clock evolution, maintaining the corresponding GNSS time scale using atomic clocks, and updating the navigation messages. 3. User Segment: This segment comprises the GNSS receivers. The receivers' main function is to receive the GNSS signals, determine pseudoranges and other observables, and solve the navigation equations to provide the user with coordinates, velocity, and precise timing. GNSS technology, and the associated data processing and analysis, can target high-accuracy positioning goals, requiring accurate modelling of measurements down to the centimetre level or better.