The Global Navigation Satellite System comprises a constellation of satellites and a ground segment that determines the satellite coordinates. The main idea behind GNSS is to measure distances between satellites and users located on the surface of the Earth or in the lower atmosphere.

Satellites move around the Earth along precisely known orbits, so we can define a satellite’s position precisely at any given moment. GNSS satellites transmit signals on frequencies between 1 and 2 GHz. These signals can be received by using a GNSS receiver. When the receiver acquires the satellite signal, the user can estimate the time required for the signal to propagate to the user from the satellite. By referring to information about satellite orbits and measured distances to the satellites, a user can calculate receiver position as an intersection of four spheres in the four-dimensional space-time continuum. If the receiver time could be perfectly synchronized with the system or satellite time frame, only three satellites would be required to determine receiver position in three-dimensional space (Figure 3.1).