Global Navigation Satellite Systems (GNSS) at the time of writing comprise four systems, two of which are fully operational and two of which are on their way (see Table 1.1). A brief history of GNSS is given in Chapter 10 along with a timeline of application development and prospects for this development, especially concerning mobile applications.

Each GNSS comprises a constellation of satellites, called a space segment, and a ground segment (Figure 1.1). The main idea behind GNSS is to measure distances between a satellite and a user located on the surface of the Earth or in a lower atmosphere. Satellite coordinates can be calculated at any moment of time. The information that allows the calculation of satellite position is uploaded to and then broadcast from satellites to the user. The ground segment is responsible for determining satellite orbits, which it then uploads to the satellites, and also for defining the coordinate frame and time frame in which satellite and user positions are estimated.

Having received the information on satellite orbits and measured distances to the satellites, a user can calculate receiver position as an intersection of four spheres in a four-dimensional space-time continuum. If the receiver clock is perfectly synchronized with the satellite time frame, only three satellites would be required to determine receiver position in three-dimensional space (Figure 1.2).