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Figure 2.1 illustrates the key parameters that may be used to describe a GNSS satellite that orbits the Earth.

Various configurations may be used to complete a GNSS constellation. Various design parameters may be considered regarding the number of satellites, their location in the constellation, location on ground infrastructure components, and user segment service area.

GPS satellites occupy six orbital planes in an MEO that are inclined 55° from the equatorial plane, as illustrated in Figure 2.2. Each of the six orbit planes in Figure 2.2 contains four or more satellites. The European Galileo and Chinese BeiDou use three orbital planes separated in longitude by 120°. Galileo has plans for 10 MEO satellites in each orbital plane. Glonass also used three orbital planes while maintaining a nominal 24 satellite constellation. GNSS augmentation or regional satellite‐based navigation systems are most often placed in GEO or IGSO over the region to be services; these satellites will have an orbital period of 24 hours.

Figure 2.1 Parameters defining satellite orbit geometry.

Figure 2.2 Six GPS orbit planes inclined 55° from the equatorial plane.

Regardless of the type of radionavigation (satellite‐based or terrestrial‐based) system, most often the solution method requires the determination of a unknown user antenna location with known transmitter locations and measured range (or pseudorange) observations.