GNSS determine object positions, expressed for example, by latitudes, longitudes and height.
It accomplishes this task by a simple trilateration process using range differences measured to satellites.
The satellite positions in space are known; this kind of information is computed from the ephemerides broadcast. The receiver on the ground defined by its geocentric position vector employs a clock that is set precisely to system time. The receiver determines its range from each satellite by comparing a code generated by a satellite with the same code generated in the receiver. The time difference between the two codes multiplied by the speed of light in a vacuum gives the range. Each of these ranges(for each satellite) defines a sphere with center at the satellite position. Range to just three satellites would be needed since an intersection of 3 spheres yields 3 unknowns(latitude, longitude, & height). A fourth satellite is needed to help calculate time.
Exterior orientation parameters can be obtained from a GNSS by attaching a GPS on an aircraft acquiring photogrammetric data. The GPS uses the trilateration concept to give the location of the sensor onboard the aircraft, based on the known location of at least 4 satellites in the constellation. The position is given in 3 dimensions(latitude, longitude and height). The three represent the first 3 exterior orientation parameters (X,Y,Z)of the acquired data.
