GNSS establishes object positions, expressed for instance, by latitudes, longitudes and heights.
It achieves this task by a basic trilateration process utilizing range differences measured to satellites.
The satellite positions in space are known; this sort of data is registered from the ephemerides broadcast. The receiver on the ground characterized by its geocentric position vector utilizes a clock that is set exactly to framework time. The receiver determines its range from each satellite by comparing code generated by a satellite with a similar code generated in the receiver. The time distinction between the two codes multiplied by the speed of light in a vacuum gives the range. Each of these ranges(for each satellite) characterizes a sphere at the satellite position. Range to only three satellites would be required since an intersection point of 3 spheres yields 3 unknowns(latitude, longitude, and height). A fourth satellite is required to help compute time.
Exterior orientation parameters can be obtained from a GNSS by mounting a GPS on an aircraft securing photogrammetric data. The GPS utilizes the trilateration concept to give the location of the sensor locally available the aircraft, based on the known positions of no less than 4 satellites in the constellation. The position is given in 3 dimensions(latitude, longitude and height). The three represents the initial three orientation parameters (X,Y,Z)of the captured photoogrammetric data.
