In determining the positions of points on Earth from satellite observations, three
different reference coordinate systems are important. First of all, satellite positions
at the instant they are observed are specified in the “space-related” satellite
reference coordinate systems. These are three-dimensional rectangular systems
defined by the satellite orbits. Satellite positions are then transformed into a
three-dimensional rectangular geocentric coordinate system, which is physically
related to the Earth. As a result of satellite positioning observations, the positions
of new points on Earth are determined in this coordinate system. Finally, the geocentric
coordinates are transformed into the more commonly used and locally
oriented geodetic coordinate system. The following subsections describe these
three coordinate systems.