Coordinate Systems

Satellite positions are specified using a coordinate system: that is, the position is given relative to a particular point (the origin), in each of three dimensions, by a set of three coordinate numbers.
Coordinate systems can be the familiar Cartesian type, with three axes at right angles; Spherical, with two angles and a radial distance; or other more complicated types.
Here are some of the common coordinate systems:
* Geodetic Cartesian (called "Earth-Fixed Greenwich" - EFG - by NASA)
* Inertial (Celestial) Cartesian (called "Earth-Centered Inertial" - ECI - by NASA).
Both of these systems use the center of the earth as the origin, and the axis of the earth as one axis (Z, or G axis. But the inertial system has the other two axes (X, Y) fixed with respect to the stars, while the geodetic system (E, F) keeps them fixed with respect to the surface of the earth.
* Latitude/Longitude/Altitude - a spherical system
* Right Ascension/Declination - another spherical system
* Orbital