In MaxQ and SPICE, the origin of a coordinate system is often defined by an “Observer” location. Coordinates and state vectors of “Target” objects are computed relative the the Observer (at the coordinate system’s origin).
There are also a few specially designated Observer/Target locations in MaxQ/SPICE.
|Observer / Target||It is|
|MOON, MARS, etc||Celestial bodies|
|[body] BARYCENTER||The point in space multiple bodies mutually orbit|
|SOLAR SYSTEM BARYCENTER||Solar System “Barycenter”. The point in space the Sun and Planets etc mutually orbit. This is “home” to everything else.|
|SSB||Short form for above|
|EARTH BARYCENTER||The point in space EARTH and MOON mutually orbit.|
|EMB||Short form for above|
An Observer location combined with a Reference Frame fully describe a coordinate system’s spatial and rotational positions. The only detail left to agree on is the exact coodinate system type - rectangular, spherical, or otherwise.