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.