Ephemeris/attitude reference determination using on-board optics and other satellite ephemeris

The present invention provides a system (200) for determining the ephemeris and attitude for a spacecraft based on optical payload pointing directions and the ephemeris of other spacecraft. An ephemeris determining subsystem (300) obtains ephemeris data (308) for one or more reference spacecraft. Optical payload pointing information (306) is obtained for the optical payload pointed to the reference spacecraft. The spacecraft ephemeris (312) is then calculated with ephemeris determination algorithms (302) based on the ephemeris data (308) and optical payload pointing information (306). Other available information, such as stored results of previous measurements and calculations, time (310), spacecraft attitude (304), information from non-payload sensors and ranging information may be used to enhance the accuracy or decrease the complexity of the ephemeris calculations. Spacecraft attitude can also be determined using reference spacecraft ephemeris data and optical payload pointing information.