Refsmmat

REFSMMAT is a term used by guidance, navigation, and control system flight controllers during the Apollo program, which carried over into the Shuttle Program. REFSMMAT stands for "Reference to Stable Member Matrix". It is a numerical definition of a fixed orientation in space and is usually (but not always) defined with respect to the stars. It was used by the Apollo PGNCS as a reference to which the gimbal-mounted platform at its core should be oriented to. Every operation within the spacecraft that required knowledge of direction was carried out with respect to the orientation of the guidance platform, itself aligned according to a particular REFSMMAT. During an Apollo flight, the REFSMMAT being used, and therefore the orientation of the guidance platform, would change as operational needs required it, but never during a guidance process—that is, one REFSMMAT might be in use from launch through Trans-Lunar Injection, another from TLI to Midpoint, but would not change during the middle of a burn or set of maneuvers.