IMU Realign checklist (Virtual AGC)
Among others the Inertial Measurement Unit (IMU) is used to determine the CSM and LM attitude with respect to an inertial system, the IMU platform, also called "Stable Member". Independent from the spacecraft's manoevers the IMU platform's attitude is kept fixed with respect to any inertial system, especially the Apollo Basic Reference Coordinate SystemCSMGSOP. The coordinate system the IMU platform is aligned to actually can vary throughout the mission for various reasons (to avoid gimbal locks etc.)
At launch and during earth orbit insertion (and often much longer) the Local Vertical Coordinate system at the pad is used, the IMU prelaunch alignment. In order to locate the earth, the moon or the stars the AGC knows the current attitude of the IMU platform with respect to the Basic Reference System by storing the Reference to Stable Member Matrix (REFSMMAT) in it's erasable memory.
The Virtual AGC program P52 is used to check the IMU orientation. Both in reality and in Orbiter the IMU platform doesn't maintain it's attitude perfectly, but it drifts a little bit and that causes errors in the calculations of the AGC. To check and realign the IMU to its previous alignment orientation sightings on two stars in the sextant are used. Additionally to the original procedureA15_CSMGC_CHECK the checklist below explains the usage of the sextant in Orbiter.
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