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.)

IMU prelaunch alignmentTrainingGN

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 change the IMU platform orientation, however this funtionality will be explained later.

The Virtual AGC program P52 is also 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.

Apollo navigation stars

Not every star is usable for star sightings but the AGC carries a catalogue of the positions of 37 prominent stars distributed across the sky, the Apollo AGC navigation stars. The Apollo astronauts needed to know this stars and trained to find them within the celestial sphere, thanks to Orbiter's star marker feature this is not necessary in Project Apollo - NASSP:

• Press F4
• Choose "Visual helpers..."
• Check "Planetarium mode" in the "Visual helpers" window
• Check "Celestial" in the "Markers" box and press the "Config" button nearby
• Select "Apollo AGC navigation stars" in the "Configure celestial markers" window
• Close all the windows

Now the navigation stars are marked with a green cross and labeled with their number and so they are very easy to find.

P52 (REFSMMAT) checklist

Additionally to the original procedureA15_CSMGC_CHECK the checklist below explains the usage of the sextant in Orbiter.

Procedure	Panel	Remarks
Preparation   CMC - on ISS - on SCS - operating BMAG MODE (3) - RATE 2	1	As these systems are running usually, nothing is to do.
Zero optics   OPT MODE - MANUAL then ZERO (about 15 sec) OPT MODE - CMC	122	Optionally use V16 N91E to monitor the current shaft (.01°) and trunnion (.001°) angle in R1 and R2.
Start program P52   Key V37E 52E F 04 06 Option Code ID: 00001 Align Option: 00002 Key V22E F 22 06 Key 0003E F 04 06 00001 00003 Key PRO	DSKY	P52 has several alignment options, option 2 "nominal" is the default option. Change it to option 2 "REFSMMAT" to check the current REFSMMAT.
Star selection   F 50 25 00015 Key PRO		"Request Celestial Body acquisition" checklist code. The AGC will select two available stars.

If the AGC fails to select two suitable stars, a PROG alarm occurs and the DSKY shows "F 05 09" with error code "00405" (two stars not available) in R1. In this case press RSET, change the spacecraft's attitude to a better position (for example pitch up 90°) and key in "V32E" to return to Star selection and to try again. Otherwise go on with the Mark sequence.

Procedure	Panel	Remarks
Mark sequence   F 01 70 000XX OPT MODE - CMC (verify) CONTROLLER SPEED - HI Key PRO 06 92 Shaft (.01°) Trunnion (.001°) OPT MODE - MANUAL F 51 Switch to telescope (KEYS!) Adjust position with the W,A,S and D keys. CONTROLLER SPEED - LO Switch to sextant (KEYS!) Adjust position with the W,A,S and D keys. Press Q key to mark the star F 50 25 00016 Key PRO F 01 71 000XX Key PRO	DSKY   122

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