IMU Realign checklist (Virtual AGC)

The Inertial Measurement Unit (IMU) is used to determine the CSM and LM attitude with respect to an inertial, star-fixed system. Independent from the spacecraft's manoevers the IMU platform, also called "Stable Member", 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 sometimes 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 orientation 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 functionality 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 these 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..." in the "Main" window
• 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 can be found very easily.

Sextant and Scanning Telescope

Navigation star 23 in the CSM Scanning Telescope.

The CSM has two optical instruments, the Sextant (SXT) and the Scanning Telescope (SCT). Both have a panel of it's own in Project Apollo - NASSP. To switch to the Scanning Telescope panel, press <CTRL><DOWN> twice from the main panel. Press <CTRL><LEFT> to go to the Sextant panel from the Scanning Telescope panel. The Scanning Telescope has a fixed field of view of 60°, the Sextant of 10°, so use the Scanning Telescope to coarse align to the star to be marked and use the Sextant to do the actual mark. The following keys are used to operate the Sextant and the Scanning Telescope:

Key	Usage
W	Move up
A	Move left
S	Move down
D	Move right
Q	Mark star
E	Reject mark (to cancel a previous mark)

The speed of the shaft and trunnion movement of both the Sextant and the Scanning Telescope can be selected with the CONTROLLER SPEED switch on panel 122.

P52 (REFSMMAT) checklist

Additionally to the original procedureA15_CSMGC_CHECK the checklist below is adapted to the usage of the Sextant and the Scanning Telescope 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 00003E F 04 06 00001 00003 PRO	DSKY	P52 has several alignment options, option 2 "nominal" is the default option. Change it to option 3 "REFSMMAT" to check the current REFSMMAT.
Star selection   F 50 25 00015 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 PRO 06 92 Shaft (.01°) Trunnion (.001°) OPT MODE - MANUAL F 51 Center the star in the Scanning Telescope CONTROLLER SPEED - LO Center the star in the Sextant Mark the star F 50 25 00016 PRO F 01 71 000XX PRO	DSKY   122   DSKY       122 DSKY SCT 122 SXT   DSKY	XX is the selected star code. The Scanning Telescope and the Sextant move to the supposed star position automatically. Use the keys explained above. Use the keys explained above. Press "Q" to mark. Reject mark ("E" key) and mark again, if the mark wasn't fine. XX is the selected star code.

When the first mark sequence is completed, a second star sighting is to be done, so the mark sequence is repeated with another star. After the second mark sequence go on with the IMU orientation adjustment.

Procedure	Panel	Remarks
IMU orientation adjustment   F 06 05 Star angle difference (.01°) PRO to accept or key V32E to reject and bypass the next step	DSKY	The difference of the angle between the 2 stars and the angle to between the 2 marks to check the marking quality. Allowable limits are +00011 for the SCT and +00003 for the SXT.
F 06 93 Torquing angles (OG, IG, MG .001°) CMC MODE - FREE PRO	1 DSKY	The IMU is torqued to the corrected position via fine align. Optionally key V32E to bypass.
F 50 25 00014 ENTR F 37 Key 00E CONTROLLER SPEED - HI OPT MODE - ZERO	122	Optionally press PRO to recheck and return to star selection.

Now the IMU orientation with respect to the Apollo Basic Reference Coordinate System matches the REFSMMAT stored in the AGC again.

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