IMU Realign checklist (Virtual AGC)

The Inertial Measurement Unit (IMU) is used to determine the CSM and LM attitude with respect to an inertial 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.)

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 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..." 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

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.

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 00003E
	F 04 06
	00001
	00003
Key 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
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.