See list attachedFebruary 25, 196969-PA-T-34APA/Chief, Apollo Data Priority CoordinationD Mission LM PGNCS IMU drift checks
This memo is to backup a telephone conversation with Will Fenner. I hope it doesn't just add confusion but I thought it might be worthwhile to put into writing my latest with regard to the D Mission LM PGNCS IMU alignments and gyro drift checks. I am pretty sure if limits are approached or slightly exceeded, the guidance officer is going to be forced to exercise some real time judgment and I don't envy him in this particular case. I would recommend he reference this memo if it supports his judgment but if he doesn't use these numbers, I certainly will not call anyone's attention to it.
Marty James, TRW, has spent a considerable amount of effort in determining the magnitude of the various error sources contributing to our uncertainty in the relative orientation of the two nav bases. I spent a good bit of time talking to him and my feeling is that he has done a good job and these numbers are probably okay. The following table shows the contribution of each of the error sources:
Values listed are the 1 sigma misalignment uncertainty estimates between the listed spacecraft components
Around x-axis Around y and z-axes
CSM IMU ½ min ½ min CSM NAV BASE 10 10 CSM SPACECRAFT AXES 20 8 CSM DOCKING RING (OR INDEX) 15 4 LM DOCKING RING (OR INDEX) 5 4 LM NAV BASE ½ ½ LM IMU RSS 14 min 28 min
If you RSS these values, we find the 1 sigma uncertainty around the y and z-axes is about ¼ degree and around the x-axis is about ½ degree. That is, The PGNCS alignment against the CSM IMU should be within better than 3/4 degree around the y and z-axes and 1½ degree around the x-axis. If we add to this the maximum gyro drift we are willing to tolerate (i.e., 1.5 degree per hour) for the 2 hours between alignments, we can obtain the largest tolerable gyro torquing angles beyond which we say the IMU is broken. It seems to me then that 4 degrees should be that limit. However, since we have no real experience with LM IMU alignments of any sort this number must be tempered by real time judgment and thus becomes more of a guideline value than a limit.