See list attachedJanuary 10, 196969-PA-T-2APA/Chief, Apollo Data Priority CoordinationSome decisions regarding lunar landmark tracking on the F and G missions
We had an Ad Hoc Mission Techniques meeting on January 9 to talk over lunar landmark tracking. In particular, we wanted to discuss what we thought had been learned from the C' mission and what we want to do on the F and G missions. This memo is to outline all that briefly. The specific things we were trying to decide were:
a. Whether special tests of any sort should be included on the F mission which might permit us to broaden the acceptable sun elevation angle constraints associated with the lunar landing and
b. To decide if optical observations (SCT or SXT) of the landing site are required on DOI day for descent targeting and if so how many, when should they be taken, and how should they be used?
Jack Schmitt has probed extensively into the landing sun elevation angle constraints problem both before and after C' and probably has a better understanding of this overall situation than anyone else I know. He has intensely debriefed all of the C' crewman on this specific subject and is confident that the visibility will be accept- able for landing if the sun elevation angle is no less than about 3 or 4 degrees. The upper constraint he feels is in excess of 20 degrees and the actual limit will probably be based on heating considerations on the spacecraft or the crew during EVA rather than visibility during descent (we'll find out what that limit is). In other words, it looks like we have a sufficiently wide band of acceptable sun elevation angles that this imposes no real constraint on G launch opportunities! Further- more, there appears to be no reason to provide special tests on F designed to broadened these limits or give us greater confidence in them. One interesting point he emphasizes, though, is that we should avoid landing with a glide path within about 2 degrees of the sun eleva- tion angle since there is a definite degradation in visibility along that line which would impair the crew's capability of evaluating the landing site. This means that we should avoid sun elevation angles between about 14 and 18 degrees – a little band of unacceptable light- ing conditions within the much larger acceptable limits. He feels that this band may be avoided in the few instances we encounter it by delay- ing launch somewhat or by adding an extra revolution or two in lunar orbit. It is also evident that by the use of the hybrid flight plan we can extend the translunar coast time with the same effect.
In summary, it appears that the sun elevation angle constraint on G mission launch opportunities is not significant at this time and there is no need to provide special tests on F to confirm this opinion.
The question of optical tracking of the landing site is not so clearly understood. However, the consensus is that it would be a serious mistake at this time for the flight plan not to include optical obser- vations of the landing site as part of the descent targeting operation. But, based on the ease with which the C' crew located and tracked the landmark on their first opportunity there seems to be no reason not to eliminate the first series of landmark tracking, which we had previously included primarily for on-the-job training. Accordingly, we intend to utilize the tracking plan and ground targeting operations previously developed in our Descent Mission Techniques meetings except that the first of the two tracking periods will be deleted or moved to LOI day if it can be conveniently included in the timeline. Since the landing site will be in darkness at that time, this particular session would have to be on some other landmark located 5 or 10 degrees to the east of the landing site.
I would like to discuss briefly the reasons for retaining the optical observations. Basically, they reduce to two things neither of which could be described as mandatory – but they are certainly not just “nice to have” things either. The first, of course, is to significantly improve the accuracy of the descent targeting which will make the descent trajectory more nearly nominal. In line with this, it also makes it more likely the landing radar can return the trajectory to within acceptable limits. The second benefit is that they provide a complete, independent check on the overall targeting system in the same sense that the star check confirms burn attitude or the horizon check confirms retro attitude on other mission phases.
Our discussions included numerically defined MSFN and spacecraft systems performance (expected and/or experienced) compared to descent targeting requirements which, you see, I have not included at all on this memo. However, they support the above conclusions substantially and could be made available to you if you want to see them. I left them out here simply because it is too complex a matter to discuss clearly in a memo such as this. What I am trying to say is that I feel these are well- founded conclusions which may be applied to both the F and G missions and we are going to press on based on them.