See list attachedJuly 14, 196969-PA-T-109APA/Chief, Apollo Data Priority CoordinationHow we will handle the effect of mascons on the LM lunar surface gravity alignments
What do we do if one of those big damn lumps of gold is buried so near the LM that it screws up our gravity alignment on the lunar surface? Without exception, the calculations of all the various far-flung experts predict that mascons should have no significant effect on our lunar sur- face gravity alignments. In fact, based on this we have chosen to use gravity alignments nominally as opposed to star alignments. They are easier to do and probably more accurate. A few of us got together the other day, though, to figure out what to do if, contrary to expectation, some sort of weird gravity effect is noted, which appears to be acting on the LM on the lunar surface. This memo is to tell you about that.
As you know we have several sources of data for determining the LM's position on the lunar surface (RLS). One of these is through the use of data obtained from LM platform measurements of the direction of the lunar gravity and from AOT observations of the stars. If this determination, using the LM data, disagrees substantially with the other data sources, we must consider the possibility that it's due to gravity anomalies. The sort of difference we are willing to tolerate is 0.3° in longitude, which is more or less equivalent to 0.3° pitch misalignment in the platform. True alignment errors in excess of that could present ascent guidance problems. Since 0.3° is equivalent to about five miles, you'd expect the crew's estimate of position could probably be useful in determining the true situation. All they'd have to do is tell us they are short or over-shot the target point a great deal.
If uncertainty still persists, it seems we must believe the gravity and use it for our alignments – both PGNCS and AGS. That is, we have more faith in it than in our other sources of RLS determination. However, if examina- tion of all these sources convince us that the gravity does have some fun- nies greater than 0.3° associated with it, we would have to modify the crew procedures in real time such that the ascent platform alignment is done using the stars (Alignment Technique 2) rather than gravity.
Consideration was given to hedging our bet by aligning the PGNCS to the stars and using the lunar gravity alignment in the AGS. Further considera- tion, however, revealed an interesting and somewhat sad thing. What we actually discovered was that the ground trajectory processing during ascent is also affected by downrange position error – that old demon that seems to be plaguing us in so many ways recently. The fact is that throughout ascent we would never know which system was right and so we would never have the intelligence to switch over from one system to the other. In other words, there is no point in using different Alignment Techniques for the two guid- ance systems.
The problem noted above is primarily in support of Ascent 1 rev after land- ing. After that, additional very accurate sources of RLS determination become available. Specifically CSM sextant tracking of the LM is always the prime source and if Mike has trouble on one try, he should try again on later revs – there are plenty of opportunities and little else to do. If he still fails and the uncertainty noted above exists, we have the situation in which LM rendezvous radar tracking of the CSM becomes manda- tory. You recall we deleted this from the timeline with the understanding it would be reinserted if we could determine RLS in no other way and this is that case. We sure don't expect this to happen, but if it does RR will be needed.
In summary then:
a. We should always align both AGS and PGNCS to the same data source, gravity or stars.
b. We use gravity unless we have some concrete reason to question it – such as all data sources including the crew estimate of RLS are in dis- agreement with it by more than 0.3° in longitude (pitch). In that case, use the stars (both AGS and PGNCS).
c. Naturally longitude initialization error louses up the ground ascent trajectory monitoring just like it does descent.
d. If RLS uncertainty persists, either CSM sextant or LM RR tracking of the other vehicle becomes mandatory.