See list belowDEC 18 196767-PA-T-118APA/Chief, Apollo Data Priority CoordinationNo need to perform lunar landmark orbit determination
1. A number of Trajectory Control Data Priority meetings have been held on the Descent phase of the lunar landing mission. A basic question arose early in these meetings regarding lunar orbit land- mark tracking with the command module sextant and/or telescope. It was necessary to decide whether or not such observations are required for use in improving our knowledge of the spacecraft orbital elements, otherwise determined using MSFN S-Band observations only. And, if so, at what time must they be obtained and how would they be used? Accord- ingly, a special meeting of all parties known to be interested in this subject was held on November 30 in order to answer these questions.
2. This memorandum is to report the results of that meeting, which I may briefly summarize at the onset by saying it was the consensus as of this time that performance of the MSFN orbit determination capabil- ity is expected to be adequate and landmark tracking for purposes of navigation does not appear to be required. Therefore, in order to make the overall operation itself and development of the systems to be used as simple and unsophisticated as possible, it is our recommen- dation that we should not include this activity in the mission at least until it is proven to be necessary. That is, in establishing the manner in which the various guidance systems are initialized, verified, moni- tored and used during the lunar landing mission, it is reasonable to assume that tracking of lunar landmarks will not be done. Furthermore, development of the crew timeline, RCS propellant budgets, mission plans, crew procedures, RTCC programs, etc., may also omit this activity. It is very important, however, to emphasize that spacecraft optical obser- vations of the landing site are essential at various times in the lunar landing mission in order to establish its location with respect to the spacecraft orbital elements as determined by MSFN.
3. At out meeting, Math Physics Branch people described in some detail the current status of the MSFN lunar orbit determination capability. As you probably know, we have recently obtained a considerable amount of actual tracking data on Langley's Lunar Orbiter spacecraft which have revealed that the performance of MSFN is not quite as good as previous preflight analysis had indicated it would be. Processing of all this data is far from complete, but there apparently is good reason to believe that the only significant error source associated with MSFN is a lack of pre- cise knowledge of lunar potential. The effect of this on MSFN orbit deter- mination accuracy shows up as an uncertainty in the orientation of the spacecraft orbital plane, the uncertainty being almost entirely a rotation of the orbital place about the earth/moon line estimated to be in the order of ½° in magnitude.
4. Associated with this, work is underway in two areas. The first is by Math Physics Branch people, both here and at JPL, attempting to define with more precision and confidence exactly what the characteristics of the orbit determination uncertainty really are. Results of this should be available within a couple of months for use in planning what to do if it doesn't get any better before we go to the moon. The other major effort is being carried out by the Langley Research Center with assistance from Boeing. These people are concentrating on improving our knowledge of the lunar gravitational potential which, if successful, should significantly improve MSFN accuracy. It is anticipated that results will be available in the March-April-May time period.
5. This discussion was followed by a brief definition of the current RTCC orbit determination capability. Specifically, at present there is no way we can directly utilize observational data in determination of spacecraft ephemerides other than S-Band data obtained from the earth tracking sites. Moreover, to add in the RTCC the capability of processing optical observa- tions of lunar landmarks obtained onboard the spacecraft would have a significant impact on the development of RTCC programs. Therefore, we concluded we should not recommend doing so. It was noted, however, that both command module optical observations and LM-on-the-lunar-surface rendezvous radar observations can be processed to improve the RTCC's knowledge of the relative position of the landing/launch site based on the assumption that the MSFN ephemeris is perfect.
6. There was also some discussion about the onboard procedures and pro- grams which, in general, sounded pretty good. There was one significant exception, however, dealing with the manner in which the spacecraft systems take into account the offset of the desired landing site location from the identifiable landmark associated with it. It is probably that a Colossus Program Change Request will be initiated from the Descent Phase Data Priority Panel to improve the manner in which this job is carried out.
7. The discussions noted above were primarily to make sure everyone had an understanding of the expected capability of the systems we are dealing with. Following that we spent the rest of the time on how best they might be used and in determining if they were acceptable. These discussions brought out that as long as accurate onboard optical observations of the landing site were made fairly soon before initiation of descent, the mission could be carried out with no apparent extra cost of propellant. This is because, like rendezvous, it is not necessary to have an under- standing of the absolute, inertial position and velocity in space but only to have a precise knowledge of the target's relative position with respect to the current estimate of the ephemeris.
8. A much more difficult problem is involved in preparing for the Ascent phase. Again, it is almost certainly necessary that observations of one spacecraft with respect to the other must be made shortly before lift off. Furthermore, if a command module plane change is required to insure ??? being within the LM propulsion capability, it will be necessary to make observations of the LM/command module relative position prior to targeting that maneuver. There is a choice of using either the command module optical device and/or the LM rendezvous radar for this purpose. Since operating the LM radar will probably conflict with other lunar stay time activities when this timeline is eventually established, it is anticipated that the command module optical device will be prime for this purpose backed up by the LM radar. But only if it is impossible to obtain the optical observation for some reason. The RTCC has been programmed to accept both of these data sources for this purpose.
9. There is one feature or the Ascent phase which remains troublesome. Ascent, unlike Descent, demands that at the end of the burn the LM not only be in the right place (in the CSM orbital plane) but, in addition, must have its velocity vector in the right direction, namely in or parallel to the plane of orbit. Targeting for this additional constraint may be difficult to do unless MSFN performance is substantially improved and it is not entirely certain that adding more landmark tracking will help the situation very much.
10. Studies are continuing in this area to define exactly what our problem is. However, pending results of these analyses and those by the people attempting to improve the lunar potential model, there does not seem to be sufficient justification to make landmark tracking a requirement at this time, other than those of the landing/launch site. We do not propose to delete the currently planned capability anywhere and can add this activity back into the planning and procedures develop- ment later on if it proved to be worthwhile.