See list attachedApril 4, 196969-PA-T-54APA/Chief, Apollo Data Priority CoordinationRLS Determination
On April 2 we had a Mission Techniques meeting to discuss how we should handle the determination of the LMs position on the lunar surface (RLS). Specifically, we were concerned with how to deter- mine its values and, after improved values are determined, when they should be loaded into the spacecraft computer. One obvious conclu- sion, if anything can be called obvious coming from this discussion, is that we have many excellent data sources for determining RLS, each of which is estimated to be of a quality much better than we need to support the operation.
“RLS” is actually the LM position vector on the lunar surface consist- ing of three components. It is moon fixed – that is, rotates with the moon – and is simply the latitude, longitude, and radial distance of the LM from the moon's center.
Prior to landing it is necessary to establish the values of RLS to be used in Descent targeting. For the first lunar landing, where the F mission will have thoroughly surveyed the landing site, the consensus is that we should use the RLS determined on the F mission and only use in-flight mission G measurements as a system check similar to the horizon check made before retrofire. For landings at sites which have not been surveyed previously, the RLS must be determined in real time based on the MSFN/sextant tracking done pre-DOI. The Math Physics Branch (MPB) of MPAD proposes that this be handled in the following way and I think everyone finally agreed it was logical, at least pend- ing results of the F mission:
a. The CSM/LM state vectors will be a so-called single pass MSFN solution based solely on data obtained during the sextant tracking pass. Orientation of the orbital plane of this solution will be con- strained by the pre-LOI plane plus confirmed maneuvers. (In fact, MPB proposed that we use this technique throughout lunar orbit from LOI through TEI. Data Select and MPB people have the task of establishing the technique for monitoring rev by rev single pass solutions with the orbital plane unconstrained to confirm that the pre-LOI value falls within the scatter of these determinations and of establishing the limits beyond which they would abandon the pre-LOI plane orientation.)
b. Having established the CSM state vector as described in “a,” the sextant tracking data is given full weight in the determination of RLS. That is, the landing site location will be based entirely on the sextant data determination of its position relative to the CSM state vector. But I would like to iterate that this RLS determination is only used as a system check for a surveyed site such as planned on the nominal mission.
After landing we have five good data sources for determining various components of RLS. (MPB has the task of establishing their relative accuracy.) We have decided to put off figuring out how we will actually use them in real time until after the F mission since it is anticipated that it will impact our choice tremendously. The various data sources are as follows:
a. The crew observations made during descent and after landing referenced to onboard maps – This is simply a matter of the crew informing the ground of where they think they landed in terms of longitude and latitude based on their visual observations. In addition to relaying latitude and longitude, they should also express an opinion of how certain they are about where they are.
b. The position is determined by use of star observations and the gravity vector data obtained during the first IMU alignments on the lunar surface. This data will be processed both onboard the space- craft and at Mission-Control Center. It is also only capable of determining latitude and longitude – not radius.
c. The Lear powered flight processor which uses MSFN-doppler data during descent is expected to have outstanding accuracy in deter- ming the change in LM position from PDI to touchdown, provided we do not encounter sustained periods of data dropout. The problem in determining LM position on the lunar surface with this data, of course, depends on the accuracy of our knowledge of the LM position at PDI to which we will add the position change measured by Lear. According to MPB it is possible to obtain a very accurate estimate of LM position at PDI using a MSFN short arc solution with the orbital plane constrained as discussed previously. (They emphasized, however, that the short arc solution is only accurate in the determination of position – not velocity – and would only be obtained during post-landing processing of tracking data obtained on the LM between AOS and PDI.) RLS then is found by determining the LM position at PDI using the short arc solution and manually adding to it the change in latitude, longitude, and altitude as measured by the Lear Processor during powered descent. Note that this yields all three components of RLS.
d. PGNCS telemetry data may be used in a similar manner to the Lear Processor. That is, by taking the PGNCS estimate of position at PDI and at landing we are able to determine its measurement of change in latitude and longitude during descent. They may also be added to the short arc solution described above to get RLS. It is to be emphasized, however, that PGNCS acceptance of landing radar destroys the capability of determining the change in altitude as measured by the PGNCS.
e. We can do the same thing with the AGS state vectors as described for the PGNCS. Again, since altitude updates are currently planned during descent, only latitude and longitude can be obtained.
The question now is which of these sources do we use?
a. For RLS radius our preferred source is the pre-flight determined value if we land at a surveyed site. If not a surveyed site, we would either use the radius determined by the MSFN/sextant observation obtained pre-DOI or from the Lear Processor plus short arc solutions. These two sources are currently estimated to be roughly equivalent.
b. For latitude and longitude all of the sources noted above (i.e., crew/map, AOT/g, Lear, PGNCS, and AGS) are all considered competitive and their priority must await F experience. It should be noted that Lear, PGNCS, and AGS are not completely independent in that they are all initialized from the same source.
Flight Dynamics, Data Select, and MPB people were given the task of establishing the precise technique for obtaining the Lear, PGNCS, and AGS solutions for RLS latitude and longitude. This is not some- thing that falls automatically out of the RTCC but will require a considerable amount of manipulation of many different state vectors stored in it and a bunch of manual (simple) computations.
You will note that all of the above data sources are available within an hour after landing and, as far as we are concerned, should provide all of the data ever needed to carry out the operation. However, we have currently planned to obtain rendezvous radar and sextant tracking of each spacecraft by the other, both two hours after touchdown and two hours before lift-off. Based on our discussions at this time, the consensus is that this tracking is by no means mandatory. In particular, if rendezvous radar tracking by the LM becomes even slightly problematic, it can easily be dropped. For example, if it conflicts with other crew activity, uses too much LM power, presents thermal problems, or wears out the rendezvous radar we can eliminate it from the timeline. Of course, if in real time our other data sources get noodled up in some way, it would have to be added back in at that time. In fact, I should emphasize that we are not proposing that it be dropped from the timeline, but rather that it could be dropped if necessary – so can the sextant tracking for that matter, although no reason for dropping it occurred to us.
In summary, we have many excellent data sources for RLS determination. How we will use them will be established after the F mission. Rendezvous radar tracking by the LM on the lunar surface is no longer a requirement. And, a couple of new MSFN facts are that a short arc solution yields a good position vector and it is proposed that the pre-LOI determined orbital plane plus confirmed maneuvers be used throughout the lunar orbit activity.