See list attachedApril 16, 196969-PA-T-64APA/Chief, Apollo Data Priority CoordinationHow the MSFN and sextant data are used to target DOI and Descent
We had a meeting on April 9 which was extremely interesting to me. We discussed and settled on how the MSFN tracking and sextant land- mark observations would be used in the MCC/RTCC to produce optimum DOI and Descent targeting for the LM. The big new factor that had to be taken into account somehow was the propagated state vector errors resulting from our inaccurate modeling of the lunar potential. This has forced us to change our planned techniques somewhat from those proposed before the C' mission. Most of what we now plan to do is just as the Math Physics Branch (MPB) of MPAD proposed to us at this meeting. I feel they should be commended for a pretty fair piece of work.
I would first like to describe the manner in which MPB proposed that the RTCC orbit determination consistency checks be made during the flight. As you recall, in a previous memo I noted that they feel it is best to use the orientation of the orbital plane determined pre-LOI to which they add the in-plane orbital elements based on new MSFN tracking. Of course, it is necessary to continuously monitor and confirm that the plane established in this way is right. They intend to do this by performing single-pass MSFN solutions after each lunar orbit and comparing the resulting inclination with that established pre-LOI. It is expected that the single-pass solutions will show a random variation about the pre-LOI value indicating it is safe to continue using it. If they detect a bias or trend in these single- pass inclinations away from the pre-LOI value, they will have to update it.
In addition to the inclination check performed continuously, they also plan some discrete consistency checks made in revs 6, 7, and 8. These checks will be made by processing MSFN tracking just as will be done later for the DOI and Descent targeting. That is, they will determine the orbit based on rev 3 and 4 data and propagate it to rev 6. They will make a “plane-free” single-pass solution in rev 6 based on rev 6 tracking. They will compare the three position components in local vertical coordinates (that is, downtrack, altitude, and crosstrack) at 20 minute intervals throughout rev 6 and will plot the differences vs. time. These plots should show the propagated error from the older solution as a function of time throughout rev 6. They will do the same thing using revs 4 and 5 data propagated to rev 7 and compared with a single-pass rev 7 solution. They will do the same thing with revs 5 and 6 propagated to rev 8. These position difference plots determined for revs 6, 7, and 8 will be superimposed upon each other to make sure there is consistency on determination of propagated state vector errors. This consistency, incidentally, has been demonstrated on C' and we expect to reconfirm it on the F mission prior to G. If it works as expected, it should be possible to determine the propagated error in all three components as a function of time on a state vector propagated ahead two revs. The significance of this, of course, is that the DOI and descent targeting is performed with a state vector which is two revs old and if we are able to determine the propagation error, bias may be applied to compensate for them. That is a description of a rather complicated process. The important thing for you to understand is that a technique appears to be available for determining and compensating for propagation error in real time.
The manner in which we intend to use sextant tracking of the landing site has not changed since before C'. That is, we intend to determine the landing site position by applying the measured relative displace- ment in all three components – latitude, longitude, and radius – to the current MSFN solution at the time of the sextant observations. Thus, the targeting solves the relative problem compensating for errors in both MSFN state vectors and the preflight estimate of the landing site location. We have established that the change from the preflight value in each of these components based on the real time data must not exceed the following values:
a. Latitude must not be changed more than 12,000 feet.
b. Longitude must not change more than 6,000 feet.
c. Radius must not change more than 6,000 feet.
These values are based on our current 3 sigma estimates of preflight map accuracy RSSed with the MSFN orbit determination accuracy. It is felt that corrections larger than these must indicate some sort of gross failure demanding either that the sextant tracking be redone by delaying DOI one rev or that the sextant tracking be ignored and the Descent targeting be based on the preflight values. Incidentally, the mission rule defining which of these choices to pursue is a significant open item which must be resolved.
Now I would like to describe how the propagated errors are compensated for.
a. Crossrange, which is essentially latitude, will not be com- pensated for propagation errors at all. Since we are using the frozen plane technique, by definition, no propagated error can occur.
b. Error in spacecraft altitude is compensated for by changing the radius of the landing site by an amount equivalent to the propagated state vector error in the altitude direction. The empirical correction is determined from the propagation state vector plots described above by reading out the error in altitude associated with a time in orbit equivalent to touchdown time. The point is that the state vector is not corrected, but rather compensation is applied to the landing site radius since this is a much cleaner procedure.
c. Downrange error is more-or-less equivalent to landing site longitude and presents special problems. Consideration was given to compensating downrange propagation errors by changing landing site location in a manner similar to the radius bit just discussed. That would work fine for Descent, but can result in a serious problem in Descent aborts. Specifically, downrange error in the state vectors during powered flight act in a way equivalent to a platform alignment error in inertial space. Specifically, 10,000 feet downrange error is equivalent to 0.1° IMU misalignment. Therefore, if we were to leave the propagated downrange error in the state vector, all powered flight by the inertial guidance system would be carried out with 0.1° error and, in the event of a Descent abort, would cause the system to aim for the wrong insertion conditions by that amount. Of course, the AGS, which is initialized from the PGNCS would also have this error. Although we don't expect the downrange error to exceed about 5,000 feet, we have no assurance of this and conservatively feel that an alternate approach for compensating downrange error is preferable. The alternate approach we adopted is to change the time tag on the state vectors such that the downrange error at touchdown time is zero. Changing a state vector time tag is not a simple thing to do in the RTCC. It has not yet been “automated.” As a result, it is necessary for the Data Select Officer to manually enter the entire state vector into the RTCC using his type- writer like input device. This is a time consuming process because it must be very carefully checked. (It is recognized that the RTCC program for the lunar landing mission has been frozen, but it was suggested to the Data Select people that they consider automating this input since it is becoming part of the nominal operation.) It is to be emphasized that this time tag compensation is applied to both the LM and CSM state vectors in all three computers – RTCC, LGC, and CMC. We may eventually establish a lower bound in this downrange compensation below which it is considered acceptable to live with the error. For example, if the downrange error is less than 5,000 feet, we may choose to apply that small correction to the landing site longitude and leave the state vectors time tag alone since that is a much simpler thing to do. But that's not the current technique.
One significant open item I failed to mention in passing is that there is still a controversy raging on whether a single-pass or two- pass MSFN orbit determination should be used for Descent targeting . That is, the sextant tracking is done on rev 11 and the MSFN tracking on that rev is certainly used. The question is, should rev 10 MSFN tracking be incorporated in as well? The solution to this depends on ironing out inconsistencies between two computer programs which are given conflicting results. The answer could come at any time. Once the one-rev vs. the two-rev decision is reached, of course, it will not only apply to orbit determination techniques for Descent targeting but will also be incorporated in the MSFN propagation error determination techniques described above.
It is currently planned that these G mission operations will be carried out on the F mission exactly as if that flight were a lunar landing. This obviously means that to the maximum extent possible these techniques will also be used in the F mission simulations. There is some question, however, if changing the state vector time tag to compensate for propagated downrange error is a reasonable thing to do on the F mission. Accordingly, this must be discussed with the F mission operations people before we naively assume they will do it.
Much of the preceding discussion deals with the landing site location to be used in the LGC during Descent. The landing site position (RLS) to be loaded in the command module computer should be the preflight map values of the prime landing site landmark and there is no reason to go through this “mickey mouse” of updating the CMC values from the MCC before the LM lands.
The time tags on the state vectors transmitted to the spacecraft computers on G are essentially the same as on the F mission. The LM state vector sent to both the LGC and CMC will be time tagged at DOI -10 minutes. The CSM state vector sent to both spacecraft will be time tagged at PDI + 25 minutes, which should be close to the initia- tion of rendezvous navigation in the case of a late Descent abort.
Except for the open items noted above, I think this pretty well establishes how we plan to do the targeting for DOI and Descent on the lunar landing mission, at least until F mission results come in.