See list attachedMay 5, 196969-PA-T-70APA/Chief, Apollo Data Priority CoordinationDescent Monitoring Mission Techniques – a status report
I think we are beginning to see the light at the end of the Descent Monitoring Mission Techniques tunnel. At the April 24 meeting on that subject we thoroughly discussed the integration of the onboard techniques with the activity at the MCC during powered descent and I feel the resultant is as reasonable and complete as possible, con- sistent with practical operational constraints.
One thing we have finally been able to get under control was this squirmy idea that there is some way for the crew to compare the output of the AGS and PGNCS onboard the spacecraft with the objective of making abort and/or switchover decisions. Obviously there is no question that a massive system failure will be obvious to them and their course of action will be clear. Obvious too, is the fact that the crew will be monitoring both of these systems as well as many other data sources throughout powered descent. But, now known to everyone, is the fact that there is no way for the crew to compare AGS and PGNCS such that they are able to detect which system is mal- functioning, if that malfunction is of a slow drift degradation type, at least not with the assurance necessary to take any action. There- fore, just as in the case of ascent, not only is the MCC prime for carrying out the task of slow drift malfunction monitoring, but we now recognize that MCC is the only place this can be done. That, my friends, is a fantastic event – the death of a myth we have been haunted by for two years. Don't get the idea I'm happy with the situa- tion. What I am pleased about is that everyone now agrees it is the situation.
There is another thing about powered descent crew procedures that has really bugged me. Maybe I'm an “Aunt Emma” – certainly some smart people laugh at this concern, but I just feel that the crew should not be diddling with the DSKY during powered descent unless it is absolutely essential. They'll never hit the wrong button, of course, but if they do, the results can be rather lousy. Therefore, I have been carrying on a campaign aimed at finding some way to avoid the necessity of the crew keying up the on-call displays. This campaign has not been alto- gether successful. I guess partly because not everyone shares my concern.
Although, I started out by saying the end is in sight, we still have quite a batch of unresolved issues which I would like to list here so that everyone can continue to think about them.
a. There is still a wide open question concerning what is considered our real time minimum landing radar data requirement in order that descent can be continued. There are many of us who feel that failure to obtain a certain amount of good landing radar data by some point in the powered descent is sufficient justification to abort – for example, landing radar altitude updating by 13,000 feet has been suggested as a require- ment. The crew apparently feels that this constraint is not real and that their observations – visual, I suppose – are an adequate substitute. Just how we are able to integrate in these real time crew observations to overcome the landing radar deficiency has not been established yet and I am not sure who, if anyone, is working on it.
b. Although, a month or so ago, the decision was made that the crew is to manually backup the automatic switching of the landing radar antenna position during a nominal descent, there is still substantial concern that this is not the right thing to do. For example, the LM systems people point out that the switch the crew uses to do this must be cycled from “auto” through the old landing radar position to get to the new landing radar position and a switch failure could override a perfectly operating automatic signal and send the antenna scurrying back to the position it just came from.
c. I am still not content with the AGS altitude update techniques. That is, how many times and when during powered descent should this be done?
d. There is some point in powered descent after which it should be possible to continue the landing with an inoperative gimbal drive actuator. Procedures for handling this situation in real time remain to be established.