About frequency of LM radar observations

See list belowOCT 15 196767-FM-T-81FM/Deputy ChiefAbout frequency of LM radar observations

1. I learned something the other day about how the LM computer uses rendezvous radar data that is different than I thought. It had been my understanding, and I think most other people's too, that rendezvous radar observations were obtained and processed by the computer every 60 seconds. In fact, there has been some concern that perhaps that is not frequent enough. Well, it turns out that the time between observations is probably more like 1¼ minutes. What actually happens is the computer obtains an obser- vation from the radar and processes it (i.e., redetermines the spacecraft state vector). This processing takes about 15 seconds. Once processing is completed, an internal timer is set in the computer to obtain the next observation one minute from that time. In some cases, it waits even longer if some other higher priority task is underway.

2. In addition to reducing the amount of radar data used during a mission, processing in this way prevents the crew from using the computer obtained radar observations in their backup compu- tations as conveniently as they did on Gemini. On Gemini, you recall, observations were obtained and stored in computer memory exactly once per minute which allowed the crew to read them out at their convenience and be sure that they were obtained at precisely 60 second intervals – a convenient delta t to use in their backup computation.

3. This memo is just to pass on this discovery to you. It is not our current intention to change this program, although I suppose it would have been nice if we could.