Spacecraft computer program design to process VHF ranging for rendezvous navigation

See list belowNOV 14 196767-FM-T-104FM/Deputy ChiefSpacecraft computer program design to process VHF ranging for rendezvous navigation

1. On November 1, 1967, a team of MSC people were at MIT to discuss, among other things, use of the VHF ranging data in the spacecraft com- puter. Specifically, we reviewed the work MIT has done in this area since our “tiger team” meeting on October 5. Steve Copps is the guy at MIT responsible for incorporating this new requirement into Chapter 4 of the GSOP. He has nearly finished this but has not reviewed it in any detail with the programming people. He did make a point of emphasizing some assumptions he has made and I am writing this note to make sure you are aware.

2. The first matter deals with getting the data on the downlink. The only channel for sending VHF ranging data to the ground is through the spacecraft computer. However, Steve Copps is setting up the program such that, unlike the LM, there will be no special program in the computer solely for getting the ranging information from the VHF device and putting it on the downlink. It will only get on the downlink if the rendezvous navigation program is in operation. Doing it this way is said to be the easiest and least complex but means that VHF range data will not be trans- mitted directly to the ground when “average g” is running (during the final phase of the rendezvous and during burns) or when in the platform alignment programs.

3. The second point Steve emphasized was that precisely the same data acceptability test is used on all rendezvous navigation data sources— sextant, telescope, reticle and VHF range. As you recall, each individual observation is processed and changes in the state vectors are computer. If this change is in excess of some test values (preset in erasable memory) the crew is given the option of rejecting that data. The test values are the same for all data sources.

4. Steve also noted that there is no VHF maximum range test governing the computer's use of the data. This function is left to the crew to make sure that the computer does not receive data in the second range interval, that is, beyond approximately 400 miles since it will be wrong by about that amount. Supposedly, the VHF is only designed to work to 200 miles away.

5. Finally, MIT will attempt to display the number of sextant and VHF range observations separately as two digit integers in the same DSKY register separated by a zero or a blank. If anyone sees anything wrong with the above design assumptions, he better inform FSD personnel immediately as well as supplying justification for going in some other way. As of now, MIT has been told that these assumptions are acceptable to MSC.