The purpose of this memorandum is to inform you of two special features of the Apollo spacecraft rendezvous guidance computer program you may not be aware of since we just added them to the system.

First of all, you recall that both spacecraft–the CSM and the LEM– have rendezvous guidance systems. In order for the computers to determine what maneuvers are required to bring about rendezvous, the basic thing each of the computers needs is the state vectors–that is, orbital elements–of both vehicles. Up until now, all thought has apparently been given to the LEM program. Since the CSM is supposed to be passive, all radar data is used to update the LEM state vector, based on the assumption that the CSM it is tracking is in a well known, unchanging orbit. Also, as the LEM makes maneuvers, the guidance system senses them and so there is no need for a pilot input to the computer to inform it that they were made. However, when we consider what’s going on in the CSM, or in the LEM during a CSM rescue, this doesn’t look so hot.

First of all, the computer may really have a better defined state vector for its own spacecraft, making it more desirable to update the state vector of the other vehicle. Therefore, pilot control is needed over which spacecraft state vector should be updated based on the radar and optical observations. This will allow the pilot to exercise his best judgment as opposed to providing some sort of automatic logic built into the computer program. Also, if the other vehicle maneuvers, the computer won’t know it unless informed by some external source, like the crew. For this reason and others, it is also necessary to include in both the CSM and LEM computer programs the capability for the pilot to input to the computer the fact that the other spacecraft is making a maneuver such that it can be taken into account in maintaining the best current state vector of each spacecraft in each spacecraft’s computer.

Accordingly, both of these options are being provided; that is, the crew will, inform the computer which spacecraft state vector should be updated and he shall also input to the computer all necessary information when the other vehicle makes a maneuver. Associated with this latter capability is the need to assure that the observational data is not improperly used. Therefore, in order to avoid complex and sophisticated computer logic, we have decided to again utilize the crew’s capability to understand the situation and control the computer processing in the following way. The pilot will interrupt the computer program at the time it is planned that the other vehicle will make the maneuver, which will cause the computer to reject all tracking data until the actual ΔV of the maneuver is input. He will have to get this information by voice from the other spacecraft after the maneuver is executed, of course. This procedure will assure that the quantities which are input are the most accurate available and should assure that the observational data is used properly.

Terms & Abbreviations


Command-Service Module.


see LM


Lunar Module. Earlier it was known as the Lunar Excursion Module and abbreviated “LEM.” Even after the name change, it continued to be pronounced “lem.”