Rendezvous terminal phase guidance program in the Apollo spacecraft computer

On July 7, 1966, a team of MSC and MIT “experts in rendezvous” (including Paul Krammer, Ed Lineberry, John Dahlen, and Norm Sears) met at MIT to discuss and review the preliminary Guidance System Operation Plan (GSOP) which MIT has unofficially distributed, covering the terminal phase and External ΔV programs for the AS-207/208 mission. This meeting was sort of a mile-pebble in the accelerated program development sequence we have established in an attempt to get all this business on schedule. That is, we are obtaining bits and pieces of the GSOP as they come off the MIT press rather than awaiting receipt of the formally published, final document.

It is our hope that, by reviewing and commenting on these pieces as they become available, the GSOP should be virtually acceptable without modification on the date of its publication and should permit the computer program development to proceed much more quickly than it has in the past. We had previously discussed these mission programs and our pilot input and display requirements for them in detail a month or so ago with MIT, and the pieces of the GSOP I am talking about here reflected that input very well. Therefore, most of the discussion was for purposes of clarification to assure a firm understanding on both MSC’s and MIT’s part as to what this program was really going to do and how we were going to operate it. Basically, very few modifications were considered necessary.

In my opinion, this meeting was highly successful; and, since these processors–the terminal phase and External ΔV–are the most significant new requirements and the most controversial of the mission programs, I feel we are probably over the hump as far as defining the program for the AS-207/208 mission.

I would like to point out here the two items given the most attention at this meeting since they serve well to describe the character of the terminal phase rendezvous guidance philosophy:

  1. One of the capabilities of most interest which we have provided was the display of range, range rate, and the angle the spacecraft X-axis makes with respect to the local horizontal. It was decided to make these three quantities available at crew request at any time the data was available. (This stuff is used for carrying out the crew backup procedures.) Contrary to one of my previous reports, these quantities will all be computed based on the current best estimate of the two spacecraft state vectors. (We had previously expressed an intention for the computer to display raw radar range and range rate in the LEM). Our action in this case was based on our desire to make the CSM and LEM computer programs as much the same as possible, and, since the raw radar data is available on what is said to be a highly accurate analog display in the LEM we have not really lost anything. In order to make this particular feature of the program as independent as possible from the automatic guidance system processing, we have divorced the display of these quantities from the activity associated with the primary guidance system to the maximum extent.

  2. Based on Gemini experience, the crew has emphasized that there is no requirement for automatic execution of the braking maneuvers by the G&N system. As previously reported, it is felt that this task can be carried out just as well, if not better, by the crew if they are provided the proper information; namely, the range and range rate data. At least this is true in the case of the nominal mission and most contingency situations, and we want to take advantage of that. However, there are occasions when automatic control of these maneuvers by the G&N might be mandatory. For example, visual acquisition is required for the crew to carry out this task, and under some abort situations lighting conditions can be unacceptable. Also, there are abort cases in which the closing velocity is too high for effective manual control. Recognizing that procedures are available for utilizing the remaining computer processors to carry out the G&N controlled braking maneuvers by proper pilot manipulation of the computer, we deleted the requirement for automatic computer logic for this task. The point is, we felt that there was insufficient justification to carry out the extra programming, debugging, verification, and documentation, as well as using some 50 to 100 words of precious computer storage, for a program which was not needed, except in rather remote contingency situations, as long as procedures were available to handle all situations. And, they are.

The final GSOP shall reflect these characteristics; otherwise, it was accepted pretty well as is.

In the course of our discussions, I learned some rather interesting things about the command module which I must say didn’t impress me very favorably. In fact, I really wonder (i.e., doubt) if it is possible for one crew member to carry out a rendezvous in the CSM. For example, the only observational data available to the computer is from the sextant, and that requires manual tracking and input of observations into the computer. (The LEM has automatic radar tracking with its data available to the computer as it periodically requests it.) And, of course, in order for the pilot to use this system, he has to be down in the navigation area of the spacecraft, which means he has to quit making observations sometime before any SPS maneuver to get strapped into his seat. On top of that, the sextant apparently can’t be oriented along any of the major spacecraft axes, which makes it necessary to orient to some attitude not consistent with making RCS midcourse maneuvers.

I’ll bet that when we finally get a crew timeline on a CSM one-man rendezvous, he has to do it without any observational data available to the computer after about 15 minutes before TPI. If my guess is right, in effect we have provided practically no CSM G&N rendezvous guidance system, and thus the job will end up being carried out pretty much using the crew backup procedures. Boy!

Terms & Abbreviations


AS-207/208 (also known as AS-278) was to have been the first test of the LM in Earth orbit. It was also to have be a dual mission with the command and lunar modules launched on separate Saturn 1Bs. The mission was cancelled after the Apollo 1 fire and the Saturn 1Bs were used to launch Skylab 3 (AS-207) on July 28, 1973 and Skylab 4 (AS-208) on November 16, 1978. The LM was first tested by Apollo 5 in January 1968.


Command-Service Module.

Dahlen, J. M.

Member of the MIT Instrumentation Lab. Director, Systems Engineering Division, Apollo Guidance and Navigation Program.


Guidance and Navigation.


see Guidance System Operations Plan

Guidance System Operations Plan

The GSOP was essentially the specification for how the guidance computer and its software where required to work for a specific mission. Many of GSOP’s are available online including the GSOP for the cancelled AS-207/208 mission

John Dahlen

see Dahlen, J. M.


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.”


Massachussets Institute of Technology. In these memos, MIT is shorthand for the MIT Instrumentation Laboratory, created and led by avionics pioneer Charles Stark Draper. It is now known as the Charles Stark Draper Laboratory and became independent of MIT in 1973.


Manned Spacecraft Center. Now known as Johnson Space Center.


Reaction Control System.


Service Propulsion System, the large engine of the Service Module that was used to enter and exit lunar orbit, as well as make course corrections while going to and from the moon.


Transfer Phase Initiation (also known as Terminal Phase Initiation). One of the maneuvers performed by the LM after ascent from the lunar surface to rendevouz with the CSM.