I am afraid there is a bit of confusion as to what navigation modes are being provided in the AS-503 and AS-504 Apollo spacecraft computer programs. I am sure I have contributed to this confusion myself, and the purpose of this memorandum is to try and clear it all up.

According to Norm Sears, it is intended to provide the following navigation; that is, onboard orbit determination programs in the AS-504 command module computer program:

a. During earth orbital operations there shall be no onboard navigational capabilities at all.

b. During the trans-lunar and trans-earth phases the navigation program is being formulated to process both star/landmark and star/horizon measurements. The landmarks and horizon may be either earth or lunar at the choice of the flight crew. That is, there is no interlock governing which is used depending on position of the spacecraft relative to those two bodies. The pilot must manually key in location of the earth landmarks and it is probable that he will also have to key in lunar landmarks since those stored for lunar orbit navigation are likely to be of a size not readily observable during these phases of the mission.

c. In lunar orbit the navigation program will utilize only lunar landmarks referenced to the platform. Twenty-eight landmarks will be stored in the computer program, but I am certain others may be keyed in if the crew desires.

For the AS-503 mission, it is currently intended to have only one navigation mode - namely, use of star/landmark or star/horizon observations. The landmarks and horizon used are restricted to earth only since it is not intended to have such routines as the lunar ephemeris, lunar rotation, etc., programs available. Earth landmarks must be keyed in manually by the crew. Norm Sears (MIT) points out that use of this data in orbits of the type currently planned for AS-503 may actually result in degradation of the onboard state vector, and as a result it may be necessary to restrict this process to a spacecraft system test rather than an operational procedure in support of the mission.

I suppose, to make this entirely complete, I should also list here the processing of the command module sextant data for rendezvous navigation, which will be in all Block II computer programs currently planned.

Other than rendezvous navigation utilizing the spacecraft radar, there is no navigational capability planned to be included in the LGC program for any mission.

We are currently in the midst of an exercise to make the AS-278 programs identically the same as AS-503. Since we have a difficult schedule situation on AS-278, there may be implications on the navigation modes available for the AS-503 mission as noted above; however, at this time I do not expect that to be the case and will certainly inform you if the situation changes.

Terms & Abbreviations


see AS-207/208


see AS-504


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.


see AS-207/208


Before the Apollo 1 fire, the mission referred to as AS-503 was originally scheduled for October 1967. AS-503 eventually launched as Apollo 8, December 23 1968.


Before the Apollo 1 fire, the mission referred to as AS-504 was originally scheduled for December 1967. AS-504 eventually launched as Apollo 9, March 3 1969.


Lunar Module Guidance Computer.


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.