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Sunday, May 5, 2013

The AGC.

Many commented on my previous post. You can see the heart rate of Neal Armstrong go up as the AGC (Apollo Guidance Computer) starts throwing 1201 and 1202 alarms and really wants to put the Eagle LM in a crater. 

Here is all you need to know about the AGC.  

Here is what happened during the landing. 

PGNCS generated unanticipated warnings during Apollo 11's lunar descent, with the AGC showing a 1201 alarm ("Executive overflow - no vacant areas") and a 1202 alarm ("Executive overflow - no core sets").[9] The cause was a rapid, steady stream of spurious cycle steals from the rendezvous radar, intentionally left on standby during the descent in case it was needed for an abort.[10][11]
During this part of the approach the processor would normally be almost 85% loaded. The extra 6400 cycle steals per second added the equivalent of 13% load, leaving just enough time for all scheduled tasks to run to completion. Five minutes into the descent Buzz Aldrin gave the computer the command 1668 which instructed it to calculate and display DELTAH (the difference between altitude sensed by the radar and the computed altitude). This added an additional 10% to the processor workload causing executive overflow and a 1202 alarm. After being given the "GO" from Houston Aldrin entered 1668 again and another 1202 alarm occurred. When reporting the second alarm Aldrin added the comment "It appears to come up when we have a 1668 up". Happily for Apollo 11, the AGC software had been designed with priority scheduling. Just as it had been designed to do, the software automatically recovered, deleting lower priority tasks including the 1668 display task, to complete its critical guidance and control tasks. Guidance controller Steve Bales and his support team that included Jack Garman issued several "GO" calls and the landing was successful. For his role, Bales received the US Presidential Medal of Freedom on behalf of the entire control center team and the three Apollo astronauts.[12]
The problem was not a programming error in the AGC, nor was it pilot error. It was a peripheral hardware design bug that was already known and documented by Apollo 5 engineers.[13] However because the problem had only occurred once during testing they concluded that it was safer to fly with the existing hardware that they had already tested, than to fly with a newer but largely untested radar system. In the actual hardware, the position of the rendezvous radar was encoded with synchros excited by a different source of 800 Hz AC than the one used by the computer as a timing reference. The two 800 Hz sources were frequency locked but not phase locked, and the small random phase variations made it appear as though the antenna was rapidly "dithering" in position even though it was completely stationary. These phantom movements generated the rapid series of cycle steals.

A shout out to Michael Collins. He was the command module pilot orbiting the moon while all of this went down.


  1. Such a very cool time in our history. I was teaching conversational english in a Tawianeese military school during the landing. The commander invited me to his quarters to watch the landing. It was a world event - not just a US event. On a family note, our "claim to fame" . . . my Uncle Victor designed the attitude control rockets for the lem. They are the cone shaped rockets around the outside of the lander. Pretty cool time . . . . I miss it.

  2. That was the time when our government had some semblance of fostering the expansive spirit of man and freedom, instead of putting their boot on our neck.

  3. Yep, and that was ALL done without computers as we know them... LOTS of analog on Apollo!