Sunday, July 20, 2014

Forty Five Years Ago Today

David Woods, video editor
(his books)

Apollo 11 Lunar Surface Journal, The First Lunar Landing
Corrected Transcript and Commentary Copyright © 1995 by Eric M. Jones
All rights reserved.

Blog notes:

The first moon landing was forty five years ago, today.  As a child, I watched many of the moon landings and was fascinated with the details of the radio communication.  In this video, you can hear the challenges the Apollo 11 mission faced with radio communication and with different kinds of background noise.  (Some of it is also engine noise.  Today, with radio and telephony, you rarely hear radio noise because it is filtered out using digital and analog filters and other noise and error rejection schemes.)

For most of the descent, the Eagle lunar lander is controlled by the flight computer, the Apollo Guidance Computer (AGC), which among other things, is fed data by a radar system which can "see" the lunar surface. 

The full transcript of the lunar descent is available at the NASA site Apollo 11 Lunar Surface Journal, The First Lunar Landing (see above for the link).

The video starts at 102:32:35 into the transcript.

At about 5:17 into the video, or 102:38:04 into the transcript, you can hear Aldrin say "we got good lock on", meaning that the radar and AGC have acquired information about the lunar surface and are flying under computer control with the radar information.

The "P" annotations on the video refer to the computer programs that are being run during the descent.  The AGC programs were:

P63 - Landing maneouver braking program
P64 - Landing maneouver approach phase
P65 - Landing phase - auto
P66 - Rate of descent landing
P67 - Manual landing phase

From the Apollo Guidance Computer wiki page:

At about 5:40 in the video, "Buzz Aldrin gave the Apollo Guidance Computer (AGC) 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 work load 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". "

"Luckily 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."

"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. 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 AGC cycle steals."

Again at about 9:30 into the video, there are low priority alarms, '1201' and '1202', which are again over-ridden by the AGC.  The AGC continues to control the flight profile, processing high priority tasks, and ignoring low priority tasks.

At 10:30 into the video, Armstrong takes the AGC out of the controlled P64 approach phase program and into a P66 rate of descent manually controlled landing.  He pitches the profile of the Eagle forward, to maintain speed, in order to fly across Crater West boulder field.  Once across the boulder field, at about 11:05, he pitches back to slow down.

They land with 20 seconds of fuel remaining.

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