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    Re: Apollo navigation
    From: Frank Reed
    Date: 2013 Oct 22, 12:11 -0700

    Yes, Sean and Norm, it is fascinating. BTW, while I don't want to suggest that "everything has already been posted" or "it's in the archives", a link to this video was posted in a NavList message by Don Seltzer just seven weeks ago: http://fer3.com/arc/m2.aspx/Sextants-Space-Seltzer-aug-2013-g25009. The ink isn't even dry on that message. ;)

    In fact, the link in Don's message may be preferable for some folks in a couple of ways (simplest being that you don't have to go through 'youtube hell' to see it): http://kottke.org/13/08/the-apollo-guidance-computer

    Note that the description of the navigation in this video may be misleading since, by the time of the Apollo flights, the sextant had largely been superseded for navigation. The sextant was used on one mission for "real" navigation to the Moon: by Jim Lovell on Apollo 8 (though even then, the ground's vectors were preferred in nearly every case), and it was tested out occasionally as a backup on each succeeding lunar flight. But by then it was redundant and un-necessary for actual navigation. The primary function of the sextant on the landing missions was as an astro-compass for orienting the spacecraft. It was also employed as a computer-guided telescope used for locating the lunar modules from orbit. The computer-driven sextant in the Apollo spacecraft was a marvel in the early 1960s. The navigation computer contract was the first contract written in the entire Apollo program, and they got right to work designing it. But the world had changed, both in political terms and technological terms by 1968, and autonomous on-board navigation, apart from the astro-compass role, was no longer a requirement by the time of the landings.

    The rest of the video discussing the computer's construction is really fascinating. It's amazing how much of it was done by hand. And it's fascinating, too, to see the clear division of labor by gender in that period. Women did much of the actual work building the Apollo computers.

    There are several good books on the Apollo guidance computer. One of my favorites, which has more analysis and less technical detail, is "Digital Apollo" by David Mindell. You should be able to find it as a pdf online. Progressively more technical are "Journey to the Moon: the History of the Apollo Guidance Computer" by Eldon Hall (he was chief designer of the AGC, and if I remember correctly, he's in the video), and finally "The Apollo Guidance Computer - Architecture and Operation" by Frank O'Brien. Are there any others?

    Although the technology seems primitive, the Apollo guidance computer was a very sophisticated integrated, embedded computer system. The computer navigated and also FLEW the spacecraft. The astronauts' control inputs did not directly fire the thrusters, as they appeared to. Rather, they provided "joystick" input which the computer used to generate the proper impulses. The astronauts also entered simple keyboard commands. If they built one today, they're wouldn't be much difference in operations. Except of course it would be voice operated... "Computer, fly me to the Moon... And while we're on the way, could you tell me what Spring is like on other planets... let's say, Jupiter and Mars."


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