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    Re: celestial navigation on Gemini and Apollo flights
    From: Frank Reed
    Date: 2008 Sep 22, 23:19 -0400

    Paul H, you wrote:
    "Not that the sextant worked any better, in practice. Module VI was a
    failure, and the crew used data from Mission Control to make the
    rendezvous."
    
    Yeah, that seems to have been the case with all attempts to do "traditional"
    celestial navigation on manned spacecraft, and in practice it has always
    been the case that the navigation intelligence has remained on the ground.
    With only one or a small number of manned spacecraft up there at once, it's
    much easier to track them from down here. Even among unmanned spacecraft,
    the navigational intelligence usually stays "down here" except where the
    light travel time is greater than the reaction time (can't navigate to a
    landing on Mars in a three-minute descent from orbit when it takes twelve
    minutes for a signal to get to Mars from Earth...).
    
    You added:
    "The Apollo spacecraft used star observations too." And quoting Collins,
    "The spacecraft platform, with its three gyroscopes isolated from spacecraft
    motions, could then be aligned in relation to the stars, providing a fixed
    frame of reference."
    
    And this is an important point: the Apollo sextant was really used primarily
    as a three-dimensional (actually bi-directional) astrocompass. Determining a
    spacecraft's position from ground observations is relatively
    straight-forward, but the orientation can change, especially with astronauts
    bouncing around inside, and you can't easily detect that from the ground. So
    you need a system on-board to keep the thing lined up. The Apollo spacecraft
    had a guidance system "auto-pilot" that fired thrusters as appropriate
    whenever the orientation changed beyond some limits. This all depended on
    accurate alignment of the inertial platform hence the occasional recourse to
    star observations. In practice, these rarely were necessary since the
    inertial system was so good, but they were a priceless backup. And note that
    in the one true emergency, the Apollo 13 mission, the sky around the
    spacecraft was so filled with particulate debris and ice crystals that it
    was nearly impossible to see the stars. Even when the stars did become
    briefly visible, Mission Control advised the astronauts to skip the sextant
    alignment of the platform since they trusted the gyros.
    
    And you quoted Collins who wrote:
    "a star sighting using the wrong star would be embarrassing at best, and
    could easily be disastrous..."
    
    Also nearly impossible given the narrow field of view of the instrument and
    the careful selection of stars.
    
    
    Quoting Collins again:
    "My next task involves realigning our inertial platform for the second time,
    and again, with help from the computer in pointing the sextant, it goes
    swiftly and well. Five balls! How about that, sports fans?"
    
    This was the normal case: the star observations yielded zero error in
    orientation. The inertial gyroscopes were extremely stable.
    
    
    And Collins continued:
    "measuring the angles between five selected stars and the earth's horizon. A
    couple of stars I can see fine, like Altair, but with them I have difficulty
    finding that spot on the horizon which is directly below them, the
    substellar point as it is called. In other cases, such as Enif, the star is
    not bright enough to be readily seen. Finally, I wade through it all, but
    the results are not very accurate and I am discouraged. This exercise is for
    practice, really, as we would not have to rely on such measurements unless
    we lost radio contact with the ground"
    
    Yeah, these tests led nowhere. As far as I have been able to determine, the
    only time these experiments (never used, merely tested) in actual navigation
    ever worked out more or less as planned were on Apollo 8 when Lovell was
    doing the observations. He was a Navy man, and he liked celestial
    navigation. By the way, you'll notice that these are observations where a
    star is brought to the limb of a bright object. They're 'lunar distance'
    observations, or better yet 'terrestrial distance' observations yielding a
    position fix (two distances yield a position "ray", some third observation,
    giving distance along that ray or a surface cutting the ray, is required for
    a true fix in 3d space).
    
    
    Regarding the computer, you wrote:
    "The same chapter also has lot of information on the Command Module
    Computer, since a large part of its job was guidance and navigation. It was
    a 16 bit device with 38,912 words of storage in core memory. If you watched
    the movie "Apollo 13" you got a look at the control panel. Operation was
    unusual by modern standards: you entered numbers representing data ("nouns")
    and the action to take ("verbs")."
    
    Also, if you enjoy this, you might like the episode about Apollo 14 from the
    HBO miniseries "From the Earth to the Moon". On that mission, a software
    patch had to be written for the LEM computer at the last minute to lock out
    the effect of a bit of loose solder in the abort button switch which was
    intermittently sending a signal which would have jettisoned the descent
    stage and fired up the ascent stage in the middle of the landing run.
    
    And you wrote:
    "Most people would laugh at it today, but the Apollo missions would have
    been impossible without this machine."
    
    And you could still do an Apollo-type moon landing using a computer that
    today would be smaller than your thumb. No one would, of course, and in
    truth the Apollo missions were very risky, and it was mostly luck that the
    astronauts were able to solve the problems that went beyond the little
    computers' standard scenarios.
    
     -FER
    
    
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