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    Celestial navigation on Gemini and Apollo flights
    From: Paul Hirose
    Date: 2008 Sep 15, 13:25 -0700

    While reading "Carrying the Fire", by 1960s astronaut Michael Collins, I
    enjoyed his description of a navigational experiment on his rookie
    flight on Gemini 10 in 1966. It was a "very cumbersome procedure,
    whereby I was to use a portable sextant to measure the angle between a
    star and the earth's horizon (repeating the process at carefully
    precribed intervals with stars selected for their positions with respect
    to our orbit) and feed the angles and times into Module VI [software in
    a memory expansion module] of our computer. Combining Module VI data
    with a variety of charts and graphs carried on board, we would be able
    to determine our orbit and predict where we would be at a given future
    time relative to our Agena target."
    
    The objective was to rendezvous and dock with an Agena upper stage
    (already in orbit), using Module VI and no help from the ground, then
    use the Agena to boost themselves into a higher orbit Then they would
    rendezvous with a second Agena (but not dock). Collins would spacewalk
    to the second Agena and remove a micrometeoroid experiment from its
    exterior, for return to Earth.
    
    Collins and crewmate John Young practiced the rendezvous interminably in
    the simulator with the Module VI engineers. It was a tough grind,
    "pushing me to my limits of time and understanding... for once a mistake
    was made, there was no way to cross-check it. Instead all computations
    downstream of the mistake were poisoned, and the ultimate result was
    invariably incorrect."
    
    One problem didn't show up until they got into orbit and Collins shot
    the first star, Schedar. "My first shock comes when I try to find the
    horizon: somewhere in that homogeneous void black sky changes to black
    water -- that I can see, grossly, by the fact that the stars stop at a
    certain point, but they seem to ooze into oblivion without any sharp
    line of demarcation... After some experimenting, I find I can watch the
    image of the star through my sextant as it approaches the horizon. There
    is a faint muddy band, the airglow, into which the star, descending,
    almost disappears, but if I watch closely enough, it re-emerges into a
    very narrow zone of visibility before being snuffed out by the
    atmosphere. It is this instant of snuffing that I must capture, as
    accurately as possible. When I do I yell MARK, and John pushes a button.
    Then we must turn up the lights long enough to read the sextant angle,
    and manually punch it into the computer, digit by digit. I am getting
    the hang of it, but the process is slow, and we are running behind by
    the time we have finished with Schedar and are ready for Hamal, our
    second star... I finally get one measurement at the last possible
    moment. Then a quick and sloppy try at Vega and Altair before the night
    ends and we break into brilliant sunshine near the western coast of
    Australia. John is jubilant. 'Isn't that beautiful!' Deep in Module VI
    calculations, I can only grumble, 'Don't even tell me about it, babe.
    I'll look tomorrow.'"
    
    Collins has some results by the time they reach Hawaii. It's a mixed
    bag. His predicted thruster firing times look fairly good compared to
    Mission Control's numbers, but he's way off laterally and cannot figure
    out where the error occurred. They decide to use the data from the
    ground to fly the rendezvous.
    
    Although the celestial navigation experiment was disappointing, the
    mission overall was a success. Both rendezvous were accomplished and the
    experiment was retrieved.
    
    On Collins' next (and last) flight he was the command module pilot on
    the Apollo 11 mission with Armstrong and Aldrin.
    
    On that mission one of his duties was to take star shots to keep the
    inertial navigation platform aligned. "I had made several trips to the
    Massachusetts Institute of Technology near Boston, and had tried my
    level best to suffer through a couple weeks of 'simple' explanations of
    the system by their experts, but I always came away shaking my head.
    They didn't speak my language and I didn't speak theirs."
    
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