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    Re: Apollo navigation
    From: Paul Hirose
    Date: 2013 Nov 03, 10:22 -0800

    The Apollo 13 air to ground voice transcription can be found here:
    The emergency technique that used the Earth and Sun to establish a known
    attitude is described on this page:
    The link to the "navigation report" didn't work for me. I got only a
    partial file. But the report is also available here:
    "Gimbal lock" is frequently mentioned in the report. Because the
    inertial measurement unit's gyro and accelerometer platform was
    supported by only three gimbals, there were forbidden attitudes where it
    was unable to maintain a fixed orientation independent of spacecraft motion.
    Grumman, the Lunar Module contractor, had insisted on four gimbals to
    assure total freedom of maneuver. But the designer of the navigation
    system, MIT, said that wasn't necessary. A system with three gimbals
    would have a size, weight, and reliability advantage.
    As recounted in "Digital Apollo" (David A. Mindell, 2008), in early 1964
    NASA got both parties together in a meeting and demanded the issue be
    explored until one side admitted defeat. MIT came very well prepared and
    the decision was for three gimbals. But when Apollo began flying,
    everyone soon realized Grumman had been right. The requirement to avoid
    gimbal lock imposed an irritating workload on mission control and the
    astronauts. Sometimes winning an argument merely proves you're the more
    skillful arguer.
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