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    Re: Admiral GAGO Coutinho
    From: Douglas Denny
    Date: 2010 Oct 19, 16:36 -0700

    Dear Gary,

    The sextant you mention is that used on the Gemini programme, and I believe, on later Apollo flights. There were only a few made and they are without a shadow of doubt the most accurate sextants ever made.

    The sextant was for the astronauts to have the capability to be able to monitor and evaluate their trajectory independently of the automatic systems.
    One of the expected uses was to assess the orbit transferring from an 80 nm altitude circular orbit to a point approx 50000ft above the lunar surface in the lunar landing module.

    The repeatability of the mechanical arrangement for the index mirror for example was within 0.4 arsecond. The normal telescope used was tested to be within 7.5 arcsecond, and the sextant had a designed overall error envelope of accurately measuring less than 10 arcseconds.

    This is why irradiation became a very important aspect of the manual navigation exercise part of the Gemini project as it was an unknown quantity and swamped any of the standard corrections needed to obtain that 10 arcsecond accuracy desired. They went to a lot of trouble at the Ames laboratory to assess irradiation and other aspects of observation effects with manual navigation with a sextant. Each astronaut using this sextant had to be individually 'calibrated' too, using it in the Ames laboratory.

    The results obtained from the Gemini XII spacecraft showed a standard deviation of measurement error of less than 10 arcseconds. I believe it was designatd the TOO2 experiment and the sextant made by Kollsman.

    It's a fascinating story in its own right I think.
    There are lots of amazing technical notes available on line from NASA including a lot on this subject.

    All this effort for the American space programme in the 1960s regarding manual navigation has to be the very zenith - the ultimate - of what has been achieved and what can be achieved with hand-held sextants and "manual navigation". It will not be bettered I think.

    See:-

    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680015040_1968015040.pdf

    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710026298_1971026298.pdf

    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690005000_1969005000.pdf


    Douglas Denny.
    Chichester. England.
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