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    Aircraft bubble sextants
    From: Brian Jones
    Date: 1997 Jun 20, 14:55 -0400

    RE: Bubble sextants
    ...I was wondering if anyone has had any experience in using an
    aircraft sextant for land navigation.
    When I was in Nav school (Air Force--early 70's), we were issued hand held
    bubble sextants to take home for practice.  Although I did not attempt to
    navigate over land with it, I was able to resolve fixes to within a couple
    of miles of my actual location.  The biggest problem I had with the hand
    held bubble sextant was that it was difficult to keep steady, the bubble
    would bounce around making it difficult to keep the body centered.  In the
    aircraft, however, there is a sextant mount into which a periscopic sextant
    is installed.  If you were unfortunate enough to not have a sextant mount,
    but instead a sextant dome and a hand-held sextant, then you would connect
    one end of a stabilizing strap to your belt and the other end to a D-ring on
    the floor.  Pushing yourself up against the strap during your shot would
    help to stabilize it--some such rig could help if using it on land.  As a
    shot taken in the aircraft requires observing the body for two minutes (must
    average out the motion of the autopilot--a two minute sine wave), and
    performing a three-star fix; that's six minutes straining against the
    The aircraft hand helds have a 2x eyepiece making it suitable for
    Sun/Moon/Venus fixes.  They also have progressive filters for observing the Sun.
    As a note for those not familiar with these sextants, they have a built-in
    two minute averaging timer.  To take a shot, open the shutter, center the
    body within the bubble then start the timer one minute prior to the time for
    which you want to resolve your fix.  Once the timer starts, continue to
    center the body within the bubble for the two minutes using the height
    adjustment knob.  At the end of the two minutes the shutter will close.
    Once the shutter has closed, center the averaging indices and read the HO.
    Also, the size of the bubble is adjustable.  If one decides to buy one of
    these sextants make sure that the bubble chamber has not leaked any fluid.
    If it has then the bubble size is not adjustable and the sextant is useless
    as you have lost your artificial horizon.
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