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Gordon,

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
strap--phew!!!

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.

Brian


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