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In June I brought my A-7 sextant to the Navigation Weekend for "show and
tell" in connection with my talk about the navigation on Amelia
Earhart's flight since it is a newer version of the Pioneer sextant that
was being used by Noonan. On the way home I decided to do something I
had never done before, take sextant shots from a passenger seat on an
airliner. It turned out that the A-7 was the ideal instrument to use for
doing this. The Pioneer sextants are unique among sextants in that the
eyepiece can swivel about 130 degrees each side of straight ahead which
allows you to take shots in an arc from over your left shoulder to
straight ahead to over your right shoulder.
 (See the photos at:
http://sites.google.com/site/fredienoonan/topics/pionneer-octant ).
This facility made it possible to take shots of the sun even when it was
only 30 degrees from the tail of the plane. When you consider how
limited you are in your movements when strapped into your passenger seat
it would not have been possible to take these sights with any other kind
of sextant.

I had planned to compare the LOPs with the GPS positions taken at the
same time as the shots but my GPS stopped working. (I had planned to
bring two GPSs with me but only brought one so I have to kick myself in
the butt.)  So far  I have had three GPSs fail, lending support to the
idea of staying current with your celnav. To deal with the problem of no
GPS I took shots while flying over airports that I knew I would be able
to identify on aviation charts and I drew diagrams of the runways to be
sure. When I got home I located the airports on my charts, looked at
them with Google Earth to be certain and then compared the LOPs with the
ground landmarks. Because of the coarseness of this method I could not
compare them to the level of precision I could have done with a GPS but
I am sure they were all within 10 NM.

Two days ago, on the way to Paris, I took two more sun shots with the
A-7 and worked them out with my Bygrave. (This time I have two GPSs with
me.) I was surprised with the accuracy, one had an intercept of 2.3 NM
and the other was 2.0 NM. After landing I recomputed them using the Navy
website and got intercepts of 0.3 NM and 2.0 NM, the Bygrave computation
of the first shot had been off by 2 minutes but was perfect for the
second shot. I have to admit that even I am surprised by this level of
accuracy. The plane was traveling at about 480 knots so the coriolis
corrections (applied mathematically to each shot) were both 5.6' since
the relative bearings on both shots were 49 degrees and the latitudes
were 36 and 38.5 degrees.

I have a window seat reserved for the return flight and will try to take
many sun shots to get a larger sample, but so far, so good. This also
again confirms that the sextant being used by Noonan was certainly
accurate enough to get them to Howland Island and that Noonan would have
been able to get shots on all bearings since he was less confined than I
was.

gl

   

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