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    Re: Celesital Navigation Through Clouds
    From: Gary LaPook
    Date: 2009 Dec 20, 23:23 -0800

    I accidentally left out the dates of January 9-10 in my  prior post
    which are the dates I thought might be too soon for someone planning a
    trip to California. But it is three weeks away and I am up for those
    On Dec 20, 2:37�pm, Gary LaPook  wrote:
    > I have given the weight question some thought. SZP (Santa Paula) has
    > only 2700 feet of runway but OXR (Oxnard) has 5900 feet so if we go with
    > four in the plane we will take off from OXR. This won't add any time to
    > the mission since we were going to land there each time to switch
    > shooters. I also considered that it might be possible for one person in
    > the back seat to swing a sextant but too crowded for two. But that is
    > not very important as my original idea was that only the navigator in
    > the co-pilot's seat would be a shooter and the possibility of back seat
    > shooters was only an afterthought that might save a little bit of time
    > and money.
    > Regarding the dates, I can see that the beginning might be too soon
    > especially for someone who might want to come from a distance. January
    > 23-24 �would also work astronomically but I will probably be out of town
    > that weekend (but not for certain.) I have no problem with putting it
    > off til March but then the sunny clime of California looses some of its
    > allure for distant navigators.
    > Regarding the issue of refraction through the windshield I am attaching
    > some of my prior writings about this:
    > -----------------------------------------------------------------------------------------------------
    > Another one of my old posts:
    > �>
    > �> Re: [Earhart] Noonan's Navigation
    > �>
    > �> I thought that refraction would be a problem when I first started
    > �> shooting stars through uncalibrated aircraft windows and windshields
    > �> based on what I had read concerning astrodomes, etc. I started shooting
    > �> fixes over land and would get a VOR radial and a DME reading in the
    > �> middle of the two minute observation period so that I had a fix to
    > �> compare with the celestial fix so that I could figure the accuracy of
    > �> the celestial. I was pleasantly surprised to find that there was no
    > �> problem. The fix accuracy was well within the 10 nm standard required on
    > �> the Flight Navigators flight test. That's just my experience and I did
    > �> lots and lots of fixes this way. .
    > �>
    > I flew up to the symposium on friday with a friend in a C-182. He did
    > the flying and I used my MA-2 sextant to find OAK. I precomputed the
    > altitudes and azimuths and plotted them on graph paper so it would be
    > easy to compare the sextant altitude with the precomputed altitude, find
    > the intercept and plot the resultant LOP. I use an Air Force issue
    > Polhemus Celestial Computer, CPU-41/P which makes plotting the LOP very
    > fast. I aimed 30 NM south of OAK and intercepted the sun line at 0021Z
    > and flew a 001 True heading into OAK Worked perfectly. Approach even
    > gave us a clearance through the class bravo with out a hassle. I shot 5
    > sun lines in an hour and a half and the worst one was 8 NM off when
    > compared to GPS fixes. The old system still works perfectly.
    > �> Flying up to the symposium I was shooting sun lines through the
    > �> uncalibrated windshield of the Cessna 182 starting with the sun about 60
    > �> degrees left of the nose and about 35 degrees high, then some straight
    > �> ahead and finally out on the left wing tip and down to about 32 degrees.
    > �> The landfall procedure worked perfectly and took us within a couple of
    > �> miles of OAK. Since this is now the 21st century I was also getting GPS
    > �> fixes to compare with the sun lines and the worst one was within 8
    > �> miles. Maybe with an astrodome the accuracy would have been 3 miles but
    > �> nevertheless it was certainly accurate enough to find OAK or Howland.
    > �>
    > �>
    > gl
    > -----------------------------------------------------------------------------------
    > I was flying to an Amelia Earhart symposium at Oakland airport in May 2002.
    > ----------------------------------------------------------------------------------
    > �>
    > �> Regarding the lack of an astrodome in a C-172 I am attaching something I
    > �> posted on a website devoted to the disappearance of Amelia Earhart.
    > �>
    > �> gl�> Gary writes:
    > �>
    > �> Rollin, we've discussed the possible refraction error from shooting
    > �> through an uncalibrated window before. It only becomes a problem if
    > �> you are shooting through a curved piece of glass or plastic such as in
    > �> an astrodome or a highly curved portion of a windshield. If you look
    > �> through the windshield of any Cessna up near the wing roots where the
    > �> window plastic is highly curved you can see the distortion as you move
    > �> your head around so you don't take sights through that part. But in
    > �> the vast majority of the windshield, if you move your head around, you
    > �> do not see any distortion. If you look at the windshield of the
    > �> electra you will see that it is made up of two pieces of flat glass or
    > �> plastic with no curvature so no problem with refraction.
    > �>
    > �>
    > �> I wrote before:
    > �>
    > �> "The reason that there is even this table of refraction for a standard
    > �> astrodome is because the dome is curved. The purpose of having a
    > �> curved dome was to allow the navigator to shoot stars at lower
    > �> altitudes (elevation as measured with the sextant above the horizontal
    > �> and not the height of the airplane above sea level) ) than would be
    > �> possible with a flat plate glass window in the roof of the plane.
    > �> Curving the dome allows the sextant to be positioned above the top of
    > �> the airplane allowing the taking of low altitude shots but also causes
    > �> refraction. Using a dome is what causes the refraction problem in the
    > �> first place. There would be no refraction with an optically flat
    > �> glass plate no matter what the angle that you shoot through it. You
    > �> can consult any text on optics or telescope making to confirm this
    > �> fact. Optically flat glass is glass that has both surfaces parallel
    > �> and flat to within 1/8 of the wavelength of light which is the same
    > �> standard used for grinding lenses and telescope mirrors. Regular
    > �> plate glass comes very close to this level of precision and can be
    > �> used except for very precise purposes at high magnifications and is
    > �> perfectly fine for astro navigation purposes."
    > �>
    > �> I reported recently my test of three sextants (including an A-7,
    > �> similar to the A-5 used by Noonan) in flight in a Cessna 172 aircraft
    > �> with the resulting accuracy of the sights of seven, five and three
    > �> nautical miles. All of the shots were taken through the uncalibrated
    > �> plastic windshield. The accuracy reported included the total of all
    > �> the errors inherent in a sextant shot in flight so any contribution of
    > �> unknown refraction from the window must have been a very small
    > �> component of the total error. (I have attached that post below.) I
    > �> have taken hundreds of sights through uncalibrated plastic windshields
    > �> in aircraft down to the size of a Cessna 150 and have always been able
    > �> to achieve the normal accuracy the navigation textbooks say is the
    > �> achievable level of accuracy in flight so there must be no problem
    > �> shooting through uncalibrated windshields.
    > �>
    > �> gl
    > �>
    > �> >
    > �> > Gary wrote:
    > �> >
    > �> > Sunday I finally had the chance to test the accuracy of my Pioneer
    > �> > sextant in flight. I only got this sextant a couple of months ago and
    > �> > the weather on weekends had prevented this test before. This sextant
    > �> > is similar to the one being used by Noonan.
    > �> > I rented a Cessna 172 and had a friend of mine come along to handle
    > �> > the controls while I shot the sun with the sextant. He is a private
    > �> > pilot and was not familiar with flying from the right seat. The most
    > �> > critical thing when taking an observation, which took one minute and
    > �> > 45 seconds, is to maintain the exact same heading during the entire
    > �> > period. Every slight deviation causes the bubble in the sextant to
    > �> > move about and you have to turn the altitude adjustment knob on the
    > �> > sextant to keep the image of the sun next to the bubble, this causes
    > �> > incorrect readings. This is the reason that a number of shots are
    > �> > taken and the average of the shots used in determining the LOP, to
    > �> > eliminate these random variations. My pilot didn't do a perfect job
    > �> > but I understand his difficulty since the directional gyro was on my
    > �> > side of the instrument panel and not right in front of him.
    > �> > I was able to take 15 observation during this shooting period by
    > �> > using the averager on the sextant. I pushed the button on the GPS in
    > �> > the middle of the shooting period so that it recorded the position of
    > �> > the airplane at the middle of the shooting period which corresponds to
    > �> > the position as determined with the average measured altitude of the
    > �> > sun. After comparing the position computed from the observation of the
    > �> > sun with the recorded GPS position the difference was 7 nautical
    > �> > miles. And this was shooting through an uncalibrated aircraft plastic
    > �> > windshield, not a carefully manufactured flat glass plate such as the
    > �> > one installed in NR16020. And the shots were accomplished with some
    > �> > unwanted deviations in the heading.
    > �> >
    > �> > I also took a two minute observation with my MA-2 sextant and its
    > �> > error was only 5 NM. It has a more sophisticated averager which
    > �> > automatically records constantly any changes in the altitude during
    > �> > the shooting period amounting to several hundred observations going
    > �> > into its average altitude rather than the 15 shots obtained with the
    > �> > Pioneer instrument so it is not surprising that it would give a more
    > �> > accurate sight.
    > �> >
    > �> > I also took a two minute observation with my A-10A sextant which also
    > �> > has an automatic averaging mechanism which recorded 120 individual
    > �> > sights during the 2 minute period which went into determining the
    > �> > average measured altitude. The accuracy of this sextant turned out to
    > �> > be only 3 NM.
    > �> >
    > �> > So my conclusion is that the the Pioneer instrument is certainly
    > �> > capable of an accuracy that should have allowed them to find Howland
    > �> > even shooting through an uncalibrated window and with deviations in
    > �> > heading during the shooting period. NR16020 had an autopilot and
    > �> > autopilots are much better at maintaining a...
    > read more �
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