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    Re: Celesital Navigation Through Clouds
    From: Ken Gebhart
    Date: 2009 Dec 22, 21:11 -0600

    Gary,
    
    FTIW I got my FN License with 3 other guys in a C-310.  We took out  
    some rear seats, built in a quick nav table, and added a Loran C to  
    check the celnav fixes (and also because taking an electronic lop was  
    a requirement of the test).  We hung an A-10 (or A10A) under the rear  
    wrap-around ceiling windows.  This gave us 310 deg observation.  We  
    calibrated the windows by swinging the airplane on the tarmac and  
    taking successive sun shots through different parts of the windows.   
    We marked the correction with grease pencil on the windows in the  
    area where the shot was aimed.  (BTW I did the same for the front  
    seat of other SE Cessnas before oceanic ferrying.)  We took off from  
    San Jose (SJC?) , went 50 miles off shore, and went a couple of  
    hundred miles north before reversing course. We did this twice, and  
    got everyone qualified in the one flight.
    
    I know you may not want to spend the extra money for a twin, but it  
    would save having to carry life rafts.
    
    Ken Gebhart
    On Dec 21, 2009, at 1:23 AM, glapook---.net wrote:
    
    > 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
    > dates.
    >
    >
    >
    > gl
    >
    >
    > 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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