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    Re: Celesital Navigation Through Clouds
    From: Gary LaPook
    Date: 2009 Dec 20, 14:37 -0800

    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 constant heading than are
     > > human pilots so I would expect that Noonan, shooting through the flat
     > > glass window with the plane flown on autopilot, would have had a
     > > higher level of accuracy than I achieved, more like 2 to 4 NM. Worst
     > > case is he achieved the 7 NM accuracy that I achieved which,
     > > considering the prevailing visibility at Howland, should have allowed
     > > them to find the island.
     > >
     > >
     > > (I am starting to suspect that I may be the last person on earth to
     > > actually take celestial observations in flight.)
     >
     >
     > gl
    
    
    --------------------------------------------------------------------------------------------------
    
    Greg R. wrote:
    > "Gary LaPook"  wrote:
    >
    >
    >> Here are my thoughts. I can rent a Cessna 172 at Santa Paula airport,
    >> near Ventura California for about a hundred bucks an hour and we will
    >> have to share this cost. It holds four people, one pilot and three
    >> navigators. We fill all the seats and take off and fly out over the
    >> ocean and take sights.
    >>
    >
    > Not wanting to second-guess your piloting skills, but you might want to get
    > a weight-check on the prospective navigators first - I don't know of any 172
    > that will hold 4 "average-sized" people + full fuel (or maybe you can
    > arrange with the FBO to not re-fuel it on the last flight before this one?),
    > not to mention the relatively-short runway at SZP.
    >
    > Also, what sort of refractive correction would need to be made for the
    > Plexiglas windscreen and windows, or is it insignificant enough not to worry
    > about? Though I guess it could also be determined "empirically" using the
    > known GPS position as a reference, and then factoring in whatever extra
    > correction is needed to make the celestial LOP agree with that.
    >
    >
    >> I will try to get out to the airport next week and see if it is possible
    >> to use a sextant in back since I have never tried this before.
    >>
    >
    > It's been a while since I was in the rear seat of a 172, but if I remember
    > right the top of the passenger windows slopes downwards from the front -
    > might be problematic for 2 people trying to take sights together, but might
    > work OK for one person if they had to crouch down to get a clear sight
    > through the window.
    >
    >
    >> Did I mention that it was 77 degrees here today?
    >>
    >
    > And people wonder why we put up with all the "inconveniences" to live
    > here.....  ;-)
    >
    > --
    > GregR
    >
    >
    >
    > ----- Original Message -----
    > From: "Gary LaPook" 
    > To: 
    > Sent: Sunday, December 20, 2009 6:00 AM
    > Subject: Re: [NavList 11293] Celesital Navigation Through Clouds
    >
    >
    >
    >> Here are my thoughts. I can rent a Cessna 172 at Santa Paula airport,
    >> near Ventura California for about a hundred bucks an hour and we will
    >> have to share this cost. It holds four people, one pilot and three
    >> navigators. We fill all the seats and take off and fly out over the
    >> ocean and take sights. You can bring your own bubble sextant and I can
    >> supply a variety of bubble sextants, MA-1, MA-2, A-10A, A-7 and the
    >> navigators can take a number of sights each using different instruments
    >> if they choose. The navigator siting in the right seat can take the
    >> sights while the navigators in the back seats can record data. We will
    >> push the button on the GPS at the mid time of each sight so we can
    >> determine the accuracy of the sights, the navigator with the worst
    >> average buys the beer. Since the plane won't allow the navigators to
    >> change seats in flight we will land at the Oxnard airport, right next to
    >> the beach, to allow the navigators to change seats and a new navigator
    >> to start taking sights. I estimate that it will take each navigator
    >> about a half hour so the plane cost should be about fifty bucks each. It
    >> may be possible to lower this cost somewhat if it is possible to take
    >> sights from the back seats through the back window as this would
    >> eliminate the necessity of landing to change shooters. I will try to get
    >> out to the airport next week and see if it is possible to use a sextant
    >> in back since I have never tried this before. If more than three
    >> navigators are interested in participating we can switch out crews at
    >> the Oxnard airport.
    >>
    >> I believe the best dates for this would be January 9-10; February 6-7;
    >> or February 20-21 or possibly later in the year.  We should plan on
    >> flying on a Saturday and keep Sunday as a backup in case of bad weather
    >> on Saturday. The reason I suggest these dates is that the sun and the
    >> moon will both be visible with good cuts for daytime fixes.
    >>
    >> If anybody is coming from afar, the Burbank (BUR) airport is the most
    >> convenient. LAX is a bit farther and Long Beach (LGB) is about as
    >> convenient as LAX (it might not look like it on a map but a map doesn't
    >> show the traffic coming from LAX on the 405 over the Sepulveda pass.)
    >> Ontario (ONT) is also doable so shop for the best airfare. Things to do
    >> in the area include Santa Barbara for wine tours, an hour drive up the
    >> coast. San Diego is about a three hour drive down the coast or one can
    >> take a train. You can go aboard the Star of India and the carrier
    >> Midway. Long Beach for the Queen Mary and a Russian submarine. Near Long
    >> Beach in San Pedro is the Lane Victory (a victory ship) and a good
    >> maritime museum. One can also drive five hours and see Yosemite which is
    >> beautiful in winter. A different five hour drive up the coast takes you
    >> to San Francisco or you can take a train or fly.You can visit the
    >> Pampanito submarine (SS-383) and several historic vessels including a
    >> liberty ship, the Jeremiah O'Brien. A four hour drive from here is
    >> Vegas, baby. There are plenty of flights from BUR to Vegas also. A three
    >> hour drive takes you to Palm Springs. A two hour drive takes you to the
    >> ski slopes.
    >>
    >> Did I mention that it was 77 degrees here today?
    >>
    >>
    >> http://www.wunderground.com/cgi-bin/findweather/getForecast?query=93021
    >>
    >>
    >> gl
    >>
    >>
    >> frankreed{at}HistoricalAtlas.com wrote:
    >>
    >>> Peter Monta, you wrote:
    >>> "Wouldn't a UAV be the logical solution here? I'm not sure units with
    >>> 40000ft capability are all that cheap yet, though. A very small
    >>> payload might suffice for sun sights (small camera). Establishing the
    >>> offset from UAV position to ship would probably come free with the
    >>> overall control scheme to get the thing back, and the 2D offset would
    >>> only be a mile or two anyway. Fixed wing might be best for smallest
    >>> platform jitter when taking the sight."
    >>>
    >>> Nice! That's a very clever solution to the problem of the 40,000 foot
    >>> mast. And if the sensor package is cheap enough (a camera and a radio
    >>> with a ten-mile range?) then you could make them expendable and launch
    >>> on weather balloons. The price of the balloon might turn out to be
    >>> greater than the cel nav package. Whether that's more economical than
    >>> a mini/micro-UAV or not would depend on the cost of the "toy plane"
    >>> and the expected loss rate. In any case, a system like this means no
    >>> sextant and no navigator holding said sextant, so it certainly takes
    >>> the charm out of it, but at least it would still be real celestial
    >>> navigation.
    >>>
    >>> -FER
    >>>
    >>> --
    >>> NavList message boards: www.fer3.com/arc
    >>> Or post by email to: NavList@fer3.com
    >>> To unsubscribe, email NavList+unsubscribe@fer3.com
    >>>
    >> --
    >> NavList message boards: www.fer3.com/arc
    >> Or post by email to: NavList@fer3.com
    >> To unsubscribe, email NavList+unsubscribe@fer3.com
    >>
    >
    >
    
    --
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
    To unsubscribe, email NavList+unsubscribe@fer3.com
    

       
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