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    Re: Celesital Navigation Through Clouds
    From: Gary LaPook
    Date: 2009 Dec 23, 09:38 -0800

    I think getting fixes will be more interesting than just getting a
    series of sun lines. But if someone wanted to take more shots we could
    do more sun lines after the moon sets. But my original plan was to
    minimize the costs so more guys can give it a try. If someone wanted
    more shooting time we can do it but then his costs will go up but it
    might be worth it for such a "once in a lifetime experience."
    
    So for those who are interested please let me know so I can reserve a
    plane. I already have one vote for January 9-10 but the dates are not
    finalized yet.
    
    gl
    
    On Dec 22, 8:40�pm, Gary LaPook  wrote:
    > Those are good ideas Ken but we are not planning anything so elaborate.
    > I think the guys are just looking for an opportunity to try their hands
    > with their bubble sextants in flight. In an effort to keep the costs
    > down I am contemplating each guy having about 30 - 40 minutes in the
    > co-pilot's seat of a C-172 and then a return to the airport to switch
    > out navigators. �I am planning to fly on a heading approximately
    > bisecting the azimuths to the sun and moon so that sights can be done on
    > the sun and the moon without changing heading. This will allow for a fix
    > by advancing the first line. I think we can get in the sights for two
    > such fixes in fifteen minutes on heading then a one eighty turn and back
    > to the airport to switch navigators. Since the fix will be worked out
    > after landing we might be able to get by with only one minute between
    > shots (depending on the type of sextant and the averager setup) so we
    > might be able to take six shots in eighteen minutes producing three
    > fixes.Then the data can be evaluated on the ground. If someone wanted
    > more eyepiece time we can probably do it too. I will plan the track so
    > we don't get very far offshore (or we could just stay over land if that
    > is more comfortable) or far from the airport to minimize time on the
    > airplane and costs. I will push the button on the GPS at the mid time of
    > each shot so their accuracies can be evaluated when we crunch the
    > numbers over lunch.
    >
    > We will have a limited window of time, between two and three hours,
    > which is the period when both the sun and the moon are high enough above
    > the horizon on the weekend days that I have suggested for the flight so
    > each shooter must be limited in time so that others can get their turns
    > so the time each guy gets depends on how many people want to do it. If
    > we get a big turnout we could fly on both days which would allow each
    > guy more time and shots.
    >
    > gl
    >
    > Ken Gebhart wrote:
    > > 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, glap...---.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
    >
    > ...
    >
    > read more �
    
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