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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Celesital Navigation Through Clouds
From: Gary LaPook
Date: 2009 Dec 20, 23:23 -0800
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 dates. gl On Dec 20, 2:37�pm, Gary LaPookwrote: > 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 � -- NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com