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Author Topic: Questions for the Celestial Choir  (Read 12347 times)

Mona Kendrick

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Questions for the Celestial Choir
« on: May 09, 2011, 11:59:03 AM »

How long does it take to work up a position fix after taking a sextant sight?  How freqently do you take sightings on a long flight?  And how much free time do you have between sightings, drift readings, and computations?  Are you constantly busy?

LTM,
Mona
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h.a.c. van asten

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Re: Questions for the Celestial Choir
« Reply #1 on: May 09, 2011, 01:17:45 PM »

Reduction of sextant sight to position fix and plot takes 3-6 minutes in the air (20 minutes , ships @ sea) , but the time needed depends on the before flight home work having been done. Precomputation is of paramount importance in air navigation and is done with the aid of special air navigation tables and an air almanac. On long range flights the navigator checks the DR in a 20 minutes sequence plan by taking sights on heavenly bodies. Generally there is little or no leisure time during flight. These features valid for "navigation old fashion" , although , also with advanced navigation methods , operating and checking the nav instruments is an all time consuming business.
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Mona Kendrick

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Re: Questions for the Celestial Choir
« Reply #2 on: May 09, 2011, 01:39:43 PM »

Thank you.  Anyone else?

Mona
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Ric Gillespie

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Re: Questions for the Celestial Choir
« Reply #3 on: May 15, 2011, 07:07:39 AM »

Although I can't speak to celestial navigation, having never used it, I can confirm that "old fashioned" navigation keeps a pilot (or navigator is you have one) very busy.
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Gary LaPook

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Re: Questions for the Celestial Choir
« Reply #4 on: May 23, 2011, 12:26:31 PM »

Reduction of sextant sight to position fix and plot takes 3-6 minutes in the air (20 minutes , ships @ sea) , but the time needed depends on the before flight home work having been done. Precomputation is of paramount importance in air navigation and is done with the aid of special air navigation tables and an air almanac. On long range flights the navigator checks the DR in a 20 minutes sequence plan by taking sights on heavenly bodies. Generally there is little or no leisure time during flight. These features valid for "navigation old fashion" , although , also with advanced navigation methods , operating and checking the nav instruments is an all time consuming business.

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Using the navigation tables that Noonan said he always used, H.O. 208 (U.S. Navy Hydrographic Office Publication number 208) commonly called "Dreisonstok" after it author, along with the American Nautical Almanac takes about four minutes to do one computation. If you are doing the computation for two sights then it takes only about seven minutes since there is some savings in time in extracting the data from the almanac. If you are doing a lot of computations for drafting a graph of altitudes versus time, assembly line fashion, then you get the computation down to about two minutes per computation. Reviewing Noonan's chart work on the way to Hawaii shows that he never did a three star fix  (as is modern practice for error detection) so for him to do the computations, in flight, for a two star fix should have taken about seven minutes. Using the more modern tables, H.O. 249 you can do the computation for three sights commonly used for the fix in just ten minutes.

A marine navigator does these computations after taking the sextant observations since he has leisure but a flight navigator does all of these computations prior to picking up the sextant so that he can plot his fix very quickly after taking the observations because the plane is moving along quickly compared to a ship. Using a bubble sextant you must take a number of observations and average them in order to eliminate random errors caused by accelerations on the fluid in the bubble chamber. Using a sextant like Noonan's it takes about three minutes to take 10 shots in flight which is a convenient number of shots for averaging. Allowing two minutes between shots to reset the sextant and get lined up on the second star means that shooting a two star fix takes about eight minutes and averaging the shots (which can only be done after the observations) takes about one and a half minutes each so from start to finish it would take about eleven minutes. To compare his observations with the precomputed altitudes and plot the LOPs on the chart would take about another three minutes. Total for the process, (if all the work was done in flight about 20 to 22 minutes. If the pre-computations were done on the ground (as was commonly done) then you can subtract the 7 minutes computation period and get the time down to 13 minutes.

For taking just one shot for determining the location of the plane in relationship to the LOP through Howland and with the pre-computations having already been graphed, then these sights and this determination could be made about every 6 to 7 minutes. 

See:

http://www.oceannavigator.com/content/celestial-air

http://www.avweb.com/news/avtraining/IFR_bySunAndStars_200781-1.html

http://www.fer3.com/arc/m2.aspx?i=116311&y=201104

https://sites.google.com/site/fredienoonan/other-flight-navigation-information/recent-landfall-approach


https://sites.google.com/site/fredienoonan/other-flight-navigation-information/in-flight-celestial-navigation

Gary LaPook
« Last Edit: August 27, 2012, 12:38:53 AM by Gary LaPook »
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Martin X. Moleski, SJ

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Re: Questions for the Celestial Choir
« Reply #5 on: May 23, 2011, 05:30:26 PM »

A marine navigator does these computations after taking the sextant observations since he has leisure but a flight navigator does all of these computations prior to picking up the sextant so that he can plot his fix very quickly after taking the observations because the plane is moving along quickly compared to a ship. Using a bubble sextant you must take a number of observations and average them in order to eliminate random errors caused by accelerations on the fluid in the bubble chamber. Using a sextant like Noonan's it takes about three minutes to take 10 shots in flight which is a convenient number of shots for averaging. Allowing two minutes between shots to reset the sextant and get lined up on the second star means that shooting a two star fix takes about eight minutes and averaging the shots (which can only be done after the observations) takes about one and a half minutes each so from start to finish it would take about eleven minutes. To compare his observations with the precomputed altitudes and plot the LOPs on the chart would take about another three minutes. Total for the process, (if all the work was done in flight about 20 to 22 minutes. If the pre-computations were done on the ground (as was commonly done) then you can subtract the 7 minutes computation period and get the time down to 13 minutes.

Are an octant and a sextant the same thing?  Noonan seems to use the term interchangeably here:

"In describing the techniques he used to navigate the 1935 Pan Am China Clipper flight, Noonan says: 'Two sextants were carried. A Pioneer bubble octant and a mariner’s sextant. The former was used for all sights; the latter as a preventer'" "The Noonan Project."

"Production of Brandis sextants ceased in 1932 by which time Pioneer had developed the aeronautical bubble octant Noonan referred to in his letter to Weems. Obsolete Brandis instruments were sold as surplus to retailers like Negus Instruments of New York. It is, therefore, hardly surprising that Noonan’s back-up “preventer” in the mid-1930s would be a Brandis, and it is the Brandis brand that points toward the box found by Gallagher" ("Sextant Boxes: Numbers Game").

"A bubble octant is not listed in the inventory. Harry Manning had signed for Navy Pioneer Bubble Octant, Serial No. 12-36 from the Naval Air Station in San Diego and retained possession of it rather than have it shipped back with the damaged aircraft. Noonan then signed a receipt for the octant on Matson Line stationery and gave it to Manning following the Luke Field accident. We do not know whether Noonan used that instrument on the second attempt or whether he returned it and used something else" ("Air Navigation: State of the Art in 1937").


Forum Highlights, January 2 through 8, 2000.
Subject:    Noonan's Sextant
Date:    1/3/00
From:    Doug Brutlag

While I cannot find the reference book I got the info from, I believe FN used an A-5 model bubble sextant manufactured by Pioneer. Accuracy is good---for a bubble sextant. Generally speaking if you can shoot a 2 or 3 line fix and get within 10 miles you are doing pretty good. Besides the problems of shooting through a moving platform you will get an error imposed on the bubble itself by aircraft movement. Personally I say if you can shoot 10-15 miles with a hand-held bubble sextant you are doing good. Holding the thing steady in your hand is also a challenge---in turbulence, forget it!

I mainly use the A-10 series which was used by the AAF in WWII, but I may be coming into possession of an A-7 Pioneer sometime in the future (late Christmas present). The A-7 was the next model after the A-5-not much difference to the best of my knowledge. It's accuracy is about the same. I'm getting the manual to it soon and then I'll be able to tell what kind of averager it had. Understand it was very reliable---all of them are precision instruments to be cared for. I've never tried using a marine sextant at all. You would need an artificial horizon of some sort. I can't comment on how well it would work. In any case, the real key to successful celestial is PRACTICE-PRACTICE-PRACTICE. I've been practicing for 12 years and I am still learning. Fred had been doing it since he was a teenager when he went off to sail square riggers I believe. Fred and Nathaniel Bowditch were the best that ever sailed & flew.

Happy New Year to all. Am going off to go aviate & navigate again. I'll let everyone know if Y2K shut my airplane down. The A-10 is along just in case.

Doug Brutlag

From Ric

You may be entirely correct about Noonan using a Pioneer A-5. The instrument Manning borrowed from the Navy was a Pioneer bubble octant (serial number 12-36) and if the A-5 was current in March of 1937, then it was probably an A-5.
LTM,

           Marty
           TIGHAR #2359A
 
« Last Edit: May 23, 2011, 05:38:20 PM by Martin X. Moleski, SJ »
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Ricker H Jones

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Re: Questions for the Celestial Choir
« Reply #6 on: May 23, 2011, 06:27:27 PM »

Weems wrote in his 1938 "Air Navigation":
     "For a great many years practically all instruments used by marine navigators for measuring altitudes have been sextants, and the term sextant has become so generally used that it is now applied to all instruments for measuring altitudes of heavenly bodies whether they are actually quadrants, sextants, or octants." (p. 301)
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Martin X. Moleski, SJ

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Re: Questions for the Celestial Choir
« Reply #7 on: May 23, 2011, 06:42:58 PM »

Weems wrote in his 1938 "Air Navigation":
     "For a great many years practically all instruments used by marine navigators for measuring altitudes have been sextants, and the term sextant has become so generally used that it is now applied to all instruments for measuring altitudes of heavenly bodies whether they are actually quadrants, sextants, or octants." (p. 301)

Thanks, Ricker!
LTM,

           Marty
           TIGHAR #2359A
 
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Gary LaPook

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Re: Questions for the Celestial Choir
« Reply #8 on: May 23, 2011, 09:15:50 PM »

A marine navigator does these computations after taking the sextant observations since he has leisure but a flight navigator does all of these computations prior to picking up the sextant so that he can plot his fix very quickly after taking the observations because the plane is moving along quickly compared to a ship. Using a bubble sextant you must take a number of observations and average them in order to eliminate random errors caused by accelerations on the fluid in the bubble chamber. Using a sextant like Noonan's it takes about three minutes to take 10 shots in flight which is a convenient number of shots for averaging. Allowing two minutes between shots to reset the sextant and get lined up on the second star means that shooting a two star fix takes about eight minutes and averaging the shots (which can only be done after the observations) takes about one and a half minutes each so from start to finish it would take about eleven minutes. To compare his observations with the precomputed altitudes and plot the LOPs on the chart would take about another three minutes. Total for the process, (if all the work was done in flight about 20 to 22 minutes. If the pre-computations were done on the ground (as was commonly done) then you can subtract the 7 minutes computation period and get the time down to 13 minutes.


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Re accuracy of cel nav in small planes see:


http://www.fer3.com/arc/m2.aspx?i=111467&y=201001

http://www.fer3.com/arc/m2.aspx?i=111510&y=201001

http://www.fer3.com/arc/m2.aspx?i=111496&y=201001

These were the results obtained by three guys who had never before used a bubble sextant in flight. One of the guys got one shot with a 2 nm accuracy and another of 3 nm accuracy. Noonan was not a first time navigator.


gl

« Last Edit: August 27, 2012, 12:41:29 AM by Gary LaPook »
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