A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2010 Oct 08, 18:45 -0700
On 10/8/2010 5:24 PM, Gary LaPook wrote:
The U.S. Air Force's Kollsman sextants were updated in the late 1980s with electronic averagers which must be the penultimate development of aircraft sextants. The U.S. Air Force worked on the assumption that all electronic navaids would be turned off by both the Russians and by the U.S. if World War III ever got started. Also, when penetrating air defense, you don't want to turn your own emitters on, such as ground mapping radar, as these types of emissions tend to attract missiles.
As to the use of celnav on commercial oceanic flights, Francis Rogers in his book Precision Astrolabe describes the celestial navigation done by the navigator on Portuguese TAP airline's flight TP 322 on June 24, 1970 from Lisbon to Santa Maria, Azores (hey, I landed there in my Cessna 172) and on to Boston, flying in a Boeing 707. The navigator used a Kollsman sextant and H.O. 249 (A.P. 3270) for sight reduction. Even though LORAN A was available, there were (and are, even with LORAN C) gaps in the coverage. The aircraft also had Doppler but that is just an aid to dead reckoning and needed to be updated by actual fixes, either LORAN or celestial. The use of LORAN and Doppler and, obviously celestial, still required a navigator onboard so they were not eliminated until the development of INS installed in Boeing 747s that first entered service in 1970. INS is also a form of dead reckoning but its rate of drift is less than one nautical mile per hour so was precise enough for oceanic navigation. But it required three independent INS's on each aircraft for redundancy so was an expensive proposition but must have been less expensive that paying he salary of the navigators.
On 10/8/2010 3:01 PM, Douglas Denny wrote:
".. I consider the periscopic Kollsmans as the perfection of (or at least the
highest development of) the bubble sextant since they were continued in
use in great numbers by the U.S. Air Force into the 21st century , more
than 50 years after other bubble sextants were abandoned.."
The Hughes periscopic sextants were continued to be developed from the MkIX variants after WW2 and used as far as I know by the RAF into the 1970's too (I think) in the Vulcan bombers when the latter were still in service in 1986 when the Falklands war was in progress.
Aircraft sextants would only be on board for backup I guess as electronic navigation aids had superceded celestial navigation for military work by the end of WW2. Interestingly, the long-haul aircraft passenger flights post war were done with persicopic sextant however, but I am not sure when these would have been phased out for use of radio aids such as Loran and later Omega. Consol was still available in Western European areas up to the 1980's.
Hughes produced bubble sextants in later variants with pendulous references, but the bubble chamber type was used until quite late.
The main improvement was not in the bubble or pendulous reference as this had already been perfected to be as good as possible; but was in the averaging device. Improvements could only be made with better averaging-out of the side accelerations inherent in an aircraft. The optics and bubble (or pendulous) reference was as good as possible for the job.
Previously 'averaging' started with the 'median Octant' type of approach as in the A10 and A12; or other mechanical single-shot summations before the clockwork type which integrated readings every second with a rotating 'catch' method on a divided (toothed) wheel in the MkIX; then later with a motor driven continuous reading ball-integrating mechanism.
I doubt if there is anything to choose between the later (say 1970) Kollsman or Hughes periscopic sextants in ultimate accuracy obtained. The problems had all been solved by the 1960's except how to stop the side accelerations - which you cannot of course!
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