A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2019 Oct 27, 08:10 -0700
Hewitt Schlereth you wrote: David, thanks for the info. My experience is with the Navy Mk 5, the British Mk IX, and IX-A. The averagers sure help. I gather Fred Noonan had an A-6 or A-7 which get an average with a trigger operared pencil that marks a drum. I think at least one member of the forum has used an A type in flight.
Until the late 1930s pencil mark averaging was used in aircraft sextants on both sides of the Atlantic, and the median mark was chosen as the bodies measured altitude. It wasn’t possible to allocate an exact time to this particular altitude if more than one was taken in a sequence, so I would imagine the mid time of the whole sequence was used. The RAF MkIX sextant introduced mean averaging although the observer still chose when to press the trigger, and the mid time of the entire sequence was used. The RAF MkIXA introduced automatic mean averaging. Once the trigger was pressed, 60 individual values/60, each two seconds apart were recorded (chosen apparently to match the dynamic response of the Sunderland flying boat) whether the observer liked it or not. The time used was the mid time of the sequence. There was never any ‘finish early’ facility on the clockwork Kelvin Hughes Smiths sextants as on some Kollsman sextants.
So mathematicians, first of all, bearing in mind the large number of variables affecting the accuracy of airborne bubble sextant altitudes, is it worth attempting any meaningful analysis of median versus mean averaging, and if it is, which is likely to be the most accurate? DaveP