NavList:

Jean you wrote:

The manual unit was use onbard bommers from 1939 to 41. I found out that these manual unit are more accurate when well calibrated but they need about 6 to 8 minutes to be use. It takes 3 to 4 times more time to have a fix thats why the RAF as commision an automatic averager. It is a little less accurate in my opinion but more faster to get several sights reading.

Jean
However it might appear on the ground, automatic averaging is definitely more accurate in the air, because, in addition to any statistical advantage of averaging 60 readings compared to around six, it also avoids the natural tendency of the navigator to only press the trigger when the bubble appears steady.  This is often the worst thing you can do.  The dynamic response of an aircraft in flight causes the bubble to be displaced fore and aft by the ‘phugoid’ and athwartships by ‘Dutch Roll’.  Both these responses are sinusoidal.  However, it’s not simple displacement of the bubble that the navigator notices, it’s change of displacement, so you have to differentiate these sinusoids to see what’s really happening.  The rate of change of bubble position is actually least when the displacement is greatest and greatest as the acceleration slices back through zero.  In WW2 Celestial was much more useful during maritime operations than for bomber operations over Europe, where one has to try hard to find it being used at all, and the apocryphal story is that the two minute sequence for the MkIXA sextant was chosen because it best suited the dynamic response of the Sunderland Flying Boat.  Incidentally, there’s nothing sacrosanct about being restricted to six trigger pulls with a MkIX.  There are formulae in AP1234 for getting an average from fewer or more than six trigger pulls.  However, whist these formulae might be just about understandable in a warm well lit classroom, they would probably tax the mental agility of a professor of advanced mathematics from Oxford University when applied in a noisy, bumpy, dimly lit, boiling hot or freezing cold aeroplane while wearing a tight flying helmet.  DaveP