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    Re: Averaging
    From: Gary LaPook
    Date: 2004 Oct 20, 03:29 -0700
    Doing celestial navigation in aircraft requires the averaging of many sights to eliminate the errors caused by random accelerations of the aircraft that displaces the bubble (which acts as an artificial horizon)  in the field of view. A commonly used aircraft sextant, the A-10, has a disk (which rotates in synchronization with the index prism) on which a timer mechanism makes pencil marks every second and you usually track the body for 120 seconds. You then visually estimate the center of the  marked area  and rotate the index wheel to center this on the pencil index. You then are able to read out the average (actually median) of the 120 sights. You use this altitude with the mid time of the sights to compute the LOP.

    Gary LaPook

    George Huxtable wrote:
    Chuck Taylor sent this message-
    
      
    George Huxtable wrote, concerning the concept of
    averaging observed sextant altitudes over time:
    
    "But I don't see how you would apply that
    technique to a quantity that was changing
    systematically, in the way that observed altitudes
    change rather steadily with time (either
    increasing or decreasing), with a bit of random
    scatter superimposed."
        
    
    ==========================
    
    No. Chuck has that quite wrong; perhaps I didn't make myself clear..
    
    That's what I wrote, in response to a posting by Federico Rossi, but it
    wasn't "concerning the concept of averaging sextant distances over time."
    
    Federico Rossi had written, about averaging an odd number of observations-
    
      
    The explanation I've found for this recommendation is that this way you
    can consider the central value of the odd series (i.e. the so called
    median) which is definitely less affected by abnormal values than the
    average value.
        
    
    And I replied-
    
    "Selecting a median that way seems to be an alternative to averaging. It's
    a rather extreme case of discarding outliers, in that it discards
    EVERYTHING but the one median observation.
    
    I can see that it might be usable (if not ideal) for dealing with a
    quantity that fluctuates in a random manner. But I don't see how you would
    apply that technique to a quantity that was changing systematically, in the
    way that observed altitudes change rather steadily with time (either
    increasing or decreasing), with a bit of random scatter superimposed."
    
    =================
    
    What I didn't see, was how you could apply the technique of CHOOSING A
    MEDIAN VALUE (as Federico suggested) to a series of measurements that
    varied with time, as altitudes do.
    
    I see no problem, none at all, in applying an AVERAGING technique to such a
    series of measurements.
    
    George
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
      
       
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