NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Averaging
From: Gary LaPook
Date: 2004 Oct 20, 03:29 -0700
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:
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@huxtable.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. ================================================================