# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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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."

```
```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
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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