[amsat-bb] Re: How to calibrate the azimuth angle?

Jim Sanford wb4gcs at amsat.org
Thu Dec 27 15:01:47 PST 2007

I have to agree with Franklin . .. . compasses are HARD.   Starting 
with, it's difficult to get accurate variation & deviation information 
(delta between local magnetic & true North) for land-based locations.

Mariners, for centuries, used a now obsolete method of navigation called 
"celestial" -- which meant using sun, moon, stars, tables, and 
calculations to determine location based on observations.

One of those techniques was known (at least 35 years ago, when I first 
studied celestial Nav) as "Local apparent Noon".  You started with an 
estimate of your latitude, determined a time offset from the center of 
the time zone based on assumed (dead-reckoning) position, and then 
started some time PRIOR.  Using a sextant, you would start "swinging" 
the sun, and click a stopwatch (synchronized to WWV) at the highest 
elevation of sun.  You have now determined the time when the sun's 
elevation is highest, or Local Apparent Noon.  From that, you could 
determine longitude, and latitude using other techniques.

This process lends itself nicely to reverse engineering as follows:
1.  Using your handy GPS, determine the most precise coordinates for 
your tower/antenna.
2.  Synchronize your laptop or computer clock with any of the internet 
time servers.
3.  Synchronize your watch with the laptop/computer.
4.  Enter the pre-determined coordinates into your favorite tracking 
program (InstantTrack, Nova, SATPC32 . . . .)
5.  Using the "fast forward" feature of that tracking program, move 
until you get sun azimuth of 180 degrees.  This should coincide with 
maximum elevation.
6.  Write down the time of max elevation/180 degree azimuth.
7.  Stand "north" of your tower/antenna at about that time, with a stick 
or other marker.
8.  When your watch indicates the appointed time, put the stick in the 
ground at the far extent of the shadow from your antenna/tower.
9.  You just marked true North at your location. 

Obviously the taller your antenna, and longer your shadow, the more 
accurate your marking will be.  For any reasonable antenna height, this 
will be well within the accuracy of any reasonable amateur antenna system.

I have done this, and it matched PRECISELY with the direction determined 
using (difficult to determine) local variation and deviation vs. 
magnetic.  I had the luxury of an open field, so suspect very little 
interference to the magnetic compass from "local" effects.

Frankly, I find the celestial method easier, and more repeatable, 
particularly at Field Day sites, etc.

Good luck & 73,
wb4gcs at amsat.org

Ollie Eisman wrote:
> Franklin Antonio wrote:
> ...
>> Compasses are very difficult to use to get an accurate 
>> direction.  
> I've always had excellent results with a Brunton compass.  It takes a 
> few minutes to learn how to use one but it's well worth it.
> http://en.wikipedia.org/wiki/Brunton_compass
> Ollie, AJ1O
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