[amsat-bb] Re: How to calibrate the azimuth angle?
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
2. Synchronize your laptop or computer clock with any of the internet
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
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
> 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.
> Ollie, AJ1O
> Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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