[amsat-bb] Re: How to calibrate the azimuth angle?
antonio at qualcomm.com
Thu Dec 27 12:29:26 PST 2007
At 09:08 PM 12/26/2007, Chiu-Teng Tsai wrote:
> I am Chiu-Teng Tsai (BM6ERA) from Taiwan. Now we are building a
> mobile ground station. One of our problem is how to calibrate the
> azimuth angle. Use compass only, or any better solution? Thanks for your help!
I agree with the comments of others re two methods that work
well: The stick in the ground, and tracking the sun.
Compasses are very difficult to use to get an accurate
direction. First there's the issue of magnetic declination or
magnetic variation, caused by the fact that the magnetic north pole
is not at the north pole. You look up a number on a map or
something, but most people frankly can't remember whether you're
supposed to add the number or subtract it. This is a mess. But
that's not all. Local iron (nearby cars, pipes in your house, or
even the mast of your antennas can affect the reading more than you
might imagine. (This is called magnetic deviation.) Sure, mariners
have been using compasses for a zillion years, but they've studied
variation and deviation, and calibrated their compasses for the
magnetic effects on their boats, and have done it routinely over many
years to keep themselves reminded of the details. The weekend
antenna putter-upper has not. (With the advent of GPS etc, fewer and
fewer mariners every day know how to use a compass.) Compasses on
boats are nailed down, so that it is possible to calibrate out the
effects of other (also nailed down) metal. When you're walking
around with a compass, you don't have this luxury. I once tried to
use a compass on an outing to figure out where to point an antenna,
and as I walked around I kept getting different readings. Finally I
realized that if I was anywhere near my car, the compass pointed
toward my car. With a handheld compass you just have to stay far
away from cars, houses, antenna masts, etc. You can't hold it in
your left hand as you hang on the side of the steel tower tightening
the rotor bolts, and expect success. With enough care, a compass can
work, but getting an accurate reading is more difficult than most
I say just say no to compasses. Use the stick in the ground or
tracking software that can point your antenna at the sun.
I suggest you start with the stick. That will give you enough info
to assemble the system, tighten down the bolts, etc.
Pointing the antenna at the sun is the best final check, because it
double checks so many pieces of your system. If you put the stick in
the ground, that may make you know which way is North, but it doesn't
mean you tightened down the screws with the mast in the right
position, or calibrated the rotor control software right, or have the
right timezone in your computer, etc.
With a Yagi antenna, because it is long and skinny, its shadow is a
really accurate indication.
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