[TowerTalk] [Fwd: Beam headings]
Wed, 15 Mar 2000 20:09:15 -0500
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Date: Wed, 15 Mar 2000 20:02:53 -0500
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Subject: Beam headings
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Since this thread seems to be generating a lot of interest, I thought I
had better put in my two cent's worth.
Several years ago I decided I would establish a baseline down my poperty
that could be used for all time for any antenna I wanted to aim,
including an 800 foot long rhombic. Within a half hour one evening I
established this baseline to an accuracy of 2 or 3 minutes of arc!
Now I know that this accuracy is completely unnecessary for any antenna,
and my only excuse was the challenge - i.e. "it was there".
I borrowed a surveyors transit, obtained a copy of software called
'astrocalc' which gives the position of the planets, etc. as a function
of time, a copy of the Astronomical Almanac, a conversion of sidereal to
EDST for that day, and an accurate watch.
At dusk I sighted on Saturn (an unmistakable sight, even with the modest
telescope on the transit), and recorded its position on the transit's
scale at a precise time. This gave a reading of 51 degrees, 46.8
minutes for the baseline.
I then sighted on Polaris, and using the corrections in the Astronomical
Almanac, determined a bearing of 51 degrees, 42.9 minutes.
I rounded the average of these to 51 degrees, 45 minutes, plus or minus
3 minutes, as the bearing of my baseline.
Finally, the bearing was taken using the compass on the transit.
Applying the magnetic declination correction for this location and date,
I obtained a bearing within about three quarters of a degree of the
I make no claim that this exercize was necessary, but as I said, I did
it "because it was there". The compass reading would have been quite
Incidentally, contrary to what has been said, a correction for sidereal
time AND latitude is necessary to obtain an accurate reading for
Polaris. This star circles about a point in the sky directly above the
axis of the Earth, with a radius of about a degree. But the difference
of the bearing from true north can exceed this. Consider an extreme
case, observing Polaris from a location within a degree of the pole. At
times it will be due south! This would require a correction of 180
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