[amsat-bb] Re: PC clock
Edward R Cole
kl7uw at acsalaska.net
Tue May 11 13:58:16 PDT 2010
At 12:07 PM 5/11/2010, i8cvs wrote:
>----- Original Message -----
>From: "Greg D." <ko6th_greg at hotmail.com>
>To: <w7in at montana.com>
>Cc: <amsat-bb at amsat.org>
>Sent: Tuesday, May 11, 2010 7:47 AM
>Subject: [amsat-bb] Re: PC clock
> > Hi Larry,
> > Ok, I understand the need for an accurate clock, though I believe you're
> > expectations for being able to track an overhead pass is pushing the
> > limits of orbital prediction pretty hard. Another ham I know locally
> > tried this, and ultimately gave up. His issue was not one of clock
> > accuracy, but of Keps and the mathematics behind them.
> > Greg KO6TH
>Hi Greg, KO6TH
>When the software calculate the Az and the El of the satellite at the right
>time with an accurate clock then it send a command to the rotators but the
>antennas takes a certain time to go in that calculated position and when the
>antennas are finally there then the satellite is already in another position
>far advanced in it's orbital path particularly when the satellite pass is
>I dont think you want to run the motors every 5 or 10 seconds othervise your
>control relays will work as a machine-gun
>So the issue is not on clock accuracy or Keps or mathematics behind them but
>it is on the tracking system that we normally use to mimichaise the
>satellite position with a phase difference between the satellite calculated
>position and the actual antenna position when the traching command is sent
>to the motors.
>If you go over the roof and you follow the ISS by naked eye you will realize
>that your antenna pointing is always a little bit behind the ISS position in
>the sky and so a very accurate clock to track a LEO satellite is meaning
>less particularly using high gain antennas with a narrow main lobe.
>Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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Good point! I guess if we had software that could provide a position
slightly in advance of the satellite so the tracking could be a
series of short drifts thru the beam of the antennas...that would be
ideal. Also much more complicated since the lead time/angle would
vary by satellite pass, and ground station antenna
parameters. Tracking Leo satellites on s-band with a two-foot dish
can be demanding considering the narrow beamwidth. Fortunately, one
does not need that much gain for Leos.
When I track the Moon, manually, this is what I do. But the Moon
apparent motion in the sky is about 15-degrees/hour in azimuth so
with 16-degree 2m antennas it only requires repointing 2-3 times per
hour. At 1296 my eme antenna beamwidth is 3-degrees so keeping
peaked within 1-dB requires much more frequent movement (10 or more
times/hour). Fortunately, most auto-track sw has input for how often
to command the rotators.
My 16-foot dish actuators rotate the dish fairly slow (90-deg/5-min),
so it takes some time for repointing. I can resolve 0.1 degree movement.
73, Ed - KL7UW, WD2XSH/45
BP40IQ 500 KHz - 10-GHz www.kl7uw.com
EME: 144-600w, 432-100w, 1296-60w, 3400-fall 2010
DUBUS Magazine USA Rep dubususa at hotmail.com
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