[amsat-bb] Re: PC clock

[Edward R Cole]

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
>overhead.
>
>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.
>
>73" de
>
>i8CVS Domenico
>
>
>
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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
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