[amsat-bb] Re: PC clock
ko6th_greg at hotmail.com
Tue May 11 21:47:23 PDT 2010
The local ham I referred to was trying to drive his rig's Doppler open-loop, based only in time and keps, and found that he could get close enough for FM, but not quite good enough for some of the more frequency-sensitive modes. Antennas (his and mine) aren't sharp enough to need such precise tracking, but I understand about the antenna lag. My own system has a granularity of 6 degrees Az (10 El), and even so it gets busy around TCA. But fortunately, few passes are so high as to cause motor or relay problems.
My bigger problem, actually, is that my controller doesn't (yet) know about flipped operation, so passes that cross the Zenith to North line have that annoying 60-second gap. But the fix is a matter of software, and some time to write it.
Thanks for your words of advice. Always appreciated.
> From: domenico.i8cvs at tin.it
> To: ko6th_greg at hotmail.com; W7IN at montana.com
> CC: amsat-bb at amsat.org
> Subject: Re: [amsat-bb] Re: PC clock
> Date: Tue, 11 May 2010 22:07:22 +0200
> ----- 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.
> 73" de
> i8CVS Domenico
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