At 12:07 PM 5/11/2010, i8cvs wrote:
----- Original Message ----- From: "Greg D." ko6th_greg@hotmail.com To: w7in@montana.com Cc: amsat-bb@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 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 144-600w, 432-100w, 1296-60w, 3400-fall 2010 DUBUS Magazine USA Rep dubususa@hotmail.com ======================================