At 12:07 PM 5/11/2010, i8cvs wrote: >----- Original Message ----- >From: "Greg D." <ko6th_g...@hotmail.com> >To: <w...@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 > > > >_______________________________________________ >Sent via amsat...@amsat.org. Opinions expressed are those of the author. >Not an AMSAT-NA member? Join now to support the amateur satellite program! >Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
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 dubus...@hotmail.com ====================================== _______________________________________________ Sent via amsat...@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
