[amsat-bb] Re: probably simple

Greg D. ko6th_greg at hotmail.com
Sat Jan 9 11:53:03 PST 2010

Hi Mark, Pete, all,

So, please forget my suggestion about TCA as being an easy way to predict where one should set their dial when setting up for a footprint-stretching sked.  That only works if the satellite is passing directly between the two stations, at 90-degrees to a line that runs between them.  Being on the West Coast, most DX for me is to the East, and since most of our satellites are in polar orbits, the model kind of fits.  But it's a special case for most everyone else.

You are absolutely right that the rate of change in Doppler shift is greatest at TCA, and is proportional to the frequency.  When manually tuning the 2.4 ghz downlink on an overhead AO-51 pass, one needs to continually spin the dial.  It's a physics demonstration at its finest.

Greg  KO6TH

> Date: Fri, 8 Jan 2010 13:28:10 -0500
> Subject: Re: [amsat-bb] Re: probably simple
> From: marklhammond at gmail.com
> To: ptrowe at yahoo.com
> CC: ko6th_greg at hotmail.com; amsat-bb at amsat.org
> I think part of the subject that confuses most of us (me included!) is
> that while at TCA the offset is 0, the RATE of change on either side
> of TCA is higher than at AOS or LOS.    So, the actual frequency
> offset is BIGGEST at LOS and AOS, but the rate of change is slow (it
> doesnt' change much).  It's easier to tune.    At TCA you should hear
> the satellite at the "actual" frequency, but not for long because the
> change in frequency is fastest right before and right after...you tune
> like crazy to keep up.
> Maybe that will make sense to somebody...but I won't promise :)
> Also---don't forget---a 10kHz offset at 2M is 30kHz at 70cm (and one
> is increasing in freq while the other is decreasing...)
> Use SatPC32 and you just about forget about having to do anything, and
> watch it happen--good chance to see what is really going on.
> 73,
> Mark N8MH
> On Fri, Jan 8, 2010 at 1:11 PM, Pete Rowe <ptrowe at yahoo.com> wrote:
> > Hi Greg
> > I'm confused. It seems to me that the place when Doppler is zero is when the satellite is about half way through the pass. This is when the Doppler goes from positive (coming at me) to negative (going away from me). It seems like this place would be the time of closest approach (TCA) and not way out on the horizon when I see maximum Doppler.
> > What am I missing?
> >
> > 73,
> > Pete
> > WA6WOA
> >
> > --- On Thu, 1/7/10, Greg D. <ko6th_greg at hotmail.com> wrote:
> >
> > From: Greg D. <ko6th_greg at hotmail.com>
> > Subject: [amsat-bb] Re: probably simple
> > To: glasbrenner at mindspring.com
> > Cc: amsat-bb at amsat.org
> > Date: Thursday, January 7, 2010, 7:34 PM
> >
> >
> > Hi Drew,
> >
> > SatPC32 is probably an excellent program (I'm on Linux here, so can't use it), and if you've got the automation available, that's certainly the best way to go.  And, by definition, the lower the elevation, the farther away the satellite is, so your DX contacts are going to be at the edges of the pass.
> >
> > But any pass where you are really stretching the footprint is going to be a low elevation pass.  The more you stretch, the lower the pass.  In the limit, I think Bob's ultimate pass has a peak at .001-degrees for both stations.  If you're doing that, then you're at TCA, and zero doppler.
> >
> > That's all I meant to convey,
> >
> > Greg  KO6TH
> >
> >
> >> Date: Thu, 7 Jan 2010 05:53:13 -0500
> >> From: glasbrenner at mindspring.com
> >> To: ko6th_greg at hotmail.com
> >> CC: w7lrd at comcast.net; amsat-bb at amsat.org
> >> Subject: Re: [amsat-bb] Re: probably simple
> >>
> >> Greg D. wrote:
> >> > Hi Bob,
> >> >
> >> > Whatever the satellite, if you're trying to stretch the footprint, your sked is always going to be at the peak of elevation, for that fleeting moment when the satellite is a few degrees above the horizon.  That means that you're always going to be at zero doppler shift, and the math will always be the same.  Find yourself once at TCA on any pass, and lock them in.  The numbers will be the same for your sked.
> >> >
> >> > Good luck,
> >> >
> >> > Greg  KO6TH
> >> >
> >> >
> >> I wouldn't agree with that statement at all. Most of my long haul
> >> contacts on AO-7, FO-20 and 29, and now HO-68, are right after AOS or
> >> just before LOS, certainly not at TCA. Use my recent QSOs on HO-68 with
> >> Argentina as an example. Even when I work Europe on AO-7 it is at the
> >> beginning or end of a pass...not the middle.
> >>
> >> Bob, SatPC32 will show you the frequency with Doppler shift, and the
> >> Doppler shift. A little subtraction or addition and you have what you want.
> >>
> >> 73, Drew KO4MA
> >
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