[amsat-bb] Re: probably simple

Greg D. ko6th_greg at hotmail.com
Thu Jan 7 22:16:02 PST 2010

Hi all,

Here's my (updated!) answer:

I think you're going to need to
depend on a computer prediction program.  I use Predict on Linux, but
there is also a PC version.  Predict, at least, can tell you what the
doppler-adjusted up and downlink would be for a specific satellite-perceived frequency pair.  I have mine set
up for mid-bandpass up and down on the linear birds, i.e. what the zero
doppler values are for the satellite itself.  The program then tells me
what I should tune the radios to, so that the satellite hears my
signal at mid-bandpass, and I hear the return at the correct pitch. 
Then, if your sked is at some other frequency you'll need to add or
subtract to compensate.  For example, if the sked is set up for 10 khz
above mid-passband, I think you can just add 10 khz from your
computer-predicted uplink, and subtract 10 khz from the predicted
downlink (for an inverting passband), and be really close.  If the sked
is 10 khz below mid-band, subtract 10 from the up, and add 10 to the
down.  Both you and the other station will need to do the same thing,
both focusing on the satellite's perception of your signals.  Your
individual numbers could be very different.

I hope I've got it right this time,

Greg  KO6TH

Date: Fri, 8 Jan 2010 05:58:39 +0000
From: w7lrd at comcast.net
To: glasbrenner at mindspring.com
CC: amsat-bb at amsat.org; ko6th_greg at hotmail.com
Subject: Re: [amsat-bb] Re: probably simple

Hi Greg, Drew, etc

This conversation is starting to make my head hurt!  My example is to connect with some of our European friends.  Some of them I share maybe a 60 second window, with PA1TNO it is less than 30 seconds.  I have had several false starts in that I heard Paul 2E1EUB, by the time I located my downlink on his, he was gone.  I want to predetermine where to set my uplink for a given downlink.  There just is no the time be looking around.  Like I said this is probably simple I'm just have trouble getting my thinking around the concept.  Also if say I am planning on a downlink of 145.950 will Paul be looking at the same frequency?  In the past these concepts never concerned me as there was always plenty of time to yak it up with the locals on AO-7.

73 Bob W7LRD

CN87 Seattle, Wa.




----- Original Message -----
From: "Andrew Glasbrenner" <glasbrenner at mindspring.com>
To: "Greg D." <ko6th_greg at hotmail.com>
Cc: w7lrd at comcast.net, amsat-bb at amsat.org
Sent: Thursday, January 7, 2010 9:30:44 PM GMT -08:00 US/Canada Pacific
Subject: Re: [amsat-bb] Re: probably simple

> 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
I understand, but with all due respect your assumption is incorrect that 
long distance QSOs are always at TCA and zero Doppler, even 
theoretically . This is only the case when the two stations are at near 
right angles to the track of the satellite.

Use the following example. If I want to work LU5BOJ/O in FG75 from EL88 
on HO-68, my only windows are at either LOS or AOS, depending on whether 
it is an ascending or descending pass. Neither pass will be a low pass 
for either station, and neither QSO will occur at TCA or zero Doppler.

In Bob's case, let's look at his next possible window with Paul, 2E1EUB 
in IO92. Bob is in CN76. At the beginning of the 1 minute window 
tomorrow at 1251Z, Bob's Doppler shift on 432 is -3.68 khz.

On the next mutual window at 1452Z, the beginning Doppler is -7.93 khz. 
On the next, at 2020Z, it's -8.3 khz. None of these windows are over 2 
degrees elevation, and none are at TCA for either station.

When you have 60s to make the QSO, being right dead on frequency is 

Drew KO4MA

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