[amsat-bb] Re: FM sat frequencies?

Bob Stewart bob at evoria.net
Mon Sep 10 11:40:37 PDT 2007

It's pretty clear from tracking the doppler effect during
multiple passes that these frequencies are not correct.
I believe I read somewhere that AO-27 was being quoted
at 436.797, which is pretty much what my measurements
showed.  Of course, my measurements are going to be
off by however much my old FT-726R is wrong.  But I
have reason to believe that's less than 1kHz.

Here's how I'm coming up with my figures:  In the gpredict
program, there is an area where the doppler shift is
calculated.  As posted recently, I hacked this to show
the actual frequency that should be on my dial.  I added
the following line just after the doppler was calculated.
I also added an if statement to select a specific satellite,
but that is irrelevant to this discussion.


In the program, the doppler is calculated in hz for a 100mHz 
signal.  My idea was to simply rescale that to 436.7950 by 
multiplying by 4.367950.  I need to move the decimal point
over two more so that I can display the 100kHz and down
frequency readout, so I divided by 100.

The problem was that the results were wrong by different
amounts for the three satellites.  The only way to get
the right figure was to start moving the satellite's implied
frequency.  e.g.:


The result from this calculation tracks SO-50 pretty well, so
unless my idea is fundamentally wrong, I believe that SO-50
is transmitting near 436.7920.


--- John Kopala <jkopala at gmail.com> wrote:

> The frequencies given for each of the easy satellites are:
> AO-27    436.795
> SO-50    436.795
> AO-51    435.300
> AMSAT gives the details for the individual satellites and their uplink and 
> downlink frequencies.  Go to the following site and then simply click on the 
> satellite you are interested in. 
> http://www.amsat.org/amsat-new/satellites/status.php
> With doppler shifts as the satellites make a pass the actual receive 
> frequency could vary by as much as about 10 KHz plus or minus the nominal 
> frequency.  The only time you would ever receive the satellite exactly on 
> its downlink frequency would be when the satellite heading is exactly 90 
> degrees from your bearing to the satellite.  At that point the satellite is 
> tangential to a circle whose center is your location and is neither moving 
> toward or away from your location.
> I am using an IC-910H.  For manual tuning at the beginning of a pass, I 
> simply tune about 5 to 9 KHz higher (depending on the elevation of the pass) 
> than the nominal frequency.  With the AFC turned on, the radio will 
> automtically lock into the signal when it becomes strong enough and track 
> the frequency as the doppler shift changes the receiving frequency during 
> the pass.  At loss of signal, the receive frequency will be below the 
> nominal frequency by about the same amount it was above at the start.
> John Kopala
> N7JK
> > Date: Sun, 9 Sep 2007 19:44:15 -0700 (PDT)
> > From: Bob Stewart <bob at evoria.net>
> > Subject: [amsat-bb]  FM sat frequencies?
> > To: Amsat-BB <amsat-bb at amsat.org>
> > Message-ID: <869490.76831.qm at web909.biz.mail.mud.yahoo.com>
> > Content-Type: text/plain; charset=iso-8859-1
> >
> > Does anyone know the actual downlink frequencies of the
> > easy sats?  From messing around with my hack for gpredict,
> > I get the values below, but it would be nice to know the
> > correct values as a check against what I'm doing.
> >
> > AO-27     436.7972
> > SO-50     436.7920
> > AO-51     435.3008
> >
> > Bob - AE6RV
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