[amsat-bb] Re: Question about doppler correction software

[OZ1MY]

Yes I think Paul is right :-)
Some satellites change frequency with temperature and
for other reasons.
What comes into the old mind is AO-07.
SO-50 even if it is a FM transponder satellite, jumps
5 kHz every now and again. Right now 5 kHz up on the
downlink.
Therefor is important to be able to adjust frequencies
in the program in an easy way.
And yes again - full doppler tracking is very nice and the
right way to do it.
73 OZ1MY
Ib
----- Original Message ----- 
From: "Paul Williamson" <kb5mu at amsat.org>
To: <amsat-bb at amsat.org>
Sent: Friday, August 31, 2007 7:25 PM
Subject: [amsat-bb] Re: Question about doppler correction software


> > If I can tune the transmitter such that the
> >frequency at the satellite is one of the listed frequencies, how sure can
> >I be that the receive is as published?
>
> Not very sure at all. The published frequencies are usually nominal, and
satellite designers don't make extraordinary efforts to match them exactly.
What's worse, satellite designers also don't make much of an effort to make
the oscillator(s) completely stable, so there's some drift to contend with.
Both issues vary from satellite to satellite, and some future satellites
(Eagle) are supposed to be better. And of course the user's own rigs can
have the same issues, though with modern radios in benign hamshack
environments this is not a big problem.
>
> You need to allow the user a way to adjust the conversion constant. As a
minimum, this should be something like a configuration file entry. Ideally,
there would be a nice easy way to do it from the radio's front panel by
tuning the receiver manually to match the downlink and then telling the
program to save that value.
>
> If you really want to get fancy, use a soundcard interface to transmit a
test signal through the satellite and determine the frequency error on the
received signal. Just be aware that a naive implementation of this scheme is
going to be fragile, so you will want to use every trick. You'll also want
to use every trick to make sure your transmissions are very short and as low
in power as possible, to avoid annoying other users.
>
> It would be a nice touch if your resulting database of satellite frequency
information is in a format that could be shared from station to station, so
a new user would have a good starting point for his own configuration file.
You can provide a default config with your software.
>
> >Can I extend that to the point where I would be able to automatically
> >determine frequencies (at the satellite) of the downlink for a given
> >uplink and then compute doppler from there?
>
> Yes, in theory, except for the spacecraft oscillator imperfections
mentioned above. And that will be enough if every operator does it. But
every operator won't, so you need something much more flexible. You want the
user of your software to be Super Operator who's never confused and always
on frequency but you don't want to insist that every other operator live up
to those standards. So you need a nice, easy way for your user to
interactively adjust things and follow around the other operators on the
passband.
>
> Longtime readers will anticipate that I'm now going to plug my old essay
on the subject: http://www.amsat.org/amsat/features/one_true_rule.html -- 
even though it hasn't been updated with a discussion of any of the newer
software options for Doppler tuning.
>
> 73  -Paul
> kb5mu at amsat.org
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