[amsat-bb] Re: Doppler Tuning

[Paul Williamson <kb5mu@xxxxxxxxx>]

I wrote about getting InstantTrack to display "NET Doppler shift" by 
just entering the frequency difference as the "beacon frequency". 
John AA1YE called me on an important detail I had glossed over:

>	You can do this for the downlink repeat of YOUR signal but not for the
>other station's signal. Unless he is located next door to you, his Doppler
>will not be the same as yours.

That's exactly right.

The "net Doppler" I was talking about is only useful for locating 
your own signal. With the aid of this kind of display, you can put 
your transmitter on the right frequency to answer a CQ or join a QSO 
or call on a clear frequency, without sweeping through the 
transponder. This kind of one-time coarse adjustment is about the 
most you can hope to do manually, based on a number displayed on 
InstantTrack's screen.

Predicting the other station's downlink frequency isn't so easy. You 
need to know his location and his transmitter frequency. Usually, you 
don't know either one, at least at the beginning of the QSO. There 
wouldn't be much practical use for a hybrid "net Doppler" calculation 
that combines the other station's uplink Doppler with your own 
station's downlink Doppler, even though that's what happens when 
you're listening to another station through a linear transponder.

If you're going to get fancy and go beyond a coarse manual 
correction, you'll probably want to go to full computer control, 
which should be designed to keep a constant frequency at the 
satellite. This technique doesn't require any knowledge about the 
other station, as long as he's doing it too. Of course, 
InstantTrack+InstantTune supports this.

I can't let this conversation end without plugging my old (1994) 
writeup on The One True Rule for Doppler Tuning. See 
http://www.amsat.org/amsat/features/one_true_rule.html if you haven't 
already read it.

73  -Paul
kb5mu@amsat.org
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