[amsat-bb] Doppler Correction?

John Papay john at papays.com
Sun Jul 24 19:00:21 PDT 2011

The purpose of full doppler correction is to keep your signal in
the exact same spot on the receiver in the satellite.  It has a finite
passband and if you don't correct your uplink, you move within that finite
passband.  Theoretically a single linear transponder can support many
conversations but it will not support the maximum unless everyone stays
put at the satellite receiver.  Those who don't doppler correct slide 
within the
passband and run into those who are staying put.  Of course if you adjust
your uplink so you stay put at the satellite, you have to adjust your
receiver to hear yourself because of the doppler shift in the other direction.
Both uplink and downlink require doppler correction in this case.

If you don't have a rig that can be computer controlled or if you are
operating portable and can't readily incorporate a laptop into your
setup for whatever reason, then by all means manually adjust the highest
frequency since that is the one that changes three times faster than the
lower frequency in a v/u satellite.  You simply do the best you can.

However, if you have a home rig that can be computer controlled, then by all
means use a computer to adjust your doppler.  Stay in one spot in the
satellite's passband.  You can make faster contacts since you only have
to pick up the mic to talk or use the key to send cw.  If you are working
dx and the window is short, being on frequency and staying put will help
put that distant grid in the log.

The program does all the work but you must be sure that your keps
are current AND your computer clock is accurate to the second.  On a
high pass when the satellite is overhead, the doppler changes very fast
and even a few seconds of clock inaccuracy will have you not hearing
yourself in a normal voice.  At lower elevations the doppler is not as
severe so a few seconds won't make much difference.

Use the HO-68 beacon to test your doppler correction.  Set your radio for
CW with a 500 Hz or better bandwidth.  Pick a high pass where the bird will
get up over 45 degrees.  Tune the beacon in at your AOS and then don't touch
the dial.  If the CW note stays constant over the entire pass, your system
is operating perfectly.  Make sure your program settings are such that there
are frequent frequency updates.  I have mine set at 1 second no matter what.
Some older radios will not accept frequency changes while you are 
transmitting.  In
that case, make sure you only key down for short periods of time so the
radio frequency can be updated.

Of course you must adjust your uplink calibration on SatPC32 so that you
hear yourself on the downlink.  Hearing yourself in a normal voice will
be an indication that you have your calibration set properly.

Most of the birds stay on frequency, but I've noticed that AO-7 Mode B will
be off by 1700Hz when it changes from Mode A to Mode B.  Normally we would
not observe this phenomenon but since lately we have had this mode change while
over North America we are able to experience it.  Within a few minutes, the
satellite is down to a 1200 Hz offset.  I'm not sure what happens on the next
pass because it is out of my footprint.  The next morning, however, the bird
is back to my normal setting.  This means the frequency drifts about 1700 Hz
back down to it's steady state value.  The other birds may have some slight
drifting, probably due to spacecraft temperature but it is minimal, less than
a few hundred Hz.  So when you are trying to make a contact on the first
AO-7 pass in Mode B, be prepared to readjust your uplink calibration so you can
hear yourself.  It may be as much as 1700Hz at the start.

In my opinion, everyone that can control their radio for doppler should do so.
Doppler correction gives us the maximum usage out of our linear transponders.
Why go through the agony of fiddling with your radio when your computer can
make your life so much easier.  You will never have to apologize for not being
on frequency.  If I wasn't able to have doppler control on my radio, I wouldn't
spend much time on the linear birds, unless of course there was a rare grid
to be worked. <grin>

John K8YSE  

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