[amsat-bb] Re: Doppler Correction?

Michael Schulz mschulz at creative-chaos.com
Mon Jul 25 06:30:37 PDT 2011


Very good explanation and after that I played with it a bit more last night and got it working almost fine
as you can attest when we had a QSO on VO-52. So I stand corrected, and will from now on use full 
doppler computer control whenever possible :).

73 Mike K5TRI

On Jul 24, 2011, at 9:00 PM, John Papay wrote:

> 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>
> 73,
> John K8YSE  
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