[amsat-bb] DopplerPSK

aflowers at frontiernet.net aflowers at frontiernet.net
Thu Jun 13 02:58:05 UTC 2019

 Scott, Roy, Umesh, et al;
Part of the problem with PSK31 is that it required the adjustment for changes in Doppler shift to be to be phase-continuous.  I think the Doppler rate (delta-Doppler shift) at even 10m is too fast for most radios--what you often get with CAT control, even if you can update while transmitting, is PSK31 that jumps more than 31Hz per update, and that totally destroys the phase continuity required to tell a '1' from '0'.  This was the "fun" problem that spurred me to take a swing at it over a summer vacation.
When DopplerPSK applies "anti-doppler" on the transmitted signal, it does so at the baseband audio level.  This works fine for 3Khz passbands on a 10m uplink, but if you want to do it with SSB transponders and VHF and UHF you'll need an SDR where you aren't limited by an IF filter.   The same algorithm in DopplerPSK could be used on a complex signal in the SDR architecture, and it could work equally on TX and RX in the processing chain.   To the accuracy of the orbital elements everyone uses, if everyone corrects their uplink and downlink properly there is no perceived frequency drift by anyone using the satellite.  Just what you need for a GHz-range LEO with a narrow-band transponder, eh? 
Since the downlink of NO-84 is FM there is no Doppler shift in the baseband audio of the downlink--you hear what the satellite hears.  If we get a mode-K satellite it might be worthwhile to do a baseband demodulator in a soundcard since the total change in Doppler shift will be small enough to fit in a conventional HF radio's IF passband in both directions.
If the antiquated hosting site doesn't come back I'll get off my duff and find a "proper" place for it.  It's nice to see people playing around with it. (Thanks for bridging the gap in the meantime, Peter.)
Andy K0SM/2

    On Wednesday, June 12, 2019, 9:24:12 PM EDT, Scott via AMSAT-BB <amsat-bb at amsat.org> wrote:  
 While it might certainly be possible to put together a solution that would
compensate your uplink so that its corresponding downlink through the
transponder holds stationary on a single spot on the downlink audio
spectrum, there are two reasons (at least) that I can think of to not go to
that trouble:

#1, the DopplerPSK app takes care of it perfectly already.  The work has
been done.

#2, if memory serves, some radios do not respond to CAT frequency
adjustments when transmitting.  In this mode, your uplink radio is
transmitting non-stop for the entire ~10 minute pass.  If you have a
hardware/software setup that can only change RF frequency when in receive
mode, that will never happen during an NO-84 pass.

Lastly, like much satellite work, the uplink and downlink are two
completely separate discussions in this mode.  While the 10m uplink works
extremely well with the DopplerPSK software and the HF radio of your
choice, receiving & demodulating the PSK31 downlink can be done by any
antenna/FM radio(or SDR), and computer combination of your liking.  Just as
one example, my setup is:

TX:  Windows PC > DopplerPSK > Signalink > FT-857d > MyAntennas EFHW-8010

RX:  70cm X-Quad > SDRPlay RSP1 > HDSDR on Windows PC (doppler track w/
Orbitron) > FLDigi

-Scott,  K4KDR


On Wed, Jun 12, 2019 at 8:20 PM k6vug at sbcglobal.net <k6vug at sbcglobal.net>

> I'm very curious about this, thanks for sharing the links.
> Looks like the program adjusts the PSK modulation within the pass band of
> the uplink, and at 10 meters, that would be relatively small.  It makes
> sense, however would this work if SatPC32 manages the uplink ?
> Secondly, I came across this on the website:
> "DopplerPSK does not provide any PSK31 demodulation; you should use a
> separate program for that purpose. DopplerPSK does not provide any
> compensation for downlink Doppler (which you would need for fully linear
> transponders)."
> Question 2 - why not use something like HRD for everything (doppler, psk,
> rotator, logging, etc) ?
> (I believe I tried it sometime back by routing of the downlink audio back
> into HRD)
> 73!
> Umesh
> k6vug
> On Wednesday, June 12, 2019, 8:10:11 AM PDT, Scott via AMSAT-BB <
> amsat-bb at amsat.org> wrote:
> I originally used the link on 2M0SQL's excellent how-to page:
> https://www.2m0sql.com/2018/10/29/using-the-no-84-psat-psk-transponder/
> ... which is:
> http://www.frontiernet.net/~aflowers/dopplerpsk/dopplerpsk.html
> If that link isn't available, you're welcome to the copy I downloaded which
> has just been put at:
> https://www.qsl.net/k/k4kdr//downloads/DopplerPSK_0.2.zip
> -Scott,  K4KDR
> ==========================
> On Wed, Jun 12, 2019 at 10:53 AM Roy Dean via AMSAT-BB <amsat-bb at amsat.org
> >
> wrote:
> > With all this talk of NO-84, and me finally having a semi-operational
> > ground station - I went looking for DopplerPSK, but every link I found
> was
> > broken.  Where can I get this software?
> >
> > --Roy
> > K3RLD
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