[amsat-bb] DopplerPSK

[Pedro Converso]

Hello Phil,

Nice application for SDR !

You can also use for check these satellites that emit continuous carrier.

http://amsat.org.ar/pass?satx=lo-19,co-55

73, lu7abf, Pedro

On 6/19/19, Phil Karn via AMSAT-BB <amsat-bb at amsat.org> wrote:
> On 6/12/19 19:58, aflowers--- via AMSAT-BB wrote:
>> 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?
>
> That's exactly what I do in the SDR I've been working on. It uses a
> Funcube dongle and Raspberry Pi (or any general purpose Linux or OSX
> system) and applies an optional frequency rate to the digital local
> oscillator. The Funcube samples at 192 kHz, so you have a lot of room.
> Right now it can do a quadratic phase function, i.e., it can sweep at a
> constant rate in frequency, which is a constant acceleration with
> continuous phase and frequency. (I'll have to think about which order
> PLL this corresponds to.)
>
> It would be easy to extend this to higher orders, but because you can
> update everything very rapidly (it's all in software) I've seen no real
> need to do so.
>
> Originally the SDR read the velocity data from a pipe, but more recently
> I've changed over IP multicasting for everything. A completely separate
> program, which can be on a different computer, multicasts the tracking
> data to the LAN and the SDRs (there can be more than one) pick it up and
> make the frequency computations based on the current frequency.
>
> It all works very well. I tested it on one of the older Japanese
> cubesats that transmits a CW beacon on 70 cm. At first I thought I had a
> bug in my program because the CW shifted every time it started. Then I
> realized it was just a soft power system in an old satellite; the beacon
> remained rock steady in a 200 Hz CW filter from horizon to horizon.
> Naturally this requires accurate elements and clock time. It was a
> little weird hearing miscellaneous spurs sweep through the passband as
> the signal stayed in the middle.
>
> All said, PSK31 still isn't the best choice for a LEO satellite with a
> lot of Doppler...
>
> Phil
>
>
>
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