[Ground-station] Question for ORI:

[Douglas Quagliana]

All,

We're already got several solutions for compensating for Doppler. If
compensating for Doppler is a concern, I'll point out that there are ways
to remove the Doppler shift in software from the received signal.  SATNogs
already does this on their recordings, and there's at least one github
repository with code to remove Doppler using the TLEs (
https://github.com/cubehub/doppler - " Command line utility that takes IQ
data stream as input and produces doppler corrected output stream based on
TLE. Firstly it was written in C (last commit to C version
<https://github.com/cubehub/doppler/commit/e6df4d271ece09a88b8dba9b054bb10bdcb996ce>),
however now it is rewritten in rust <http://www.rust-lang.org>." )  SATNogs
removes the Doppler, and many of their ground stations are running a
Raspberry Pi and an SDR Dongle such as a RTLSDR dongle.  This combination
(Rpi+RTLSDR) is NOT expensive.  I've been able to take their FalconSat-3
9600 baud recordings and pull out hundreds from frames from a pass using my
9600 baud brute force demodulator.

As a second example, Phil's BPSK1200 downlink that we used on ARISSat
searched the receive bandpass for the signal, and if the signal was
present, then it was decoded and demodulated with minimal tuning.  This
along with a large interleaver meant that you could easily retune in the
middle of receiving a frame and the software would re-acquire the signal,
start decoding and demodulating again, and the forward error correction
would take care of any errors induced by retuning so you received the whole
frame without any errors.  If you were used to 100Hz retuning on the old
PACSATs, then this was completely different.

Bruce writes : " We should be evangelizing all Radio Amateurs *by our own
example*"
We have.  ARISSat.  FUNCube.  Fox's DUV (coarse FM tuning is fine for DUV
as long as you are receiving the FM signal).  I was able to receive the
AO-40 beacon at apogee, with many errors corrected, with just an S-band
patch antenna cut out from the top lid of an Altoid's tin. This says more
about the forward error correction than the patch antenna.)  We have solved
this problem already multiple times.  We should be shouting "Here's several
ways to do it!" and do the shouting where we will be heard by the builders
and designers.

73,
Douglas KA2UPW/5





On Thu, Apr 1, 2021 at 11:04 AM Bruce Perens via Ground-Station
<ground-station at lists.openresearch.institute> wrote:

> On Thu, Apr 1, 2021 at 8:30 AM Pierros Papadeas <pierros at libre.space>
> wrote:
>
>> We, as radio amateurs, need to seriously take a strong stance against
>> LoRa unless Semtech is willing to lawfully allow an open SDR-based
>> complete implementation of LoRa PHY.
>
>
> We should be evangelizing all Radio Amateurs *by our own example* not to
> accept hood-welded-shut technology. That is why we formed ORI in the first
> place, why we worked on Codec2, why we successfully insisted that we should
> be able to receive PACTOR I through IV so that we could police our own
> airwaves without paying for an expensive device. We must be able to make
> our own implementations without restriction, or we should not be using the
> technology.
>
> And from a practical basis, being constrained by intellectual property law
> from being able to make a flying version that can work with cheap ground
> chips successfully seems to be a pretty big technical problem. We would
> have to deal with the problems that the ground stations don't: compensating
> for doppler on both the uplink and downlink, improving S/N over the cheap
> implementation, etc.
>
>     Thanks
>
>     Bruce
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openresearch.institute/pipermail/ground-station-openresearch.institute/attachments/20210401/0eea7a4a/attachment-0001.html>