[amsat-bb] Outernet L-Band now carries AMSAT and ARISS weekly bulletins

[Dani EA4GPZ]

El 14/03/18 a las 20:40, Daniel Cussen escribió:

> I would hesitate to recommend this system, as it is vapour-ware at the
> moment, and all the previous kits/hardware released have been dropped
> and made obsolete by changes to the broadcast. Previous broadcasts
> were 12Ghz geostationary, but decoded using a DVB-S tuner demodulator,
> then they changed to L-band low earth orbit immarsat and now they are
> back on geostationary, this time with a new modulation scheme making
> pointing easier. They are using the "LoRa" standard, basically just
> using a protocol that allows low signal margin decoding ( Chirp Spread
> Spectrum modulation (CSS) which trades data rate for sensitivity
> within a fixed channel bandwidth. ), similar to WSJT/PSK31 and other
> low data rate weak signal modes.
> 
> It is interesting they thing that a bare LNB (about 80 degree beam
> width) that seems to be doing the trick, although they also show
> pictures of patch antennas which presumably are designed for narrower
> beam width more suited to this.

Hi Daniel,

I also find it a bit hard to believe that the current Outernet goal can
be made to work: a 30kbps stream from a GEO Ku-band satellite that can
be received with a bare LNB or small patch antenna. This is not
necessarily impossible if you run the numbers, but its feasibility is
pretty borderline. I'll believe it when I see it working.

As I understand, one of the main issues they're having is co-channel
interference. This goes as follows: if you look at link budget alone
(free space path loss, the gain of an LNB and so on), maybe things can
look OK. However, in the real world what happens is that your LNB has a
wide beam, so you receive the signals from over a dozen different GEO
satellites. The signal you want to receive is now interfered by DVB-S
transponders (or other signals) from many different satellites and now
you have a problem (as before the main contribution of noise was the LNB
noise figure, and now you notice that the noise floor is much higher due
to interference).

This is not a problem when using a dish, since the beamwidth is rather
narrow and your dish only sees a few satellites at a time, so
interference is unlikely. But when you look at many satellites instead,
the spectrum is extremely crowded.

The fun thing about this story is that they claim that the are
experimenting with LoRA to fight co-channel interference (since LoRA is
spread spectrum). In my last talk about Outernet I commented that this
is nonsense and that they don't understand properly how spread spectrum
works.

If you think about it, spread spectrum (in comparison to a narrowband
signal) works very well against narrowband interference, but it doesn't
make any difference against wideband interference. In this case, the
co-channel interference is DVB-S and other wideband signals. For all
practical effects, they just look as an elevated noise floor and there
is no way to fight against them, spread spectrum or not.


Since the Outernet topic has come to this mailing list once again, I
take the liberty to remind you that while Outernet can be interesting
from the technical point of view, they have always kept secret their
modulation, coding, protocols and so on, and the key parts of the
receiver are closed-source. This is no good for Amateur Radio and
experimentation in general.

Thanks to my work and the help of some other people, now there is a
fully open-source receiver for the (now defunct) L-band signal, as well
as public specifications for everything. This was done by reverse
engineering, without any support from the Outernet team (which don't
seem to like this open-source receiver).

Some references:

https://github.com/daniestevez/free-outernet

http://destevez.net/tag/outernet/

http://destevez.net/talks/


73,

Dani EA4GPZ.