[Ground-station] LoRa on our GEO sat ?

[Phil Karn]

To paraphrase a favorite saying, "The great thing about modems is that
there are so many to choose from".

But there actually are very good reasons for having so many modems. Some
links require the highest possible radio energy efficiency, others have
to fit narrow bandwidths, and still others have to be small and cheap.
You can't have all those things at once, so you have to choose.

LoRa was designed for IoT devices so it's in that third category: small
and cheap. This comes at the expense of both energy and bandwidth
efficiency, which are much more important on satellite links.

Fortunately, modems designed specifically for satellite links are now a
lot smaller and cheaper than they used to be (if still not as small and
cheap as LoRa). One is DVB-S, which stands for Direct Video Broadcast -
Satellite. As the name implies, it's designed specifically for a
satellite downlink, which traditionally requires the best possible
energy efficiency. DVB-S actually gives you a whole set of options to
trade between bandwidth and power because many direct broadcast
satellites are now even more limited by bandwidth than in power.

For its downlink the groundstation project chose DVB-S2X, the latest
revision of DVB-S. I think it's exactly the right choice; not only are
the applications very similar, but DVB-S is already widely used in
relatively inexpensive consumer devices so there's already a good supply
of the necessary parts.

(The uplink is a different story.)

Phil