[Ground-station] Satellite program

[Phil Karn]

I won't be at Dayton this year, but I do have some thoughts.

Michelle is absolutely, positively 100% on the right track in developing
digital microwave ground station equipment. That's the ONLY possible
future for the amateur satellite service; unfortunately, it's been
impossible to convince AMSAT of this fact. They'll probably keep flying
analog 2m/70cm FM repeater satellites until the last member dies off.

I do *not* assume most or even *any* of our satellites will be
geostationary orbit; all or nearly all will be in low and relatively
short-lived LEO orbits of the type most commonly used by cubesats. If
our opportunity actually materializes to fly a payload on an upcoming
GEO platform, great. But I ain't betting on it, nor do I think we'll get
many more opportunities like that in the future. OTOH, we do know there
will be many opportunities to reach LEO.

So the usability of any digital microwave satellites we build implies
developing two closely related items of technology: active spacecraft
attitude determination and control (ADaC), and accurate real-time
antenna pointing on the ground.

You can buy complete off-the-shelf ADaC subsystems for cubesats, but
they are very expensive. Much more expensive than I think they should
cost, but maybe there are good reasons. Momentum wheels seem necessary,
along with torquing coils and magnetometers. I am familiar with the
latter two items but space qualified momentum wheels seem like a
mechanical engineer's nightmare -- and I'm an EE, not a Mech E.

AMSAT-type stations have long used az/el rotors to point VHF/UHF arrays
to LEO satellites. In principle they could be used for microwave
antennas but I don't think they're very well suited for the purpose. For
one thing, they're oversized for the job of pointing small (< 1m)
dishes. For another, they're very expensive. For a third, they aren't
designed for precise pointing. We're going for a much larger user base
than AMSAT has traditionally had; that means small, inexpensive and
reliable. (Do we only get to pick two?)

I am not sure what's already been discussed here about reproducible, low
cost ground station antenna pointing systems. I do know a very little
about the SATNOGS project but their design seems rather complex. This
will be a key part of the project so easy reproduction is essential.
Again, this seems more in line with a mechanical engineer's skills than
my EE skills so I'm wide open to ideas.

73, Phil