[amsat-bb] Re: Turn off AGC when receiving BPSK-1000

Phil Karn karn at philkarn.net
Wed Aug 17 18:32:43 PDT 2011

On 8/17/11 1:36 PM, Mark L. Hammond wrote:

> In any event, Phil...THANK YOU for making this code real.  I have
> seen it print data when the signal was visibly "in the dirt" which is
> impressive and fun to see.

You're most welcome. It was a lot of work mainly because there were so
many options in the design that were hard to nail down. Nobody really
knew how fast the fading would be, or how long the data frames should or
would be, or how fast the telemetry values would change, or the
frequency and phase response of all the SSB receivers people would use
to receive the telemetry, or the accuracy of the A/D clocks in their
computers, or the type and speed of their CPUs, or the experience level
of the operators and whether they'd used automatic or manual tuning.

I did know that there would be fading, probably very deep. I knew the
orbit would be that of the ISS so I knew the passes would be short with
fierce Doppler. I knew that the beacon would operate in a broadcast mode
so latency wasn't a major concern.

So I went for a conservative, robust design. I didn't try to maximize
the data rate or minimize latency as I might have on a Pacsat being used
for interactive and store-and-forward user communications. Those things
might have made the signal much less robust especially by impairing its
fade resistance.

I think my emphasis on fade resistance has definitely paid off, but I'm
less happy with its tolerance (or lack thereof) of various frequency
errors, from Doppler correction to off-frequency A/D clocks in computer
sound interfaces. But that's one of the reason we fly these things, to
get that kind of experience for the next time.

Another thing this mission (and many previous amateur satellites) shows
is that the one thing we really, really need on our small satellites is
a good attitude determination and control system. One would make *so*
many problems just go away:

We could mount microwave antennas on a nadir-facing surface and provide
consistent, predictable, strong, wideband signals to ground stations
during a pass.

We could mount our solar panels on rotating booms to track the sun and
generate far more power from a given number of (very expensive) cells
than we now get by hedging our bets and covering every outside surface.

We could predict and control spacecraft heat flows and temperatures far
more easily.

We'd know where our cameras are pointing and we could take pictures of
predetermined targets.

The problem consists of two parts: attitude determination and attitude
control. For determination I keep thinking that we should be able to do
a lot with small, light and inexpensive CCD cameras. With proper light
baffling it should be possible to see stars even in the daytime, and
onboard software with a star chart could figure out which they are.

For attitude control, I think control moment gyros are the way to go.
(They're somewhat different from momentum wheels in that they operate at
constant speed.) This is largely a mechanical problem: designing
flywheels and motors that are small, lightweight, can store a lot of
angular momentum, draw minimal power, and be precisely moved around to
control the direction and magnitude of the overall spacecraft angular
momentum vector. We'd still need magnetorquing coils to dump excess
momentum, but the cm gyros would provide quick and accurate control of
spacecraft attitude.


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