[amsat-bb] ANS-199 AMSAT News Service Special Bulletin - AMSAT Fox-1C Launch Opportunity Announced

[Phil Karn]

On 07/19/2014 01:42 PM, Andrew Glasbrenner wrote:
> 
>> Getting a launch opportunity is difficult and expensive. Going digital is
> not.
> 
> The Fox satellites each have four designated experiment cards. Assemble a
> team and pitch a proposal for 1D, or 1C if you can do it in a hurry. 
> 
> 73, Drew KO4MA
> 

I see no reason to consider a digital satellite an "experiment". Not in
2014, anyway.

Besides, there are several other critical technical hurdles AMSAT must
first overcome if we are to do anything interesting with a cubesat.
First and foremost among them is

ATTITUDE CONTROL!

Passive bar magnets (or nothing at all, as on ARISSat-1) no longer cut it.

The lack of attitude control forces us to use simple omnidirectional
antennas, which in turn keeps us on the crowded and narrow VHF/UHF
bands. Worse, there's really no such thing as an "omnidirectional
antenna" so our links are plagued by frequent deep fades of unlimited
(or at least unknown) duration. Fading has driven every one of my
modulation/coding designs for AMSAT telemetry links -- at the expense of
making them much less power-efficient.

And "power efficient" means smaller ground antennas, and that means a
cheaper and more accessible ground station for the average ham. And THAT
means a much larger potential AMSAT membership.

With attitude control, our satellite could use directional antennas on
the microwave bands. Directional antennas on higher frequencies mean
much better link budgets. You could easily do MEGABITS PER SECOND
between LEO and small ground antennas!

A lack of attitude control also plagues thermal design. I learned this
from Dick Janssen KD1K's talk at the Symposium a few years ago when he
said Fox-1 was the most difficult thermal design he'd ever done for
AMSAT. And he's done many.

The problem is that without attitude control you have to cover every
available surface with solar cells to guarantee power in any attitude,
and solar cells are excellent thermal radiators. Those not facing the
earth or the sun face dark sky, radiating away much of the spacecraft's
heat. I think he found the equilibrium temperature of a 1U cubesat to be
something like -30 or -40 C! The team had no choice but to add
resistance heaters to keep the payload (especially the batteries) warm.
I can't think of a more painful use of scarce, hard-earned, expensive DC
power in space.

Attitude control would completely solve this problem too. In LEO the
most common form of attitude control is "local vertical/local
horizontal", the mode the ISS uses most of the time. You'd designate one
face as nadir-pointing; here you would mount antennas and earth
observation cameras. With circularly polarized antennas, you'd still
have a degree of freedom around the yaw axis, i.e., you could rotate
around the local vertical with no effect on the RF links.

The opposite (zenith-facing) side would have a solar cell, as would one
of the four remaining sides. The remaining three would be covered with
thermal blankets to insulate the spacecraft from the dark sky they'd
face all the time. (Not being a thermal designer I haven't calculated
what the new equilibrium temperature would be. But it would obviously
solve Dick's problem. He might even have to leave part of those surfaces
exposed to radiate excess heat!)

You see how this works? At low beta angles, when the sun is in or near
the orbital plane, you point the side with the solar cell forward at
local sunrise. At local noon you yaw 180 degrees so the cell faces the
sun as it sets behind you. At higher beta angles you simply yaw to track
the sun as it passes off to one side; in the limiting case of a beta
angle of 90 degrees (continuous sunlight) you'd simply keep the
spacecraft yawed at a constant +90 or -90 degrees.

And you only need to buy two expensive solar cells instead of (nearly) six.

Attitude control systems for cubesats already exist. You can buy one for
$200,000 from suppliers serving the burgeoning cubesat community (most
of which considers ham radio completely irrelevant except as a source of
free spectrum). But that's not the AMSAT way. I am sure that if we
brought enough clever minds together (along with some good mechanical
engineers and craftsmen) we could design and build our own attitude
control system for far less.

But to attract all those people to do all these new (in AMSAT) things,
you have to stop doing the same old thing over and over. You have to
make a conscious choice to stand back and actually innovate at the
systems level instead of relegating digital to the status of an
"experiment" on the same old primitive spacecraft bus.

Look at all the attention and excitement generated by the ISEE-3 Reboot
project. That's because they're actually doing something new and
challenging. They say they've gotten donations from many people far
outside the usual "space groupie" segment of the population. It's also
caught the imagination of at least a few hams, including the AMSAT-DL
gang which has been using their 20m Bochum dish to receive telemetry.
And me, as I wrote the software they're using to demodulate and
FEC-decode the ISEE-3 signal.

Pose a really interesting technical challenge and make it clear you
really want it to happen, and the necessary technical volunteers will
come out of the woodwork. Do the same old thing over and over, and
you'll find it hard to even pay people to do the work.

--Phil