[amsat-bb] ANS-199 AMSAT News Service Special Bulletin - AMSAT Fox-1C Launch Opportunity Announced
bruninga at usna.edu
Sun Jul 20 04:23:00 UTC 2014
> 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.
Yes, but with who? 95% of everyone in view is more than 45 degrees OUT of
the main beam. Directional antennas have zero value on LEO birds that need
to serve everyone in view at the same time. And if you only serve those in
the main beam, then the duration is under 1 minute.
> A lack of attitude control also plagues thermal design....
> I think he found the equilibrium temperature of a 1U cubesat
> to be something like -30 or -40 C!
I cannot believe that. The equilibrium of a nominally black (solar panels
on all sides) spacecraft is something like about 0 to 30 C (32F to 90F) a
very benign operational range. The only time you DO have thermal issues is
when you DO have attitude control and have things that are not equally over
time seeing the sun and dark sky.
PCSAT is now 13 years in orbit and the above range is what it sees. And
that range is over a 2 month period. THe orbit-by-orbit temperature
changes are less than +/- 10 degrees C. The extremes are due to the
"seasons" of the orbit. When it is seeing eclipses it averages to about
10C and when it is in full sun for weeks at a time, it gets up to 30 C (90F)
I do agree that attitude control is nice to have, but my point is that it
only makes the thermal problem much worse and that "gain" is of no value
for a LEO where it must see everyone in a footprint at the same time.
(Remmebr you cannot have wide beamwidth and gain at the same time).
Gain for HEO's of course is another matter! (think AO-10, AO-13 and AO-40)
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