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

Phil Karn karn at ka9q.net
Mon Jul 21 01:36:06 UTC 2014

On 07/19/2014 09:23 PM, Robert Bruninga wrote:

> 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.

Run the link budgets. With reasonable numbers (transmit power 100 mW,
range 1,000 km, Rx T = 50K) you only need a few dB of transmit gain to
get megabits/sec into a 60 cm DBS dish at 10 GHz. A few dB can easily
cover an entire hemisphere, though you might want to squash the pattern
to cover it more uniformly (as the GPS satellites do).

The big reason to use attitude control isn't high on-axis antenna gain,
it's AVOIDING ANTENNA NULLS. Fading was the major factor in every one of
the modulation/coding schemes I've designed for AMSAT. It forces me to
use noncoherent (DBPSK) modulation, which even with FEC can cost as much
as 4-5dB over coherent BPSK. And that's WITHOUT fading.

Fading is a major headache in coding design because you need to know how
long a fade will last to know how long to make your interleaver, and you
simply don't know when the spacecraft is unstabilized.

At least I had a good idea with AO-40 because the fading was caused by
the spacecraft spin so it was quite predictable. But ARISSat-1 was
completely unstabilized and I didn't even have an antenna pattern, so I
basically had to pull a maximum fade duration out of my butt.

Same with FOX-1, although it's somewhat less severe in the
data-under-voice mode since it's the fade duration relative to the data
rate that matters, and the data rate in that mode is so extremely low.
But it's a serious problem in the high speed (data only) mode.

And for truly interesting data rates (hundreds of kilobits/sec and up),
slow fading is simply intolerable. Attitude stabilized antennas are the
only way.


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