🚀 go-pugleaf

In article <ewright.726175018@convex.convex.com> ewright@convex.com (Edward V. Wright) writes:
>In <1993Jan1.165738.24729@ke4zv.uucp> gary@ke4zv.uucp (Gary Coffman) writes:
>
>>Redundancy is always desirable if it's affordable, but there is a practical
>>difficulty with having *cold* spares in orbit. Will they work when we need
>>them? 
>
>A cold spare is unlikely to be completely cold.  More likely it will
>operated in a low-power mode so that you can still run diagnostics
>and self-tests on the hardware.
>
>>They have to be cold spares if they share the same orbital slot and
>>frequencies. 
>
>Not true.  You can put two birds in the same slot but only use
>half the transponders on each one.  If the transponder frequency
>X transponder fails on bird one, you switch to the frequency X
>transponder on bird two.  If bird one fails entirely, bird two
>can take over the entire service til a new backup is launched. 

That might work, but you'd have to swap transponder *pairs* over
from one bird to another due to frequency reuse on opposite
polarizations. Trying to keep two independent satellites perfectly
crosspolarized would be quite a strain on stationkeeping. The
transponders also have their local oscillators phaselocked together 
and that would be tougher with two separate birds.

I think the worst problem though isn't really technical. That's
the waste of 50% of each satellite's capacity while it's still
fresh. The longer the birds remain in orbit, the more radiation
damage their solid state electronics accumulate, the more degradation
in output of the solar cell arrays, the more aging on the batteries,
and the more stationkeeping fuel expended. Keeping the spare in another
slot allows all of each satellite's capacity to be used from the 
beginning, with secondary services subject to being "bumped" should
a primary service fail.

That's the way NBC is using K2 now. The primary feeds (east and west
coast) are on K2-1 and K2-9 respectively. Less critical news feeds
are on K2-7 and K2-13. They can reconfigure the prime network to the
alternates without repositioning the Earth stations by a single software
command transmitted to the Earth stations. If the bird fails in a way
that loses all transponders, they have bumping rights on SBS 3 and 
Westar IV. That requires repositioning of all the Earth station antennas
though. About half the network has secondary antennas at the Earth
stations that are normally kept pointing at one of the backup birds,
though they can be individually pulled off to other targets as need
arises.

Gary
 
-- 
Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | uunet!rsiatl!ke4zv!gary
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