[PLUG-TALK] DBS eclipse

Keith Lofstrom keithl at gate.kl-ic.com
Mon May 7 02:33:02 UTC 2012


On Sun, May 06, 2012 at 09:11:12AM -0700, Denis Heidtmann wrote:
> On Sun, May 6, 2012 at 12:39 AM, Russell Johnson <russ at dimstar.net> wrote:
> 
> > ... Even when you are in the shadow, there's still a lot of light....
> > Russell Johnson.
> 
> 
> I wonder.  In the absence of scattering due to atmosphere, how can there be
> much light in the direction of the sun?  In the direction of the earth is a
> different story, but I would doubt the collectors are pointing toward the
> earth.  I expect the question is how long would a satellite be in full
> shadow in the case when it is in the path of a moon shadow.

Moon eclipse:
As always, "it depends".  The moon is 3480km in diameter, and moves
at 1km/sec .  A GEO satellite moves at 3070m/s.  If both are on the
same side of the earth, and the moon is at the same ecliptic angle,
the velocity difference migh be about 2.1km/s, so the transit time
behind the moon, from half-penumbra to half-penumbra (and thus 1/2
solar array power), would be about half an hour.  If the relative
inclinations are different, the shadow time and depth will be less.
This will happen at oddball times all year.

Earth eclipse:
Much bigger effect.  In the spring and fall, because the ascending
or descending node of the orbit points away from the sun, the
satellite will spend up to 70 minutes (2*6378)/(3.07*60) between
half-penumbras.  And it will do so around midnight, once a day,
for about two weeks around the equinoxes.  

Behaviorally, much depends on how well the satellite can handle a
partial powerdown.  I imagine it would have to shut down most of
the transmit channels in half shadow, and all in full darkness.
The satellite uses battery power in complete darkness, so it can
continue to receive commands and send telemetry.  There are
probably portions of the satellite (like frequency references)
that must be kept heated, because without power and sunlight
some modules will cool down fairly quickly.

The solar cells go through deep thermal cycling (perhaps 100C),
so all the thousands of little welds that hook them together
will get stressed. 

Keith

P.S.: Current reading:  Handbook of Geostationary Orbits by
E.M.  Soop, 1994, borrowed via interlibrary loan from Embry
Riddle University in Prescott Arizona (where it hasn't been
checked out since it was purchased 17 years ago).


-- 
Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs



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