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RE: AO-40: why did they have to rotate it?

Thank you for the patient, clear, concise, de-jargoned answer.  It is always
nice to have things put forward in such a straightforward manner.  That kind
of response is what amateur radio/satellites are all about.

Ron, ag5rs

> -----Original Message-----
> From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
> Behalf Of Stacey E. Mills
> Sent: Monday, February 18, 2002 1:20 PM
> To: amsat-bb@AMSAT.Org
> Subject: [amsat-bb] AO-40: why did they have to rotate it?
> >Hi all,
> >
> >Can anybody point me to an article that explains why AO-40's
> antennas were
> >off pointed in the first place.  Something to do with the Sun but exactly
> >what?Maybe some drawings?
> >
> >Thanks and 73's,
> >                                 Tracy K7KCS
> I've heard it said on the AO-40 passband that we did it just to be
> mean!  However, it's really just straightforward geometry derived
> from two
> facts.
> 1. A spun object always points at the same spot in inertial space.
> 2. The earth goes around the sun once a year.
> Another way of stating the above is that, from the satellite's
> perspective,
> the sun appears to rotate around the satellite once a year.  Thus, if the
> satellite were to remain earth pointing at apogee (ALON/ALAT = 0/0), the
> sun would, during a full year, alternately illuminate the sides
> -> back ->
> sides -> front-> sides of the satellite.  Since the solar panels
> are on the
> sides, as the sun moves towards the front or back, illumination on the
> solar panels decreases to unacceptable levels and the satellite must be
> rotated.  We can generally tolerate solar angles to +/- 45 degs (90 deg
> total range) on the solar panels.  Therefore, during a year, there are
> roughly six months of good orientation (the two "good" 90 deg
> ranges), and
> six months of compromise (the two "bad" 90 deg. ranges).  If the
> inclination of the satellite is substantially out of the earth's orbital
> plane, which is not the case for AO-40, then the illumination situation
> becomes considerably better.  If the orbit did not precess, then
> the times
> of bad illumination, would be exactly the same each year.
> However, because
> the orbit slowly changes over time, the times of good/bad
> illumination will
> vary as well.
> You can visualize all this fairly easily with some simple drawings or a
> crude model on a desktop.  Also, if you search the amsat.org archives,
> there are some good articles by James Miller dealing with the
> illumination
> of AO-13.  This is exactly the same for AO-40 in its current
> configuration.
> --
>   ________________________________________________________________________
>   Stacey E. Mills, W4SM    WWW:    http://www.cstone.net/~w4sm/ham1.html
>     Charlottesville, VA     PGP key: http://www.cstone.net/~w4sm/key
>   ________________________________________________________________________
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