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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 currrent 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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