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



Thank you Stacey,

OK, I hear you say we have poor illumination twice a year 
approximately 6 months appart and precessing over time. 

Then can we expect that command must rotate the bird 
twice a year also and that we can expect bad squints 
because of this as a routine?

By the way, are all of you on the team retired?  How do 
you find the time to manage the bird and reply to our 
e-mails?  We certainly do appreciate it.

73's  Tracy K7KCS 
> 
> >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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