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



On Monday 18 February 2002 14:19, Stacey E. Mills wrote in "[amsat-bb] 
AO-40: why did they have to rotate it?":
>>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...

Just to elaborate on a few other things Tracy might not know:

There is always *some* offpointing execpt for some (temporarily) lucky 
spot on earth at which the high-gains might happen to be directly 
pointing...the amount of off-pointing or "squint angle" is not the same 
for all points on the earth at the same time, especially near perigee 
when the satellite is closest to the Earth.

And Stacy alludes to the fact that the current configuration of AO-40 
is not the intended final configuration. Currently the satellite spins 
deliberately to keep it from tumbling, and to keep it pointed in a 
known direction. Magnetic coils onboard (called "magnetorquers") allow 
the command stations to influence the spin rate and change the 
direction the spin axis points in (described by the ALAT and ALON 
numbers), although the effect of the coils allows only small changes 
over a long period of time.

There is also a set of three electric gyroscopes called "momentum 
wheels" that, when spinning under the control of the onboard computers, 
should allow the satellite's attitude to be controlled much more 
easily. It's hoped that the spin on the satellite will eventually 
stopped, and control of the attitude given over to the intertial wheel 
system. The satellite will then be in a "three-axis stabilization" 
mode, allowing the control ops to select attitudes giving an optimum 
combination of sun angle and antenna pointing thoughout each orbit. 

Also, once the spin is stopped, the solar panels currently wrapped 
around the outside of the satellite can be unlatched and allowed to 
spread like the wings of a bird, allowing the entire set of panels to 
be pointed in more-or-less the same direction all at once.   

 73 de Maggie K3XS

-- 
-----/___.   _)   Margaret Stephanie Leber    / "The art of progress  /
----/(, /|  /| http://voicenet.com/~maggie   / consists of preserving/
---/   / | / |  _   _   _    `  _AOPA 925383/ order amid change and /
--/ ) /  |/  |_(_(_(_/_(_/__(__(/_  FN20hd / change amid order."   /
-/ (_/   '  K3XS  .-/ .-/    ARRL 39280   /___ --A.N.Whitehead ___/
/____ICQ 7161096_(_/_(_/__AMSAT 32844____/ <maggie@voicenet.com>

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