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Re: commutating antennas (was Re: Canted S-band antennas on satellite)

>One approach is to only sense the position of the sun.  The software would 
>note the time that the sun goes by the sun sensor, and lock to the spin 
>rate.  Assuming a roughly circular orbit, if the software also knew the 
>orbit period and mean anomaly, then it could trivially compute the 
>direction of the earth.

Doesn't that assume that the spin axis is already oriented normal to
the orbit plane? The bigger problem is in first getting the spin axis
where you want it. Two numbers (RA/DEC or LAT/LON) define the
orientation of the spin axis in space, so you need two independent
measurements to determine it, e.g., sun and earth directions.

Once the spin axis is where you want it, I agree it's easy to figure
out where the earth is relative to the sun, given knowledge of the
orbit. Except when you're in eclipse, where the sun sensors don't work.

>Sun & earth sensors have operated on several AMSAT spinning spacecraft, so 
>nothing new about the hardware required, right?

I believe all the earth sensors flown on AMSAT spacecraft have
operated in the visible region, so the terminator (and eclipses)
complicate the measurement.  Infrared sensors avoid this problem, but
they're either expensive or require cooling, or both.

That's why I think it would be interesting to look into using the
uplink receivers as a radiometer to detect the earth. If the earth's
natural thermal emissions aren't strong enough, just transmit a homing
beacon from the ground. Or make use of all those L-band radars...


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