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



>The technical challenge is having accurate, real-time knowledge of
>which side of the spacecraft if facing the Earth.  I've been
>pondering implementations of inertial sensors to make such a
>determination.  Such a setup would, however, require a backup set of
>omni antennas for cases in which the switching electronics wasn't
>working correctly.

Why make it so hard? Chances are you're going to want spin-switched
receive antennas as well as transmit antennas to minimize required
uplink powers (L-band amplifiers being expensive, and all that).

So just pair the receive antennas with the transmit antennas, look for
the receive antenna with the most signal, and route it to its paired
transmit antenna. Voila, an L-band earth sensor.

If the receivers are quiet enough, you *might* even be able to detect
the earth's ~300K thermal emissions vs those of deep space as each
receive antenna sweeps by. So this trick might work even without any
uplink signals. This would despin the transmit antennas for the sake
of the beacon.

BTW, this scheme would not eliminate the need for attitude control, as
you'd still need to align the spacecraft spin axis to the orbit normal
for it to work throughout each orbit.  For low inclination orbits,
you'd put the solar panels on the same surfaces as the antennas,
parallel to the spin axis. To work year-round in a high inclination
orbit, you'd also need solar panels on the top and bottom surfaces,
perpendicular to the spin axis.

Phil



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