[amsat-bb] commutating antennas (was Re: Canted S-band antennas on satellite)

[Franklin Antonio <antonio@xxxxxxxxxxxx>]

At 01:52 PM 5/2/2002, Ronald Long wrote:
>On a spin-stabilized satellite like AO-40
>why couldn't we have multiple S-band transmit
>antennas mounted at different angles to the boresite.

Oh, you could.

This is related to something I have been advocating for awhile now.  AO-40 
(while in its present spin mode), like its cousins AO-10 and AO-13 before 
it, has the antennas pointing parallel to the the spin axis.  That causes 
the operators put the spin axis near the plane of the orbit.  That causes a 
geometric problem at some times where the sun has bad angles to the solar 
panels (which are on the sides).  It also makes the antennas point toward 
Earth during only one part of the orbit.

A different approach would be to put antennas on the sides, (perhaps 4 or 6 
sides), and let the satellite spin axis be roughly perpendicular to the 
orbit plane.  For a low inclination orbit (like AO-40's orbit), this would 
give good sun angles all the time.

Those side faces will be spinning past the earth.  One therefore needs 
multiple identical antennas, on each face (4 or 6 faces), and would need to 
electrically commutate between them.  An earth sensor and software phase 
locked loop would control the commutating so that the antenna that was 
pointed toward earth would be used at any given time.  This set of antennas 
and the software controlled switch would work together to make a commutated 
antenna that would point toward earth from all points in the satellite's 
orbit.

This way you get the benefits of 3-axis stabilization (ie antenna always 
points toward earth), without the difficulties of 3-axis stabilization 
(need for wheels, need for instantaneous earth sensors to control the 
wheels, thermal management difficulties, etc.)

For a satellite with a lot of different bands, like AO-40, this would be a 
big hassle, but for a satellite with lesser goals, with perhaps only one 
xmit band, this would not be difficult to do.


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