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

```At 09:34 AM 5/3/2002, Ken Ernandes wrote:
>The technical challenge is having accurate, real-time knowledge of which
>side of the spacecraft if facing the Earth.  I've been pondering

I think this can be done easily.

Note that this is a spinning satellite, and I've proposed a low
inclination, and spin axis roughly normal to the orbit plane.  That means
that simple sun & earth sensors can be made by putting a photocell in a
tube behind a slit pointing out one of the side faces.  The sun and earth
spin by once per rotation.

The more difficult attitude determination problem occurs in NONSPINNING
spacecraft, such as AO-40 in 3-axis stabilized mode.  (Damn near
impossible, in the case of AO-40 because it doesn't have attitude sensors
designed for 3-axis stabilized operation.)

We're only dealing with one dimension here.  I presume that the spin axis
is fixed over the short term.

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.  The precision requirements here are not
difficult.  Suppose we want to switch within 3 degrees of the correct
angle.  If we allocate half of the error budget for the error in knowledge
of orbit, then that's 1.5 degrees error in propagated mean anomaly.  That
seems like it might be workable.

If one were willing to sense the edges of the lit earth as well, then need
for precise knowledge of the orbit period and mean anomaly mostly goes
away.  The software still needs to know a little bit about the orbit,
because unfortunately the whole earth disk is seldom lit.  One edge of the
illuminated region is a real edge of the earth, and the other is
not.  Software needs to know enough to decide which edge of illuminated
region is the real edge of the earth and which is not.  One could just
sense the edge of the earth, and use a fixed offset from that as the
direction of the center of the earth.  If orbit is noncircular, then a
little calculation would be used to do something a little better than just
applying a fixed offset.

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

Is there something I've missed that you believe makes this difficult?

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