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Re: Preferred Phase 3 Orbits



>Why is a highly inclined orbit desirable for a Phase 3 satellite?

Several reasons:

A satellite operating primarily at high latitude (which a Molniya
orbit makes possible) allows a mechanically simpler spacecraft. Its
antenna beams can parallel to the solar panel surfaces and yet still
maintain good sun angles while the antennas are pointed at earth.

Spinning spacecraft in equatorial orbits generally require
mechanically despun antennas to track the earth. This adds cost and
complexity and can be unreliable. You also can't fully utilize your
solar array as only part of it is in sunlight at any moment.

The trend in commercial geostationary spacecraft is to 3-axis
stabilized bodies carrying the antennas and electronics, and external
flat solar "wings" that can be more fully utilized. But the arrays
still have to mechanically track the sun.

The AMSAT Phase IV proposal for geostationary orbit resembled P3D
except the antennas were mounted at right angles, and the solar arrays
did not deploy. This avoids the despin bearing, but it still has the
power penalty associated with a cylindrical solar array.

Visualize AO-40 directly over the north pole, with its solar panels
deployed and the antennas pointing directly at the earth. No despin
bearing, and no motors on the solar arrays; it can track the sun by
simply rolling on its Z axis.

The other main argument for a high latitude spacecraft is its ability
to provide direct communications between stations farther apart in
longitude than is possible with a geostationary satellite. The
argument is that because most of the earth's population lives in the
temperate latitudes of the northern hemisphere, a single spacecraft in
a northern Molniya orbit can better serve them than one in an
equatorial orbit.

Phil
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