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The Question

(Submitted June 13, 1997)

What are the conditions and requirements for a Sun-synchronous satellite orbit? To which orbits with which orbital parameters does this fit. Don't hesitate to go into mathematical details! Are there some references on the web?

The Answer

A Sun-synchronous orbit is an interesting orbit. Since the Earth is not an exact sphere (it has a slight bulge at the equator) satellite orbits can be effected by the extra gravitational pull. A Sun-synchronous orbit uses this feature of the Earth's shape. An orbit that is close to being polar (passing within about 1000 km of the North and South poles of the Earth) will be affected asymmetrically by the bulge at the equator. This asymmetry acts to slowly rotate the plane of the orbit about the axis of the Earth. When the inclination is chosen just right (about 8 degrees off the polar orbit) the motion matches the motion of the Sun across the sky. The plane of the orbit executes one full rotation about the axis of the Earth in one year. They are typically orbiting about 800-1000 km above the Earth.

Regards,

Padi Boyd,
for the Ask an Astrophysicist

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