Page 58 - Intro to Space Sciences Spacecraft Applications

Orbital Principles
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Pacific oceans could allow direct communications between countries on
either side or ships upon the seas. Many more uses exist, and the popu-
larity of this operating position is producing a crowded area in space.

Polar Orbits

Strictly defined, polar orbits are orbits with an inclination of 90” which
would pass over the earth’s poles each orbit. In actual use, many orbits with
inclinations near 90” which pass over earth’s higher latitudes are also
called polar orbits. The benefit of such an orbit, besides being able to view
the higher latitudes, comes from the fact that the orbital plane is fixed and
the earth rotates continuously beneath this plane. If the orbital period is a
nonintegral multiple of the sidereal day, then eventually all areas of the
globe will pass beneath the orbiting satellite. Of course, integral multiples
of the sidereal day can allow a satellite to pass over the same point on the
surface at regular intervals. This type of orbit is used regularly by satellites
gathering information about the earth and its environment and resources.

Sun-synchronous Orbits

The sun-synchronous orbit can be considered a special type of polar
orbit. Our discussion of orbits so far has ignored the small, but present,
perturbing effects that affect the motion of an orbiting satellite. In most
cases, these perturbations are accepted or simply compensated for by
small station-keeping propulsion systems on board the spacecraft. In the
case of a sun-synchronous satellite, though, one of these perturbing forces
is used to produce a desired effect.
As depicted in Figure 2-9 (greatly exaggerated), the earth is not a per-
fect sphere, having more mass at the equator than at the poles. For a satel-
lite in an orbit inclined greater than 0” but less than 90”, this situation
imparts a force that attempts to “pull” the orbital plane toward the equa-
tor. Due to gyroscopic properties, though, this force results not in a change
in orbital inclination, but in a precession of the orbital plane around the
equator as indicated in the figure.
The effect of the perturbing force depends on the satellite altitude and
the inclination of the orbit. A combination of these two can be arranged to
produce a rotation of the orbital plane around the equator equal to about