ORBITAL PERTURBATIONS
Theoretically, an orbit described by Kepler is ideal as Earth is
considered to be a perfect sphere and the force acting around the Earth
is the centrifugal force. This force is supposed to balance the
gravitational pull of the earth.
In reality, other forces also play an important role and affect the
motion of the satellite. These forces are the gravitational forces of Sun
and Moon along with the atmospheric drag.
Effect of Sun and Moon is more pronounced on geostationary earth
satellites where as the atmospheric drag effect is more pronounced for
low earth orbit satellites.
As the shape of Earth is not a perfect sphere, it causes some variations
in the path followed by the satellites around the primary. As the Earth
is bulging from the equatorial belt, and keeping in mind that an orbit
is not a physical entity, and it is the forces resulting from an oblate
Earth which act on the satellite produce a change in the orbital
parameters.
This causes the satellite to drift as a result of regression of the nodes
and the latitude of the point of perigee (point closest to the Earth).
This leads to rotation of the line of apsides. As the orbit itself is
moving with respect to the Earth, the resultant changes are seen in the
values of argument of perigee and right ascension of ascending node.
Due to the non-spherical shape of Earth, one more effect called as the
“Satellite Graveyard” is seen. The non-spherical shape leads to the
small value of eccentricity at the equatorial plane. This causes a
gravity gradient on GEO satellite and makes them drift to one of the
two stable points which coincide with minor axis of the equatorial
ellipse.
Working satellites are made to drift back to their position but out-of-
service satellites are eventually drifted to these points, and making
that point a Satellite Graveyard. Atmospheric Drag For Low Earth
orbiting satellites, the effect of atmospheric drag is more pronounces.
