Satgen293  Satellite Problem No.6  Gravity   by GM4IHJ         5th Nov
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In theory an earth orbiting satellite might be expected to follow the same
orbital circuit , for ever. But Nature is never that simple, and, there
are all sorts of reasons why this does not happen. Most of these reasons
concern the force of gravity.

The first disturbing feature is caused by the equatorial diameter of the
earth being greater than the polar diameter. Every time a satellite goes
over the mass bulge at the Equator it is pulled slightly east or west
depending on its orbital inclination angle

Less obvious than the first reason is the second disturbing feature . This
is caused by the different composition and hence mass of the earths crust.
Oceanic crust is heavier than Continental crust. So gravity is generally
greater over the ocean than over the continents. This induces an out of
balance situation whereby say, over the Atlantic, the force over the
centre is greater than that nearer the continents. As a result of this
"sideways " tug, geostationary satellites are very rarely stationary. Left
to themselves, most geosats would wander off station within a month or so,
unless their controllers pushed them back into position every few weeks
using their small attitude thrusters.

That is not even the end of the geosat problem. Any geosat is out at
36,000 kms from the earth , and out there it is subject to both earth
gravity , and the gravitational attraction of the Sun and the Moon. This
has the effect that geosats are pulled to higher latitude northwards above
the equator in the northern summer when the Sun gets up around Latitude 23
N, and, pulled back down below the equator in the northern winter when the
Sun drops down to around latitude 23 S. Lunar effects are also present but
are much less easy to predict .

The above Solar and Lunar effects are also present on low earth orbit
satellites , but they are usually too small to be of any great concern.
But they cannot be ignored for the high altitude elliptical orbiters such
as Oscar 10, Oscar 13 and the proposed Phase 3D satellite. Because of
their constant movement from apogee to perigee twice a day, these high
elliptical orbiters can develope severe perturbation of their orbit, if as
happened with Oscar 13, they synchronise their visits to apogee orbit high
point furthest from earth, with the Sun being in the same part of the sky,
slowly day after day pulling the apogee height upwards.

Further out in space the situation gets even worse. Seen from Earth, the
Moon looks like a perfect sphere . But do not be fooled by this. The Moon
is locked with the same face pointing at the earth continuously, and we
therefore only see its end view. Seen from the side the Moon is more like
a lemon pip, or a rugby football in shape, with its long axis pointing
away from the earth. Just imagine therefore the perturbation induced by
trying to maintain a long term safe orbit around such a variable shape .
73 de GM4IHJ @ GB7SAN