Satgen317   In Orbit   by GM4IHJ                             22 April 95

A satellite takes up an orbit appropriate to its orbital velocity and, the
mass,of the planet or moon,around which it is travelling. Thereafter,
subs
quent events depend on the various forces which can perturb the
orbit. In the case of a satellite of the Earth, these perturbation
agencies will be :-    The ATMOSPHERE for any satellite coming below 1000
kms altitude for part or all of its orbit. Drag in the atmosphere slows
the satellite down , forcing it to fall to a lower altitude in order to
maintain orbital velocity. Drag is variable, being greatest at Solar 11
year cycl
 maximum when the atmosphere supplemented by material from the
Sun is at its densest. Drag also varies spasmodically , when Solar Coronal
Mass Eject ons briefly increase the upper atmospheres density, what ever
the phase of the Solar sunspot cycle.
The EQUATORIAL BULGE of the Earth, which exerts a force on the satellite
pulling the plane of its orbit, so that the satellites track with respect
to the stars changes gradually with time.
The GRAVITATIONAL PULL of the Sun and the Moon, which is particularly
noticable on satellites going more that 20000 kms from the earth for part
or all of their orbit. This effect is therefore not of great interest in
respect of low earth orbit satellites , but it can make a great deal of
difference over months to the orbits of both geostationary satellites out
at 36000 kms, or Molinya elliptical orbiters like Oscar 13 which go out to
36000 kms or more at the apogee high point of their orbit.

In order to predict where a satellite is at any time , we can predict
the effect on it of the Earths mass, and measure the parameters of its
orbital ellipse at a particular EPOCH time. Then making allowance for the
effect of the earths equatorial bulge, we can calculate where it will be
at some future time.  Provided that is we remenber the problem posed by
atmospheric drag, and accept that we will have to measure a full set of
new parameters every month or so , if the satellite is at low altitude
such as Mir. Or about every 3 months if it is below 1000 kms around solar
cycle maximum. For satellites at solar cycle minimum we can stretch the
time between providing new orbital parameters to two or three times those
given above, but for longer periods new parameters must be measured if we
are to avoid errors due to Solar and Lunar gravitational effects on the
satellites at higher altitude.

For many years now the North American Radar NORAD, has measured and
supplied the orbital information for near earth orbit satelcites. With
NASA being the public contact point for distribution of this data, which
we call Keplerian Elements. Satgens over the next few weeks will discuss
thele elements and their usage , together with a look at the very
different orbital data and methods of calculation which we must use for
objects outside the Earths gravitational sphere of influence , such as the
Sun, Moon, Planets , Comets and Deep Space craft.