# Orbital Prediction Algorithms

*Models* (jargon for a set of equations and the computer program
that computes them, sometimes called an *algorithm*) are used to predict
the position of the satellite at any time, given the keplerian elements (also
called the *model parameters*). **WinOrbit** allows choosing from 4
different models (each of which can incorporate or neglect a drag or decay
term):

**Ideal Keplerian Model** (Ideal): This model assumes that the Earth is a
point in space, and that the sun, moon, etc. have no influence. The satellite
orbit is then a perfect ellipse whose orientation in space is fixed forever,
while the earth rotates underneath.

**Basic Model** (Basic): The original Clark algorithm (with a name borrowed
from the original article title). Similar models have been published by Karl
Meinzer (DJ4ZC) and James Miller (G3RUH), I believe. Real orbits drift slowly
with time because the earth is not a sphere. This is called *precession*.
Both the orbital plane (represented by the right ascension of the node) and
the orientation of the orbit in that plane (represented by the argument of
perigee) change with time. The rate of change is influenced by the
eccentricity and inclination of the orbit. The earth is represented as a
simple ellipsoid, and other bodies (sun, moon) are ignored. (The elliptical
representation of the earth refers to its gravitational effects, not to the
location of the observer on the surface, which is a second issue).

**Simplified General Perturbation Model** (SGP): this series of models is
the most accurate one that I am aware of. There are several levels (SGP, SGP4,
SGP8, and "deep space" versions SDP4 and SDP8). The *perturbations* of
the title are due to the influence of the sun and moon, as well as to more
complicated distortions of the earth's gravitational field. **WinOrbit**
incorporates both SGP and SGP4 at this time. Both models were adapted from the
FORTRAN listings provided by Tom Kelso in the **SpaceTrack** documentation.

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