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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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