An orbit propagator is a computer program that is used to compute the position and
velocity of an earth-orbiting satellite using a compatible element set. All orbit
propagators represent "solutions" to the equations of motion for two or more
bodies moving relative to each other subject to a variety of forces, the primary one being
gravity. An exact solution to this problem can only be achieved in the special case of two
point-mass bodies. All real world solutions involve the use of either simplifying
assumptions (the General Perturbations methods) or numerical integration (the Special
Perturbations, SP, methods) or some combination of the two (the Semi-analytical Theories).
These are defined below.
Two-Body
Exact solution to equations of motion for any two mutually attracting bodies. A two-body
propagator is adequate for modeling and educational purposes, but should not be used for
any realistic application (such as satellite viewing).
General Perturbations
The perturbing forces are modeled as series expansions which are terminated after a
certain number of terms. An exact (or nearly exact) solution is then obtained,
analytically, for the simplified equations of motion. Examples of this type are the three
predominant propagators used for satellite viewing, SGP, SGP4 and SDP4. All three were
developed for use by the US Air Force and are still in general use by that agency for
satellite tracking. SGP and SGP4 are intended for low earth orbit (periods less than 225
minutes). SDP4 includes additional terms to model deep space satellites. These propagators
run much faster than SP and Semi-analytical theories and produce results that are valid
for most non-precision applications. This makes them ideal for satellite viewing.
SatSpy 3.0 supports all three general perturbation orbit propagators.
Special Perturbations
SP propagators use exact, or very nearly so, representations of the perturbative forces.
The resulting equations of motion are then integrated numerically to obtain an
"exact" solution to the equations of motion. Depending on the type of
integration performed, SP solutions can take many hours of execution time on the fastest
main-frame computers. For this reason, these propagators are used only for very
specialized applications which require exact representations of satellite motion.
Semi-analytical Theories
Propagators in this class represent an attempt at a compromise between the need for high
precision and rapid computation. This means that some of the perturbative forces are
modeled analytically while others are numerically integrated.
Want to Learn More?
There are a variety of books that you can use to learn more about astrodynamics, orbit
propagation and element sets. Here are a few:
"Methods of Orbit Determination", Pedro Ramon Escobal, John Wiley & Sons, Inc. 1965.
"Orbital Motion", A. E. Roy, Adam Hilger Bristol and Philadelphia, 1988.
"Observing Earth Satellites", Desmond King-Hele, Van Nostrand Reinhold Company, 1983.