Simplified perturbations models are a set of five mathematical models
(SGP, SGP4, SDP4, SGP8 and SDP8) used to calculate orbital state
vectors of satellites and space debris relative to the Earth-centered
inertial coordinate system. This set of models is often referred to
collectively as SGP4 due to the frequency of use of that model
particularly with two-line element sets produced by NORAD and NASA.