  The algorithms used in the construction of a semi-analytical propagator for
the long-term propagation of Highly Elliptical Orbits (HEO) are described. The
software propagates mean elements and include the main gravitational and
non-gravitational effects that may affect common HEO orbits, as, for instance,
geostationary transfer orbits or Molniya orbits. Comparisons with numerical
integration show that it provides good results even in extreme orbital
configurations, as the case of SymbolX.
