Two-Line Elements (TLEs) continue to be the sole public source of orbiter
observations. The accuracy of TLE propagations through the Simplified General
Perturbations-4 (SGP4) software decreases dramatically as the propagation
horizon increases, and thus the period of validity of TLEs is very limited. As
a result, TLEs are gradually becoming insufficient for the growing demands of
Space Situational Awareness (SSA). We propose a technique, based on the hybrid
propagation methodology, aimed at extending TLE validity with minimal changes
to the current TLE-SGP4 system in a non-intrusive way. It requires that the
institution in possession of the osculating elements distributes hybrid TLEs,
HTLEs, which encapsulate the standard TLE and the model of its propagation
error. The validity extension can be accomplished when the end user processes
HTLEs through the hybrid SGP4 propagator, HSGP4, which comprises the standard
SGP4 and an error corrector.Comment: 18 pages, 4 figure
