SGP4

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SGP4 (Simplified General Perturbations Satellite Orbit Model 4) is a NASA/NORAD algorithm of calculating near earth satellites. Any satellite with an orbital time of less than 225 minutes should use this algorithm. Satellites with orbital times greater than 225 minutes should use the SDP4 or SDP8 algorithms. The choice of 225 minutes for selecting the propagation model (near-Earth or Deep-Space) appears somewhat arbitrary, but is thought to relate to the original range of the NORAD tracking radar system.

In theory SGP4 is less accurate than SGP8 for conditions of high drag (e.g. a satellite near re-entry) but in practice SGP4 is the best choice as that is the model believed to be currently in use by NORAD/AFSPC for generating the Two-Line Elements (TLE).

TLE data should be used as the input for the SGP4 algorithm. The accuracy of SGP4 is typically 0.1º longitude and 0.1º latitude from the ground. TLE data older than 30 days is considerably inaccurate due to perturbations in the orbit.

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