SGP4

SGP4 (Simplified General Perturbations Satellite Orbit Model 4) is a NASA/NORAD algorithm of calculating near earth satellites (i.e. calculating their orbital state vectors relative to the Earth Centered Inertial coordinate system). 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); in practice, however, SGP4 is often the best choice as it 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 about 1 km in position. At a distance of for example 300 km this could cause a pointing error of up to 0.2 deg. TLE data older than a few days are likely to be considerably less accurate than this.

External links

* [http://www.celestrak.com/NORAD/documentation/spacetrk.pdf Spacetrack Report No. 3] , a really serious treatment of orbital elements from NORAD which also documents the SGP4 algorithm (in pdf format)
* [http://www.centerforspace.com/downloads/files/pubs/AIAA-2006-6753.pdf Revisiting Spacetrack Report No.3] Recent in-depth paper on SGP4/SDP4 (1.2MB, in PDF format)
* [http://www.centerforspace.com/downloads/files/sgp4/AIAA-2006-6753Code.zip SGP4 Code and TLE set interpretation] written in C++, FORTRAN, Pascal, and MATLAB.
* [http://www.kefa.it/satellites/default.htm SGP4 algorithm in VB.Net] , vb.net implementations of SGP4 algorithm with source code