SDP4

SDP4 is a NASA/NORAD orbital model used with deep space satellites. Satellites with orbital times less than 225 minutes should use the SGP4 or SGP8 algorithms. [Hoots, Felix and Ronald Roehrich "SpaceTrack Report No. 3"]

In theory SDP4 is less accurate than SDP8 for conditions of high drag (e.g. a highly elliptic satellite orbit with very low perigee height or orbital decay and reentry), but in practice SDP4 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).

The two key differences between SGP4 and SDP4 are:

(1) In SDP4 only the 'simplified drag' equations are used. This is not such an accuracy issue, as high drag satellite cases do not remain in "deep space" for very long as the orbit quickly becomes lower and near circular.

(2) SDP4 adds Lunar-Solar gravity perturbations to all orbits, and Earth resonance terms specifically for 24 hour geostationary and 12 hour Molniya orbits.

TLE data should be used as the input for the SDP4 algorithm. Accuracy is usually of the order of 10km and TLE should be updated every few days if this level of accuracy is to be maintained (or following any engine burn, etc).

Most newer software implementations have merged SGP4 and SDP4 in to a single code base, and generally just refer to both just as 'SGP4'.

References

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 SDP4 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 SDP4 algorithm in VB.Net] , vb.net implementations of SDP4 algorithm with source code