Working with TLEs and the SGP4 Propagator

Two-Line Element Sets (TLEs) are general perturbation mean elements constructed by a least squares estimation from observations of a satellite's orbit. The descriptions of the Two-Line Orbital Element Set format is covered in the Two-Line Element Set File page. TLE data for existing satellites is available from numerous sources including celestrak.com and space-track.org. If a TLE does not already exist for the desired satellite, one can be generated from observation data provided by sources such as the Space Surveillance Network or commercial entities; or the data can be generated by using FreeFlyer's simulated observation capabilities. FreeFlyer can use any of these real or simulated observation data sources to generate TLEs.

The mean state values (referred to as an SGP4 state in this guide) are constructed by removing periodic variations in the force model due to the oblateness of the Earth, atmospheric drag, and lunar and solar gravitational effects. In order to obtain good predictions from the state, the periodic variations must be reconstructed in exactly the same way they were removed. FreeFlyer's SGP4 propagator (which implements the SGP4 algorithm) provides the means of reconstructing the variations, thus enabling the proper propagation of SGP4 states. Therefore, if you are loading a Spacecraft state from a TLE, you should use the SGP4 propagator when running your Mission Plan.

This section covers the following topics about working with TLEs and the SGP4 Propagator:

See Also

•Two-Line Element Set File

•SGP4 Properties and Methods

•TwoLineElement Properties and Methods

•TLECatalog Properties and Methods

•SGP4StateEstimator Properties and Methods