Norad Two-Line Orbital Element Set File
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The US Air Force provides data for Earth-orbiting satellites in Two-Line Orbital Element Sets (TLE). A derivation of the Two-Line Element set format called the Three-Line Element set exists where the first line contains the name of the satellite. These element sets are general perturbation mean elements constructed by a least squares estimation from observations. The Air Force provides the observations taken from the Space Surveillance Network, and FreeFlyer provides the capability of simulating observations to generate TLEs.
The mean values 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 TLE, the periodic variations must be reconstructed in exactly the same way they were removed. FreeFlyer's Norad propagator type (which implements the SGP4/SDP4 algorithm) provides the means of reconstructing the variations, thus enabling the proper propagation of TLE states.
Two-Line Orbital Element Sets are provided in an ASCII formatted text file and each file could contain TLEs for multiple objects. A typical TLE entry can contain the name of the object on the first line (line 0), however this is not a requirement. The next two lines (lines 1 and 2) contain 69 characters which describe the orbit of the object in the NORAD Mean Element set.
Below is a template of the common format of a Three-Line Element Set entry. In the template shown below, 'A's can represent any character between A-Z or a space and 'N's can represent any number between 0-9 or a space. The last line is not part of a TLE entry; it was added to help identify the individual columns.
Format Definition, Line 0
Line 0 is provided in a Three-Line Element Set file, and is not included in a Two-Line Element Set. The line contains the name of the object, up to 11 characters.
Format Definition, Line 1
Line 1 of a TLE entry contains general descriptive information of the object itself (classification, origin, etc.), along with information describing the force model parameters (drag/solar radiation pressure, time derivatives, etc.).
Format Definition, Line 2
Line 2 of the TLE entry contains the state data of the object.