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This set of models is often referred to collectively as SGP4 due to the frequency of use of that model particularly with two-line element sets produced by NORAD and NASA. These models predict the effect of perturbations caused by the Earth’s shape, drag, radiation, and gravitation effects from other bodies such as the sun and moon.
A two-line element set (TLE, or more rarely 2LE) or three-line element set (3LE) is a data format encoding a list of orbital elements of an Earth-orbiting object for a given point in time, the epoch. Using a suitable prediction formula, the state (position and velocity) at any point in the past or future can be estimated to some accuracy.
The Satellite Catalog Number (SATCAT), also known as NORAD Catalog Number, NORAD ID, USSPACECOM object number, is a sequential nine-digit number assigned by the United States Space Command (USSPACECOM), and previously the North American Aerospace Defense Command (NORAD), in the order of launch or discovery to all artificial objects in the orbits of Earth and those that left Earth's orbit. [1]
For orbit propagation purposes, there is a field for BSTAR drag in two-line element set (TLE) files, where it is to be given in units of inverse Earth radii. [2] The corresponding reference air density is given as 0.15696615 kg / ( m 2 ⋅ R Earth ) {\displaystyle 0.15696615{\text{ kg}}/(\mathrm {m} ^{2}\cdot R_{\text{Earth}})} . [ 3 ]
Keplerian elements parameters can be encoded as text in a number of formats. The most common of them is the NASA / NORAD "two-line elements" (TLE) format, [4] originally designed for use with 80 column punched cards, but still in use because it is the most common format, and 80-character ASCII records can be handled efficiently by modern databases.
NASA maintains civilian databases of GP orbital elements, also known as NASA or NORAD two-line elements. The GP element sets are "mean" element sets that have specific periodic features removed to enhance long-term prediction performance, and require special software to reconstruct the compressed trajectory .
Celestrak provided by Dr. T.S. Kelso, includes visible objects, openly available [22] SpaceTrack maintained by the United States Strategic Command provides orbital information on unclassified satellites requires an account but is available for educational and hobbyist use as well as military, government and spacecraft and payload owners.
Samples of three GPS satellites' orbits over a five-year period (2013 to 2018) USA-242 · USA-239 · USA-151 · Earth As of 22 January 2025, 83 Global Positioning System navigation satellites have been built: 31 are launched and operational, 3 are in reserve or testing, 43 are retired, 2 were lost during launch, and 1 prototype was never launched. 3 Block III satellites have completed ...