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WGS and NORAD models Orbital elements per se are not in any way referenced to a geodetic datum, they are referred to an inertial reference frame. However, TLEs are not classical orbital elements. They are massaged in a special and undocumented way to give the best possible results over a certain time span with the NORAD models. This becomes clear by inspecting the code which begins by recovering the original mean motion and semi-major axis. This reconstitution of two variables (which in fact are not independent of each other) involves the WGS72 values of the Earth's equatorial radius and angular velocity. If one changed this to WGS84 one would probably introduce an error and not recover the original mean motion and semi-major axis. The NORAD models give the position of a satellite in an earth-centered inertial reference system in units of earth radii. The algorithm uses the J2, J3 and J4 spherical harmonics of the gravitational field. It is at this point where it could make sense to use their WGS84 values. I have made a test of SGP4 with all WGS72 constants replaced by their WGS84 values. The position of a satellite, expressed in units of Earth radii, changes in the 7th decimal place only. This is in the 100 m range. I recall having read that the intrinsic accuracy of the TLEs at epoch is in the order of 500 m. The error introduced by using WGS72 instead of WGS84 is smaller than that, so in summary, it does not seem useful to replace WGS72 constants by those of WGS84 which are very close to each other anyway. There are far more important sources of error elsewhere in the algorithms, notably in the atmospheric density model. I'd appreciate any comments. Bruno Tilgner ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html

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