Kinematic Orbit Determination of Low Earth Orbiter using Triple Differences

Jay H. Kwon, Dorota Grejner-Brzezinska and Chang-Ki Hong

Abstract:	Among numerous applications of GPS, the precise orbit determination of a low earth orbiter (LEO) has been one of the most actively developing fields in the past decade. The precise orbit determined from GPS is needed in many space science applications such as radar altimetry, satellite gravity/gradiometry, and atmospheric sounding. The LEO needs very precise mathematical model for the dynamic forces acting on it, because of its low altitude (below than 700 km). With errors induced by the imperfectness of the model, the orbit calculated by the dynamic approach degrade with the length of the arc. Thus, force model parameters need to be adjusted during the orbit determination (OD) procedure, adding more unknowns to the solution. Therefore, under the assumption of a complex dynamic behavior of LEO satellite, the kinematic strategy could potentially generate an orbit with the accuracy comparable to the dynamic approach with the force model estimated, provided good geometry (PDOP) is maintained. In addition, the procedures for the kinematic OD are much simpler and more efficient since neither dynamic nor physical models are necessary, and the number of unknowns is limited to the LEO position coordinates. In this paper, a kinematic OD procedure for LEO using GPS triple-differenced observables is presented. Ion-free triple differenced phases are used as observables, and the coordinates at the initial epoch are held fixed, due to the fact that the triple difference method can only provide coordinate change from epoch to epoch. To achieve the computational efficiency, the precise orbits published by the International GPS Service (IGS), and well-distributed ground stations data/coordinates are utilized, leaving the satellite coordinates at each epoch as the only unknowns. Currently, the 3D accuracy of the developed algorithm applied to the German CHAMP satellite (CHAllenging Minisatellite Payload, launched on July 15, 2000) shows better than 30 cm fit to the published rapid orbit solution (good to ~10 cm per coordinate) from GFZ (GeoForschungsZentrum Potsdam), and much better accuracy can be obtained for the portions of the trajectory with good satellite and ground station configuration. Because the ultimate goal of this research is to provide rapid orbits (with 2-3 hour latency) to support GPS occultation data analysis, where the velocity information is required, the velocity is also derived from the orbit with a 3D accuracy of 1.2 mm/sec, which still needs to be improved, to meet the accuracy requirements in supporting the atmospheric sounding. Since the kinematic POD highly depends on the strength of GPS satellite and ground station geometry, the issue of optimal configuration of the ground stations is also discussed. Keywords: kinematic POD (precision orbit determination), LEO (low earth orbiter), orbit accuracy analysis.
Published in:	Proceedings of the 2002 National Technical Meeting of The Institute of Navigation January 28 - 30, 2002 The Catamaran Resort Hotel San Diego, CA
Pages:	762 - 770
Cite this article:	Kwon, Jay H., Grejner-Brzezinska, Dorota, Hong, Chang-Ki, "Kinematic Orbit Determination of Low Earth Orbiter using Triple Differences," Proceedings of the 2002 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2002, pp. 762-770.
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