LEO Dynamic Orbit Enhancement Using Atmospheric and Empirical Force Modeling

Tae-Suk Bae

Abstract:	In recent years, many Low Earth Orbiters (LEOs) have been launched for scientific purposes, such as Earth gravity recovery and atmospheric profiling for GPS meteorology, and new missions are expected in the next few years. CHAMP (CHAllenging Minisatellite Payload) is one of the recently (2000) launched missions designed for static Earth gravity field recovery, magnetic field mapping and atmospheric/ionospheric profiling. The CHAMP satellite has been placed in an almost circular, near polar orbit with an initial altitude of 450 km, and the mission is managed by GeoForschungsZentrum (GFZ). To meet the scientific mission objectives of LEOs, a precise orbit determination of the LEO satellites must be provided, although the required accuracy may be different for each application. LEOs, however, experience highly complex dynamics in their orbits due to the significant impact of high frequency gravity field components and atmospheric effects, which make it complicated to achieve an accurate orbit solution for a LEO. Numerous efforts were undertaken to measure the non-conservative forces with the CHAMP onboard accelerometer data provided by GFZ (http://www.gfzpotsdam. de/pb1/op/champ/index_CHAMP.html). These data, however, do not represent all nonconservative forces properly. This is because the accelerometer data provided to the analysts are smoothed, based on a moving average of up to 10 data points. Therefore, the published accelerometer data are not sufficient for LEO Precise Orbit Determination (POD) since there might still be some dynamics between the consecutive epochs of data. Of all the nonconservative forces, the atmospheric drag is the most dominant in the LEO POD; therefore, its effect on the orbit solution is studied and presented in this study, based on the CHAMP satellite. In this study, the parameters of the atmospheric drag are estimated on an hourly basis using two different atmospheric models, that is, NRLMSISE-00 and Jacchia 1971, and the results are compared to the solution obtained with a single drag coefficient estimated for the entire arc. It is shown that by estimating the nonconservative forces in the orbit determination (OD) process, the orbit can be substantially improved (compared to the case with no orbit improvement performed), resulting in ~13 cm 3D RMS (root mean square) error with respect to the Rapid Science Orbit (RSO) provided by GFZ. This orbit can be further improved up to 8 cm 3D RMS when the empirical force modeling is included. The software used in these analyses is the new OD software package developed by the author.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	1219 - 1226
Cite this article:	Bae, Tae-Suk, "LEO Dynamic Orbit Enhancement Using Atmospheric and Empirical Force Modeling," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 1219-1226.
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