AAS 98-190

ESTIMATING ATMOSPHERE DENSITY VARIATIONS TO IMPROVE LEO ORBIT PREDICTION ACCURACY

A. I. Nazarenko - Center for Program Studies, Russia; P. J. Cefola - C. S. Draper Laboratory; V. Yurasov - Space Research Center Kosmos, Russia

Abstract

Atmosphere density models such as Jacchia or MSIS provide insufficient accuracy for short-arc orbit determination applications involving strongly drag-perturbed orbits. Therefore, operational orbit determination procedures for these orbits employ a "solve-for" vector which consists of an element set (or the six equivalent components of position and velocity) and a ballistic coefficient or related drag factor. The drag factor adjusts the density model to better represent the real conditions experienced by the satellite. Thus, the observed ballistic coefficient associated with a particular fit span reflects the "average" solar activity and geomagnetic disturbance. When an orbit prediction into the future is made, the most recent element set and ballistic coefficient are employed. Significant orbit prediction errors occur because the "average" solar activity and geomagnetic disturbance in the predict interval is not identical to that in the fit interval. In this work, an atmosphere density tracking process that operates in parallel to the orbit determination process is introduced. The atmosphere tracking process employs data from multiple satellites. The atmosphere tracking process includes separate procedures for constructing the atmosphere density variations and for estimating the true ballistic coefficient of the satellites employed. The process also includes procedures for forecasting the atmosphere density at future times. Practical implementation of this process allows construction of a unique system for the upper atmosphere state monitoring. This work was supported by a purchase order issued to the Scientific-Industrial Firm "NUCLON" under the Draper Laboratory IR&D Program during DFY 97.

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