<h2>AAS  96-127</h2><h2>AN EVALUATION OF JACCHIA 71 AND MSIS90 ATMOSPHERIC MODELS WITH NASA ODERACS DECAY DATA</h2><h4>C. C. Chao and G.R. Gunning, The Aerospace Corporation; K. Moe, Space and Missle Systems; T. J. Settecerri, Lockheed-Martin Engineering & Sciences</h4><h2> Abstract </h2>This paper presents the results of  an orbit decay study of four of the six ODERACS spheres using two widely used atmosphere density models, Jacchia 71 and MSIS90. The NORAD 2-line elements were processed by a semi-analytical propagat or, LIFETIME, which differentially corrects the ballistic coefficient. Drag co efficient, Cd, were deduced from the ballistic coefficients for an altitude range from 100 km to 350 km.  The consistently lower values of Cd suggest that the Jacchia 71 density model is 5 to 20 % too high in predicting density. The findings of this study are valuable in improving the accuracy of orbit decay predictions and orbit sustenance fuel estimation.<br><br>