AN EVALUATION OF JACCHIA-71 AND MSIS90 ATMOSPHERE MODELS WITH NASA ODERACS DECAY DATA

The Space Shuttle Discovery (STS-60) deployed six Orbital Debris Radar Calibration Spheres (ODERACS 1) on February 9, 1994. A year later on February 3, 1995, three more spheres and three small dipoles (ODERACS 2) were placed in orbit from Discovery (STS-63). The orbital lifetimes of these metal spheres ranged from 8 to 13 months. The continuous NOR AD 2-line orbital elements and the known dimension and mass of each sp here provided a good opportunity for the evaluation of atmospheric den sity models. This paper presents the results of the evaluation of two commonly used atmospheric density models, Jacchia 71 and MSIS90, based on decay histories of these spheres. The NORAD 2-line elements were p rocessed by a semi-analytical computer program LIFETIME, which differe ntially adjusts the ballistic coefficient used with one of the two atm osphere models, so that the computed orbit agrees with the observed or bit. The daily measurements of solar flux, F-10.7, and geomagnetic ind ex, A(p), were used by LIFETIME for atmosphere density prediction. Two sets of drag coefficient, C-d, were deduced from the ballistic coeffi cients for four altitude bins covering an altitude range from 100 km t o 350 km. Physical drag coefficients for spheres were compared with th e observed drag coefficients deduced from the NASA data. Results of co mparison strongly suggest that the Jacchia 71 density model is 9 to 22 % too high in predicting density at the above mentioned altitude range at a time of low solar activity. The results of this study further su pport the estimated 15% error in Earth atmosphere density models.