<h2>AAS 99-189</h2>
<h2>Precise Orbit Determination of the Low Altitude Spacecraft TRMM, GFZ-1, AND EP/EUVE Using Improved Drag Models                                   </h2>
<h4>C.M. Cox*, F.G. Lemoine**, E.C. Pavlis***                                                                                                                                           </h4>
*Raytheon ITSS Corp, Lanham, MD, **NASA GSFC, Greenbelt, MD, ***Univ. of Maryland, Baltimore                                                                              
<h2> Abstract </h2>
The launch of the Tropical Rainfall Measurement Mission (TRMM) provided a unique opportunity to improve geopotential models, which lacked tracking data from satellites at such low altitude (351 km) and inclination (35.0 degrees). TRMM is tracked  by NASA's Tracking and Data Relay Satellite  System, which proved coverage over the Western Pacific and Mid Atlantic regions. A long standing obstacle to low altitude orbit determination has been limitations in drag modeling, particularly for complex spacecraft. Application of thermospheric wind, free molecular flow, and, and "box-wing" models, to account for temporal variations in the lift and drag coefficients, have improved the TRMM orbit determination. These models are also shown to have application to spherical satellites at low altitude, such as the GFZ-1 geodetic satellite at 350 km and 51.7 degree inclination, and other large spacecraft such as the Explorer Platform/Extreme Ultraviolet Explorer (EUVE) at 517 km and 28.5 degrees inclination.