<h2>AAS  96-179</h2><h2>ORBIT DETERMINATION OF THE ORBITAL RE-ENTRY EXPERIMENT (OREX) SPACECRAFT BY GPS</h2><h4>M. Murata, M. Harigae, and T. Tsujii, National Aerospace Laboratory, Japan</h4><h2> Abstract </h2>The OREX (Orbital Re-entry EXperiment) spacecraft carrying a Japanese first spaceborne GPS receiver was launched on February 4, 1994. The objective of the OREX/GPS experiment was to demonstrate the navigation performance of the GPS receiver in space. An inertial measurement unit (IMU) of stable-platform type was aboard the OREX. Both GPS and IMU data were telemetered to the ground, which enabled orbit determinations in a post-mission mode.  Three orbit determination strategies were evaluated: 1) standalone GPS, 2)GPS/IMU hybrid navigation, and 3) differential GPS between OREX/GPS and the ground receiver. An extended Kalman filter was tailored with emphasis on modeling de-orbit burn. The navigation accuracy was assessed by the comparison with a reference orbit which was derived from standalone GPS using precise ephemerides and SV clock corrections.<br><br>