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ABSTRACT
The ROCSAT-1 satellite was successfully launched on January 27, 1999. The satellite is orbiting the Earth at an altitude of 630 kilometers with an inclination of 35 degrees. The ROCSAT-1 satellite is mainly designed to carry out ocean color imaging and scientific research missions. The launch date of ROCSAT-2 satellite is scheduled at the middle of 2003, and the satellite will orbit the Earth at an altitude of 891 kilometers with an inclination of 99 degrees. The ROCSAT-2 satellite is mainly designed to perform near real-time high resolution remote sensing of the ocean and landmass in the vicinity of Taiwan. The accuracy demand of the ROCSAT-2 satellite is higher than the ROCSAT-1 satellite for that the external orientation parameters are needed for ROCSAT-2 missions. There are no original GPS observation data downlinked from the ROCSAT-1, ROCSAT-2 satellites for postprocessing applications. As a result, the goal of this study is to calculate the systemic errors of pseudorange observations, including GPS satellite clock error, GPS broadcast ephemeris error and ionospheric delay, by using IGS tracking stations distributed uniformly around the world. The above systemic errors are turned into a correction to orbital position. Test results indicate that with a good geometry distribution of tracking stations and known PRN values of observed GPS satellites when SA effect is on, the accuracy of the navigation position can be improved from 60 meters to 10 meters, and when SA effect is off, PRN values of observed GPS staellite are not important to the results, and the accuracy of the navigation position can be improved from 20 meters to 10 meters.
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