| In the last decade GPS technology has gained rapid development, and has been widely applied to many fields such as surveying and mapping, navigation, earth rotation determination, and crustal deformation monitoring. It has played a unique role in the research of continental tectonic deformation and geodynamics in particular. Applying GPS in the precise geodetic surveying, the precise ephemeris is indispensable in post-processing of GPS data. The IGS precise orbits have been widely used in the world ever since the beginning of their broadcast in 1994. However, what is their accuracy? Is their formal accuracy representative of the real accuracy? Are there systematic biases with them? If so, what are the origins? Is it possible to improve their accuracy? If so, how? It is vitally important and urgently needed to answer these questions in order to improve the accuracy of GPS technique for the applications in, for example, earth rotation determination, research on tectonic plate motion, tectonic deformation of continents, and earthquake monitoring.In order to answer the above questions, this work reprocesses GPS data from the IGS global tracking network to obtain precise GPS orbits from 1994 to 2004. Uniform data processing strategy is adopted, consistent orbital and Earth rotation parameters are prescribed, the most recent geophysical models and appropriate error correction models are used, and the ITRF2000 station positions and velocities of the IGS sites are used to define the reference frame. We expect to evaluate the accuracy of IGS orbits through comparison between the ICS and our reprocessed orbits.Systematic biases and random errors exist in the IGS precise orbits and the reprocessed orbits. In order to differentiate the systematic biases and the random errors for better analysis of their origins and magnitudes, I estimate transformation seven parameters (3 for translation, 3 for rotation, and 1 for scale) between the two sets of orbits. The result verifies that the seven-parameter-transformation can eliminate much of the systematic biases between the two orbits. Then I research thesystematic biases and their origins through analyzing the time series of the transformation parameters, and investigate the random errors through comparing the IGS precise orbits with the reprocessed orbits from which the systematic biases have been eliminated.The findings and conclusions of this thesis are as follows:1. The systematic biases exist in different ITRF reference frames, whose differences cannot be solely removed by applying the transformation relationships between the reference frames published by IERS. The remaining systematic biases are still significant.2. Significant differences are found between the IGS claimed and actual orbital accuracies, especially for the orbits of the early years.3. The systematic biases of the IGS orbits vary with time resulted mainly from adopting different ITRF reference frames over the years. Analysis of the seven transformation parameters between the IGS precise orbits and the reprocessed orbits shows that the systematic biases of the IGS orbits are closely related to the ITRF reference frames adopted. Especially, the differences between the origin, orientations, and velocity fields of the ITRF92 and ITRF93 reference frames and other ITRF reference frames after ITRF94 have been reflected in the systematic biases of the orbits. To be specific:? The influence of the mass center definition: The origin of a reference frame is defined as the mass center of the earth, which, for the ITRF92 and ITRF93 reference frames, is obtained from the analysis result of SLR, derived by the Space Research Center of the University of Texas. On the other hand, origins of the ITRF sequences after 1TRF94 are obtained from a weighted average of SLR and other geodesic results. The obvious change of the trends of the translation parameters w and \) around 1996.5 reflects the systematic bias caused by this change of definition of the earth origin.? The influence of the model constraints on the ITRF reference frames: The maintenance of the ITRF reference frames relics heavily on the knowledge of the reference stations' motion. The orientations of the ITRF92 and ITRF93reference frames are constrained using station velocities predicted by the NUVEL1A plate motion model. The difference between this plate motion model and reality has an impact on the definition of the earth origin, which is evident as demonstrated in the approximately linear trend of the translation parameters &x and ar from 1994.0 to 1996.5? The influence of the orientation constraints on the ITRF reference frames : The significant jumps of the rotation parameters Qxand Qyin 1995, 1996, and 1997 take place at the moments of transition between the ITRF reference frames, which reflect the change of orientation parameters of the ITRF reference frames. The orientation parameters of ITRF93 differ from the parameters of other ITRF reference frames by 1.5-2.0 mas. Accordingly, Q, and Qy obtained for the time span of ITRF93 differ significantly from those of other time periods.4. GPS data analysis by CMONOC reveals that since 2001 a scale factor change has emerged, ranging from ~1 x 10"9 in 2001 to ~3 x 10"9 in 2004. This observation agrees with the observations of GPS community abroad (e.g. Dong Danan, personal communication). However, this change is not reflected in the comparison between the seven transformation parameters, suggesting that the effect must commonly exist in both orbits and therefore will not show up in the calculation of the transformation parameters between the orbits.5. Relative to the reprocessed orbits, the random errors of the IGS precise orbits diminish along with time: they are about 15-20 cm in 1994, reduced to 6-8 cm in 1998, and are less than 5 cm after 1998.6. The accuracy of the IGS precise ephemeris is improving steadily, which benefits mainly from the following aspects: ? The accuracy of the ITRF reference frames adopted in IGS precise ephemeris keeps improving. ? In the last decade, the GPS satellite condition, positioning technique, and receiver technology have gained significant progress, and the accuracy of the GPS observation lias been improved gradually. (3)The geophysical models and data error models adopted in data processing have also been improved. The coordinates o\' the tracking stations havebeen refined .The logs of the tracking stations and device updating records have become more and more complete. The data processing methods have been optimized and converged. (4) The number of IGS tracking stations has increased and the distribution of these stations become more and more uniform, so that the influence of random errors of the tracking stations and accidental gross errors of individual stations is minimized.To study crust deformation using early-year GPS observations with its network covering a large area, it is possible to reduce the effect of inaccurate orbits by using relaxed IGS precise orbits to obtain loosely constrained solutions and convert that to the 1TRF global reference frame by performing seven parameter transformations. However, the influence of systematic biases in the IGS precise orbits on the position results can not be eliminated by such transformations, because the differences between the ITRF reference frames are not totally systematic, thus their influence on the positioning results can not be systematic as well.Reprocessing the GPS satellite orbits and improving the accuracy of the orbits of the early 1990s will be helpful in improving the baseline-mode positioning results. For the high-accuracy GPS applications, such as the earth rotation determination, tectonic plate motion, and earthquake deformation monitoring which usually require long time span of observational data, the need for reprocessed GPS orbits is more urgent. In this regard, the systematically reprocessed orbits for the last decade presented in this thesis have made an important contribution to this aspect.
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