| dc.description.abstract | With the advancement of technology, Global Positioning System (GPS) observations have been used in a variety of scientific and technological disciplines requiring high-precision positioning. Hence, how to improve the GPS positing accuracy is constantly considered as an important research topic. In the GPS data processing schemes for general purposes and of most advanced, the positioning accuracy is much higher in horizontal coordinates than in the vertical coordinate. One of the major causes is in that the uncertainty and variation resulting from in atmospheric water vapor is very much significant.
In this dissertation, we investigate the relationship between tropospheric delay due to water vapor and the GPS positioning accuracy in the vertical coordinate. The tropospheric delay is obtained by three ways, standard atmospheric value (SAV), surface meteorological measurement (SMM), and water vapor radiometer (WVR) that are incorporated into the parameter estimation and external correction methods, respectively. Results based on thorough and complete investigations are consequently presented.
Several major findings are obtained from our investigations and stated behind. The vertical coordinate from static GPS positioning by use of parameter estimation method remains almost the same when SAV is replaced by SMM with deviation only on the order of mm. In contrast, the baseline derived from static GPS positioning may differ by an order of cm by use of external correction method when SAV is replaced by SMM. In addition, repeatability of the baseline is improved. Similarly, the vertical coordinate may vary on the order of cm, the average daily standard deviations vary unexpectedly large over a period of several days. Results from the use of WVR are similar to those from the use of SMM for the external correction method. Under clear sky conditions, results from WVR appear better when they are incorporated into external correction method than parameter estimation method. This indicates the significant impact of the tropospheric delay on the vertical coordinate determination by GPS positioning. Apparently, it is important to acquire accurate tropospheric delay in order to assure high-accuracy of vertical coordinate determination. Meanwhile, it is crucial to utilize high-quality data when external correction method is implemented.
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