Shanghai Observatory, Chinese Academy of Science, 80 Nandan Road, Shanghai 200030, P.R.China
GPS measurements are considered as a new revolution in navigation and geodetic positioning. The accuracy for relative positioning is of the order of 10ppb and even of 1ppb. However,the propagation delay due to atmosphere have been the major errors source in GPS technique.
In the paper, the atmospheric features are reviewed. Several important models of tropospheric refractive correction are discussed. Using the improved SHAGAP software established by us, many computations and analyses are done. Some parameter estimation methods to correct the wet zenith delay, including constant parameter, piecewise linear and stochastic estimation manners,are compared with each other by direct comparison of their effects on relative positioning with the data of the Chinese GPS 94 campaign. Using the observations of the 15 stations distributed in China, 105 baselines are determined. The accuraey of baseline is about 6mm+3ppb. the repeatability of the vertical component is comparable with that of the east-west component, if the stochastic process or the approximation methed of mult-parameters estimation with appropriate functions is used to model the zenith tropospheric delay correction.