Research On Ocean Tide Loading Problem In GPS Precision Surveying

With the vigorous development and wide application of modern space positioning technology represented by GPS (Global Positioning System), a large amount of valuable raw data has been accumulated in the past20years, which opens up the broad prospects for geoscience’s research such as geodesy and geophysics. High precision and high reliability of GPS data processing result is regarded as a basis for geoscience’s research, ocean tide loading displacement corrections to improve the estimation accuracy of GPS has been adopted widely, it not only provides a effective means to separate the effect of ocean tide loading from GPS coordinate time series, but also can test the accuracy and adaptability of ocean tide model using GPS coordinate time series, so analyzing ocean tide loading effect in GPS precision measurement hold the important theoretical meaning and reference value in better service for geodetic and geophysical research.In this paper, the development trend of ocean loading effect in GPS precision measurement is analyzed and summarized, the different ocean tide loading models are focused on, the relationship between ocean tide loading displacement and the zenith tropospheric delay (ZTD) variation is established, and the spurious long period signal character in GPS height time series is further investigated, which is caused by unmodeled ocean tide loading displacement. The main contents and achievements are as follows:(1) A brief overview of basic theory for modeling ocean tide and ocean tide loading displacement is given. The tide-raising potential and its harmonic expansion are studied, and then the expression analysis methods to ocean tides and ocean tide loading are derivate in detail.(2) The measurement models of zero-difference PPP(Precise Point Positioning) is introduced, including GPS time systems and the transformation between them, GPS coordinate systems and the transformation between them, code pseudo range measurement, phase pseudo range measurement and parameter estimation models. At last the error sources of zero-difference PPP are analyzed and the correction models and methods are summarized.(3) The spatial distribution characteristics in China Sea area is obtained by the M2constituent of the global ocean tide model of FES2004. The result reflects small amplitude region in the South China Sea, the larger amplitude region in the Bohai Sea, the Yellow Sea and the East China Sea, amplitude anomaly region near the coastal areas. On the basis of this study, the ocean tide loading displacements are computed across China, the magnitude of the vertical displacement are3-4times as large as that of the horizontal displacements, meanwhile ocean tidal loading displacement change is great in the coastal and offshore areas.7kinds of global ocean tide models are chosen to analyze the effects of different ocean tide loading displacement in different regions of China, the results show that the root mean square of different ocean tide loading displacement is larger in the offshore area, that indirectly testifies that global ocean tide models have a lower accuracy in the offshore region in China.(4) Using the global ocean tide model and local ocean tide model in China Seas, the ocean tide loading displacements are calculated and analyzed for13IGS stations in China and the surrounding areas. GPS daily and sub-daily solutions are investigated when ocean tide loading displacements are modeled and then not modeled. According to the comparisons, the accuracy of local ocean tide model and those from global ocean tide mode is in the same level, between which ocean tide loading displacements are identified. For GPS daily solution, on the whole the ocean tide loading displacements have no effect on the station coordinate, but have influence on the vertical coordinates of the coastal stations; for GPS sub-daily solutions, ocean tide loading displacement correction can improve2.5mm accuracy on the coastal stations in China.(5) GPS-derived ZTD estimates are researched when the mapping function and the horizontal gradient are taken into account. The results show that GMF is better than NMF, which increases ZTD estimation precision. While horizontal gradient can also improve the accuracy of ZTD estimation, especially under the conditions of low cut-off elevation angle and weather dramatic changes. The effect of ocean tide loading vertical displacement on GPS-derived ZTD estimates is investigated in China and the surrounding areas, the linear relation between ocean tide loading vertical displacement and ZTD is found, the scaling factor is in the range of2-5for a7°cut-off elevation angle. In order to meet the requirements of GPS meteorology application that precipitable water vapor (PWV) is better than1mm, ocean loading effects must be taken into when estimating ZTD from GPS data, especially in the coastal area.(6) The aliasing signal propagation mechanism in GPS height time series is systematically studied. Spurious long period signal characters are investigated by analyzing GPS time series at8stations in China and surrounding areas at which the ocean tide loading displacement is not modeled. It is found that spurious long period signals rang in period from fortnight to one year, mainly focus on fortnight, semiannual and annual periods. It is shown that the unmodeled height component periodic ground displacement can propagate to GPS height signal with admittance of12%, whereas the horizontal admittance maximum reaches a maximum of100%. At the same time the spurious long period signals caused by unmodeled three-dimensional component are obviously different from that caused by unmodeled height component. It is reflected that the horizontal load displacement values are very small, but those can not be ignored for spectrum characteristics interpretation. At last a concise evaluation method is provided for accuracy assessment of ocean tide loading models through the peak amplitude of specific frequency in GPS time series.