High-Rate GPS Data Non-difference Precise Processing And Its Application On Seismology

The 12 May 2008 Ms 8.0 Wenchuan earthquake caused a large amount of casualties and property losses. Global Navigation Satellite Systems (GNSS), as a primary technique of modern geodetic observation, can obtain the accurate crustal deformation. Futhermore, with the high-rate GNSS data, we can obtain the transient crustal deformation and seismic wave. The high-rate GNSS provides a new tool to study the earthquake rupture process, seismic wave characteristics, the position of earthquake epicenter inversion etc. Therefore GNSS seismology has become the forefront topic in the present research field of GNSS applications.Compared with traditional seismic measurements, High-rate GNSS have a lot of advantages.1) when recovery of displacements is desired, GNSS directly estimates them, but seismic data must be integrated once or twice in order to recover displacement. Integration is an often error-prone process and has the potential to amplify noise and distort the true signal.2) Seismic instruments can saturate or clip with sufficiently large ground motions so that the instrument does not record the full amplitude of local velocity or acceleration. GNSS observations will not saturate in amplitude because, unlike seismometers, no instrument response limits the observation capability of the receiver.3) Seismometers operate based on the theory of gravity, and tilt of the instrument can produce artificial horizontal acceleration, but GNSS instruments are not affected in this way.The Crustal Movement Observation Network of China (CMONOC) is a network of large scale and high precision that covers over the whole China mainland, the sampling rate of GNSS can be up to 50Hz. In addition, many provinces in China have built the Continuously Operating Reference Station (CORS) network, which can also provide high-rate GNSS data. All of them will provide a large scale and high-rate GNSS data for researching GNSS seismology.However, there are many shortcomings while using the high-rate GNSS technology for GNSS seismology. Firstly, the approaches and software of high-rate GNSS data processing are not fully meet the high precision application requirements of the dynamic deformation analysis. To date, there are two approaches in estimating station positions with high-rate GPS data: network solution and Precise Point Positioning (PPP) technique. In the former approach, at least one station must be fixed or tightly constrained to its known values, although it is normally also displaced by the seismic motions. Therefore, the displacements estimated for the other stations are biased by the displacement of the fixed station. In order to obtain displacement with respect to a reference frame, stations which are not affected by the earthquake should be included as reference stations into the data processing. As the position accuracy in the relative positioning degrades usually along with the baseline length, the inter-station distance is limited to several tens to hundreds kilometers. In the PPP approach where satellite clocks and orbits are fixed to pre-estimated precise values, and the coordinates can be estimated station by station in the reference frame defined by the orbits and clocks. However, PPP approach induces systematic errors such as multipath error, unmodeled antenna phase center varitions, and errors of the satellite orbit, so the position are significantly influenced by these errors. Secondly, the seismic waves obtained by high-rate GNSS positioning are different from that obtained by seismograph, how to make full use of GNSS seismic wave for seismology research is a critical topic.The paper aims at the improvement of high-rate GPS non-difference dynamic positioning acurracy and application to seismology, and focuses on the following research: A method to obtain the high precision transient crustal deformation by non-difference positioning with high-rate GPS data is proposed. It is based on network solutions PPP ambiguity fixing, sidereal filtering, and S-transform denosing. After getting the GPS seismic wave, some inversions are researched in seismology. Main constents and results in this paper include the follow parts:1) The paper first summarized the development trends of high-rate GPS technology, the construction of GPS observation network in global area, and the GPS data processing software. According to the application requirements of high-rate GPS positioning approach and software, it clarified that the method to obtain the high precision transient crustal deformation by non-difference positioning with high-rate GPS data need to be developed urgently.2) Two major non-difference GPS positioning methods are introduced, PPP and non-difference network solution. And two critical issues of GPS dynamic positioning are investigated. Finally, the positioning accuracy of the PPP and non-difference network solution method are compared.3) The problem of ambiguity fixing in dynamic positioning with PPP is further studied and realized. Analysis with examples illustrates that ambiguity fixing is able to enhance positioning precision in East direction, but the effects will not be so obvious when the precision of float solution higer than lcm. With regard to North direction, there is no significant improvement in positioning precision even after ambiguity fixing.4) Sidereal filtering technique is used to eliminate systematic errors (such as multipath error) related to station environment and geometric structure of satellites. Meanwhile, effects of different period on sidereal filtering are analyzed. Results show that compared with directly using sidereal period, actually-calculated mean satellite orbit repetition period can improve about 10% in positioning precision,5) S-transform denoising method is studied, realized and applied to denoising processing of GPS seismic wave, which leads to more accurate seismic wave arrival time. Considering the existence of longer-period error terms in GPS seismic signals, S-transform plus trend correction approach is proposed, which helps GPS seismic signals contribute in seismic analysis.6) To verify the positioning precision of high sampling rate GPS technology, experimental analysis is conducted with auto-designed GPS-Seismometer simulation platform. It is illustrated that sound consistence exists between vibrations recovered from GPS and that from seismometer.7) Epicenter inversion method based on dynamic processing result of high sampling rate GPS observables, and mathematic model for inversion of epicenter and original time with GPS data is deduced. Based on this method, GPS data during Wenchuan earthquake is used to inverse the epicenter and earthquake time. Compared with the results issued by China Seismological Bureau, inversed epicenter and earthquake time have errors of 12.5km and 7s, respectively. GPS data during Chile earthquake is used to inverse the epicenter and earthquake time, the inversion errors are 27.4km and 5s compared with the results provided by United States Geological Survey (USGS).