Theory And Methodology Of Earth Gravity Field Determination Using Satellite-to-Satellite Tracking

Since 2000, research of earth gravity field with Satellite-to-Satellite Tracking techniques has been in a notable development period. The development of SST techniques can be traced back to more than 30 years ago. Various schemes, including GRAVSAT, GRM, etc., were discussed and analyzed from then on, but the applied SST data were not available until the launch of CHAMP and GRACE. The two projects make SST be the highlight of Geoscience. The predictable potential of SST makes a series of SST projects and joint SST projects (GOCE, COSMIC etc.) be put in schedule. These SST missions may bring great achievements in earth gravity field determination. On the other hand, with highly precise space-borne GNSS (GPS, Galileo, etc.), accelerometer and KBR system payload techniques, the long-wave and middle-wave length part of precise static gravity field and the large-scale gravity field information varying with time can be mapped in low cost. The abundant information of earth gravity would not only meet the requirements of Geodesy, but also provide more earth information for Geophysics, Oceanography, water resource research, and other fields. At the same time, the space-borne GNSS occultation technique plays an important role in the numerical weather prediction (NWP) system, troposphere and ionosphere study. SST is also regarded as a prime technique for the seismic satellite project.SST techniques has two configuration types: one is Satellite-to-Satellite Tracking in high-low mode (SST-hl), the other is Satellite-to-Satellite Tracking in low-low mode (SST-11). In 1960's, the SST-hl principle was proposed and some experiments were carried out. Instead of a high orbit satellite and a low orbit one as required in SST-hl, SST-11 needs two satellites that would follow each other along much the same orbit, keep in special distance range, and at as low an altitude as atmospheric friction would allow. Wolff proposed the principle of SST-11 in 1969. Since then, some organizations (NASA, ESA, etc.) have carried out a series of pre-studying projects and some of them have been put in practice. But for various reasons, almost all of these projects were withdrawn or replaced at last. In 2000 and 2002, the launch of CHAMP and GRACE inspired the research upsurge again. In China, the study of SST techniques is still in preliminary step.In this dissertation, based on international and internal research work, the theories and methods of SST are summarized systematically. With CHAMP and GRACE as examples, the SST data are simulated and the perfomance analysis of SST are carried out. All the methods and conclusions are applicable for other SST projects. The main research work and related conclusions are as following:1. The development of satellite gravity techniques and their applications are reviewed. These techniques include ground tracking of satellite, satellite altimetry, SST andsatellite gravity gradiometry. The characteristics of SST are compared with those of traditional terrestrial gravity determination method and classical satellite gravity method. The import of highly precise space-borne GNSS (GPS, Galileo etc.), accelerometer and KBR system payload techniques makes SST be able to provide gravity information with precision high enough to fulfill the requirements of Geoscience.2. Time system, coordinate system and force model system related to SST are introduced. Data numerical simulation, perfomance analysis and data processing of SST are all based on the above three systems. Force model system is the key part in the simulation of SST-11 observations. A rigorous way to simulate SST observation data is to use precise orbit integeral method, taking into account all gravitational forces and nongravitational forces. The simulated observation data are then combined with simulated accelerometer data to carry out the analysis of SST data. In this dissertation, to highlight the research focus, the nongravitational forces are not taken into account in the simulation of SST observation data. The impact of these forces can be considered being deducted with the accelerometer data.3. Some concepts of the accelerometer characteristics and the basic principles of space-borne acceleration are introduced. With CHAMP space-borne accelerometer as an example, the processing and transformation methods of altenative acceleration data are introduced in detail. The way to transform the acceleration data to CIS using quaternions and combining star sensor data is described. CHAMP practical Orbit and Gravity (OG) level-2 data (From No. 001,2002 to No. 100, 2002) are processed. The results are analyzed and corresponding conclusions are drawn.4. The definitions of three types of SST measurements, including relative line of sight distance (LSD), relative line of sight velocity (LSV) and relative line of sight acceleration (LSA), are given. The relationship between the SST measurements and earth gravity potential spheric harmonic coefficients is deducted. The observation fuctions can be established and the coefficients can be calculated conveniently this way. The spectrum relationships between the recovered field errors and the errors of KBR, ACC and GPS measurments are established. Based on these relationships, the resolution and precision of gravity field recovered with SST data can be analyzed easily. To carry out the spectrum analysis efficiently, the maxmium entropy specturm method is used. The terrestrial covering of SST-11 observation is discussed.5. Based on the theories introduced in chapter 2 and 4, the SST observation simulation system and the performance analysis systems are established. The simulation system can produce SST observations for analysis. The functions of the performance analysis system include: KBR system performance analysis, satellite altitudeanalysis, the seprated distance of two satellites analysis, the gravity field recovery performance analysis (geoid error or gravity anomaly error), ACC performance analysis and GPS performance analysis. After the practical analysis, some conclusions are drawn.6. The basic configuration, objectives and data products of the CHAMP project are introduced in detail. The rapid science orbits of CHAMP provided by GFZ and JPL are compared. As another representative of SST-hl technique, the basic configuration and objectives of the GRACE project are introduced. Pitch angle of SST-11 is analyzed and a reformed pitch adjustment method, which can efficiently reduce the range of the pitch angle adjustment to ensure the KBR measurement of two along satellites, is proposed.,7. With the simulation and performance analysis systems, the simulating SST data are produced and analyzed. The impact of the degree and order of gravity field model and the impact of orbit parameters on the simulated observations are analyzed. The spectrum analysis of SST observations are carried out using the maxmium entropy specturm method. After the analyses, some conclusions are drawn. A way to improve the SST performance through adjusting the RAAN of two satellites is proposed. The ground distribution figures of SST observations are presented and analyzed.8. Considering the requirements of Geodesy, Geophysics, Oceanography and other fields on earth gravity field, an optimal SST scheme is provided by the simulation system. The scheme can map the earth gravity field in 200km resolution. The precision of geoid and gravity anomoly provided by the scheme can be 20cm and 3mgal respectively. Such precision is almost equivalent to that of the GRACE project, and the scheme asks for lower payload performance. All the optimal results can be used as references in future work.9. In this dissertation, the plentiful tables and figures are able to show the study results directly. All the programs are written in standard Fortran 90 language. These programs are easy for maitenance and updating and can be transplanted between different operation systems conveniently.