| The Earth’s gravity field is a fundamental physical field for Earth science research and analysis,and its spatial and temporal variations are closely related to the mass distribution and migration of the Earth.Therefore,accurate observation of the Earth’s gravity field is essential for understanding the material structure distribution and mass change of the Earth’s system.The implementation of satellite gravity measurement technology has ushered in a new era of high-precision observation of the Earth’s gravity field and greatly improved our understanding of the Earth’s gravity field and its variation over time.The GOCE(Gravity Field and steady-state Ocean Circulation Explorer)gravity satellite is the first gravity gradiometric-type satellite that provided valuable information for the recovery of the Earth’s static gravity field model during its 4 years of flight.Compared with other gravity satellite missions,the GOCE satellite is more sensitive to gravity field shortwave signals due to the electrostatic gravity gradiometer on board.The related products solved based on the satellite’s observations have been widely used in oceanography,seismology,and other research fields,improving our knowledge and understanding of the solid Earth and sea level changes.However,the complex noise characteristics of the gradiometer observations in the low-frequency region limit the accuracy of the gravity field model from GOCE observations.How to further improve the observation accuracy of this type of satellite has become a research hotspot in the field of Earth’s gravity field recovery.On the one hand,it needs to do refinement for the different noise characteristics of the gradient data,and on the other hand,as the key load that constitutes the gravity gradiometer,the optimization of the observational performance of the accelerometer will be a breakthrough to improve the accuracy of gravity gradiometric-type satellite observation.The development of new-generation accelerometers(including MicroSTAR,HybridACC,cold atom interferometry accelerometers,etc.)provides new opportunities for future satellite gravity gradient measurement missions.Given this,this dissertation systematically conducts research to address the above two needs and achieves the following research results:(1)Effects of different filters on the recovery of Earth’s gravity field using GOCE satellite gravity gradient dataWe used the time-domain method to recover the Earth’s static gravity field model and evaluated the applicability of different filters to the 1/f error,the orbital frequency error in the observation errors,and the time variability of the error.The experimental results show that the cascade filter,consisting of a differential filter and an autoregressive moving average(ARMA)filter,provides the best accuracy for the 1/f low-frequency errors.For the processing of orbital frequency errors,the introduction of empirical parameters can reduce the effect of errors,and the introduction of trap filters makes the inverted results worse.Frequent orbit changes and other changes in the observing environment have little impact on the solution of the new version of the data.However,a comparative analysis of the old version of the data shows that the influence of this factor on the calculation results is not negligible and can be effectively eliminated by shortening the estimation period of the filter.(2)Recovery of the Earth’s static gravity field model based on GOCE satellite dataSeparate and joint inversions based on GOCE satellite orbit and gravity gradient observations were performed to solve the Earth’s static gravity field model.The maximum degree is 250.In addition to obtaining two independent gravity field models that are complementary in the wavelength part,five generations of gravity field models were obtained based on the inversion of the observation data segments adopted by the official model for five-time spans(November 2009～January 2010,November 2009～June 2010,November 2009～April 2011,November 2009～June 2012,and November 2009～October 2013).The cumulative degree error RMS of the geoid height relative to the GOCO06s model for the five combined models are 4.62,3.01,2.27,1.68,and 1.07 cm below the 200 d/o,respectively,which are comparable to the accuracy of the officially released TIM series models.(3)Contribution of satellite gravity gradient measurements based on nextgeneration accelerometers to the observ ation of the Earth’s gravity fieldWe evaluated the contribution of the new generation accelerometers(MicroSTAR,HybridACC,and cold atom interferometry accelerometer)to the Earth’s gravity field in the scenario of satellite gravity gradient measurements through numerical closedloop simulations.Compared to the simulations with the GRADIO gradiometer carried in GOCE,the MicroSTAR-type gradiometer solutions for the three main diagonal components are more accurate up to degree 100 and they are of comparable accuracy after degree 100.This type of gradiometer can significantly improve the accuracy of the joint solution in the high degree part(after degree 50)by making equal accuracy observations of the off-diagonal components The accuracy of the gravity field model recovered from the HybridACC-type gradiometer is affected by the noise level of the cold atom interferometry(CAI)accelerometer.A HybridACC-type gradiometer with low CAI performance(1×10-9 m·s2/(?))can only guarantee the accuracy of the higher degree coefficients since it has a higher measurement accuracy within the measurement bandwidth(5 mHz～100mHz).A better CAI performance up to 1×10-11 m·s2/(?)will significantly improve the retrieval performance.For the CAI gradiometer,the solved gravity field model can show higher accuracy at all degrees and orders if the satellite considers the orbital rotation compensation mechanism.Otherwise,the solution accuracy based on this type of gradiometer only shows the advantage below 50 d/o.(4)Effects of different parameters choices in the CAI gradiometer on the gravity field solutionWe evaluated the effects of different parameter choices of the CAI gradiometer(including different orbital rotation compensation schemes,different sampling rates,and satellite observation modes)on the gravity field observations.The experimental results show that the orbital rotation compensation system can effectively reduce the influence of satellite rotation on the CAI gradiometer in the nadir pointing observation mode,and thus improve the accuracy of the gravity field model solution.The difference between different schemes is negligible,and adjusting the Raman reflector in a twomonth cycle is sufficient to meet the gravity field solution requirements.For the sampling rate of the CAI gradiometer,the accuracy of the solved model can be improved by increasing the sampling rate.However,regardless of whether single-axis,dual-axes,or triaxial-axes observations are employed,the CAI gradiometer only outperforms the GOCE satellite results for an overall solution at a resolution of 1s or even higher.In the inertial pointing mode and when only the residual angular velocity is considered,the effect of the angular velocity error on the solved results still exists.When the satellite orbital plane rotation is added,the joint solution outperforms the GOCE satellite results at all degrees.Comparing the results of the two observation modes,it can be found that the single-axis CAI gradiometer in the nadir pointing mode with the orbital compensation scheme,the Vzz component has a higher accuracy of the component solution.The two-axis CAI gradiometer in the inertial pointing mode with the orbital plane rotation and the two combined modes Vxx+Vzz and Vyy+Vzz have the best accuracy.The observation accuracy of the three-axis CAI gradiometer is comparable in both observation modes,and both are better than the results of the GOCE satellite. |
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