Zou Xiancai

The Fast Analysis of the GOCE Gravity Field

The semi-analytical (SA) approach is proposed for the fast determination of gravity field from the GOCE (Gravity Field and steady-state Ocean Circulation Explorer) observations. In this paper, we discuss the principle and the characteristics of the SA approach in detail, and point out that based on the application of FFT and the introduction of the nominal orbit (circular, exact repeat orbit, uninterrupted measurement time series, and constant inclination), the SA approach can effectively recover the potential coefficients from large observations. This method can give a fast diagnosis of the GOCE system performance in parallel to the running of the GOCE mission by comparing the estimated noise characteristics of the SGG (Satellite Gravity Gradiometry) time series with the gradiometer error PSD (Power Spectrum Density) (prior). The performance of this method is evaluated using the simulated observations generated in different cases (ideal and practical). The results in this paper show that this method can be applied to the practical cases of non-circular, non-repeat orbits and non-constant inclination in the time series of observations by the iteration algorithm which can eliminate or weaken the approximate errors in the observation model. The test examples show that to recover the high degree gravity field model, the optimal regularization parameter should be selected in the Kaula regularization technique

Research on the Calibration of Onboard Accelerometer by Dynamic Method

For the new generation of satellite gravity missions, the observations of the onboard accelerometer can be calibrated by the dynamic method. In this paper, the strategy to calibrate the onboard accelerometer is studied in detail using the simulated precise orbit, and the impacts of the reference earth gravity model on the accelerometer calibration are discussed specifically. To evaluate the impacts of different strategies on the accelerometer calibration, the satellite orbit is integrated again with the calibrated accelerometer observations and the related dynamic models, and compared with the precise orbit. When the reference earth gravity model is fixed in the accelerometer calibration, the standard deviation of the differences between the re-integrated orbit and the simulated may be up to the order of meters, which shows that the reference earth gravity model has a significant effect on the accelerometer calibration. When the geopotential coefficients and the accelerometer calibration parameters are estimated simultaneously (simultaneous solution method in short), the accelerometer calibration parameters are independent of the reference earth gravity model. The results in this paper show that the simultaneous solution method is suitable for the calibration of the onboard accelerometer. It is also illustrated that to determinate the earth gravity model and the calibration parameters more reliably and accurately, we should choose an optimal maximum degree of the recovered earth gravity model

Accuracy analysis of geopotential coefficients recovered from in-situ disturbing potential by energy conservation method

The characteristics of the normal equation created in recovering the Earth gravity model (EGM) by least-squares (LS) adjustment from the in-situ disturbing potential is discussed in detail. It can be concluded that the normal equation only depends on the orbit, and the choice of a priori gravity model has no effect on the LS solution. Therefore, the accuracy of the recovered gravity model can be accurately simulated. Starting from this point, four sets of disturbing potential along the orbit with different level of noise were simulated and were used to recover the EGM. The results show that on the current accuracy level of the accelerometer calibration, the accuracy of the EGM is not sufficient to reflect the time variability of the Earth’s gravity field, as the dynamic method revealed.