Li Jiancheng

Bathymetry predicted from vertical gravity gradient anomalies and ship soundings

Abstract: In this paper, the admittance function between seafloor undulations and vertical gravity gradient anomalies was derived. Based on this admittance function, the bathymetry model of 1 minute resolution was predicted from vertical gravity gradient anomalies and ship soundings in the experimental area from the northwest Pacific. The accuracy of the model is evaluated using ship soundings and existing models, including ETOPOl, GEBCO, DTU10 and V15. 1 from SIO. The models STD is 69.481m, comparable with V15. 1 which is generally believed to have the highest accuracy.

Long Distance Transference of Height Datum Across Seas

This paper focuses on studying logn distance transference of height datum across seas by combining ellipsoidal height derived from GPS with gravimetric geoid height. The Yellow Sea Height Datum is transferred to Yangshan Island which is 30 km away from Luchaogang in Shanghai. The stations heights derived in this way are compared with those determined from two independent sets of the tidal observations taken in two years, and the difference values are 1.0 cm and 6.0 cm, respectively. Moreover, the derived height differences between two sections on the island are also compared with the values derived from precise leveling with respect to the same section. The result shows that the inconsistencies are only 0.2 cm and 0.7 cm, respectively.

A local geoid determination based on ellipsoid approximation in Western China

A new methodology for precise geoid determination with finest local details based on ellipsoidal approximation is presented. This methodology is formulated through the “fixed-free two-boundary value problem” based on the observable of the type modulus of gravity intensity, gravity acceleration and gravity potential at the GPS positioned stations, with support of the known geoids potential value, W0.

A preliminary study on the establishment of ocean tide models over the South China Sea from T/P altimetry

On the basis of the characteristic of the perfect spatial distribution of the T/P altimeter data, a spatial harmonic tidal analysis is performed, which transfers tidal harmonic constantsH andg of each constituent into a pair of parameters: the cosine part U and sine partV. And each part is expanded into a polynomial. The polynomial coefficients are estimated with altimeter data upon the least squares criteria. Thus the models of principal tidal waves in the South China Sea are established. 72 cycles of T/P data from cycle 11 through 82 are included in the calculation. The models are evaluated with different approaches and data set. The conclusions are that the tide modes can provide partial tide amplitudes with 3 cm accuracy, and that phase lags deviation of those tides with amplitude large than 10 cm are within ±10°.

Discussion on some FFT problems to determine the geoid

The aim of this investigation is to study some FFT problems related to the application of FFT to gravity field convolution integrals. And the others, such as the effect of spectral leakage, edge effects, cyclic convolution and effect of padding, are also discussed. A numerical test for these problems is made. A large area of Western China selected for the test is located between 30°N∼36°N and 96°E∼102°E and includes 1 858 gravity observations on land. The results show that the removal of the bias in the residual gravity anomalies is important to avoid spectral leakage. One hundred percent zero padding is highly recommended for further research of the geoid to remove cyclic convolution errors and edge effects. 1-D FFT is recommended for precise local geoid determination because it does not use kernel approximation.

Extraction of two tsunamis signals generated by earthquakes around the Pacific rim

Abstract: As one of the ocean sudden natural disasters, the tsunami is not easily to differentiate from the ocean variation in the open ocean due to the tsunami wave amplitude is lees than one meter with hundreds of kilometers wavelength. But the wave height will increases up to tens of meters with enormous energy when the tsunami aarives at the coast. It would not only devastate entire cities near coast, but also kill miilions of people. It is necessary to forecast and make warning before the tsunami aariving for many countries and regions around the Pacific rim. Two kinds of data were used in this study to extract the signals of 2011 Tohoku tsunami and 2014 Iquique tsunami. Wave undulations from DART (Deep-ocean Assessment and Reporting of Tsunamis) buoys and SLA from altimetry could extract the tsunami signals generated by this two earthquake. The signals of Tohoku tsunami were stronger than that of Iquique tsunami probably due to the 2011 Tohoku tsunami was generated by a magnitude 9. 0 earthquake and the 2014 Iquique tsunami was triggered by a magnitude 8. 2 earthquake.

Ice-sheet surface elevation change from crossover of ENVISAT data

Understanding the current state of the polar ice sheets is critical for determining their contribution to sea-level rise and predicting their response to climate change. Surface elevation time series especially can be used to study ice-sheet dynamics and the mass or volume balance of the ice sheets, which are relevant to global climate change and sea-level rise. During the last two decades, satellite radar altimetry or airborne laser altimetry could obtain accuracy by an order of magnitude greater than the traditional airborne barometric altimetry, which has a precision of typically several tens of meters at best and only a limited coverage. The widest coverage comes from satellites, especially from the ERS1/2 and ENVISAT, which extends to 81.5° of latitude, covering almost all of Greenland and most of Antarctica. In this paper, an algorithm for time series analysis based on crossover was used to obtain 4-year (September 2002–March 2007) ice-sheet elevation changes from ENVISAT data. The height of the whole Antarctic ice sheet has a decline of about 0.4 ± 0.43 cm from September 2002 to March 2007. The time series data present clearly a seasonal and annual signal feature; that the ice sheet thickens in March. From the time series data, the seasonal and annual signal can be observed clearly.

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.