Soil moisture is an important variable in ecological, hydrological, and meteorological studies. An effective method for improving the accuracy of soil moisture retrieval is the mutual supplementation of multi-source data. The sensor configuration and band settings of different optical sensors lead to differences in band reflectivity in the inter-data, further resulting in the differences between vegetation indices. The combination of synthetic aperture radar (SAR) data with multi-source optical data has been widely used for soil moisture retrieval. However, the influence of vegetation indices derived from different sources of optical data on retrieval accuracy has not been comparatively analyzed thus far. Therefore, the suitability of vegetation parameters derived from different sources of optical data for accurate soil moisture retrieval requires further investigation. In this study, vegetation indices derived from GF-1, Landsat-8, and Sentinel-2 were compared. Based on Sentinel-1 SAR and three optical data, combined with the water cloud model (WCM) and the advanced integral equation model (AIEM), the accuracy of soil moisture retrieval was investigated. The results indicate that, Sentinel-2 data were more sensitive to vegetation characteristics and had a stronger capability for vegetation signal detection. The ranking of normalized difference vegetation index (NDVI) values from the three sensors was as follows: the largest was in Sentinel-2, followed by Landsat-8, and the value of GF-1 was the smallest. The normalized difference water index (NDWI) value of Landsat-8 was larger than that of Sentinel-2. With reference to the relative components in the WCM model, the contribution of vegetation scattering exceeded that of soil scattering within a vegetation index range of approximately 0.55–0.6 in NDVI-based models and all ranges in NDWI1-based models. The threshold value of NDWI2 for calculating vegetation water content (VWC) was approximately an NDVI value of 0.4–0.55. In the soil moisture retrieval, Sentinel-2 data achieved higher accuracy than data from the other sources and thus was more suitable for the study for combination with SAR in soil moisture retrieval. Furthermore, compared with NDVI, higher accuracy of soil moisture could be retrieved by using NDWI1 (R2 = 0.623, RMSE = 4.73%). This study provides a reference for the selection of optical data for combination with SAR in soil moisture retrieval.

This study extracts ocean tides in the China sea areas and western Pacific ocean by the ERS-2 and Envisat-1 series of sun synchronous satellite altimeter data over a tidal period of about 15 years.Orthotide response analysis is performed tocompute harmonic constants of 12 constituents along the tracks,and the accuracy of tidal parameters is assessed at the crossovers.The results show that the major constituents derived from the ERS series altimeter data are reasonable and reliable,excepting the S2 and K1 constituents.Considering the additional information from crossovers and adjacent tracks,it is possible to improve the accuracy of harmonics for coastal zones and high latitude areas.

Tidal analysis based on altimeter data,including harmonic method and orthotide response method,is one of the main methods for ocean tide modeling.In this paper,altimeter data from TOPEX/POSEIDON,Jason1,and Jason-2 has been processed to solve major constituents in the China Sea Areas and the Western Pacific Ocean using harmonic and orthotide response methods.Comparative research between the results reveals that the increase of record length will improve the precision of tidal analysis.The precision of harmonic method is almost the same as the response method when the measurement time span is sufficient for separating synodic periods of any pair of constituents.Furthermore,the response method performs better with insufficient data length.

The accuracy and resolution of marine gravity field derived from satellite altimetry mainly depends on the range precision and dense spatial distribution. This paper aims at modeling a regional marine gravity field with improved accuracy and higher resolution (1′ × 1′) over Southeastern China Seas using additional data from CryoSat-2 as well as new data from AltiKa. Three approaches are used to enhance the precision level of satellite-derived gravity anomalies. Firstly we evaluate a suite of published retracking algorithms and find the two-step retracker is optimal for open ocean waveforms. Secondly, we evaluate the filtering and resampling procedure used to reduce the full 20 or 40 Hz data to a lower rate having lower noise. We adopt a uniform low-pass filter for all altimeter missions and resample at 5 Hz and then perform a second editing based on sea surface slope estimates from previous models. Thirdly, we selected WHU12 model to update the corrections provided in geophysical data record. We finally calculated the 1′ × 1′ marine gravity field model by using EGM2008 model as reference field during the remove/restore procedure. The root mean squares of the discrepancies between the new result and DTU10, DTU13, V23.1, EGM2008 are within the range of 1.8– 3.9 mGal, while the verification with respect to shipboard gravity data shows that the accuracy of the new result reached a comparable level with DTU13 and was slightly superior to V23.1, DTU10 and EGM2008 models. Moreover, the new result has a 2 mGal better accuracy over open seas than coastal areas with shallow water depth.

Sea level changes along the coast of Vietnam were calculated and analyzed by using satellite altimetry and tide gauge data.The results showed that there is a strong correlation between sea level changes obtained from the both data.The rate of rising sea level in Vietnam calculated by the altimeter data from 1993 to 2015 was 3.18mm/a, and the rate by tide gauge data was 4.1mm/a.During the whole time span of tide gauge observation, the coastal sea level of Vietnam was on the rise, at a mean rate of about 3.02mm/a.The coastal sea level of Vietnam was characterized by strong seasonal characteristics.In the coastal areas of the Red River and the Mekong Delta, it was highly susceptible to the seasonal storms and floods.

In order to better understand the tidal characteristics and changes along the coast of Vietnam,the harmonic tidal analysis of the long-term observation data at six tide gauge sites of HONDAU,HONNGU,DANANG,QUYNHON,VUNGTAU and RACHGIA is performed to calculate the harmonic constants of the main principle constituents of O1 、P1、K1 、Q1 、M2 、S2 、N2 、K2 and long period tides,such as nodal tide Mn.The results show that the characteristics of the tide along coastline of Vietnam are complex,and the coastal zone is more vulnerable at the time when the seasonal sea level is at its highest.The observed amplitudes of the 18.6-year nodal tide Mn along Vietnam coast are uneven in spatial distribution,and significantly exceed the theoretical amplitudes,by less than 1 cm,which is relative to the changes of the non-tidal low-frequency sea level.

Spatial and temporal variations in the vertical stratification of the troposphere introduce significant propagation delays in interferometric synthetic aperture radar (InSAR) observations. Observations of small amplitude surface deformations and regional subsidence rates are plagued by tropospheric delays, and strongly correlated with topographic height variations. Phase-based tropospheric correction techniques assuming a linear relationship between interferometric phase and topography have been exploited and developed, with mixed success. Producing robust estimates of tropospheric phase delay however plays a critical role in increasing the accuracy of InSAR measurements. Meanwhile, few phase-based correction methods account for the spatially variable tropospheric delay over lager study regions. Here, we present a robust and multi-weighted approach to estimate the correlation between phase and topography that is relatively insensitive to confounding processes such as regional subsidence over larger regions as well as under varying tropospheric conditions. An expanded form of robust least squares is introduced to estimate the spatially variable correlation between phase and topography by splitting the interferograms into multiple blocks. Within each block, correlation is robustly estimated from the band-filtered phase and topography. Phase-elevation ratios are multiply- weighted and extrapolated to each persistent scatter (PS) pixel. We applied the proposed method to Envisat ASAR images over the Southern California area, USA, and found that our method mitigated the atmospheric noise better than the conventional phase-based method. The corrected ground surface deformation agreed better with those measured from GPS.

Based on variation characters of sea surface height and parameters of ICESat return pulse waveform,a method is proposed for estimating Arctic sea ice freeboard with ICESat observation.This proposed method is compared with the common ‘lowest-level’method.The results show that the lowest-level method is vulnerable to gross errors and present systematic errors in spatial distribution of freeboard. But the proposed method is more rel iable than the lowest-level method.The lowest-level method is improved by the employ of an elevation standard deviation threshold and combined with the proposed method to estimate sea ice freeboard for six ICESat campaigns between 2005 and 2006 to review the seasonal variabi l ity.The results show that the abnormal cl imate in the summer of 2005 have a profound impact on arctic sea ice,which caused the decrease of freeboard and changed the constituent of sea ice as wel l.

The Cryosat-2altimetry waveform measurements on the sea ice are retracked using the 40% threshold method in the Arctic Ocean.By combing the waveform parameters and the sea ice concentrations,the sea ice and the Lead(open water between the ice floes)are identified effectively.The sea ice freeboard is determined by along-track searching and an interpolation algorithm;it is compared with the AWI results,and the variations of the mean freeboard of the first-year and the multi-year sea ice area between 2011 and 2013in the Arctic Ocean are given.The comparison results show good correlation in the corresponding period of each year.

Using the GDR data recorded by the Jason-1,Jason-2 and Envisat altimetry satellites for three years,and on the basis of the geoid calculated by the GOCO02S gravity field model which is combined with GOCE and GRACE,the mean dynamic topography is calculated over China sea and neighbour using Pointwise approuch.The results show that different trends are reflected from mean sea surface and mean dynamic topography,which are consistent with the known results respectively.

A new selenoid( lunar geoid) is derived using the lunar gravity model CEGM02 and topography model LRO_LTM02,which approximates the physical surface as closely as possible. The estimated gravity potential value of this selenoid is 2 822 327. 8 ± 16. 2 m2/ s2,denoted as W0in usual. A triaxial level ellipsoid whose surface gravity potential equals W0above is calculated and used as a reference ellipsoid,and its semi-major axis,polar flattening,equator flattening and longitude of semi-major axis are 1 737 462 m,1 /2 579,1 /6 863 and- 76. 8 ″,respectively. Based on this reference ellipsoid,the gravity decreases about 60 × 10- 5m / s2from the equator to the poles,while the variation along the equator is about 25 × 10- 5m / s2. The selenoid undulations are in the range of- 325. 9 m to 389. 1 m.

The response function between seafloor undulation and sea surface vertical gravity gradient anomalies is derived based on the response function between sea floor velief and sea surface gravity anomaly.Accordingly,a simulation research is carried out with Gaussian seamount model.The effects of lithosphere effective elastic thickness,crustal density and the truncation wavelength on predicted results are studied.Finally,a high accuracy bathymetry model was established from actual vertical gravity gradient anomalies data.

Multi-satellite altimetry has provided a wealth of data for measuring the long-term sea level change, T/P, Jason-1 and Jason-2 construct a seamless record of global sea level change from 1993 to present. We present the results of our calibration activities, including data comparisons during the tandem period of the missions, during which we resolve the rel- ative bias between the missions, as well as comparisons to independent tide gauge data. When the entire record is assembled, global mean sea level change from 1993-2011 is 3.12± 0.4 mm · a-1 , and interannual variation is strongly correlated to E1 Nino phenomenon.

Lunar geoid(or selenoid) anomaly and topogrphy variations are the expressions of its internal density anomaly and boundary undulations.So the geoid to topography ratio(GTR) can be used to study the isostatic state of the moon and its crustal thickness.A new lunar crustal thickness model was derived by GTR technique from filtered gravity model SGM100h and topography model STM359_grid-02,with the mare fill and subcrustal mass anomalies removed.This model indicates that the lunar crust has an average thickness of 36.9 km,and the average farside thickness is 13.5 km thicker than nearside.The crustal thicknesses at Apollo 12/14 sites of our crustal thickness model are 28.3 km and 29.1 km,respectively.It also shows that the crust beneath the mare basins is generally thinner than the regions around.

A fast algorithm is proposed for the direct discrete summation of Stokes ’ and Hotine ’ s integral,the integral kernels are expressed as the functions of the latitudes of computation point and running point and the longitude-difference between them,the symmetries of the kernels are exploited so that they may only be evaluated once for all the computation points on the same latitude,the computation times of fast algorithm are far less than that of ordinary algorithm.The numerical simulation based on EGM2008 Earth gravitational model shows that the fast algorithm is far more efficient than the oridinary algorithm that the problem of numerical inefficiency can be effectively solved.Besides,the new algorithm retains the attributes of surface integral and can be used to replace 1DFFT for evaluating Stokes ’ or Hotine ’ s integral.

Due to the availability of multi-satellite altimetry and advances in numerical simulation methodologies in recent 20 years,recent global ocean tide models are well known to 2 cm RSS(root square of the sum of the squares of the 8 rms values) and with a spatial resolution of 50 km in deep ocean.The paper is aimed to validate the performance of five models by using tide gauge data.The results show that the performance of NAO99b is best in China sea areas,its RSS is 14.86 cm.Compared with other contemporary models,EOT10a fit the tide gauge data in global distribution better.

On the basis of linear relation between gravity anomaly and seafloor topography,the 2′×2′ bathymetry model in China sea and its adjacent areas would be calculated from the satellite altimeter-derived gravity anomalies and with ETOPO2v2 model.On the other hand,the calculated result is compared with the ETOPO2v2 model and shipboard depth,and then the following conclusion can be drawn that the bathymetry model calculated by gravity anomaly can reflect the characters of suboceanic structure exactly in China sea and adjacent Areas.