Coastal bathymetry inversion using SAR-based altimetric gravity data:A case study over the South Sandwich Island

The global bathymetry models are usually of low accuracy over the coastline of polar areas due to the harsh climatic environment and the complex topography.Satellite altimetric gravity data can be a supplement and plays a key role in bathymetry modeling over these regions.The Synthetic Aperture Radar(SAR)altimeters in the missions like CryoSat-2 and Sentinel-3A/3B can relieve waveform contamination that existed in conventional altimeters and provide data with improved accuracy and spatial resolution.In this study,we investigate the potential application of SAR altimetric gravity data in enhancing coastal bathymetry,where the effects on local bathymetry modeling introduced from SAR altimetry data are quantified and evaluated.Furthermore,we study the effects on bathymetry modeling by using different scale factor calculation approaches,where a partition-wise scheme is implemented.The numerical experiment over the South Sandwich Islands near Antarctica suggests that using SARbased altimetric gravity data improves local coastal bathymetry modeling,compared with the model calculated without SAR altimetry data by a magnitude of 3:55 m within 10 km of offshore areas.Moreover,by using the partition-wise scheme for scale factor calculation,the quality of the coastal bathymetry model is improved by 7.34 m compared with the result derived from the traditional method.These results indicate the superiority of using SAR altimetry data in coastal bathymetry inversion.

The Preliminary Discussion of the Potential of GNSS-IR Technology for Terrain Retrievals

The expansion of research and applications of Global Navigation Satellite Systems(GNSS)has revealed the information of reflecting surface in inherent multipath errors.GNSS signals,usually used to measure position,have been demonstrated that they can be used to retrieve water properties including water level,soil moisture,snow depth,and vegetation water content,which are important for climate analysis and water resources monitoring.Reflected GNSS signals with different azimuths can carry information of the corresponding reflecting zone,which means every reflected signal has distinct"signal-tonoise ratio(SNR)characteristics"influenced by specific reflecting zones—and the parameter named"Reflector Height(RH)"deduced from SNR frequency is focused on in this study.Thus,after interpolation of a series of reflector height by coordinates of the footprint,products describing highly detailed terrain over a reflecting footprint can be produced.Data of three GNSS sites in Earth Scope Plate Boundary Observatory,named P025,P351 and P101,was used to evaluate the terrain after calculating the terrain slopes and correcting the footprint following the slopes.A comparison of the results with a digital elevation model(DEM)showed that it is possible to retrieve terrain by GNSS-Interferometric Reflectometry(GNSS-IR);and the comparison with terrain slopes from DEMs in previous research also validated its potential.

An Edge-assisted, Object-oriented Random Forest Approach for Refined Extraction of Tea Plantations Using Multi-temporal Sentinel-2 and High-resolution Gaofen-2 Imagery

As a consumed and influential natural plant beverage,tea is widely planted in subtropical and tropical areas all over the world.Affected by(sub)tropical climate characteristics,the underlying surface of the tea distribution area is extremely complex,with a variety of vegetation types.In addition,tea distribution is scattered and fragmentized in most of China.Therefore,it is difficult to obtain accurate tea information based on coarse resolution remote sensing data and existing feature extraction methods.This study proposed a boundary-enhanced,object-oriented random forest method on the basis of high-resolution GF-2 and multi-temporal Sentinel-2 data.This method uses multispectral indexes,textures,vegetable indices,and variation characteristics of time-series NDVI from the multi-temporal Sentinel-2 imageries to obtain abundant features related to the growth of tea plantations.To reduce feature redundancy and computation time,the feature elimination algorithm based on Mean Decrease Accuracy(MDA)was used to generate the optimal feature set.Considering the serious boundary inconsistency problem caused by the complex and fragmented land cover types,high resolution GF-2 image was segmented based on the MultiResolution Segmentation(MRS)algorithm to assist the segmentation of Sentinel-2,which contributes to delineating meaningful objects and enhancing the reliability of the boundary for tea plantations.Finally,the object-oriented random forest method was utilized to extract the tea information based on the optimal feature combination in the Jingmai Mountain,Yunnan Province.The resulting tea plantation map had high accuracy,with a 95.38%overall accuracy and 0.91 kappa coefficient.We conclude that the proposed method is effective for mapping tea plantations in high heterogeneity mountainous areas and has the potential for mapping tea plantations in large areas.

Monitoring of Coastal Subsidence Changes Based on GNSS Positioning and GNSS-IR

Coastal subsidence monitoring typically employs Global Navigation Satellite System(GNSS)positioning technology.This method provides information only about subsidence below the station base.Sediments in coastal areas tend to accumulate quickly,and subsidence can change significantly due to compaction and alluvium.Therefore,monitoring subsidence above the base is essential to obtain overall coastal subsidence.A new technology called GNSS-Interferometric Reflectometry(GNSS-IR)has been recently developed,which can utilize multipath effects to monitor reflector height.Since the base of the GNSS station is deep and the base length remains constant,the height changes measured by the GNSS-IR technology can reflect subsidence above the base.Accordingly,this paper employs GNSS-IR technology to measure subsidence changes above the base.Additionally,GNSS positioning technology is used to obtain subsidence changes below the base,and the overall subsidence change is then calculated using both GNSS-IR and GNSS positioning technology.The Mississippi River Delta,known for its significant sediment thickness,was selected as the study area,and data from FSHS,GRIS,and MSIN stations was analyzed.The results demonstrate that GNSS-IR can be used to measure the subsidence rate above the base,and the corrected overall subsidence rate is equivalent to the relative sea level rise rate.

Multi-mode Multi-frequency GNSS-IR Combination System for Sea Level Retrieval

With the development of Global Navigation Satellite Systems(GNSS),geodetic GNSS receivers have been utilized to monitor sea levels using GNSS-Interferometry Reflectometry(GNSS-IR)technology.The multi-mode,multi-frequency signals of GPS,GLONASS,Galileo,and Beidou can be used for GNSS-IR sea level retrieval,but combining these retrievals remains problematic.To address this issue,a GNSS-IR sea level retrieval combination system has been developed,which begins by analyzing error sources in GNSS-IR sea level retrieval and establishing and solving the GNSS-IR retrieval equation.This paper focuses on two key points:time window selection and equation stability.The stability of the retrieval combination equations is determined by the condition number of the coefficient matrix within the time window.The impact of ill-conditioned coefficient matrices on the retrieval results is demonstrated using an extreme case of SNR data with only ascending or descending trajectories.After determining the time window and removing ill-conditioned equations,the multi-mode,multi-frequency GNSS-IR retrieval is performed.Results from three International GNSS Service(IGS)stations show that the combination method produces high-precision,high-resolution,and high-reliability sea level retrieval combination sequences.

Vertical and horizontal displacements of a reservoir slope due to slope aging effect,rainfall,and reservoir water

Landslides are common hazards in reservoir areas and significantly affect dam operation and human lives.For the prevention and management of landslides,accurate assessment of the factors influencing their generation is essential.This study evaluated the key external factors influencing horizontal and vertical displacements of Luobogang Reservoir Slope in Hanyuan County,China.Displacements had been monitored by a surface-displacement-monitoring system consisting of 118 GPS stations during 2012-2015.To identify the external driving factors,their influence zones,and slope responses,we analyzed 32 months of displacement measurements and other multi-source datasets using the empirical orthogonal function.Overall,the results show that slope aging effect,rainfall,and reservoir water levels are three main driving factors.For horizontal displacement,aging effect is the most critical factor and predominantly affects the edges of landslides,the gob cave,and the public building zones.The secondary factor is the reservoir water level,which mainly acts on the boundary between the slope and reservoir water surface.The closer the slope zone is to the reservoir water,the more significant the impact is.Regarding vertical displacement,the most important factor is rainfall.The vertical displacement caused by rainfall accounts for 56.76% of the total vertical displacements.However,rainfall induces elastic displacements that generally cause less damage to the slope.The secondary factor is aging effect,and the vertical displacement caused by aging effect accounts for 9.42%.However,seven individual zones are highly affected by slope aging effect,which is consistent with the distribution of public buildings.

Angle Dependence Analysis Method to Determine SNRArc Applied to GNSSMRSea Level Retrieval

The Global Navigation Satellite Systems(GNSS)broadcast radio signals are continuously at two or more frequencies in the L-band,and the multipath signals from sea surface recorded by off-the-shelf geodetic receivers have been demonstrated they can be used to estimate sea level,using a technology called GNSS multipath reflectometry(GNSS-MR).Before proceeding to estimate reflection parameters,the azimuth range and elevation angle range are needed to be defined,as only with suitable azimuths and elevation angles the sensing zones can be guaranteed on water.So,this study presents an angle dependence analysis method to jointly select the azimuth range and elevation angle range based on wavelet analysis which can describe the nonstationary power of different sinusoidal oscillations changed with elevation angle.The key of this method is to use one grid model to screen the spectrum power of multipath oscillation on different elevation angles and azimuths in this work.Then the elevation angles and the azimuths can be determined by searching grids with greater power.The GPS and GLONASS data of two MultiGNSS Experiment(MGEX)stations named BRST and MAYG was analyzed and used to retrieve.Firstly,the angle dependence analysis was carried out to determine the elevation range and azimuth range.Secondly,the sea levels were retrieved from individual signals.Finally,the retrievals of individual signals are combined to form a 10-min sea level retrieval series.The RMSEs of the combined retrievals are both less than 15 cm.The results show the effectiveness of the selection of angle range based on the angle dependence analysis method.

Properties of multi‑GNSS uncalibrated phase delays with considering satellite systems,receiver types,and network scales

The Wide-Lane(WL)and Narrow-Lane(NL)Uncalibrated Phase Delays(UPDs)are the prerequisites in the traditional Precise Point Positioning(PPP)Ambiguity Resolution(AR).As the generation mechanism of various biases becomes more complex,we systematically studied the impact factors of four satellite systems WL and NL UPDs from the perspective of parameter estimation.Approximately 100 stations in a global network are used to generate the UPDs.The results of different satellite systems show that the estimation method,update frequency,and solution mode need to be treated differently.Two regional networks with different receiver types,JAVAD,and Trimble,are also adopted.The results indicate that the receiver-dependent bias has an influence on UPD estimation.Also,the hardware delays can inhibit the satellite-side UPDs if these receiver-specific errors are not fully deployed or even misused.Furthermore,the temporal stability and residual distribution of NL UPDs are significantly enhanced by utilizing a regional network,with the improvements by over 68%and 40%,respectively.It demonstrates that different network scales exhibit the different implication of unmodeled errors,and the unmodeled errors cannot be ignored and must be handled in UPD estimation.

水管理对干旱区盐渍化与土地退化中和的影响

通过优化灌溉和水资源管理以减少耕地土壤盐渍化对实现土地退化中和至关重要.各种灌溉和水资源管理措施对流域尺度盐渍化的缓解作用的有效性和可持续性尚不明确.本研究利用遥感技术估算了1984-2021年干旱区耕地的表层土壤盐度.然后,利用贝叶斯网络分析比较了十个大型干旱区流域(尼罗河、底格里斯-幼发拉底河、印度河、塔里木河、阿姆河、伊犁河、锡尔河、准格尔盆地、科罗拉多河和圣华金河流域)的土壤表层盐度对水资源管理(如各种灌溉和排水方式)的时空响应.在开发水平较高的流域,管理者采用滴灌和地下水灌溉,通过降低地下水水位有效地控制了土壤盐度.对于仍采用传统漫灌的流域,经济发展和政策支持对于建立“改善灌溉系统——降低盐度——增加农业收入”的良性循环至关重要.这也是实现土地退化中和目标的关键.