Development of a flood water level estimation method using satellite images and a digital elevation model for the Mekong floodplain

We have developed a flood water level estimation method that only employs satellite images and a DEM. The method involves three steps: (1) discriminating flood areas and identifying clumps of each flood area, (2) extracting the edges of the identified flood area using a buffering technique, and (3) performing spatial interpolation to transform the extracted elevation to flood water levels. We compared the estimated flood water levels with the observed ones. The RMSE using the RADARSAT was 1.99 and 1.30 m at river and floodplain points, respectively, whereas the RMSE using the MODIS was 4.33 and 1.33 m at the river and floodplain points, respectively. Given that most errors are attributed to the DEM, the method exhibited good performance. Furthermore, the method reproduced the flow directions and flood water level changes during the flooding period. Thus, we demonstrated that the characteristics of flood inundation can be understood even when ground observation data cannot be obtained.     

基于Sentinel-1A数据在广东省近海海面风场反演应用

采用基于风条纹提取风向的方式,利用地球物理模式函数,基于Sentinel-1A数据,通过CMOD5模型反演2017年3、5、7、12月份广东省近海海域风场。将反演结果与实测数据对比,风速普遍比实测风速大,风速反演的平均绝对误差为1.98 m/s,均方根误差为2.74 m/s,相关系数为0.8。其中3、5、7月的风速较为接近,且平均绝对误差和均方根误差都＜2 m/s,而12月份平均风速＞8 m/s,实测数据与卫星过境时间不完全匹配,导致平均绝对误差和均方根误差都偏大。哨兵一（Sentinel-1A）影像反演结果整体上与实测数据相一致,验证了COMD5反演模型适用于广东省近海高分辨率海洋风场反演,可为下一步估算广东省风能资源储量提供可能。     

船载CTD仪与自动剖面浮标观测资料质量初探

海洋科学的发展离不开精确的数据,然而各种海洋观测仪器在复杂的海洋环境中作业难免产生测量误差,导致观测数据需要进行实时(或延时)质量控制。中国Argo计划在搭载多个航次布放剖面浮标的同时,对航次中获取的船载CTD(conductivity, temperature, and depth)仪观测资料、自动剖面浮标观测资料以及实验室高精度盐度计测量数据进行了实时比对。分析结果显示,利用实验室高精度盐度计对现场观测数据尤其是船载CTD仪观测资料进行质量控制,于温盐数据(特别是深层)的实时/延时校正非常重要;如某航次未经标定的船载CTD仪所测1000dbar以深范围内海水盐度,与实验室高精度盐度计的差值达到±0.1左右,远远落后于国内海洋调查规范对盐度准确度±0.02的一级测量要求,该具体实例更加突显了船载CTD仪在航次前后送往权威部门进行检测的必要性和重要性,从而确保每个航次获取的CTD资料的质量。建议有条件的情况下,在进行深海大洋船载CTD仪观测时要进行现场实验室高精度盐度计的质量控制工作及比对试验,以提高我国深海大洋观测数据的质量。     

Discrimination of species composition types of a grazed pasture landscape using Sentinel-1 and Sentinel-2 data

Species composition is one of the important measurable indices of alpha diversity and hence aligns with the measurable Essential Biodiversity Variables meant to fulfil the Aichi Biodiversity Targets by 2020. Graziers also seek for pasture fields with varied species composition for their livestock, but visual determination of the species composition is not practicable for graziers with large fields. Consequently, this study demonstrated the capability of Sentinel-1 Synthetic Aperture Radar (S1) and Sentinel-2 Multispectral Instrument (S2) to discriminate pasture fields with single-species composition, two-species composition and multi-species composition for a pastoral landscape in Australia. The study used K-Nearest Neighbours (KNN), Random Forest (RF) and Support Vector Machine (SVM) classifiers to evaluate the strengths of S1-alone and S2-alone features and the combination of these S1 and S2 features to discriminate the composition types. For the S1 experiment, KNN which was the reference classifier achieved an overall accuracy of 0.85 while RF and SVM produced 0.74 and 0.89, respectively. The S2 experiment produced accuracies higher than the S1 in that the overall performance of the KNN classifier was 0.87 while RF and SVM were 0.93 and 0.89, respectively. The combination of the S1 and S2 features elicited the highest accuracy estimates of the classifiers in that the KNN classifier recorded 0.89 while RF and SVM produced 0.96 and 0.93, respectively. In conclusion, the inclusion of S1 features improve the classifiers created with S2 features only.     

GNSS-R海面目标成像仿真方法研究

结合物理光学法及传统复合电磁散射计算模型中的"四路径"方法,以GNSS信号为信号源,提出了一种基于高频散射理论的GNSS-SAR成像模型,建模过程考虑了多路径散射、遮蔽、漫散射等效应,较好地描述了电磁散射效应对海面场景GNSS-SAR成像的影响。根据电磁散射效应,分析了海面风速对成像效果产生的干扰情况,发现在本文仿真场景下,9 m/s风速海背景下的目标能够被识别出来,17 m/s风速海背景下的目标几乎淹没在海杂波中。     

全极化雷达遥感影像的迭代优化非局部均值去噪法

相干斑的存在严重降低了全极化合成孔径雷达(polarimetric synthetic aperture radar,PolSAR)的影像质量,对相干斑进行抑制是使用PolSAR数据必不可少的预处理程序。本文提出了一种迭代优化的PolSAR非局部均值去噪方法。该方法在每次迭代去噪过程中,通过同时考虑原始影像全极化噪声统计特性和前一次迭代所得影像的全极化信息来完善像素间极化相似性的度量,从而实现对影像更精准的估计。试验部分利用模拟的PolSAR数据和真实的PolSAR影像进行了算法效果的验证。结果表明:去噪算法在显著抑制影像噪声水平的同时,也能较好地保持影像的边缘和极化特性等细节信息。     

DEM数据辅助的星载SAR俯仰向数字波束形成

俯仰向数字波束形成(DBF)处理是星载合成孔径雷达(SAR)实现高分辨率宽测绘带成像的关键。然而,在处理较大地形起伏区域的回波信号时,传统俯仰向波束扫描(SCORE)方法会出现波束指向偏差问题,导致接收回波的增益降低,影响SAR系统成像性能。针对这一问题,本文详细分析了高分宽幅星载SAR系统的俯仰向DBF接收波束扫描指向问题,提出了一种基于数字高程图(DEM)的俯仰向DBF处理方法。该方法基于星载SAR成像几何模型,首先利用成像场景的DEM数据和卫星轨道参数计算距离门单元对应的地面高程值,并进一步计算距离门单元对应的目标波达角,然后根据该对应关系计算每个距离门单元的俯仰向DBF加权矢量,从而确保在俯仰向DBF处理过程中接收波束指向正确。通过X波段的星载SAR系统进行仿真实验,结果表明(1)当地面高程值大于1.9 km时,传统SCORE方法处理得到的目标信号幅度下降都超过2.8 dB,本文方法处理得到的目标信号幅度下降都小于0.4 dB,本文方法优于传统SCORE方法;(2)由DEM数据误差导致的地面目标高程偏差对本文方法影响较小。因此本文方法能够有效改善地形起伏较大区域的回波信号接收增益。     

台风中降雨对中国高分3号合成孔径雷达观测信号的影响

Gaofen-3(GF-3), a Chinese civil synthetic aperture radar(SAR) at C-band, has operated since August 2016.Remarkably, several typhoons have been captured by GF-3 around the China Seas over its last two-year mission.In this study, six images acquired in Global Observation(GLO) and Wide ScanSAR(WSC) modes at verticalvertical(VV) polarization channel are discussed. This work focuses on investigating the observation of rainfall using GF-3 SAR. These images were collocated with winds from the European Centre for Medium-Range Weather Forecasts(ECMWF), significant wave height simulated from the WAVEWATCH-III(WW3) model, sea surface currents from climate forecast system version 2(CFSv2) of the National Centers for Environmental Prediction(NCEP) and rain rate data from the Tropical Rainfall Measuring Mission(TRMM) satellite. Sea surface roughness,was compared with the normalized radar cross section(NRCS) from SAR observations, and indicated a 0.8 correlation(COR). We analyzed the dependences of the difference between model-simulated NRCS and SARmeasured NRCS on the TRMM rain rate and WW3-simulated significant wave height. It was found that the effects of rain on SAR damps the radar signal at incidence angles ranging from 15° to 30°, while it enhances the radar signal at incidence angles ranging from 30° to 45° and incidence angles smaller than 10°. This behavior is consistent with previous studies and an algorithm for rain rate retrieval is anticipated for GF-3 SAR.     

基于理论的气旋条件下双极化C波段合成孔径雷达波参数检测

Theoretical-based ocean wave retrieval algorithms are applied by inverting a synthetic aperture radar(SAR)intensity spectrum into a wave spectrum, that has been developed based on a SAR wave mapping mechanism. In our previous studies, it was shown that the wave retrieval algorithm, named the parameterized first-guess spectrum method(PFSM), works for C-band and X-band SAR at low to moderate sea states. In this work, we investigate the performance of the PFSM algorithm when it is applied for dual-polarization c-band sentinel-1(S-1) SAR acquired in extra wide-swath(EW) and interferometric wide-swath(IW) mode under cyclonic conditions.Strong winds are retrieved from six vertical-horizontal(VH) polarization S-1 SAR images using the c-band crosspolarization coupled-parameters ocean(C-3 PO) model and then wave parameters are obtained from the image at the vertical-vertical(VV) polarization channel. significant wave height(SWH) and mean wave period(MWP) are compared with simulations from the WAVEWATCH-III(WW3) model. The validation shows a 0.69 m root mean square error(RMSE) of SWH with a –0.01 m bias and a 0.62 s RMSE of MWP with a –0.17 s bias. Although the PFSM algorithm relies on a good quality SAR spectrum, this study confirms the applicability for wave retrieval from an S-1 SAR image. Moreover, it is found that the retrieved results have less accuracy on the right sector of cyclone eyes where swell directly affects strong wind-sea, while the PFSM algorithm works well on the left and rear sectors of cyclone eyes where the interaction of wind-sea and swell is relatively poor.     

复合雷达后向散射模型与合成孔径雷达、散射计和高度计海面雷达后向散射观测的比较分析

海洋微波散射模型相比于以经验统计建立的地球物理模式函数具有不受特定微波频率限制的优势。组合布拉格散射模型和几何光学模型形成了复合雷达后向散射模型。利用南海北部气象浮标2014年海面风速风向实测值作为散射模型输入,分别比较了复合雷达后向散射模型与RADARSAT-2卫星C波段SAR、HY-2A卫星Ku波段微波散射计的海面后向散射系数,偏差分别为(?0.22±1.88) dB (SAR)、(0.33±2.71) dB (散射计VV极化）和(?1.35±2.88) dB (散射计HH极化);以美国浮标数据中心(NDBC)浮标2011年10月1日至2014年9月30日共3年的海面风速、风向实测值作为散射模型输入,分别比较了复合雷达后向散射模型与Jason-2、HY-2A卫星Ku波段高度计海面后向散射系数,偏差分别为(1.01±1.15) dB和(1.12±1.29) dB。中等入射角和垂直入射下的卫星传感器后向散射系数观测值与复合雷达后向散射模型模拟值比较,具有不同的偏差,但具有相同的海面风速检验精度,均方根误差小于1.71 m/s。结果表明,复合雷达后向散射模型可模拟计算星载SAR、散射计和高度计观测条件下的海面雷达后向散射系数,且与CMOD5、NSCAT-2、高度计业务化海面风速反演的地球物理模式函数的计算结果具有一致性;复合雷达后向散射模型可用于微波遥感器的定标与检验、海面雷达后向散射的模拟。