A semiempirical algorithm for SAR wave height retrieval and its validation using Envisat ASAR wave mode data

Many synthetic aperture radar (SAR) wave height retrieval algorithms have been developed.However,the wave height retrievals from most existing methods either depend on other input as the first guess or are restricted to the long wave regime.A semiempirical algorithm is presented,which has the objective to estimate the wave height from SAR imagery without any prior knowledge.The proposed novel algorithm was developed based on the theoretical SAR ocean wave imaging mechanism and the empirical relation between two types of wave period.The dependency of the proposed model on radar incident and wave direction was analyzed.For Envisat advanced synthetic aperture radar (ASAR) wave mode data,the model can be reduced to the simple form with two input parameters,i.e.,the cutoff wavelength and peak wavelength of ocean wave,which can be retrieved from SAR imagery without any prior knowledge of wind or wave.Using Envisat ASAR wave mode data and the collocated buoy measurements from NDBC,the scmicmpirical algorithm is validated and compared with the Envisat ASAR level 2 products.The root-mean-square-error (RMSE) and scatter index (SI) in respect to the in situ measurements are 0.52 m and 19％ respectively.Validation results indicate that,for Envisat ASAR wave mode data,the proposed method works well.

Retrieval of Significant Wave Height Under Typhoon Conditions from Gaofen-3 SAR Imagery

The objective of this paper is to propose an empirical method to inverse significant wave height(SWH)under typhoon conditions from collected dual-polarization Gaofen(GF)-3 synthetic aperture radar(SAR)imagery.The typhoon scenes were cap-tured from narrow scan(NSC)and wide scan(WSC)images,and collocated with European Center for Medium-Range Weather Fore-casts reanalysis data of(ECMWF).To improve the quality of GF-3 SAR images,the recalibration over rainforest and de-scalloping were carried out.To establish the empirical relationship between SAR-derived parameters and collocated SWH,the sensitivity analysis of typical parameters about the normalized radar cross section(Nrcs)and imagery variance(Cvar)were performed to both VV and VH polarized images.Four scenes from GF-3 SAR imagery under typhoon conditions were used for training the model by the multivari-ate least square regression,and one scene was used for preliminary validation.It was found that the joint retrieval model based on VV and VH polarized SAR imagery performed better than any single polarized model.These results,verified by using ECMWF data,revealed the soundness of this approach,with a correlation of 0.95,bias of 0 m,RMSE of 0.44 and SI of 0.01 when VV polarization and VH polarization data were both used.

The validation of HY-2 altimeter measurements of a significant wave height based on buoy data

HY-2 has been launched by China on August 16, 2011 which assembles multi-microwave remote sensing payloads in a body and has the ability of monitoring ocean dynamic environments. The HY-2 satellite data need to be calibrated and validated before being put into use. Based on the in-situ buoys from the Nation- al Data Buoy Center （NDBC）, Ku-band significant wave heights （SWH, hs） of HY-2 altimeter are validated. Eleven months of HY-2 altimeter Level 2 products data are chose from October 1, 2011 to August 29, 2012. Using NDBC 60 buoys yield 902 collocations for HY-2 by adopting collocation criteria of 30 min for tempo- ral window and 50 km for a spatial window. An overall RMS difference of the SWH between HY-2 and buoy data is 0.297 m. A correlation coefficient between these is 0.964. An ordinary least squares （OLS） regression is performed with the buoy data as an independent variable and the altimeter data as a dependent vari- able. The regression equation of hs is hs （HY-2）=0.891 × hs （NDBC）＋0.022. In addition, 2016 collocations are matched with temporal window of 30 rain at the crossing points of HY-2 and Jason-2 orbits. RMS difference of Ku-band SWH between the two data sets is 0.452 m.

The validation of the significant wave height product of HY-2altimeter–primary results

The HY-2 satellite was successfully launched on 16 August 2011. The HY-2 significant wave height （SWH） is validated by the data from the South China Sea （SCS） field experiment, National Data Buoy Center （NDBC/ buoys and Jason-1/2 altimeters, and is corrected using a linear regression with in-situ measurements. Com- pared with NDBC SWH, the HY-2 SWH show a RMS of 0.36 m, which is similar to Jason- 1 and Jason-2 SWH with the RMS of 0.35 m and 0.37 m respectively; the RMS of corrected HY-2 SWH is 0.27 m, similar to 0.27 m and 0.23 m of corrected Jason-1 and Jason-2 SWH. Therefore the accuracy of HY-2 SWH products is close to that of Jason-1/2 SWH, and the linear regression function derived can improve the accuracy of HY-2 SWH products.

Validation of Chinese HY-2 satellite radar altimeter significant wave height

Chinese Haiyang-2（HY-2） satellite is the first Chinese marine dynamic environment satellite. The dual-frequency （Ku and C band） radar altimeter onboard HY-2 has been working effective to provide operational significant wave height （SWH） for more than three years （October 1, 2011 to present）.We validated along-track Ku-band SWH data of HY-2 satellite against National Data Buoy Center （NDBC） in-situ measurements over a time period of three years from October 1, 2011 to September 30, 2014, the root mean square error （RMSE） and mean bias of HY-2 SWH is 0.38 m and （-0.13±0.35） m, respectively. We also did cross validation against Jason-2 altimeter SWH data, the RMSE and the mean bias is 0.36m and （-0.22±0.28） m, respectively. In order to compare the statistical results between HY-2 and Jason-2 satellite SWH data, we validated the Jason-2 satellite radar altimeter along-track Ku-band SWH data against NDBC measurements using the same method. The results demonstrate the validation method in this study is scientific and the RMSE and mean bias of Jason-2 SWH data is 0.26 m and （0.00±0.26） m, respectively. We also validated both HY-2 and Jason-2 SWH data every month, the mean bias of Jason-2 SWH data almost equaled to zero all the time, while the mean bias of HY-2 SWH data was no less than -0.31m before April 2013 and dropped to zero after that time. These results indicate that the statistical results for HY-2 altimeter SWH are reliable and HY-2 altimeter along-track SWH data were steady and of high quality in the last three years. The results also indicate that HY-2 SWH data have greatly been improved and have the same accuracy with Jason-2 SWH data after April, 2013. SWH data provided by HY-2 satellite radar altimeter are useful and acceptable for ocean operational applications.

Significant wave height estimation using azimuth cutoff of C-band RADARSAT-2 single-polarization SAR images

This paper proposes two simple models, look-up table（LUT） model and empirical model, to directly retrieve significant wave height（Hs） using synthetic aperture radar（SAR） azimuth cutoff（λc）. Both models aim at C-band VV, HH, VH, and HV single-polarization SAR images. The LUT model relates Hs to λc, while the empirical model relates Hs to both λc and SAR range-to-velocity（β）. The LUT model coefficients are derived by simulation under different sea states and observation conditions, which depend on incidence angle（θ）, wave direction（dw）, and βbut are independent of polarization. The empirical model coefficients are obtained by fitting the collocated data,which only depend on polarization. To fit empirical model coefficients and validate the two models, C-band RADARSAT-2 fine quad-polarization（VV＋HH＋VH＋HV） single-look complex（SLC） SAR images and collocated buoy data are collected. Retrieved Hs, using Yang model and the two models proposed in this paper from four kinds of polarization SAR data, are compared with buoy Hs. Results show that both LUT and empirical models have the capacity of retrieving Hs from C-band RADARSAT-2 co-polarization SAR data, while Yang model is not suitable for these kinds of SAR data. Moreover, the empirical model is also valid for cross-polarization SAR data showing clear ocean wave stripes.

Quality evaluation and calibration of the SWIM significant wave height product with buoy data

The significant wave height(SWH)is one of the main parameters that describe wave characteristics and is widely used in wave research fields.Wave parameters measured by radar are influenced by the offshore distance and sea state.Validation and calibration are of great significance for radar data applications.The nadir beam of surface wave investigation and monitoring(SWIM)detects the global-ocean-surface SWH.To determine the product quality of SWIM SWH,this paper carried out time-space matching between SWIM and buoy data.The data qualities were evaluated under different offshore distances and sea states.An improved calibration method was proposed based on sea state segmentation,which considered the distribution of the point collocation numbers in various sea states.The results indicate that(1)the SWIM SWH accuracy at offshore distances greater than 50 km is higher than that at distances less than 50 km,with an root mean squared error(RMSE)of 0.2444 m,scatter index(SI)of 0.1156 and relative error(RE)of 9.97%at distances greater than 50 km and those of 0.4460 m,0.2230 and18.66%at distances less than 50 km.(2)SWIM SWH qualities are better in moderate and rough sea states with RMSEs of 0.2848 m and 0.3169 m but are worse in slight and very rough sea states.(3)The effect of the improved calibration method is superior to the traditional method in each sea state and overall data,and the RMSE of SWIM SWH is reduced from the raw 0.3135 m to 0.2859 m by the traditional method and 0.1982 m by the improved method.The influence of spatiotemporal window selection on data quality evaluation was analyzed in this paper.This paper provides references for SWIM SWH product applications.

基于多层感知器和SAR参数的海浪有效波高反演方法

本文在分析雷达海面图像的多个参数与海浪有效波高相关性的基础上,应用哨兵1A卫星Level-2 SAR数据中的后向散射系数、图像强度归一化方差、截止波长、主波波长、主波波向、图像偏度和峰度参数提出了基于多层感知器(MLP)的海浪有效波高反演方法。详细讨论了输入参数组合不同时,MLP模型反演的整体海浪有效波高(SWH)、风浪有效波高(SWH_(WW))和涌浪有效波高(SWH_(S))的精度。针对极端海况下数据较少导致模型产生的系统偏差,文中采用校正函数减小该误差,从而增强了模型的适用性。与欧洲中期天气预报中心(ECMWF)和Jason3卫星提供的整体海浪有效波高数据进行对比,本文方法所得结果的均方根误差分别为0.471和0.535 m,相关系数为0.923和0.922。与ECMWF提供的风浪和涌浪有效波高相比,本文方法反演所得风浪和涌浪有效波高的均方根误差分别为0.534和0.512 m,相关系数分别为0.898和0.815。

基于准线性近似方法的SAR图像海浪参数反演研究

本文根据相干斑噪声的时间快变特征和非海浪纹理现象的时间缓变特征,基于交叉谱提出了一种对相干斑噪声和大尺度非海浪纹理的抑制的方法,进而结合SAR图像谱和海浪谱之间的准线性映射关系,基于SAR数据对海浪参数进行了反演。在反演过程中,首先仿真分析了不同海况下准线性近似法的海浪反演能力,结果表明:风浪引起的方位向截断效应会显著影响反演精度,因此该方法在低风速时的涌浪反演精度更高。通过将基于Sentinel-1卫星2020年的波模式SAR数据的反演结果与欧洲中期天气预报中心(European Centre for Medium-Range Weather Forecasts,ECMWF)提供的再分析数据进行对比,发现高海况海浪有效波高反演结果明显偏低,而且该反演误差与风速、方位向截断波长之间存在显著相关性。为了提高有效波高的反演精度,本文进一步给出了海浪有效波高反演误差与风速、方位向截断波长之间的经验校正函数模型,结果显示,通过该模型修正后的海浪有效波高反演结果与ECMWF数据和浮标测量数据具有良好一致性。

基于EMD方法的SAR海浪参数反演

【目的】采用经验模态分解(EMD)方法,基于SAR图像反演海浪参数。【方法】以东北太平洋两幅RADARSAT-2 SAR图像为例,利用谱方法和C波段地球物理模式函数(CMOD5)反演风场,在此基础上,对SAR图像进行EMD分解,得到5个波段的固有模态,由固有模态波长,结合波浪理论和风浪谱,得出研究海域波浪的主波周期和有效波高。【结果与结论】与浮标实测数据对比,反演主波周期偏差为1.9%和13.2%,有效波高偏差为12.2%和12.8%。SAR图像分解与海浪理论结合,为探测反演远海波浪信息提供了新方法。

SAR Azimuth Cut-off to Estimate Wind Speed under High Wind Regimes

In this study,the azimuth cut-off method,typically used for SAR moderate wind speed estimation purposes,is analyzed under high wind regimes.Firstly,the importance of the pixel spacing,the size of the boxes selected for Synthetic Aperture Radar(SAR)image partitioning and the image texture in terms of homogeneities are discussed by considering their influence on the azimuth cut-off(λc)estimation.Secondly,a quality control analysis of the reliability ofλc is carried out by evaluating the distance between the autocorrelation functions(ACF)and their correspondent fittings.This analysis points out the importance of filtering out the unreliable and unfeasibleλc values in order to improve the wind speed estimation.The quality control procedure is based on a x2 test,applied on a large Sentinel-1 A dataset.The soundness of the test is verified by an increment in terms of correlation betweenλc estimations and wind speed values.This approach is,then,applied under high wind regimes,i.e.,tropical cyclones.

波浪骑士数据在高度计有效波高检验中的再处理

以2011年10～11月南海现场试验得到的9次波浪骑士测量数据,进行波浪骑士再处理与默认计算结果比对。比对结果表明两者平均误差为0.16m,均方根误差为0.32m,分析产生误差的原因在于波浪骑士默认计算有效波高时间段的中心与卫星过境时间不统一和未进行数据质量控制。研究表明在卫星高度计有效波高产品检验中,波浪骑士测量的有效波高需要进行再处理,以达到减少卫星高度计有效波高检验误差的目的。

北斗反射信号海风海浪反演系统与试验

该文设计并实现了一种小型低功耗的北斗反射信号海风海浪反演系统,针对反演精度受卫星高度角影响较大的问题,提出高度角正弦值幂指数修正方法,且为降低风速突变对风速反演的影响,对风速反演模型进行了延时修正,有效提高了有效波高和风速的反演精度。为了进一步提高反演精度,提出多星联合观测和单边滤波的方法。北斗反射信号海风海浪探测试验结果表明:设计和实现的反演系统具备长期稳定运行和观测的性能;提出的反演模型和精度提升方法可以有效提高反演精度,有效波高和风速反演精度分别可达0.14 m和1.28 m/s,相比于Soulat等人提出的反演模型,分别提高了0.13 m和0.78 m/s。

时空窗口对HY-2有效波高产品检验影响模拟研究

时空窗口的选择是卫星高度计有效波高产品检验的主要影响因素。采用Monte Carlo(MC)数学模拟的方法 ,研究了时空窗口对HY-2高度计有效波高检验的影响,并采用现场浮标测量数据验证了MC模拟的可靠性。MC模拟结果表明,采用浮标测量数据对HY-2高度计有效波高检验时,必须分海况选取对应的最优空间窗口进行,并给出不同海况下的最优的时空窗口。对于高海况需采用小的空间窗口,在1 m,2 m,3 m,4 m有效波高的海况下,其理想的时空窗口为0 min,117 km,30 km,18 km和13 km。

自升式平台收放桩时桩腿撞击力时域分析研究

自升式平台收放桩时桩腿与海床间的撞击力是决定作业气象窗口的主要参数。基于能量守恒定律和刚性地基的假定推导桩腿撞击力公式,建立桩腿撞击安全因子的评估方法。应用平台运动时间历程的频-时域转换方法,通过频域和时域分析确定自主开发的350英尺自升式钻井平台收放桩作业时的许用有义波高。论文建议的桩腿与海床撞击力的时域分析方法可为其它类型自升式平台收放桩作业分析提供参考。

岸基GNSS反射信号的波浪多参数反演方法研究

针对当前GNSS-R技术利用GNSS海面反射信号仅反演有效波高一个参数的不足,基于直射信号和反射信号构成的复相干场模型,建立海面相关时间与波浪的有效波高和平均波周期的映射模型,给出了基于直射信号多普勒频移、C/A码、导航数据位参数辅助的海面反射信号处理流程和方法,确立了基于GNSS海面反射信号的有效波高和平均波周期反演流程和方法,并在青岛小麦岛海洋观测站进行了岸边实验.实验结果表明,有效波高反演结果的均方误差为0.1m,平均波周期反演结果的均方误差为0.61s.所提反演方法对海洋观测新技术研究和新装备制造,及其在我国北斗系统中的应用等扩展领域,都具有明显学术价值和重要意义.

自主HY-2B卫星北极地区风场及有效波高产品交叉验证分析

HY-2B卫星海面风场及有效波高产品在北极地区的交叉验证是实现相关产品在高纬地区推广应用的重要前提,是检验我国自主海洋动力系列卫星产品质量及应用范围的重要环节。利用2018年发射的HY-2B卫星获取的风场和有效波高数据,开展了自主海洋动力卫星与国际同类卫星产品在北极地区的交叉验证分析。近两年的观测数据表明:①HY-2B卫星与MetOp-A卫星风速风向均具有高度的一致性,数据质量稳定,精度高。风速及风向相关系数分别达0.97和0.99,均方根误差分别低于1.03 m/s和12.28°。②HY-2B卫星与Jason-3卫星有效波高数据在北极地区给定时空范围内匹配对较少,但数据的一致性及精度均较高,二者相关性达0.99,均方根误差为0.28 m。③自主HY-2B卫星风场和有效波高产品与国际同类卫星产品在北极地区均具有高度的一致性,达到业务化运行的精度要求,相关产品可满足于北极等高纬地区海面风场和有效波高的推广应用。

基于现场观测数据的卫星雷达高度计海面风速和有效波高真实性检验方法研究

卫星遥感产品定量化应用前需通过真实性检验。卫星雷达高度计可观测海面风速大小和有效波高等海洋动力环境参数,而对其进行真实性检验存在不同的时空匹配方法。使用3种不同的时空匹配方法,利用美国国家浮标数据中心(NDBC)现场观测数据对2011年的Jason-2海面风速和有效波高产品进行了检验。Jason-2海面风速大小和有效波高的均方根误差分别为1.28m/s和0.28m,同时结果表明:采用卫星高度计海面风速大小(有效波高)和同步的现场观测数据在一定时间(1h)和空间(50km)限制条件下进行对比检验,可以得到合理的高度计海面风速大小和有效波高精度评价结果。Jason-2卫星高度计海面风速大小和有效波高的精度与探测海域有关,其精度在墨西哥湾与西大西洋海域相对较高。