Measurement of ocean surface currents using a long-range,high-frequency ground wave radar

High-frequency (HF) ground wave radar (GWR) is emerging as a significant tool for monitoring ocean surface conditions at ranges well beyond the line-of-sight horizon that limits conventional systems. An experimental GWR system at Cape Race, Newfoundland, Canada that has been operational since 1991, has the ability to performing routine surveillance of oceanic surface parameters and surface target detection. Operating in the frequency range between 5 and 8 MHz, the frequency modulated interrupted continuous wave (FMICW) radar has a nominal range capability of 200 km. An experiment was performed during the period of October 20-November 21, 1992 to test the surface current measuring capability of the Cape Race system. Here, near real-time radial surface current information is extracted from the Doppler spectra of the radar time series data and a comparison is performed to the Lagrangian velocities derived from the position-time tracks of Accurate Surface Tracker (AST) drifters. A wide range of oceanic conditions were experienced during the experimental period, and favorable results were obtained from the comparison regardless of the sea state conditions. The analysis shows the standard deviation in the radar radial velocity component to be approximately 5 cm/s     

Radial velocity of ocean surface current estimated from SAR Doppler frequency measurements—a case study of Kuroshio in the East China Sea

Ocean currents are a key element in ocean processes and in meteorology, affecting material transport and modulating climate change patterns. The Doppler frequency shift information of the synthetic aperture radar (SAR) echo signal can reflect the dynamic characteristics of the sea surface, and has become an essential sea surface dynamic remote sensing parameter. Studies have verified that the instantaneous Doppler frequency shift can realize the SAR detection of the sea surface current. However, the validation of SAR-derived ocean current data and a thorough analysis of the errors associated with them remain lacking. In this study, we derive high spatial resolution flow measurements for the Kuroshio in the East China Sea from SAR data using a theoretical model of shifts in Doppler frequency driven by ocean surface current. Global ocean multi observation (MOB) products and global surface Lagrangian drifter (GLD) data are used to validate the Kuroshio flow retrieved from the SAR data. Results show that the central flow velocity for the Kuroshio derived from the SAR is 0.4–1.5 m/s. The error distribution between SAR ocean currents and MOB products is an approximate standard normal distribution, with the 90％ confidence interval concentrated between –0.1 m/s and 0.1 m/s. Comparative analysis of SAR ocean current and GLD products, the correlation coefficient is 0.803, which shows to be significant at a confidence level of 99％. The cross-validation of different ocean current dataset illustrate that the SAR radial current captures the positions and dynamics of the Kuroshio central flow and the Kuroshio Counter Current, and has the capability to monitor current velocity over a wide range of values.     

基于BP神经网络的高频地波雷达海流空间插值

高频地波雷达是海洋环境监测的重要手段,当前已经实现对海流的业务化观测,但是外部因素常引起海流空间探测的不连续性。为解决此问题,尽量保障区域数据的完整性和准确性,本文将BP神经网络技术与空间插值相结合,建立了海流的BP神经网络插值模型,并进行了针对实测数据的缺失插值仿真,通过与反距离权重法和线性插值法插值结果的对比,分析该模型在区域海流大面积缺失、流速整体较大和流速整体较小3个方面的性能。结果表明,BP神经网络插值模型的海流预测效果明显优于其他两种方法,且在流场数据大范围缺失下也取得了良好的效果。     

高频地波雷达海杂波的边界约束循环对消

在对海洋监视监测的过程中,高频地波雷达的船只目标检测能力与其对海杂波的抑制能力密切相关。但是,传统海杂波时域抑制方法存在目标回波参数与海杂波相近时难以区分其各自分量,对消时目标被误消除的问题。针对上述问题,本文提出了一种适用于高频地波雷达海杂波的边界约束循环对消方法。该方法综合海杂波频移理论和实际海杂波特性分析制定出边界条件,约束建模对消过程,实现海杂波分量的抑制。实测地波雷达数据和船舶自动识别系统(AIS)数据检验的结果表明,本文方法克服了传统方法的不足且信杂比改善更加稳定,能够更加有效地实现海杂波循环对消。     

基于分数阶傅里叶变换的水下目标速度估计

在高斯白噪声背景下,匹配滤波器作为线性调频信号的最优检测器,在水声信号处理中被广泛应用。 当发射信号为线性调频信号时,由水下目标径向速度引起的多普勒频移会造成回波和样本之间失配, 使匹配滤波器的检测性能下降,增加了目标速度估计的难度。 利用分数阶傅里叶变换对于线性调频信号的聚焦特性,提出了应用分数阶傅里叶变换的水下运动目标线性调频回波检测算法,完成对目标速度的估计, 推导目标运动速度与分数阶傅里叶变换阶数之间的关系,并对测量结果进行误差分析。 仿真测试表明,该算法可有效地估计混响背景下的目标径向速度,且具有良好的估计性能。     

Parametric estimation of ocean surface currents with HF radar

Ocean surface currents can be estimated, over a large coastal area, by utilizing the backscatter of high frequency (HF) radar waves from ocean gravity waves. Although the overall backscatter mechanism is complicated, the surface current information is contained within the spectral characteristics of two dominant Bragg components. The accuracy of the current estimate, following the usual FFT-based spectral estimate, is limited by the frequency resolution of the FFT and the time-varying characteristics of the Bragg components. This paper describes a high resolution parametric estimation of the ocean currents based on a recently proposed technique for analyzing time-varying signals. This technique, together with a time-domain ocean clutter model, allows all the Bragg signal information to be extracted from the two dominant eigenvalues and eigenvectors of a matrix constructed from the radar data. Using signals from an operational coastal surveillance radar, current estimates made using this technique are compared with those estimated by the conventional FFT-based method     

基于高频地波雷达的多时间尺度海流研究

近海海流受多种动力过程及岸线岛屿的作用呈现空间和时间尺度上的复杂变化,而地波雷达由于其探测面积广、时间分辨率高的特点成为研究这些变化的有效手段。本文利用舟山海域多年高频地波雷达资料,通过潮流调和分析、低通滤波和相关性分析对该海域海流潮周期、极端事件、季节、年际尺度的动力过程进行了解译。研究表明,舟山海域属于正规半日潮,潮流运动形式以顺时针旋转流为主,流速大小在空间上为东北方向较大,往西南方向逐渐减小,并在近岸处得到增强。余流的年际变化并不显著,但存在着明显的季节变化,例如冬季为南向流,流速减小,空间分布上近岸较外海大,而夏季与之反向,为北向流,流速较大,空间分布较为均匀。进一步分析了风与余流之间的相关性,在大风期间,风与余流的速度相关系数在0.48～0.90之间,方向相关系数在0.55～0.68之间。极端事件发生时,速度、方向的相关系数分别高达0.92与0.91。总体而言,通过分析高频地波雷达数据能够较好地反映舟山海域海流的时空特征,为海洋灾害监测和污染物、藻华的输运研究提供依据。     

Observation of the Soya Warm Current using HF ocean radar

Three High Frequency (HF) ocean radar stations were installed around the Soya/La Perouse Strait in the Sea of Okhotsk in order to monitor the Soya Warm Current (SWC). The frequency of the HF radar is 13.9 MHz, and the range and azimuth resolutions are 3 km and 5 deg., respectively. The radar covers a range of approximately 70 km from the coast. The surface current velocity observed by the HF radars was compared with data from drifting buoys and shipboard Acoustic Doppler Current Profilers (ADCPs). The current velocity derived from the HF radars shows good agreement with that observed using the drifting buoys. The root-mean-square (rms) differences were found to be less than 20 cm s⁻¹ for the zonal and meridional components in the buoy comparison. The observed current velocity was also found to exhibit reasonable agreement with the shipboard ADCP data. It was shown that the HF radars clearly capture seasonal and short-term variations of the SWC. The velocity of the Soya Warm Current reaches its maximum, approximately 1 m s⁻¹, in summer and weakens in winter. The velocity core is located 20 to 30 km from the coast, and its width is approximately 40 km. The surface transport by the SWC shows a significant correlation with the sea level difference along the strait, as derived from coastal tide gauge records at Wakkanai and Abashiri. Deceased.     

一种改进的基于逆Omega-K算法的海面场景SAR原始数据仿真方法

合成孔径雷达(SAR)海面场景原始数据仿真是研究海洋动力参数(表面波浪、风矢量和洋流)的有效工具。目前海面场景原始数据仿真方法已经基于逆Omega-K算法实现了海洋运动参数的空间变化。但是目前仅仅讨论了正侧视情况下的海面场景仿真,应用范围有限,同时没有考虑Stokes漂流以及Bragg相速度的影响,而这两者都是存在于真实海面的。通常情况下为了反演得到海面流场的二维速度矢量,雷达需要从两个不同的方位方向观察海面的同一个区域,因此这就需要考虑大斜视的雷达波束,同时Stokes漂流和Bragg相速度是SAR海表面流场观测不容忽视的两种运动。本文在不改变原有正侧视逆Omega-K算法的情况下,通过增加重新计算零方位时刻的斜视波束中心位置坐标,并据此确定SAR原始数据在多普勒域的位置来将其扩展到大斜侧视逆Omega-K算法,并通过时域Stokes漂流公式到频域内离散化Stokes漂流公式的推导来加入Stokes漂流,以及根据Bragg散射机制加入了Bragg相速度。仿真结果表明,经过聚焦成像后的SAR图像很好的体现了真实海面波浪场的形状,同时能够很好地反演出设定的雷达径向流场速度,且流速精度误差控制在6%以内。最后也证明了Bragg相速度以及Stokes漂流对于海面流场的影响不可忽视。     

GNSS-R观测下的海面飓风风速反演

利用全球导航卫星系统在地球表面的反射信号（GNSS-R）进行海面风速反演已经被广泛研究并作为一种重要的遥感手段。目前,该L波段微波信号的相关功率已可以在多普勒频率和时延码片空间进行多普勒时延图像的成像。由于该图像的图像特征与海面粗糙度有较高的相关性,因此能够用来进行海面风场反演。然而,对于该遥感手段而言,其双基雷达前向散射截面（BRCS）理论上与海面粗糙度有更高的相关性,如同目前合成孔径雷达使用后向雷达散射截面而非相关功率。所以,本文通过改进已有的GNSS-R的双基雷达散射截面方程,代替相关功率在多普勒时延空间进行成像,得出了与海面粗糙度相关的双基雷达散射截面图像（BRCS map）。基于该图像,本文提出了三种与其形状特征相关的观测量,通过2005年Dennis飓风GNSS-R机载数据生成的16000多幅图像进行地球物理模式函数建模并与经典的一维时延波形匹配方法得出结果进行对比分析,得出更为精确的风速反演结果。     

Determination of space and time sea wave structure from frequency characteristics of the radio signal scattered by the sea

A method of determining sea-wave parameters from frequency characteristics of an RF signal scattered by the sea surface is considered. The method is based on the Doppler frequency shift due to orbital velocity of motion of long sea waves. It is shown that by measuring the frequency shift ofS-band signals scattered by a small (as compared to the sea-wave length) site on the sea surface, one is able to determine both integrated sea-wave parameters (e.g., rms waveheight and mean period) and the space-time structure of energy-carrying sea components. Results of field tests are presented substantiating the efficiency of the method and allowing measurement accuracy to be evaluated. The main systematic errors of the method are discussed.     

X波段雷达海面回波谱宽特征研究

海面电磁回波频谱宽度与海浪波高密切相关,可应用频谱宽度进行海浪有效波高反演。本文应用线性滤波法仿真出了海表散射面元在雷达视向上的投影速度,建立了回波谱宽模型,分析了雷达空间分辨率、回波时间序列长度及海洋环境参数等因素对频谱宽度的影响,同时还针对如何在实际观测过程中选择回波时间序列长度、观测方位角等参数进行了讨论。最后还将理论结果与CSIR-X波段雷达实测数据谱宽估计结果进行了比较。结果表明,剔除雷达噪声以及频率泄露的影响后,基于高斯分布标准偏差的谱宽估计方法所得结果与理论结果吻合很好,这从而证明了理论结果的可靠性。本文所得结果对海浪有效波高反演具有一定参考价值。     

Distinctive features of data processing for the side-looking radar of theSich-1 satellite

We show that the parameters of primary radar images obtained from theSich-1 satellite depend on the mode of survey and that the time dependence of the coefficient of correction of amplification in the channel of formation of radar signals is nonlinear. We propose to process radar images of the sea surface by using the so-called Bragg normalization of signals, which enables one to get a physically correct dependence of specific effective scattering surface on the elevation angle. We also consider a procedure of determination of the dependence of specific effective scattering surface of the sea on the wind velocity on the basis of the data accumulated by the side-looking radar of theSich-1 satellite. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

Radiophysical investigations of sea roughness (radiooceanography) at the Ukrainian Academy of Sciences

Experiments on the scattering of radio waves in the range 200 m to 3 cm from a rough sea surface are described. Amplitude, frequency, and space-time characteristics of scattered radio signals at different states of the sea surface are presented. It is shown that the problem of the short and medium wave scattering from the sea can be solved by the perturbance method. In this case the mechanism of scattering is of "resonant" character. The intensity of the backscatter signals is proportional to the density of the spatial spectrum on the half-length of the radio waves. The high frequency radio wave scattering is well described by a two-scale model of the scattering surface, "ripple on the large wave." The intensity of scattered radio signals is also proportional to the spectrum density of "ripples" whose length is approximately equal to half a radio wave. The effect of the large waves is to modulate the amplitude of a scattered radio signal and to broaden its frequency spectrum. Methods of solution of the reverse problem were considered. This allowed determination of parameters of sea roughness by characteristics of scattered radio signals. The principles of design of the corresponding equipment are described.     

HF radar ocean current algorithm based on MUSIC and the validation experiments

Wuhan University's ocean state measuring and analyzing radar (OSMAR2000), working at around 7.5 MHz in the low region of the HF band with a 120-m-long linear receiving antenna array, can measure ocean surface current at ranges of up to 200 km. An ocean surface current algorithm based on direction finding (DF) using the multiple signal classification (MUSIC) method is developed for the OSMAR2000 radar. This paper describes the OSMAR2000 ocean surface current algorithm based on MUSIC and the validation experiments in the East China Sea. The results of the ocean surface current measurements demonstrate that the OSMAR2000 ocean surface current algorithm based on MUSIC is feasible for the long range of ocean surface current mapping with a sufficient bearing resolution.     

X-波段船用雷达观测海洋动力环境要素仿真研究

为了评估X-波段船载雷达观测海浪和海流参数的能力.基于随机海浪理论和雷达几何成像原理模拟了不同调制影响的X-波段船用雷达图像序列,介绍了估算海浪参数和海流参数的算法,对影响雷达观测海流和海浪有关因素进行了分析.同时在雷达图像中加入了随机高斯白噪声,并通过数值方法验证了雷达图像的信噪比开方和有效波高之间的线性关系.数值模拟结果说明X-波段船用雷达能够有效地估算海浪和海流参数,且带有不同噪声水平的雷达系统应具有不同的定标系数.     

基于X波段雷达海面波高估计的改进方法

船用X波段导航雷达凹波形成的海杂波图像中包含丰富的海浪、海表层流信息.借鉴合成孔径雷达(SAR)估计有效波高的方法,假设有效波高与雷达回波强度信噪比的平方根成线性关系,可由X波段雷达图像估计得到海浪的有效波高.用此方法主要分析小麦岛海域实验数据,结果显示,直流滤波后计算的信噪比估计的有效波高比不进行直流滤波结果符合得好;而按波浪浮标测得有效波高数据的高低,分两段分别进行线性拟合获取校准系数,估测的有效波高更加准确.     

HF radar data quality requirements for wave measurement

HF radar wave measurements are presented focussing on theoretical limitations, and thus radar operating parameters, and quality control requirements to ensure robust measurements across a range of sea states. Data from three radar deployments, off the west coast of Norway, Celtic Sea and Liverpool Bay using two different radar systems, WERA and Pisces, and different radio frequency ranges, are used to demonstrate the wave measurement capability of HF radar and to illustrate the points made. Aspects of the measurements that require further improvements are identified. These include modifications to the underlying theory particularly in high sea states, identification and removal of ships and interference from the radar signals before wave processing and/or intelligent partitioning to remove these from the wave spectrum. The need to match the radio frequency to the expected wave peak frequency and waveheight range, with lower radio frequencies performing better at higher waveheights and lower peak frequencies and vice versa, is demonstrated. For operations across a wide range of oceanographic conditions a radar able to operate at more than one frequency is recommended for robust wave measurement. Careful quality control is needed to ensure accurate wave measurements.     

基于相参X-波段海洋雷达的海表轮廓测量研究

X-波段海洋雷达测量所得海面散射单元的多普勒信息与散射单元的雷达视向速度密切相关。首先,基于符号多普勒估计方法,对X-波段雷达海面回波的多普勒频移信息进行了估计;在此基础上,应用各分辨单元回波的多普勒频移信息,建立了海浪表面轮廓的反演算法。该算法中,同时考虑了雷达入射角、方位角和雷达空间分辨率等因素对反演结果的影响。通过将反演结果与浮标测量数据相比较,发现雷达空间分辨率起到了类似低通滤波的作用,该作用对短重力波谱影响显著。同时,还应用加拿大麦克马斯特大学的IPIX雷达数据对海表轮廓进行了反演,并将反演所得有效波高、海浪周期与现场测量数据进行了比较,反演结果与现场测量结果吻合较好。