Measurement of Ocean Surface Currents by the CRL HF Ocean Surface Radar of FMCW Type. Part 2. Current Vector

The Communications Research Laboratory (CRL) has been developing high-frequency ocean surface radars (HFOSRs). The CRL dual-site HFOSR system can clarify the distribution of surface currents with a nominal range of 50 km. This paper presents a theoretical and experimental analysis of the measurement error of the current vector obtained by the CRL HFOSR system, using a comparison of instantaneous current vectors acquired by the HFOSR system and current meters moored at a depth of 2 m, taking account of the vertical current shear. The theoretical analysis shows that the probability distribution of the measurement error of the current vector forms concentric ellipses at a spatial scale that depends on the RMS measurement error of radial current velocity and with an aspect ratio that depends only on the azimuthal difference of the radar beams. When the azimuthal difference is a right angle, the measurement error of the current vector is at a minimum. A comparison between instantaneous current vectors measured by the CRL HFOSR system and moored current meters shows that the distribution of the difference vector between the radar current and the meter current agrees well with the theoretical measurement error of the current vector and that the RMS of difference vector length is about 10 cm s^–1 while the azimuthal difference between two radar beams is between 45 and 135 degrees. The accuracy of current measurement by the dual-site HFOSR system is therefore considered to be less than 10 cm s^–1 in this range of azimuthal difference. The theoretical analysis will be applicable for a wider range of the azimuthal difference of the radar beams.     

Extraction of Ocean Wave Spectra From Simulated Noisy Bistatic High-Frequency Radar Data

An algorithm is developed for the inversion of bistatic high-frequency (HF) radar sea echo to give the nondirectional wave spectrum. The bistatic HF radar second-order cross section of patch scattering, consisting of a combination of four Fredholm-type integral equations, contains a nonlinear product of ocean wave directional spectrum factors. The energy inside the first-order cross section is used to normalize this integrand. The unknown ocean wave spectrum is represented by a truncated Fourier series. The integral equation is then converted to a matrix equation and a singular value decomposition (SVD) method is invoked to pseudoinvert the kernel matrix. The new algorithm is verified with simulated radar Doppler spectrum for varying water depths, wind velocities, and radar operating frequencies. To make the simulation more realistic, zero-mean Gaussian noise from external sources is also taken into account     

海洋光学遥感器的辐射定标与数据真实性检验综述

遥感数据定量化应用遥感技术深入发展亟须解决的重要课题,对遥感器进行辐射定标,对遥感数据进行真实性检验的工作已在深入开展,本文介绍了国际关于定标／真实性检验的组织及分工概况,简要阐述了定标／真实性检验的概念与做法,着重讨论了与海洋光学遥感器有关的辐射定标／真实性检验,综述了国外在这方面的若干做法,并在我们工作的基础上提出了开展此项工作的几个关键问题及中能解决的途径。     

中国海洋卫星雷达高度计海上定标场建设初探

文章通过对卫星雷达高度计定标方法和定标内容的描述,跟踪调研国际上业务化的高度计定标场的配置和定标结果,提出我国海洋卫星高度计定标场建设基本思路,同时对定标场周边海区影响高度计定标的地球物理参数和水文气象参数进行初步分析,确定我国海上定标场的基本配置方案,将为我国海洋卫星雷达高度计海上定标场建设提供基本依据和参考。     

On the Development of a Second-Order Bistatic Radar Cross Section of the Ocean Surface: A High-Frequency Result for a Finite Scattering Patch

The development of a model for the second-order bistatic high-frequency (HF) radar cross section on an ocean surface patch remote from the transmitter and receiver is addressed. A new approach is taken that allows a direct comparison with existing monostatic cross sections for finite regions of the ocean surface. The derivation starts with a general expression for the bistatically received second-order electric field in which the scattering surface is assumed to be of small height and slope. The source field is taken to be that of a vertically polarized dipole, and it is assumed that the ocean surface can be described, as is usually done, by a Fourier series in which the coefficients are zero-mean Gaussian random variables. Subsequently, a bistatic cross section of the surface, normalized to patch area, is derived. The result is verified by the following two means: 1) the complete form of the bistatic HF radar cross section in backscattering case is shown to contain an earlier monostatic result that has, itself, been used extensively in radio oceanography applications; and 2) the bistatic electromagnetic coupling coefficient is shown to reduce exactly to the monostatic result when backscattering geometry is imposed. The model is also depicted and discussed based on simulated data     

X波段雷达图像提取海洋表面风场

船用X波段导航雷达海杂波图像中包含有丰富的海浪、海面流和风场等环境信息。阵风的运动在滤波后的雷达图像序列中是可见的,根据分析阵风的运动,用基于运动估计技术的光流方法从阵风的运动中测定风矢量。文章主要分析了小麦岛海域岸基试验的海杂波数据,将用雷达图像序列提取的风矢量和现场风速仪、风向标测得的风矢量做了比较,证明了光流运动估计技术提取高分率风场的可行性。     

Characterizing Observed Environmental Variability With HF Doppler Radar Surface Current Mappers and Acoustic Doppler Current Profilers: Environmental Variability in the Coastal Ocean

A network of high-frequency (HF) radars is deployed along the New Jersey coast providing synoptic current maps across the entire shelf. These data serve a variety of user groups from scientific research to Coast Guard search and rescue. In addition, model forecasts have been shown to improve with surface current assimilation. In all applications, there is a need for better definitions and assessment of the measurement uncertainty. During a summer coastal predictive skill experiment in 2001, an array of in situ current profilers was deployed near two HF radar sites, one long-range and one standard-range system. Comparison statistics were calculated between different vertical bins on the same current profiler, between different current profilers, and between the current profilers and the different HF radars. The velocity difference in the vertical and horizontal directions were then characterized using the observed root-mean-square (rms) differences. We further focused on two cases, one with relatively high vertical variability, and the second with relatively low vertical variability. Observed differences between the top bin of the current profiler and the HF radar were influenced by both system accuracy and the environment. Using the in situ current profilers, the environmental variability over scales based on the HF radar sampling was quantified. HF radar comparisons with the current profilers were on the same order as the observed environmental difference over the same scales, indicating that the environment has a significant influence on the observed differences. Velocity variability in the vertical and horizontal directions both contribute to these differences. When the potential effects of the vertical variability could be minimized, the remaining difference between the current profiler and the HF radar was similar to the measured horizontal velocity difference (~2.5 cm/s) and below the resolution of the raw radial data at the time of the deployment     

合成孔径雷达(SAR)图像的真实值定标与校验

随着合成孔径雷达（ＳＡＲ）的应用研究程度的深入，要求较为精确地掌握目标体的雷达后向散射特征，这样必须对ＳＡＲ图象进行真实值的定标与校验。本文基于基本散射理论对ＳＡＲ系统的增益进行分解，指出各个相关参数的获取方法，最后给出定标和数据处理流程图     

Combined Method of Radar Measurements of Surface Current Parameters

Izvestiya, Atmospheric and Oceanic Physics - In this paper, a combined method for measuring the velocity vector of surface currents using a modernized dual-polarization 3-cm range radar is we...     

Validation and Improvement of Satellite-Derived Surface Solar Radiation over the Northwestern Pacific Ocean

The purpose of this study is to validate and improve satellite-derived downward surface shortwave radiation (DSSR) over the northwestern Pacific Ocean using abundant in situ data. The DSSR derivation model used here assumes that the reduction of solar radiation by clouds is proportional to the product of satellite-measured albedo and a cloud attenuation coefficient. DSSR is calculated from Geostationary Meteorological Satellite-5/Visible Infrared Spin-Scan Radiometer data in 0.05° × 0.05° grids. The authors first compare the satellite DSSR derived with a cloud attenuation coefficient table determined in past research with in situ values. Although the hourly satellite DSSR agrees well with land in situ values in Japan, it has a bias of +13∼+34 W/m² over the ocean and the bias is especially large in the low latitudes. The authors then improve the coefficient table using the ocean in situ data. Usage of the new table successfully reduces the bias of the satellite DSSR over the ocean. The cloud attenuation coefficient for low-albedo cases over the ocean needs to be larger in the low latitudes than past research has indicated. Daily and hourly DSSR can be evaluated from the satellite data with RMS errors of 11–14% and 30–33%, respectively, over a wide region of the ocean by this model. It is also shown that the cloud attenuation coefficient over land needs to be smaller than over the ocean because the effect of the radiation reflected by the land surface cannot be ignored.     

太平洋北赤道流表层流速及分叉点位置

确定太平洋北赤道流表层流速及分叉点位置的变化是海洋环境研究中的1个重要问题。使用1987～1998年的WOCE浮标资料,通过估算得到了季节平均和年平均意义下北赤道流表层流速(1989～1998)。计算结果显示:北赤道流表层流的流速冬季最大,夏季最小,春秋两季相仿。在厄尔尼诺发生年的第2年,夏季平均流速往往较大。对浮标轨迹的逐年分析表明表层北赤道流分叉点的位置在11°N～14 .7°N之间,从轨迹较为密集的4年的分析可以看出,表层流分叉点的位置具有年际变化,其中,2个ElNino年分叉点偏北。     

A Method of Swell-Wave Parameter Extraction From HF Ocean Surface Radar Spectra

A new method for the extraction of swell-wave parameters from high-frequency (HF) radar spectra is presented. The method of extraction of the parameters, period, direction, and height, relies on a frequency-modulation approach that describes the hydrodynamic interaction of the swell waves with the resonant, shorter, Bragg waves. The analysis process minimizes the electromagnetic second-order interaction and a simulation model was used to validate the approach. This simplified method provides a fast means of examining swell conditions over large areas of the ocean surface. Data are acquired using a pair of coastal ocean surface radar (COSRAD) systems deployed at Tweed Heads, Qld., Australia. The radar covers a sweep (approximately 60deg) every 30 min with spatial resolution of the order of 3 km. A sample set of data from this deployment is used in a case study to show the extraction of swell direction and amplitude using these methods. The results support the use of the COSRAD HF radar for mapping swell in the near-shore zone     

高频地波雷达的发展与应用现状分析

高频地波雷达作为一种新兴的海洋探测设备,相比传统观测方式,具有大范围、全天候、低成本等优点,因而在世界各地得到了广泛使用,并在近海海洋环境监测中发挥了重要的作用。首先介绍了高频地波雷达探测海洋环境的基本原理,然后概述了阵列式和紧凑便携式两类高频地波雷达的国内外研究现状,接着介绍了高频地波雷达在海洋环境探测中的应用,最后分析了我国与世界发达国家在高频地波雷达海洋环境探测领域的差距并提出了改进建议。     

Determination of Mesoscale Surface Currents in Shallow-Water Regions According to the Data of High-Frequency Radar Measurements

We propose a new method for the evaluation of the velocities of surface currents according to the data of measurements carried out by using high-frequency land-based radars. The method is based on the representation of the velocity fields via two scalar potentials, expansion of these potentials in series in basis functions, and determination of the coefficients of expansion according to the data of radar measurements as solutions of the corresponding variational problems. The errors of the procedure of determination of the coefficients of expansion are removed by using a special regularization procedure based on information theory. The proposed method enables one to fill gaps in the space and time series of radar measurements. We illustrate the method by an example of numerical analysis of mesoscale and submesoscale (10–50 km) surface currents in the Monterey Bay (California, USA) performed on the basis of the data of radar measurements carried out in August 1994. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 2, pp. 24–36, March–April, 2005.     

星载SAR海洋表层流场反演综述

基于星载SAR获取海洋表层流场,已经成为相关海洋科学研究的重要支持。本文分析了星载SAR系统反演海洋表层流场的技术发展,重点介绍了国内外基于顺轨干涉SAR技术和多普勒频移方法估计海洋表层流场的研究进展,并对星载SAR估计海洋表层流场的研究、发展与应用作了展望。     

Target Classification and Remote Sensing of Ocean Current Shear Using a Dual-Use Multifrequency HF Radar

In this paper, we describe a high-frequency (HF) radar capable of multifrequency operation over the HF band for dual-use application to ship classification and mapping ocean current shear and vector winds. The radar is based on a digital transceiver peripheral component interconnect (PCI) card family that supports antenna arrays of four to 32 elements with a single computer, with larger arrays possible using multiple computers and receiver cards. The radar makes use of broadband loop antennas for receive elements, and a number of different possibilities for transmit antennas, depending on the operating bandwidth desired. An option exists in the choice of monostatic or multistatic operation, the latter providing the ability to use several transmit sites, with all radar echo signal reception and processing conducted at a single master receiver site. As applications for such a multifrequency radar capability, we show measurement and modeling examples of multiple frequency HF radar cross section (RCS) of ships as an approach to ship target classification. Results of using 32 radar frequencies to measure the fine structure in ocean current vertical shear are also shown, providing evidence of one edge of a 1-3-m deep uniform flow masked at the surface by wind-driven current shear in a different direction. Other applications of current-shear measurements, such as vector wind mapping and volumetric current estimation in coastal waters, are also discussed     

Poseidon-2 Radar Altimeter Design and Results of In-Flight Performances Special Issue: Jason-1 Calibration/Validation

Poseidon-2 is the dual frequency, solid-state radar altimeter embarked on the CNES/NASA oceanographic satellite Jason-1. This article gives a brief summary of the instrument design and some in-flight performances. Flight results have confirmed the very good results of the altimeter transfer function, which is very stable, and that the range noise at instrument level is less than 2 cm for a Significant Wave Height of 2 m.     

人造海洋涌升流和海洋渔场

针对渔业资源日趋减少和鱼产品需求不断增加的矛盾，探讨了建造海底山脉、人工制造海洋涌升流、从而人工制造优良海洋渔场的可行性，分析了我国建造海底山脉的水深、海底和海流条件。介绍了建造海底山脉的建筑方法和国际上建造海底山脉的最新动态。论述了我国在200n mile以内建造海底山脉的有利条件。     

Assessment of Radar Waveform Retracked Jason-2 Altimetry Sea Surface Heights Near Taiwan Coastal Ocean

This study focuses on assessing the accuracy of 20-Hz waveform retracked Jason-2 (J-2) altimetry sea surface heights (SSHs) in the vicinity of Taiwan by comparisons with the TOPEX/Poseidon (T/P) 10-Hz SSHs and sea level data from the Anping tide gauge. The study areas exhibit high, medium, and low amplitudes of ocean tides and contain diverse bathymetries with depths of 0–4000 m. The performance of Offset Center of Gravity (OCOG), threshold, modified threshold, and ice retrackers was examined by comparing the retracked SSHs with Earth Gravitational Model 2008 (EGM08) geoid via the use of the improvement percentages (IMPs). The results indicate that both altimetry measurements are significantly improved by waveform retracking techniques, with a maximum IMP of 46.6% for T/P and 82.0% for J-2, and the optimal achievement of retrackers is influenced by the characteristics of the study areas. In addition, valid retracked J-2 SSHs are much closer to shorelines than T/P. A comparison of retracked J-2 data with Anping tide gauge records reveals that applying the optimal retracking algorithms reduces the root mean squares of differences and increases the number of valid measurements.     

Leveling the Sea Surface Using a GPS-Catamaran Special Issue: Jason-1 Calibration/Validation

In the framework of the TOPEX/Poseidon and Jason-1 CNES-NASA missions, two probative experiments have been conducted at the Corsica absolute calibration site in order to determine the local marine geoid slope under the ascending TOPEX/Poseidon and Jason-1 ground track (No. 85). An improved determination of the geoid slope was needed to better extrapolate the offshore (open-ocean) altimetric data to on-shore tide-gauge locations. This in turn improves the overall precision of the calibration process. The first experiment, in 1998, used GPS buoys. Because the time required to cover the extended area with GPS buoys was thought to be prohibitive, we decided to build a catamaran with two GPS systems onboard. Tracked by a boat at a constant speed, this innovative system permitted us to cover an area of about 20 km long and 5.4 km wide centered on the satellites' ground track. Results from an experiment in 1999 show very good consistency between GPS receivers: filtered sea-surface height differences have a mean bias of ?0.2 cm and a standard deviation of 1.2 cm. No systematic error or distortions have been observed and crossover differences have a mean value of 0.2 cm with a standard deviation of 2.7 cm. Comparisons with tide gauges data show a bias of 1.9 cm with a standard deviation of less than 0.5 cm. However, this bias, attributable in large part to the effect of the catamaran speed on the waterline, does not affect the geoid slope determination which is used in the altimeter calibration process. The GPS-deduced geoid slope was then incorporated in the altimeter calibration process, yielding a significant improvement (from 4.9 to 3.3 cm RMS) in the agreement of altimeter bias determinations from repeated overflight measurements.