高频返回散射台风跟踪技术实验研究

高频返回散射海面回波Doppler谱中存在着两个明显的峰（称之为一阶Bragg峰）,研究表明两个峰值的大小与海面风向有关,根据这种关系对反演海面风向以及台风位置作了研究,利用中国电波传播研究所高频返回散射探测站的固定频率探测数据进行实验,绘制出了2007年9月台风“韦帕”的风向场图与路径图,并与气象部门实际台风数据进行比较,较好地反映出了海面风向场、台风中心位置与台风经历的路线,表明高频返回散射探测系统可以应用于海洋气象监测中。     

Numerical study on the along-track interferometric radar imagingmechanism of oceanic surface currents

The phase information in along-track interferometric synthetic aperture radar (along-track INSAR, ATI) images is a measure of the Doppler shift of the backscattered signal and thus of the line-of-sight velocity of the scatterers. It can be exploited for oceanic surface current measurements from aircraft or spacecraft. However, as already discussed in previous publications, the mean Doppler frequency of the radar backscatter from the ocean is not exclusively determined by the mean surface current, but it includes contributions associated with surface wave motion. The authors present an efficient new model for the simulation of Doppler spectra and ATI signatures. The model is based on Bragg scattering theory in a composite surface model approach. They show that resulting Doppler spectra are consistent with predictions of an established model based on fundamental electrodynamic expressions, while computation times are reduced by more than one order of magnitude. This can be a key advantage with regard to operational applications of ATI. Based on model calculations for two simple current fields and various wind conditions and radar configurations, they study theoretical possibilities and limitations of oceanic current measurements by ATI. They find that best results can be expected from ATI systems operated at high microwave frequencies like 10 GHz (X band), high incidence angles like 60°, low platform altitude/speed ratios, and vertical (VV) polarization. The ATI time lag should be chosen long enough to obtain measurable phase differences, but much shorter than the decorrelation time of the backscattered field     

高频天波返回散射回波谱实验研究

用中国电波传播研究所高频天波返回散射探测站进行的我国首次陆地/海洋回波谱的实验研究,其中包括自行研制的实验设备,实验结果.介绍了获得海洋回波谱的信号处理方法,并由海洋回波Doppler谱计算了电离层状参数和风向、海流等海洋状态参数.     

The Influence of Surface Oil on C-and Ku-Band Ocean Backscatter

A comparative study of ocean backscatter depression due to surface oil has been carried out using Ku-and C-band scatterometers supported by some X-and C-band synthetic aperture radar (SAR) imagery. The depression of radar backscatter for both C-and Ku-band HH-polarized radiation has been measured for the incidence angle range from 20° to 50° on two days, September 16 and 17, 1983 on which the average wind and wave height conditions were 3-6 m/s, 0.3 m and ~10-14 m/s, 1.2 m, respectively. Results show that the depression in Ku-band backscatter increases from approximately 3-5 dB at 20° to ~10 dB or more at 30°-40° and then decreases at the larger incidence angles. Generally, the angular dependence of C-band backscatter depression was similar to that at Ku-band, but the peak depression was shifted to slightly larger angles and usually exceeded Ku peak 1-2 dB. The Ku-band results are in good agreement with previous work [9] that tentatively explained the angular results of backscatter depression on the basis of a selective damping of the first-order Bragg resonant waves by the oil film. This explanation, however, is inconsistent with the results of the present work when both C-and Ku-band are considered. The significance of the data is discussed in the context of present and future radar systems (e. g., the ESA ERS-1 SAR and the Radarsat SAR).     

Estimation of sea-surface winds using backscatter cross-sectionmeasurements from airborne research weather radar

A technique is presented for estimation of sea-surface winds using backscatter cross-section measurements from an airborne research weather radar. The technique is based on an empirical relation developed for use with satellite-borne microwave scatterometers which derives sea-surface winds from radar backscatter cross-section measurements. Unlike a scatterometer, the airborne research weather radar is a Doppler radar designed to measure atmospheric storm structure and kinematics. Designed to scan the atmosphere, the radar also scans the ocean surface over a wide range of azimuths, with the incidence angle and polarization angle changing continuously during each scan. The new sea-surface wind estimation technique accounts for these variations in incidence angle and polarization and derives the atmospheric surface winds. The technique works well over the range of wind conditions over which the wind speed-backscatter cross-section relation holds, about 2-20 m/s. The problems likely to be encountered with this new technique are evaluated and it is concluded that most problems are those which are endemic to any microwave scatterometer wind estimation technique. The new technique will enable using the research weather radar to provide measurements which would otherwise require use of a dedicated scatterometer     

车载测风激光雷达风廓线同步观测实验

多普勒测风激光雷达能够实现高时空分辨率的大气风场测量,中国海洋大学成功研制了可以测量风廓线和三维大气风场的车载多普勒测风激光雷达,并已交付中国气象局使用。为了检验该激光雷达的测量性能,2011年春季开展了车载激光雷达与探空气球风廓线同步观测实验,获取了55组比对数据。本文介绍了此次同步观测实验,给出了激光雷达和探空气球风廓线数据的比对个例,并对所有比对数据进行了统计分析。同步比对结果显示,激光雷达与探空气球风廓线数据的风速均方根偏差为2.76 m/s。通过分析比对偏差,证明了激光雷达风廓线测量结果的准确性。     

Ku-band backscatter from the Cowlitz River: Bragg scattering with and without rain

Ku-band backscatter from the Cowlitz River in southwestern Washington State was measured for incidence angles from 0/spl deg/ to 80/spl deg/. The measurements were made for light-wind conditions with and without rain. In rain-free conditions, Bragg scattering was the dominant scattering mechanism for both horizontal (HH) and vertical (VV) polarizations out to 75/spl deg/, beyond which the SNR dropped very low at HH. When a light rain was falling on the river, the cross section increased substantially at moderate incidence angles. Doppler spectra taken during rain showed that VV polarized backscatter is primarily from Bragg scattering from ring waves, while HH polarization scatters from both ring waves and stationary splash products, depending on the incidence angle. From the VV polarized measurements, surface wave height spectrum for ring waves is inferred for light rains. Finally, a change in spectral properties was observed when rain changed to hail.     

Hydrodynamic effects in low-grazing angle backscattering from theocean

Time series of returned power, Doppler spectra and range versus time intensity (RTI) images collected from low-grazing angle radar backscattering from the ocean present features which cannot be explained solely within the framework of resonant Bragg scattering. We propose that most of the observed characteristics are a consequence of the way in which waves evolve on the surface of the ocean. We have built a model consisting of a hydrodynamic module and a radar response module. The hydrodynamics module includes most of the physics thought to be relevant to the evolution of a wavefield (i.e., nonlinear interactions, wind, and wavebreaking). The radar module computes the backscattering as the accumulation of Bragg response from every tilted facet of the reconstructed surface, except for those locations where hydrodynamic conditions leading to wavebreaking are detected. Facets involved in wavebreaking are assumed to contribute to the backscattering in a quasi-specular polarization independent fashion. The hydrodynamics module is used to simulate the evolution of a nonlinear wave field, starting from essentially monochromatic conditions. The evolution reproduces known characteristics of these systems, including the generation of sideband instabilities and downshifting. The radar response module is then exercised on the resulting surface at various stages of development. Simulated RTIs at very low-grazing angles reproduce the observed polarimetric characteristics, as well as their behavior when the grazing angle is increased. Simulated Doppler spectra reproduce the peak separation phenomenon observed in field measurements at very low-grazing angles and also show a behavior similar to that shown by field data when the grazing angle is increased     

Laboratory study of polarized scattering by surface waves atgrazing incidence. I. Wind waves

Laboratory measurements of Ku-band scattering at grazing incidence are presented. This study was motivated by the need to understand the processes which significantly contribute to scattering at grazing incidence. A dual polarized (VV, HH) coherent pulsed Ku-band scatterometer with good temporal resolution (3 ns) was used to obtain Doppler spectra and the absolute cross-section of scattered signals for grazing angles from 6-12°, and winds in the range 2-12 m/s. Wire wave gauges were used to measure the wind-wave field. Measurements of the first few moments of the Doppler spectra (cross-section, central frequency and bandwidth) showed that the data separated into two groups. The first grouping corresponded to HH scattering in the upwind look direction, and was clearly associated with scattering from the dominant gravity wind-waves. The second grouping corresponded to HH scattering in the downwind look direction, and all VV scattering, and was consistent with Bragg scattering from free higher frequency waves. This classification of the electromagnetic scattering was consistent with comparisons of direct and Doppler measurements of the kinematics of the surface wave field. The electromagnetic classification was also consistent with asymmetries in the wave field which increased with increasing wind speed     

天发舰收高频雷达一阶海杂波电磁散射特性分析

天发舰收高频雷达是对传统固定平台条件下天地波雷达系统的延伸,由于舰载平台运动和传播信道中电离层的影响,使得一阶海杂波谱呈现出更多特点.本文根据Walsh模型中一阶电场方程,推导得到了天发舰收传播模式下一阶海面散射截面积的解析表达式.然后,根据表达式对不同因素对一阶谱的影响进行仿真分析.仿真结果表明雷达工作频率、舰载平台运动和电离层水平漂移运动是引起一阶海杂波谱多普勒展宽的主要原因,风向主要影响一阶海杂波谱的形态变化,而这一特点为根据一阶海杂波谱进行海洋遥感提供了可能.     

Estimating wind speed from HF skywave radar sea backscatter

Linear expressions relating ocean surface wind speed to a theoretical estimate of the - 10 dB width of the high frequency (HF) sea echo Doppler spectrum (0.078 Hz Doppler shift frequency resolution) are derived for different radar frequencies. The -10 dB width, determined from theoretical model estimates of the Doppler spectrum as a function of radar frequency for a wide range of ocean wave conditions, changes in a complex way with the continuum of second-order echoes surrounding the stronger first-order echo. Because the amplitude of the second-order echoes is directly related to changes in the directional ocean wave spectrum, the wind speed estimates derived from these expressions are highly dependent on the wave field (both the total energy and its distribution with direction); significant differences in these expressions are also found, for example, between fetch-limited and fully developed seas, and for wind seas and swell. These expressions are extremely difficult to use experimentally to obtain reliable estimates of wind speed because a priori knowledge of the wave field is required to apply the correct expression, and yet this knowledge cannot be determined from the unresolved second-order structure at this spectral resolution. Several Doppler spectra recorded under known ocean wind and wave conditions illustrate the difficulty in applying these theoretical expressions to estimate wind speed. We conclude that the -10 dB width is not a good estimator of wind speed. Wind speed estimates are better obtained from HF radar spectra by first estimating the directional wind-wave spectrum from the second-order echoes (0.01 Hz Doppler shift frequency resolution) and then computing wind speed from a wind-wave prediction model.     

Polarimetric radar remote sensing of ocean surface wind

Experimental data are presented to support the development of a new concept for ocean wind velocity measurement (speed and direction) with the polarimetric microwave radar technology. This new concept has strong potential for improving the wind direction accuracy and extending the useful swath width by up to 30% for follow-on NASA spaceborne scatterometer mission to SeaWinds series. The key issue is whether there is a relationship between the polarization state of ocean backscatter and surface wind velocity at NASA scatterometer frequencies (13 GHz). An airborne Ku-band polarimetric scatterometer (POLSCAT) was developed for proof-of-concept measurements. A set of aircraft flights indicated repeatable wind direction signals in the POLSCAT observations of sea surfaces at 9-11 m/s wind speed. The correlation coefficients between co- and cross-polarized radar response of ocean surfaces have a peak-to-peak amplitude of about 0.4 and are shown to have an odd-symmetry with respect to the wind direction, unlike the normalized radar cross sections     

Statistical study of the spectral broadening of skywave signalsbackscattered by the sea surface: application to RMS wave heightmeasurement with a skywave radar

HF skywave signals backscattered by the sea surface are studied on a large set of data (more than 30 h of 64 independent signals) to identify the sources of the broadening of the first-order spectral line. Using high-quality signals reflected by sporadic E ionospheric layers, the natural broadening due to sea-scattering effects has been scaled to about 3/100 Hz. When the signals propagate via F layer, the total broadening due to ionospheric effects is similar in magnitude and can be attributed to two causes. The first, due to frequency modulation effects, which can be identified and corrected, scales on average to 1/100 Hz. The second, called unresolved ionospheric effects, scales on average to 2/100 Hz and is probably due to the spatial variation of the ionospheric Doppler within the ionospheric control volume. Since they are greatly variable with time and space, the influence of these unresolved ionospheric effects can be reduced by sorting spectra, according to the value of the equivalent spectral width, before averaging. Using such sorting and correcting the signals for the ionospheric frequency modulation, 70% of the considered set of data are usable to measure the root mean square (RMS) wave height     

平台运动引起Bragg海浪谱的展宽及其压缩方法

基于运动叠加原理，推导了以Ｄｏｐｐｌｅｒ频率表征的雷达平台运动条件下高频段海浪杂波的运动方程及由平台运动引起的Ｂｒａｎｎ海浪谱的展宽式，给出了受平台运动调制的Ｂｒａｇｇ浪回波与目标信号在Ｄｏｐｐｌｅｒ域里的二阶模糊关系和动用ＳＴ－ＷＤ（Ｓｈｏｒｔ Ｔｉｍｅ Ａｖｅｒａｇｅ Ｗｉｇｎｅｒ－Ｖｉｌｌｅ Ｄｉｓｔｒｉｂｕｔｉｏｎ；短时间平均Ｗｉｇｎｅｒ－Ｗｉｌｌｅ分布）技术对一阶海浪回波信号的高频相位进     

Laboratory study of polarized microwave scattering by surface wavesat grazing incidence: the influence of long waves

Laboratory measurements of microwave scattering at grazing incidence from superposed wind and weakly nonlinear (AK<0.024) regular long waves are presented. This study is an extension of previous measurements with wind waves only. A dual polarized (VV, HH) coherent pulsed Ku-band (14 GHz) scatterometer with temporal resolution of 3 ns was used to obtain Doppler spectra and the absolute cross section of scattered signals for grazing angles from 6° to 25° and winds in the range 2-12 m/s. A wire wave-gauge array was used to measure the wind-wave field. Measurements of the frequency and amplitude modulation of the scattered signal due to the long waves showed that the data separated into two groups. The first grouping corresponded to HH scattering in the upwind direction and was clearly associated with scattering from the dominant gravity wind-waves on the crests of the long waves. In this case, the wind speed clearly influences the frequency modulation due to long waves. The second grouping corresponded to scattering in the downwind direction and was consistent with Bragg scattering from higher frequency waves. In this case the frequency modulation due to orbital velocity of the long waves was found to be weakly dependent on wind speed over the range of parameters studied. This classification of the electromagnetic scattering was consistent with comparisons of direct and Doppler measurements of the kinematics of the surface wave field     

Measurement and classification of low-grazing-angle radar seaspikes

High-resolution dual-polarization X-band images of the ocean surface were obtained at a grazing angle of about 3°. Area extensive imaging allowed us to study the backscatter properties of sea spikes and to compare radar measurements with visual surface features evident from video recordings. The vertically polarized radar images consist of distributed scatter whose amplitude and Doppler velocity are modulated by larger scale gravity waves consistent with Bragg scattering and composite surface theory (CST). The horizontally polarized radar images are dominated by spatially discrete scattering centers (or sea spikes) moving at velocities comparable to the phase velocities of gravity waves beyond the spectral peak. These sea spikes also exist in the corresponding V-pol radar images, but are less prominent due to the dominant Bragg backscatter. Sea spikes are characterized by polarization ratios H/V that often exceed unity, typically by about 5 dB. Comparison of the larger spikes with simultaneous co-registered video recording of the surface indicates that approximately 30% of observed sea spikes are associated with actively breaking waves (whitecaps) while the remainder are identified with “steep” wave features. By classifying the larger sea spikes according to their corresponding surface features, we find hat the Doppler velocities for sea spikes due to whitecaps are noticeably faster (about 50%) than other sea spikes, though the distributions for both overlap significantly. We also find little measurable difference in the polarization ratios of the two classes of sea spikes as observed on the open ocean     

海面散射仿真中不同波浪谱和松弛率模型选取的对比研究

海面微波散射仿真对于实孔径、合成孔径雷达海洋遥感应用研究以及海洋监测雷达系统设计和信号处理都有很重要的意义,目前的海面散射仿真方法主要是基于复合表面模型来进行的,这些仿真方法都要用到海浪谱和松弛率模型。由于海浪谱、松弛率尤其是小尺度部分难以精确测量,不同的实验和拟合方法得到的模型有较大的差别,从而导致仿真海面散射结果往往相差较大,常常让人不知如何选取。该文比较了几种典型的海浪谱和松弛率模型的海面散射仿真结果与实测数据的差异,结果表明Romeiser提出的波浪谱在L-Ku波段,2-20 m/s风速范围内较其他波浪谱更好。而两种主要的松弛率模型的仿真结果差异不大。该结果可作为海面散射仿真中选取波浪谱和松弛率模型的参考。     

超高频雷达海洋表面流的探测结果与分析

近岸海域地形、海流复杂, 而超高频雷达系统具有较高距离分辨率、小发射功率的特点, 波长介于高频电磁波和微波之间, 能同时提取海浪毛细波与重力波信息, 与波浪作用比较敏感, 能实现近海海洋动力参数的精细测量.不同于高频雷达回波谱, 超高频雷达回波中一阶峰、二阶峰展宽严重并混叠在一起, 难以划分一阶峰区域, 因此, 文中系统设计采用数字波束形成进行流速方位提取.雷达反演的流速结果与浮标数据的相关系数为0.88, 但是均方根误差很大.通过分析, 得出海洋表面流速不仅受到潮汐效应、地球自转的影响, 当地风速在很大程度上也会影响表面流速的结论.