Measurement of oceanic wind speed from HF sea scatter by skywave radar

Remote measurements of the spatial mean ocean wind speeds were obtained using Doppler spectra resolved to 0.08 Hz from high-resolution HF skywave-radar backscatter measurements of the ocean surface. A standard deviation of 2.4 m/s resulted from the correlation of observed winds over the ocean and the broadening of the Doppler spectra in the vicinity of the higher first-order Bragg line. This broadening, for Doppler spectra unperturbed by the ionospheric propagation, is proportional to the increase in power caused by higher order hydrodynamic and electromagnetic effects in the vicinity of the Bragg line and inversely proportional to the square root of the radio frequency. A lower bound on the measure of wind speed was established at 5 m/s by the low resolution spectral processing and low second-order power. An upper limit is suggested by the steep slope in the region of the sea backscatter spectrum outside the square root of two times the first-order Bragg line Doppler.     

Mapping of North Atlantic winds by HF radar sea backscatter interpretation

This work presents preliminary results on attempts to map winds of a storm at long range (500 to 1000 nmi) over a large area (10^{5}mi²) in the North Atlantic from the U. S. Naval Research Laboratory, HF radar research facilities at Chesapeake Beach, Md. [1] It appears that the short time response of the sea surface to local winds can be mapped by the analysis of a matrix of range-azimuth records containing frequency power spectra of HF radar signals backscattered from the sea surface via the ionosphere. This paper presents such a map based upon the ratio of the strengths of the first-order contributions to the backscatter spectrum (the approach-recede first-order Bragg lines) and is compared qualitatively with a U. S. Weather Bureau map of the area.     

High-resolution mapping of oceanic wind fields with skywave radar

The direction of the mean surface wind field in the North Pacific Ocean was mapped on September 25 and 26, 1973, over an area of3 times 10^{6}(km)²by OTH-B HF radar. A spatial resolution of 60 km in range and 15 km in cross range was used at points spaced by 150 km in range and 80 km in cross range. Wind directions were inferred from the upwind/downwind first-order Bragg ratio and the measure of the maximum ratio occurring for radial winds at points near each observation.     

Time-domain adaptive beamforming of HF backscatter radar signals

Experimental results relating to the use of time-domain, continuously adapting beamformers in an HF bistatic FM/CW backscatter radar system are presented. Data for the study were obtained using the Wide Aperture HF Radio Research Facility (WARF) which is located in the central valley of California and is operated by Stanford Research Institute, Menlo Park, CA. Eastward-looking transmissions were employed with an operating frequency chosen so as to provide single-hop ionospheric propagation on both the forward and backscatter paths. Digital real-time recordings were taken at eight received subarray outputs. These recordings were then processed off-line using a large general purpose computer. The beamforming methods studied, however, are computationally simple and may be readily implemented in real-time using a commercially available minicomputer. Two adaptive algorithms were studied and in both cases it was shown that signal-to-noise ratio improvements of 10 to 15 dB are readily achieved when adaptive beamforming is compared with conventional, Dolph taper beamforming methods using identical received data in an HF backscatter environment. It was also demonstrated that the time scale of coefficient variation in an adaptive processor operating in this environment is the order of 1 s. Successful tracking of the adaptive algorithm under these conditions was demonstrated. The use of moving target indication (MTI) clutter suppression filters at the subarray outputs, prior to adaptation, was investigated. No significant improvement was observed with the use of these filters on experimental data. Finally, it was shown that the presence of fading nulls can significantly affect the determination of optimal subarray location and spacing in an HF environment. In general, the adaptive beamformer performance was found to be less dependent upon array geometry than was the case for conventional processing.     

天波超视距雷达海杂波的混沌动态特性分析

利用替代数据法对实测回波信号进行非线性检验,Cao方法进行相空间重构、Rosenstein小数据量法计算最大Lyapunov指数、改进的格拉斯伯格-庞加莱算法(GPA)计算Kolmogorov熵以及局部可预测性检验研究了高频天波雷达(OTHR)海杂波的混沌动态特性。仿真计算表明:实测的高频天波雷达海杂波吸收子具有稳定收敛的关联维数、正的最大Lyapunov指数和正的Kolmogorov熵以及具有局部可预测的特性,验证了高频雷达海杂波确实来自于一个低维混沌系统。利用方差分析初步讨论了电离层对回波混沌特性的影响,研究表明:电离层将对回波混沌特性产生显著的影响。这些结论对高频雷达目标探测和海杂波建模研究都具有重要意义。     

Estimating splash pine biomass using radar backscatter

L-band HV multiple-incidence-angle aircraft synthetic aperture radar (SAR) data were analyzed in relation to average stand biomass, basal area, and tree height for 55 slash pine plantations located in northern Florida. This information was used to develop a system of equations to predict average stand biomass as a function of L-band (24.5-cm) radar backscatter. The system of equations developed in this study using three-stage least-squares and combinatorial screening accounted for 97% of the variability observed in average stand biomass per hectare. When applied to an independent data set, the biomass equations had an average bias of less than 1% with a standard error of approximately 3%     

An on-line adaptive beamforming capability for HF backscatter radar

An adaptive-array beamforming capability has been implemented on-line in an existing over-the-horizon (OTH) backscatter radar. Inputs to the beamformer consisted of signals from eight 32- element subarrays of the 256-element, 2.5-km-long receiving array at the Wide Aperture Research Facility (WARF) located in California. Both conventional and adaptive beamforming were performed prior to the usual range/Doppler analysis used to extract radar targets from noise and clutter. The GriffithsP-vector algorithm, a recursive, time-domain adaptive technique, was implemented in all-digital fashion using fixed-point arithmetic on a 16-bit minicomputer. Desired signals utilized were aircraft targets and a fixed, ground-based radar repeater simulating a moving target, while unwanted signals were other-user interference and signals from a separate ground-based radar repeater. It was found that adaptive rejection of unwanted signals was dependent on pointing-angle alignment and that rejection was often increased by removal of the clutter by moving target indicator (MTI) filtering prior to adaptation. For some conditions, Doppler broadening can he produced by the time modulation imposed by continuous adaptation, unwanted-signal rejection with the adaptive beamformer is variable, but side-by-side comparisons obtained at WARF show that adaptive beamforming can reject off-azimuth signals up to 20 dB better than conventional beamforming with a -25 dB Dolph taper.     

Short pulse radar used to measure sea surface wind speed and SWH

A joint airborne measurement program is being pursued by NRL and NASA Wallops Flight Center to determine the extent to which wind speed and sea surface significant wave height (SWH) can be measured quantitatively and remotely with a short pulse (2 ns), wide-beam (60deg), nadir-looking 3-cm radar. The concept involves relative power measurements only and does not need a scanning antenna, doppler filters, or absolute power calibration. The slopes of the leading and trailing edges of the averaged received power for the pulse limited altimeter are used to infer SWH and surface wind speed. The interpretation is based on theoretical models of the effects of SWH on the leading edge shape and rms sea-surface slope on the trailing, edge shape. The models include the radar system parameters of antenna beam width and pulsewidth. Preliminary experimental results look promising and indicate that it may be possible to design a relatively compact airborne radar to infer, in real-time, the sea surface SWH and surface wind speed.     

Effect of wind on FM-CW radar backscatter from a wet snowcover

The most important factor affecting the microwave properties of a snowcover is the liquid water content (snow wetness). An FM-CW (26.5-40-GHz) radar has been used to investigate the influence of snow wetness on the magnitude of radar backscatter from a snowcover. The radar backscatter measurements from a wet snowcover on a windy day suggest that evaporative cooling due to the wind may reduce the amount of liquid water at the snowcover surface     

高频雷达海杂波的多重分形建模方法研究

将多重分形理论引入高频雷达海杂波建模,提出了一种新的高频雷达海杂波的时域多重分形模型。对比分析了实测数据海杂波、分数布朗运动(FBM)模型和多重分形模型的质量指数函数和奇异指数,结果表明:该多重分形模型与实测数据具有相似的多重分形性。此外,通过对韦布尔分布、对数正态分布和瑞利分布三种最常用海杂波的概率密度函数的比较分析,并利用修正的柯尔莫哥洛夫-斯米尔洛夫(K-S)统计检验,得出了高频雷达海杂波的多重分形模型具有先验的统计特性的结论,从而进一步证明了该多重分形模型的合理性和有效性。     

Directional sea spectrum determination using HF Doppler radar techniques

Second-order features in HF radar Doppler spectral data are compared with a theoretical model of the radar spectrum. The model is the corner reflector double-scatter model which employs a more realistic directional sea spectrum model than those used in earlier works. It includes a frequency-dependent angular spreading function and assumes the existence of spectral energy over a full360degarising from an apparent second-order wave-wave interaction. Comparison is made with ground wave data collected at the NRL/NOAA/ITS San Clemente Island HF radar.     

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     

Progress toward a practical skywave sea-state radar

Recent advances in propagation modeling, ionospheric diagnostics, and signal processing have helped overcome the limitations the ionosphere imposes on sea-state measurements with HF skywave radar. Wind-direction fields in tropical storms can be routinely mapped under most ionospheric conditions, but waveheight and wave-spectrum extraction is more sensitive to ionospheric distortions and requires care in signal processing and in selecting an ionospheric path. Spot measurements with a high-resolution radar have verified its ability to measure (in order of increasing difficulty) wind-direction fields, rms waveheight, and the scalar ocean-wave spectrum at ranges up to 3000 km using one ionospheric hop. Although such a radar can in principle map these quantities over millions of square kilometers of an ocean area, the time required to do so under various ionospheric conditions remains to be determined. A minimum objective of one map of rms waveheight per day seems attainable.     

HF radar wave and wind measurement over the Eastern China Sea

High-frequency (HF) radar can be employed to measure sea surface state parameters such as waveheight, wind field, and surface current velocity. This paper describes the application of the HF ground wave radar in remote sensing the surface conditions over the Eastern China Sea in October 2000. The radar, referred to as the OSMAR2000, was developed by Wuhan University. Preliminary wave spectra, waveheights, and wind fields estimated from the collected data are presented and compared with ship-recorded measurements where such are available. The range for wind direction sensing is up to 200 km. Wave information and wind speed can be provided up to a range of 120 km. The mean difference between radar- and ship-measured significant waveheight is 0.323 m; wind direction is measured within 20/spl deg/; and wind speed to within 0.6 m/s. With such agreement being fairly reasonable, the feasibility of the inversion algorithm and the ocean state real-time sensing capability of OSMAR2000 are demonstrated.     

天波超视距雷达发展综述

在天波超视距雷达系统的短基线多站联合定位中,一般假设多站点电离层反射虚高保持一致,为确定这一假设对定位结果的影响,本文进行电离层探测试验,以研究电离层虚高对多站联合定位精度的影响。试验时分别架设两个接收站模拟短基线超视距雷达系统的接收站点,再在较远距离架设目标站点,利用来自目标站点的发射信号模拟目标的返回信号。本文假设参考站点到目标站点链路的电离层反射虚高和大圆距离是已知的,对于同一工作频率,利用参考站点-目标站点链路上的电离层虚高,去解算定位站点-目标站点之间的大圆距离。参考站点和定位站点相距约90 km情况下,结果显示：目标和定位站(道孚-武汉)大圆距离约为1 260 km时,两条链路的虚高均方根误差约为5.82 km,相应的大圆距离的定位均方根误差约为5.02 km,相对误差约为0.34%;当目标和定位站(乐山-武汉)大圆距离约为1 000 km时,误差分别约为5.5 km, 5.69 km和0.46%。试验结果和理论分析表明,可以从缩短接收站点的布局和降低电离层反射虚高两个方面进一步提高目标定位的精度。本文试验结果可为短基线天波超视距雷达的建设提供较为重要的参考价值。     

Bayesian estimation of soil parameters from radar backscatter data

Given measurements m₁,m₂,...,m_J representing radar cross-sections of a given resolution element at different polarizations and/or different frequency bands, the authors consider the problem of making an “optimal” estimate of the actual dielectric constant ϵ and the rms surface height h that gave rise to the particular {m_j} observed. To obtain such an algorithm, the authors start with a data catalog consisting of careful measurements of the soil parameters ϵ and h, and the corresponding remote sensing data {m_j}. They also assume that they have used these data to write down, for each j, an average formula which associates an approximate value of m_j to a given pair (ϵ;h). Instead of deterministically inverting these average formulas, they propose to use the data catalog more fully and quantify the spread of the measurements about the average formula, then incorporate this information into the inversion algorithm. This paper describes how they accomplish this using a Bayesian approach. In fact, their method allows them to (1) make an estimate of ϵ and h that is optimal according to the authors' criteria; (2) place a quantitatively honest error bar on each estimate, as a function of the actual values of the remote sensing measurements; (3) fine-tune the initial formulas expressing the dependence of the remote sensing data on the soil parameters; (4) take into account as many (or as few) remote sensing measurements as they like in making their estimates of ϵ and h, in each case producing error bars to quantify the benefits of using a particular combination of measurements     

波束倾斜修正在天波超视距雷达中的应用

为了修正天波超视距雷达由于波束倾斜引入的方位估值偏差,基于一维相扫雷达波束扫描的原理,分析和阐述了雷达波束倾斜产生的原因,并详细推导了天波超视距雷达波束倾斜修正量的解算过程.通过对典型的场景分析,结果表明:波束倾斜修正的效果取决于对到达波仰角的估计精度.     

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.     

A method to correct HF skywave backscattered signals forionospheric frequency modulation

A method is described to correct HF signals reflected by the ionosphere and backscattered by the sea surface for ionospheric frequency modulation, which produces spectral-line smearing. The statistical mean of the time derivative of the phase of the signal, weighted by the signal energy, is proposed as an estimator of this modulation. The accuracy of the estimator is measured and the efficiency of the signal processing is tested by synthetically contaminating high-quality signals obtained via sporadic E-layer propagation. Examples of data corrected for F2-layer ionospheric perturbations are shown     

The modeling and measurement of HF antenna skywave radiationpatterns in irregular terrain

The method of moments (MoM) was used in conjunction with the geometric theory of diffraction (GTD) for predicting the elevation-plane radiation patterns of simple high-frequency (HF) vertical monopoles and horizontal dipoles situated in irregular terrain. The three-dimensional terrain was approximated by seven connected flat plates that were very wide relative to the largest wavelength of interest. The plate length along the terrain profile was the longest possible that still adequately followed the shape of the path on the azimuth of the elevation pattern of interest and no shorter than 1 wavelength at the lowest frequency of interest. The MoM model was used to determine the antenna currents under the assumption that the terrain was planar (i.e., locally flat) over the distance pertinent to establishing the input impedance. The currents thus derived were used as inputs to the GTD model to determine the gain versus elevation angle of the antennas for HF skywave when situated in the irregular terrain. The surface wave solution for groundwave was not included since this does not appreciably contribute any effect to the skywave far-field patterns at HF in this case. The model predictions were made using perfect electric conducting (PEC) plates and using thin plates made of lossy dielectric material with the same conductivity and relative permittivity as measured for the soil. These computed results were compared with experimental elevation-plane pattern data obtained using a single-frequency helicopter-borne beacon transmitter towed on a long dielectric rope in the far field on a linear path directly over the antennas. The monopoles and dipoles were situated in front of, on top of, and behind a hill whose elevation above the flat surrounding terrain was about 45 m. The patterns of all of the antenna types and sitings exhibited diffraction effects caused by the irregular terrain, with the largest effects being observed at the highest measurement frequency (27 MHz)