海洋环境监测高频地波雷达的研究现状与发展趋势

高频地波雷达作为一种海洋遥感的重要手段，已有30多年的历史，它可用于海洋表面风、浪、流等状态参数的提取，还可用于目标探测，在专属经济区的监测中，高频地波雷达具有无可比拟的优势。本文介绍了高频地波雷达的发展历史和研究现状及其应用领域，详细叙述了武汉大学研制的高频地波雷达OSMAR2000的体系结构、性能指标和信号处理方法，最后对该研究领域的发展趋势作了简要分析。     

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.     

基于高频雷达二阶海洋回波的风速反演算法研究与应用

基于高频电磁波与海洋表面的散射机制,以雷达二阶散射截面方程为理论依据。首先,进行了从高频雷达二阶海洋 回波谱中反演饱和风速的算法理论近似,讨论算法运用的前提条件;其次进行了饱和风速与二阶谱峰频率的关系式推导;然后,利用高频地波雷达模拟海洋回波谱数据对算法进行了可行性验证,证明该算法特定风速范围内有较好的反演结果; 最后,将算法应用于实测高频地波雷达海洋回波多普勒谱进行了饱和风速反演,并进行了反演结果分析与误差分析。 结果分析表明:该算法在满足理论近似条件的前提下能有效提取海面饱和风速信息。     

高频地波雷达基于海洋回波的通道幅度自校准方法

针对海洋回波的特性,以及高频地波雷达OSMAR2000的实际工作性能,提出一种基于海洋回波采样数据的自适应通道幅度软件校准方法。该方法不需要辅助信号源,能针对每一距离元进行通道幅度的实时校准,校准后的空间谱质量显著提高,并能在通道失效时给出示警信息。文中给出了现场实测数据的处理结果及比较。     

基于海洋回波的高频地波雷达阵列幅相误差校准

针对现有无源校准方法在高频地波雷达中失效的问题,该文提出了一种基于海洋回波的高频地波雷达阵列幅相误差校准算法。该算法利用相控阵中相邻天线对同一海流面元回波之间满足的关系,建立多组幅相误差方程并联立求解。通过对模拟的海洋回波的数据进行处理,分析算法的幅相误差校准值的估计偏差。将此算法应用在实测数据处理中,验证了其有效性及可靠性。     

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.     

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.     

亚帆赛间便携式高频地波雷达的海态观测

2010年汕尾亚运会帆船赛期间,便携式高频地波雷达OSMAR-S作为一种重要的海态监测手段在汕尾海域进行观测,为保障亚帆赛顺利举行提供海洋气象服务。雷达获取了汕尾海域近1个月连续的流速、流向、浪高、浪周期、风向和风速等数据,并与海监船搭载的海流计与测风仪以及现场布放浮标实测的数据进行了对比。对比结果表明:雷达与现场海洋仪器的观测数据较为吻合,初步验证了OSMAR-S探测近海海态特别是风浪参数的能力。     

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     

地波超视距雷达自适应干扰抑制的改进

针对地波超视距雷达探海工作期间高频干扰严重海杂波强的特点,在阵列自适应波束形成时考虑了海杂波约束条件,改进了地波超视距雷达探海工作时的干扰抑制方法.改进方法使得自适应干扰抑制后,回波中一阶海杂波的时域相关特性不会发生破坏而在频域上"虚假"展宽,从而有利于后续处理中进行海杂波滤除和目标检测.最后使用OSMAR2000的实测数据对本文方法进行了检验.     

海洋环境监测高频地波雷达波形参数设计

根据线性调频中断连续波（FMICW）的原理,结合OSMAR2000雷达总体设计要求,在满足无距离混迭、无Doppler频谱混迭的前提下,兼顾其它性能指标,本文重点介绍了波形参数的设计过程。     

高频地波舰载超视距雷达的双零点波束测角方法

在高频地波舰载超视距雷达中,雷达平台的运动导致了一阶海浪谱(Bragg谱线)的展宽.提取和检测落入此展宽区的目标是舰载条件下面临的主要困难.本文分析了Bragg谱线展宽的原因,提出了用单零点波束提取目标并用分裂的双零点波束测角的具体方法.在文章中用计算机对该种方法进行了仿真,仿真结果证明该方法作为高频地波舰载超视距雷达目标检测方法是有效的.     

小型化高频地波雷达舰船目标检测方法

针对OSMAR2003型小型化高频地波雷达系统的目标检测问题,讨论了对小型化高频地波雷达系统低信杂比回波信号的优化处理过程,给出了一种峰值检测后同时在距离域、多普勒域和波束域进行联合的三维联合恒虚警( CFAR)目标检测方法,利用高分辨力测向算法对检测到的目标进行方位估计,然后利用最邻近法进行目标关联得到目标点航迹,最后结合小型化高频地波超视距雷达的实测数据验证了三维CFAR检测器的有效性。     

ERS-1 SAR images of atmospheric gravity waves

ERS-1 synthetic aperture radar (SAR) images of atmospheric gravity waves over the ocean are discussed. Several case studies are presented in detail. It is shown that the well-organized long wavelength (1 to 10 km) wave phenomena which often are seen in SAR images over the ocean may be atmospheric gravity waves. The waves appear in the SAR images because they modulate the surface wind speed which in turn modulates the surface roughness and the radar cross section. The wavelength may be measured directly from the SAR image, and the mean wind speed and wind speed modulation near the ocean surface may be estimated from the observed radar cross section modulation using a wind retrieval model. The atmospheric gravity waves usually were generated by the approach or passing of a meteorological front. Atmospheric soundings and a two-layer model for the lower troposphere indicate that, in general, the observed atmospheric wave phenomena could have been supported by accompanying temperature inversion layers and wind shears     

Aircraft measurements of the microwave scattering signature of the ocean

Microwave scattering signatures of the ocean have been measured over a range of surface wind speeds from 3 m/s to 23.6 m/s using the AAFE RADSCAT scatterometer in an aircraft. Normalized scattering coefficients are presented for vertical and horizontal polarizations as a function of incidence angle (nadir to55deg) and radar azimuth angle (0degto360deg) relative to surface wind direction. For a given radar polarization, incidence angle, and azimuth angle relative to the wind direction, these scattering data exhibit a power law dependence on surface wind speed. The relation of the scattering coefficient to azimuth angle obtained during aircraft circles (antenna conical scans) is anisotropic and suggests that microwave scatterometers can be used to infer both wind speed and direction. These results have been used for the design of the Seasat-A Satellite Scatterometer (SASS) to be flown in 1978 on this first NASA oceanographic satellite.     

双基地高频地波雷达系统布站研究

由发射机(T)和接收机(Rm)共站的单基地高频地波雷达增设分置的接收系统(Rb)构成的T/Rm-Rb双基地高频地波雷达系统用于海洋环境监测时,既可测得矢量流速度,又可消除风向模糊性,是目前高频地波雷达的发展趋势之一。该文首次从T/Rm-Rb系统的探测能力与海流测量误差两方面分析最佳基线长度和最佳双基地角范围,从而确定其布站原则,即:在确定T/Rm单基地雷达的位置后,先根据T-Rb双基地雷达设计指标中的最大探测距离和确定基线长度,然后根据T/Rm-Rb系统海流测量误差最小的原则确定最佳双基地角范围,从而确定T-Rb双基地雷达接收站Rb的位置。该布站原则可为双(多)基地高频地波雷达的海洋环境监测应用提供参考和依据。     

On airborne measurement of the sea surface wind vector by a scatterometer (altimeter) with a nadir-looking wide-beam antenna

A pilot needs operational information about wind over sea as well as wave height to provide safety for a hydroplane landing on water. Near-surface wind speed and direction can be obtained with an airborne microwave scatterometer, a radar designed for measuring the scatter characteristics of a surface. Mostly narrow-beam antennas are applied for such wind measurement. Unfortunately, a microwave narrow-beam antenna has considerable size that hampers its placement on flying apparatus. In this connection, a possibility to apply a conventional airborne radar altimeter as a scatterometer with a nadir-looking wide-beam antenna in conjunction with Doppler filtering for recovering the wind vector over sea is discussed, and measuring algorithms of sea surface wind speed and direction are proposed. The obtained result can be interesting for many studies in oceanography, meteorology, air-sea interaction, and climate change and for creation of an airborne radar system for amphibious airplane safe landing on the sea surface, in particular for search and rescue operations in coastal areas.     

直接测风激光雷达频率跟踪技术及对流层平流层大气风场观测

为了实现高精度连续探测对流层和平流层大气风场,搭建了一台直接测风激光雷达系统对对流层和平流层大气风场进行探测。该系统基于双边缘法布里-珀罗标准具的瑞利散射多普勒测风原理,使用转台式探测结构,通过频率跟踪的手段对频率漂移进行跟踪,确保测风的精度。实验结果表明,该系统对对流层和平流层大气风场探测效果良好,频率跟踪的范围为±50 MHz,可以大大减小频率漂移带来的风速误差。经过系统的稳定运行和长时间的观测,在40 km处测得的径向风速随机误差为8 m/s。径向风速合成为水平风速后,随机误差在38 km处最大为10 m/s左右。该系统白天探测高度为25 km,夜晚探测高度为38 km。与探空数据对比,风速误差均小于10 m/s,其中风速误差在±5 m/s的范围内的数据量约占75.8%,探测的风向误差与探空气球的趋势基本一致,误差范围在10°～20°之间,在15°范围内的数据量约占58.6%。将实测数据与探空数据进行统计分析,结果具有良好的一致性。该系统可以为对流层和平流层大气风场的探测提供数据支撑。     

Wind vector retrieval using ERS-1 synthetic aperture radar imagery

An automated algorithm intended for operational use is developed and tested for estimating wind speed and direction using ERS-1 SAR imagery. The wind direction comes from the orientation of low frequency, linear signatures in the SAR imagery that the authors believe are manifestations of roll vortices within the planetary boundary layer. The wind direction thus has inherently a 180° ambiguity since only a single SAR image is used. Wind speed is estimated by using a new algorithm that utilizes both the estimated wind direction and σ ₀ values to invert radar cross section models. The authors show that: 1) on average the direction of the roll vortices signatures is approximately 11° to the right of the surface wind direction and can be used to estimate the surface wind direction to within ±19° and 2) utilizing these estimated wind directions from the SAR imagery subsequently improves wind speed estimation, generating errors of approximately ±1.2 m/s, for ERS-1 SAR data collected during the Norwegian Continental Shelf Experiment in 1991     

基于SVD的高频地波雷达干扰与杂波抑除算法

该文针对高频地波雷达探海工作期间高频干扰严重海杂波强的特点,干扰抑制时引入海杂波约束条件,对自适应阵列波束形成方法进行了改进,防止干扰抑制后回波中一阶海杂波时域相关特性发生破坏而在频谱上虚假展宽。然后,使用干扰抑制结果构造时域采样矩阵,由奇异值分解(SVD)方法获得积累期间一阶海杂波频率的变化历程,通过矩阵特征分析将一阶海杂波滤除。最后,使用OSMAR2000实测数据检验了本文算法的有效性。