Correcting C-band radar reflectivity and differential reflectivitydata for rain attenuation: a self-consistent method withconstraints

Quantitative use of C-band radar measurements of reflectivity (Z _h) and differential reflectivity (Z_dr) demands the use of accurate attenuation-correction procedures, especially in convective rain events. With the availability of differential phase measurements (Φ_dp) with a dual-polarized radar, it is now possible to improve and stabilize attenuation-correction schemes over earlier schemes which did not use Φ_dp. The recent introduction of constraint-based correction schemes using Φ_dp constitute an important advance. In this paper, a self-consistent, constraint-based algorithm is proposed and evaluated which extends the previous approaches in several important respects. Radar data collected by the C-POL radar during the South China Sea Monsoon Experiment (SCSMEX) are used to illustrate the correction scheme. The corrected radar data are then compared against disdrometer-based scattering simulations, the disdrometer data being acquired during SCSMEX. A new algorithm is used to retrieve the median volume diameter from the corrected Z_h, corrected Z_dr , and K_dp radar measurements which is relatively immune to the precise drop axis ratio versus drop diameter relation. Histograms of the radar-retrieved D₀ compared against D₀ from disdrometer data are in remarkable good agreement lending further validity to the proposed attenuation-correction scheme, as well as to confidence in the use of C-band radar for the remote measurement of rain microphysics     

Measurements of cloud contribution to rain millimeter wave attenuation by using a radar and radiometers

The measurement technique of cloud contribution to rain attenuation and the equipment consisted of the coherent pulse Doppler radar at wavelength λ=3.2 cm, the radiometers at λ=0.4; 0.8 and 1.35 cm and apparatus for signal recording and processing is described. The results of such measurements are given. The Doppler spectrum of the rain backscattered radar signal was used for determination of rain drop size distribution height profile then rain attenuation was calculated and cloud attenuation was determined as the difference between the total attenuation measured by using the radiometers and rain attenuation. The results of this work gave possibility to improve the known rain model of P.Misme for prediction of rain attenuation statistics for Earth-satellite links at millimeter wave.     

Backscatter and attenuation by falling snow and rain at 96, 140,and 225 GHz

The results of measurements are presented for backscatter cross section per unit volume and attenuation for falling snow and rain at 96, 140, and 225 GHz. The attenuation due to rain is almost independent of the measurement frequency, but for snow the attenuation is considerably greater at 225 GHz than at 96 GHz. The rain attenuation generally varies with the rain accumulation rate in accordance with an aR^b relationship for a Laws and Parsons drop-size distribution where R is the rain rate and a and b are constants. The attenuation at all three frequencies is about 3 dB/km for a rain rate of 4 mm/h. The attenuation due to snow varies with airborne snow-mass concentration, with the average rates of increase being 0.9, 2.5, and 8.7 (dB/km)(g/m³) at 96, 140, and 225 GHz, respectively. Generally the attenuation for snow is lower than that for rain. The backscatter cross section per unit volume for rain at 96 GHz is about -35 dB m²/m³ for a rain rate of 4 mm/h. The backscatter from snow at 96 GHz is much lower than that from rain under equivalent accumulation rates or airborne mass concentrations. Snow backscatter at 140 GHz is comparable but higher than that at 96 GHz     

W波段大功率发射机的设计与研究

毫米波雷达对云、雾、降水的形成与发展等微物理过程的探测具有明显的优势,是认识气象物理过程的重要手段。介绍了用于云雨探测雷达的W波段大功率发射机的设计方法与过程,该发射机采用分布作用速调管(EIK)作为末级放大器,介绍了EIK的工作原理、特点及应用要求,重点对发射机的聚焦极脉冲调制器、高压电源的工程实现进行了分析,高压电源采用电容钳位移相逆变器,具有快速响应及良好的稳压性能,对控制电路的实现及其功能进行了介绍,最终给出了W波段发射机的样机研制及测试结果。     

基于Ka波段毫米波云雷达多普勒谱的降雪微物理过程的分析

云中过冷水滴的识别对于预警飞机积冰及云-降水物理研究具有重要意义。本文利用美国ARM-AMF2在芬兰的35GHz测云雷达多普勒谱数据,建立了谱峰识别算法,对全局谱进行了谱分离,识别出了过冷水滴然后,经过谱矩计算得到不同类型粒子的反射率因子、多普勒速度、谱宽,最后根据经验关系反演云中液态水含量,并与微波辐射计探测结果进行对比。结果表明：(1)混合云中,雪花主导了毫米波雷达总回波强度,因此根据总雷达反射率因子反演液态水含量会造成低估;(2)冰雪晶粒子在过冷水层(SWL)中多普勒速度随反射率因子的变化梯度比在冰雪层(ISL)中大;(3)多普勒谱反演得到过冷水的液态水路径(LWP)与微波辐射计反演结果一致性较好,说明毫米波雷达能够有效估量云中液态水路径。     

Scattering of radiowaves by a melting layer of precipitation inbackward and forward directions

A melting layer of precipitation is composed of melting snowflakes (snow particles); the assumption of spherical particles along with mass conservation is used. The melting layer is studied by deriving the size distribution of the melting snow particles, the thickness of a melting layer, the density of a dry snow particle, and the average dielectric constant of a melting snow particle. Vertical profiles of radar reflectivity and specific attenuation are computed at 1-100 GHz by using the Mie theory for five raindrop size distributions at rain rates below 12.5 mm/h. The radar bright band is explained with computed radar reflectivities at 3-10 GHz. It is shown that the radar bright band can be absent in the melting layer at frequencies above 20 GHz. This agrees with radar observations at 35 and 94 GHz. The specific attenuation, as well as the average specific attenuation of the melting layer, is divided into absorption part and scattering part. The latter is increasingly significant with the increase of frequency. The total zenith attenuation due to stratiform rain is divided into the rain zenith attenuation and the additional zenith attenuation, which is the difference between zenith attenuation, due to the melting layer, and attenuation, due to the same path length of the resulting rain. The additional zenith attenuation increases with the increase of rain rate even at frequencies above 20 GHz. This should be taken into account in radar remote sensing and satellite-Earth communications     

毫米波测云雷达回波信号衰减补偿仿真研究

针对毫米波测云雷达受云雨衰减的问题,从雷达I、Q信号角度出发,模拟毫米波测云雷达回波I、Q信号并假设一定的探测环境,进行雷达回波信号衰减补偿仿真试验.试验结果表明,雷达电磁波信号穿过雨区和云区时,在远距离处衰减量很大,通过衰减补偿,信号强度明显增强,目标信号突出.     

Consistency of Constraints on Nulls and on Dynamic Range Ratio in Pattern Synthesis for Antenna Arrays

Given an antenna array of N elements, we deal with the problem of finding an array pattern that approximates a desired pattern, while: a) null constraints are imposed in M(les N - 2) assigned directions; b) an upper bound D is imposed on the dynamic range ratio (DRR) of the excitations. At first we show that the system of constraint equations has solutions if and only if D ges D₀, where D₀ is a parameter depending on the null directions, and describe a method to evaluate D₀. Then we propose two simple and fast algorithms to solve the synthesis problem and show their effectiveness by numerical examples. Statistical results show that usually D₀ = 1 when M is small compared to N, thus exact nulls can be formed for any D ges 1. This result provides an insight into the numerical results.     

用TOGA／COARE试验中机载雷达资料进行衰减效应订正的个案研究

为了蛎限度地利是生载测雨雷达，必须实现一些主要条件：高的空间分辨率、宽阔的覆盖面积，大的动态降雨测量范围以及精确的点测量。     

Remote sensing of atmospheric wind velocities using solid-state andCO2 coherent laser systems

Coherent lidar/laser radar systems have been used for the remote measurement of atmospheric wind velocity since 1966. Both CW and pulsed coherent lidars have been developed and applied to a variety of ground-based and airborne applications. In recent years, most efforts have concentrated on pulsed CO₂ and solid-state Doppler lidars for the remote measurement of atmospheric wind velocities. Issues associated with the design and application of coherent lidars to atmospheric wind measurement are discussed. Coherent lidar/laser radar systems have been applied to detection and tracking of aircraft wake vortices, measurement of atmospheric wind fields during the space shuttle takeoff and landing, airborne windshear detection, ground-based airport microburst windshear monitoring, meteorological research, and environmental monitoring. Numerous examples of measurement results are presented. An assessment of the status of coherent lidar technology as practical devices for operational and research uses is addressed     

机载雷达气象方式的降雨补偿方法设计与实现

高空降雨云层作为一种气象目标,也是对雷达波的吸收体,降雨路径上造成的衰减会对气象信息的探测带来严重的影响。文中设计了一种具有降雨路径补偿的机载雷达气象方式,引入了降雨区衰减和降雨率的关系,将近端降雨区的衰减量作为参数解算远端降雨率,并对其原理、算法和实现方法作了详细阐述。通过对实测数据处理验证表明:具有降雨路径补偿的气象方式可以更准确地反映气象环境,保证飞行安全。     

云雾中飞机尾流毫米波雷达多普勒特性研究

构建了尾流中云雾滴运动微分方程,通过求解运动方程得到了尾流中云雾滴的运动轨迹、速度分布等特性.分析了尾流中云雾滴的密度分布特性,并以典型的W波段雷达为例,分析了云雾中尾流的探测距离.在此基础上,提出了一种云雾中尾流雷达多普勒特性模拟方法,得到了典型毫米波雷达空间分辨率和观测视角下云雾中尾流的多普勒特性.研究结果表明,云雾中尾流毫米波雷达回波调制了丰富的尾流速度场信息,可为航空安全提供特征支撑.     

Cloud and Rain Effects on AltiKa/SARAL Ka-Band Radar Altimeter—Part I: Modeling and Mean Annual Data Availability

The AltiKa project, developed by the French Centre National d'Etudes Spatiales, is based on a wideband Ka-band altimeter (35.75 GHz). The technical characteristic of the instrument will offer higher performance both in terms of spatial and vertical resolutions that will lead to the improved observation of ice, coastal areas, inland waters, and wave height. An Indian Space Research Organization satellite, called Satellite with ARgos and AltiKa, will embark the AltiKa altimeter. The launch is scheduled at the end of 2010. The major drawback of Ka-band use is the attenuation of the radar signal by atmospheric liquid water. Clouds and rain effects will thus be a strong constraining factor, because the altimeter link budget imposes an attenuation of less than 3 dB. The impact of rain and clouds on Ka-band altimeter data is analyzed and quantified using an analytical model that computes AltiKa waveforms in the presence of rain or clouds. The results are then used to quantify the waveform attenuation and distortion, as well as the error induced on the altimeter geophysical parameter estimates. Because of the nonlinearity of attenuation relations, the impact of clouds/rain depends more on the cloud/rain variability within the altimeter footprint than on the mean characteristics, which makes correction using coincident rain or cloud data difficult. Small rain cell and small dense clouds can thus strongly distort the waveforms and lead to erroneous geophysical parameter estimates. The probability of 20 Hz and 1-s averaged data loss are computed from the model results and from cloud and rain climatologies. On a global scale, about 3.5% of the 20-Hz data will be lost because of rain and clouds and 2.5% of the 1-s averaged data. However, the probability strongly varies over the global ocean and can exceed 10% in the Tropics.      

基于有源照射箔条云的速度欺骗干扰方法

提出了基于有源照射箔条云对雷达速度波门进行拖引干扰的复合干扰方法.该方法运用机载电子干扰设备接收、转发敌方雷达发射的信号并照射到箔条云上,箔条云对干扰信号二次辐射,被敌方雷达接收,形成具有和载机相似径向速度的假目标,起到诱骗干扰作用.建立了复合干扰对速度跟踪系统进行干扰的相关模型,并进行了仿真实验,仿真结果表明,利用该方法对敌机测速跟踪系统进行干扰具有可行性.     

Analysis of algorithms for the retrieval of rain-rate profiles froma spaceborne dual-wavelength radar

The ability to retrieve rain-rate profiles from a dual-wavelength spaceborne radar system operating at 13.6 and 35 GHz is analyzed. The fundamental problem of extracting either the attenuation and/or the reflectivity from the backscatter echo, which contains both contributions, is addressed. Three algorithms, the backscatter, the attenuation coefficient, and the dual-wavelength methods, are examined. These algorithms are tested using four rain-rate profiles derived from radar measurements. In particular, measured (true) values are compared with calculated (retrieved) rain rates applying the algorithms with superimposed uncertainties assuming a suggested spaceborne dual-wavelength radar system. Error values of rain rates are determined where these values reflect failure of the assumptions utilized in the derivation of the algorithms, rain backscatter noise, and instrument noise     

High-resolution InSAR image simulation for forest canopies

High-resolution interferometric airborne synthetic aperture radar (SAR) images of Indonesian tropical rain forests have been acquired during the European Space Agency (ESA) Indonesian Radar Experiment (INDREX) 1996 campaign. Research efforts are directed toward development of automated canopy reconstruction algorithms. In this paper, interferometric synthetic aperture radar (InSAR) image simulation is discussed as one of the tools to support development of such inversion algorithms. First, the relevant physics, observation geometry, and radar characteristics are described. It is assumed that a forest can be modeled as a cloud of uniformly distributed isotropically scattering elements located within crown volumes. These volumes were measured during a field experiment for a 7.2 ha plot. Simulated data comprise intensity, phase, as well as coherence images. These are compared, in a statistical sense, with real data acquired in C- and X-bands. The canopy attenuation was simulated over a range of values. The normalized second intensity moment, the mean coherence magnitude, the coherence histogram, and the autocorrelation function of coherence were taken as measures for comparison. It can be concluded that simulated and real C-band images compare well for an extinction coefficient in the range of 0.15-0.3 m/sup -1/. For X-band, the selected measures of agreement lead to contradictions, indicating that the physical assumptions made may be less valid than for C-band.     

A dual-polarization rain profiling algorithm

A new attenuation correction algorithm based on profiles of reflectivity, differential reflectivity, and differential propagation phase shift is presented. A solution for specific attenuation retrieval in rain medium is proposed, which solves the integral equations for reflectivity and differential reflectivity with cumulative differential propagation phase shift constraint. The conventional rain profiling algorithms that connect reflectivity and specific attenuation can retrieve specific attenuation values along the radar path assuming a constant intercept parameter of the normalized drop size distribution. However, in convective storms, the drop size distribution parameters can have significant variation along the path. This paper presents a dual-polarization rain profiling algorithm for horizontal looking radars incorporating reflectivity as well as differential reflectivity profiles. The dual-polarization rain profiling algorithm has been evaluated with X-band radar observations simulated from drop size distribution derived from high-resolution S-band measurements collected by the Colorado Statue University CHILL radar. The analysis shows that the retrieved specific attenuation, differential attenuation, reflectivity, and differential reflectivity from the dual-polarization rain profiling algorithm provide significant improvement over the current algorithms.     

激光多普勒测速雷达技术研究现状

多普勒频率提取算法是激光多普勒测速雷达的关键技术之一,它会直接影响测速精度、作用距离、动态响应范围。本文以一款典型激光多普勒测速雷达为例介绍了测速系统的组成与关键技术,对比分析了国内外典型激光多普勒测速雷达的性能指标及技术参数,重点研究了各种多普勒频率提取算法,并对各种算法的优缺点进行概括总结。最后对多普勒频率提取算法进行了展望。     

Chirping the light—fantastic? Recent NBS atom cooling experiments

We have successfully decelerated and cooled a neutral atomic sodium beam using a counter-propagating laser beam tuned nearly resonant with the D₂ transition. In order to compensate for the changing Doppler shift as the atoms slow down, the laser frequency was rapidly scanned or “chirped”. We have observed final velocities of 6 x 10²m/s, or about 0.5 of the initial velocity. As of yet we have not been able to cool a significant number of atoms to lower velocities.