A method for estimating rain rate and drop size distribution frompolarimetric radar measurements

Polarimetric radar measurements are sensitive to the size, shape and orientation of raindrops and provide information about drop size distribution (DSD), canting angle distribution and rain rate. The authors propose and demonstrate a method for retrieving DSD parameters for calculating rain rate and the characteristic particle size. The DSD is assumed to be a gamma distribution and the governing parameters are retrieved from radar measurements: reflectivity (Z_HH), differential reflectivity (ZDR), and a constrained relation between the shape (CL) and slope (Λ) parameters derived from video disdrometer observations. The estimated rain rate is compared with that obtained from more traditional methods and the calculated characteristic size is compared with the measured values. The calculated K_DP based on the retrieved Gamma DSD is also compared with measurements. The proposed method shows improvement over the existing models and techniques because it can retrieve all three parameters of the gamma distribution. For maintaining the continuity of earlier published results, raindrop shape is assumed to be equilibrium     

Dimension statistics of rain cell cores and associated rain rateisopleths derived from radar measurements in the mid-Atlantic coast ofthe United States

Employing a multi-year radar database of precipitation in a mid-Atlantic coast region, the authors have characterized rain cell size statistics. The measurements were made with a high resolution, high power radar in which equicircle diameter contours of resolutions of 1 km and greater were identified out to ranges of 100 km from the radar. The rain cell was constructed by its core intensity isopleth and a family of enveloping contours having defined rain rate intervals in the regions between them. The isopleth statistics were extracted from a database of cores with rain level number 7 (18-24 mm/h) and greater     

电离层随机变化对OTH雷达坐标配准的影响

基于电离层实测数据,通过参数统计再取样的方法(bootstrap)建立了一种电离层随机变化模型,在此模型的基础上,用射线追踪的方法讨论了电离层随机扰动对超视距雷达坐标配准的影响.并提出了通过极大似然估计的方法改善这种扰动的影响,最后与最小方差法进行了比较,其对电离层随机扰动影响的改善要优于最小方差的估计.     

The effects of rain clutter on millimeter radar performance

The effects of rain clutter on millimeter radar performance are investigated at 35, 94 and 140GHz frequencies, including rain attenuation, radar reflectivity, maximum radar range and equivalent target cross section.     

Effects of random phase changes on the formation of synthetic aperture radar imagery

The effects of Gaussian random and linear phase change on the response of the matched azimuth processor of a synthetic aperture imaging radar is analyzed numerically.     

Double-polarisation radar measurements

This letter shows some experimental results obtained with a double-polarisation S-band radar using fixed circular polarisation at transmit and two orthogonal circularly polarised channels at receipt. The results refer to the short-time polarisation behaviour of aircraft target and ground clutter. From these results it can be inferred that adaptative polarisation techniques can be effective in improving the signal/clutter ratio.     

Predictive methods for rain attenuation using radar and in-situ measurements tested against the 28-GHz Comstar beacon signal

A program to measure the rain attenuation of the Comstar beacon signal at 28.56 GHz has been in continuous operation since March of 1977 at Wallops Island, VA. During the summer of 1977 simultaneous radar and disdrometer measurements at the site were also made and used for predicting path attenuation. The best-fit values ofaandbof the relationk = aZ^{b}were deduced for each rain period from the raindrop size measurements, wherekis the attenuation coefficient [dB/km] andZis the reflectivity factor [mm⁶/m³]. The measuredk-Zrelations and the simultaneous radar reflectivity measurements along the beacon path were injected into a computer program for estimating the path attenuation. Predicted attenuations, when compared with the directly measured ones, show generally good correlation on a case-by-case basis and very good agreement statistically after an empirical calibration adjustment is applied to the radar data. A method was also tested for predicting fade statistics at another frequency (e.g., 19 GHz) using simultaneous rain rate and fade distributions (28 GHz) in conjunction with disdrometer data. The predicted distributions showed good agreement with radar-predicted levels. The results demonstrate the utility of using radar in conjunction with disdrometer and rain gauge measurements for predicting fade events, long-term fade distributions, and establishing predictive criteria associated with earth-satellite telecommunications.     

Effect of raindrop size distribution and rain rate inhomogeneity on the relationship between attenuation and depolarization

In addition to attenuation, depolarization due to rain is another factor that degrades satellite propagation signals, especially in the higher frequency bands and in places that have high rates of rainfall. A formula to predict cross‐polarization as a function of attenuation has been proposed, and it is derived by a theoretical calculation using frequency, the forward scattering amplitude of raindrops, rainfall rates, the raindrop size distribution (DSD), and various other propagation parameters. In this paper, a formula for predicting cross‐polarization is derived on the basis of the assumption of a gamma‐type DSD up to 100 GHz. These results are compared with conventional exponential‐type DSDs, such as the Marshall‐and‐Palmer DSD. Moreover, for a more realistic propagation situation, we consider the effect on the aforementioned relationship of rainfall rate inhomogeneity along the propagation path. It is shown that, for practical purposes, this inhomogeneity does not have a significant effect on satellite propagation. Copyright © 2017 John Wiley & Sons, Ltd.     

Slant path rain attenuation and path diversity statistics obtained through radar modeling of rain structure

Single and joint terminal slant path attenuation statistics at frequencies of 28.56 and 19.04 GHz have been derived employing a radar data base obtained over a three-year period at Wallops Island, VA. Statistics were independently obtained for path elevation angles of20deg,45deg, and90degfor purposes of examining how elevation angles influences both single terminal and joint probability distributions. Both diversity gains and autocorrelation function dependence on site spacing and elevation angles were determined employing the radar modeling results. Comparisons with other investigators are presented. An independent path elevation angle prediction technique was developed and demonstrated to fit well with the radar derived single and joint terminal radar derived cumulative fade distributions at various elevation angles.     

Technique for minimising effect of detector inhomogeneity on fibre attenuation measurements

In fibre attenuation measurements by the `cut-back? method, the necessity for a homogeneous detector is implicit. We have found a commonly used germanium detector to be significantly inhomogeneous, leading to an estimated attenuation discrepancy of 0.02 dB at best and 0.4 dB at worst. In the letter, we outline a method for reducing this discrepancy to levels of less than 0.004 dB.     

随机信号雷达抗干扰性能分析

研究了采用带限高斯白噪声波形的随机信号雷达在噪声和欺骗干扰环境下的工作性能.通过研究接收机输入/输出信噪比关系和检测性能,分析了随机信号波形抗噪声干扰的性能;采用仿真方法,分析了抗欺骗干扰性能.仿真和分析结果表明,随机信号波形比线性调频脉冲压缩波形具有更好的抗欺骗干扰能力.     

Derivation of parameters of Y-Z power-law relation from raindrop size distribution measurements and its application in the calculation of rain attenuation from radar reflectivity factor measurements

The work reported herein empirically derived the coefficients of the power-law relation between the specific attenuation Y and each of two other quantities: the radar reflectivity factor Z and the rainfall rate R. The derivation was accomplished using raindrop size distributions (DSDs) measured in Montreal and Toronto using the precipitation occurrence sensor system (POSS) and a Joss-Waldvogel disdrometer (JWD). The specific attenuation was calculated for both spherical and oblate spheroidal raindrops. Prior to doing so, the effects of inaccuracies in small drop detection by the two systems on the resulting Y-Z and Y-R relations were examined. In computing the relations, the influence of grouping the DSDs according to the corresponding values of Z was assessed. The results from the two sites were then combined in a regression analysis to determine the coefficients of the power-law expressions. The final expressions were used to calculate the probability distribution of rain attenuation over several path lengths at 30 GHz. Conclusions that can be drawn from this work include that (i) the uncertainties in the measurements of small drops do not appear to be critical to the derived relations for some frequencies considered in this work, (ii) in computing rain attenuation estimates from radar reflectivity factors, different criteria for grouping the reflectivity factor measurements can be adopted without changing the statistics of the corresponding attenuation estimates, (iii) differences, which are the smallest for frequencies around 20-30 GHz, are observed in the coefficients derived from the measured DSDs compared with those recommended by the ITU-R, and (iv) the attenuation statistics at 30 GHz computed by applying the Y-Z power-law expression are resistant to spatial averaging.     

Variation with height of radar reflectivity factors due to rain and the influence of wind shear on rain cells

A 10 cm (*) radar has been used on the steerable 25 m diameter aerial at Chilbolton, U.K., to produce detailed threedimensional maps of raincell distribution and intensity. The present paper examines differences in variation of radar reflectivity with height from the model at present recommended by the Ccir.Wind shear can have a considerable influence on the distribution of rain as it falls, and may cause a 45° angle of inclination of a cell. This remains constant as the rain cell travels and is usually offset from the direction of travel of the cell. The information on the variation of radar reflectivity with height and the inclination of the rain cells are each important in the field of radio communications.     

基于OFDM随机步进频的雷达通信一体化信号模型

为了解决雷达通信一体化系统中的雷达信号与通信信号较难分离的问题,在正交频分复用(OFDM)雷达的基础上,提出了一种基于OFDM脉间随机步进频的雷达通信一体化信号模型,通过频率捷变将数据信息加载到雷达信号上,利用随机的步进频率传输数据,从而使一体化信号能同时实现雷达探测和数据通信功能,避免了信号分离。同时设计了雷达通信一体化方案,在雷达接收端,运用相关法实现一维距离成像;在通信接收端,通过带通滤波器组检测频率点解调数据。仿真实验结果表明,一体化信号能实现分米级的距离高分辨和速率为Mbit/s级的数据通信,能够满足大批量数据传输的要求。     

Compact range radar cross-section measurements using a noise radar

This paper discusses the measurement of radar cross section (RCS) with a very low-cost system that transmits band-limited random noise over the band from 1.0 to 4.0 GHz. The received signal is correlated with a delayed version of the transmitted signal. A variable delay line is used to obtain the response at various delay times. This yields the radar target impulse response as a function of delay. This can be transformed to yield both the amplitude and phase of the scattering matrix in the frequency domain     

New radar studies of slant-path attenuation due to rain

Approximately 40 h of data from the summer of 1976 were employed in a comparison of radar and radiometer estimates of slant-path attenuation due to rain. McGill Radar Weather Observatory is situated 20 km west of Montreal; the radiometers, separated by 18 km at two sites located about 90 km northwest of the radar, had fixed antennas pointed approximately southeast at an elevation of 18.5 deg. Values of radar reflectivity along the two radiometer paths were used to calculate the slant-path attenuation at 13 GHz as a function of time with a 1 min resolution for direct comparison with the radiometer measurements. It was found that the cumulative distribution of attenuation inferred by radar from each site could be made to agree satisfactorily with the radiometer distribution assuming that rain was present everywhere along the path with a Marshall-Palmer distribution and applying a 1 dB correction to the independently-determined radar calibration. This agreement, close to within a fraction of a decibel, gives confidence to the use of radar records in compiling attenuation statistics. An example is presented of a new application of such records, namely the assessment of rain-induced interference over adjacent earth-space paths.     

Prediction of slant path rain propagation statistics usingdual-polarized radar

The authors conducted a year-long experiment in which a dual-polarized S-band radar probed the volume surrounding two 11.45-GHz satellite downlink paths during rain. Accuracy was assessed by comparison to directly measured link attenuation with two 11-GHz beacon receivers 7.3-km apart at an 18.5° elevation angle, one colocated with the radar. Drop size distributions calculated from the radar horizontal reflectivity (Z_H) and differential reflectivity (ZDR) measurements were used to predict 11.45-GHz satellite beacon attenuations. The radar-predicted attenuations and those measured on the radio links agree, both on an event basis and in terms of annual cumulative distributions     

On averages of radar rain echoes in nonstationary conditions

The authors examine a problem, recently opened up by Jameson and Kostinski (1996), of statistics for radar rain echoes in a nonstationary case such as the observation of rain by a moving antenna beam. The present authors study this problem under the framework of the standard fluctuation theory of Marshall and Hitschfeld (1953). They obtain the exact nonstationary probability density function (pdf) for an average of n mutually independent samples of the received power in the square-law detection, and an approximate nonstationary pdf in the logarithmic detection. They find that the concept of bias does not appropriately describe the effect of antenna beam movement during the sampling. They introduce a new quantity to assess the effect of antenna beam movement on rain observation by radars     

Introduction to radar cross-section measurements

This paper is intended to serve both as an introduction to radar cross-section measurements and as an introduction to the several measurement papers that follow in this issue. We discuss the contribution that experimental work makes to theoretical studies. The history of cross-section measurements is reviewed. Some terms are defined, and the major measurement problems are discussed. It is pointed out that the various range geometries and measurement techniques in use today were based on the approach used to solve the basic measurement problems. Some details are given of the more popular present-day measurement systems, i.e., CW, pulsed, FM-CW, Doppler, and the standing-wave ratio method.     

Assessing the quality of SeaWinds rain measurements

While SeaWinds was designed to measure ocean winds, it can also measure rain over the ocean. SeaWinds on QuikSCAT active measurements of integrated columnar rain rate obtained via simultaneous wind/rain retrieval are evaluated via Monte Carlo simulation and the Crame/spl acute/r-Rao lower bound on estimate accuracy. Although sufficiently accurate in many conditions, the simultaneous wind/rain retrieval method used with SeaWinds on QuikSCAT data is ill-conditioned for certain wind directions and measurement geometries, sometimes yielding spurious rain rates in zero-rain conditions. To assess the validity of SeaWinds-derived rain rates, a simple empirically based rain thresholding scheme is presented, derived from simulated data. Thresholded QuikSCAT rain rates are compared to Tropical Rainfall Measuring Mission Microwave Imager monthly-averaged data, demonstrating good correlation for monthly-averaged data.