Uniting space, ground, and underwater measurements for improved estimates of rain rate

Global precipitation is monitored from a variety of platforms including spaceborne, ground-, and ocean-based platforms. Intercomparisons of these observations are crucial to validating the measurements and providing confidence for each measurement technique. Probability distribution functions of rain rates are used to compare satellite and ground-based radar observations. A preferred adjustment technique for improving rain rate distribution estimates is identified using measurements from ground-based radar and rain gauges within the coverage area of the radar. The underwater measurement of rainfall shows similarities to radar measurements, but with intermediate spatial resolution and high temporal resolution. Reconciling these different measurement techniques provides understanding and confidence for all of the methods.     

TRMM PR雷达与阜阳雷达降水资料的对比研究

选取了1998年与1999年HUBEX观测资料，将TRMM卫星上的星载降雨雷达PR与阜阳的713数字化天气雷达资料在以下3个方面做了比较：(1)强度场的分布；(2)平均反射率廓线；(3)灵敏度。分析、比较结果可知阜阳雷达在探测灵敏度和水平方向的分辨率要高于PR，但由于波束宽度、衰减和资料转化过程计算误差的原因，其远距离处的观测值与真实值有一定偏差。PR波长短，对衰减敏感，尤其是在垂直方向上经过强衰减后，订正值仍偏小。但由于探测方式的不同PR在水平方向上不存在衰减的问题，且其资料分布均匀，因此提出一种用PR资料来订正地基雷达资料的方法用以提高地基雷达探测的精确度。     

TRMM PR雷达与阜阳雷达降水资料的对比研究

选取了1998年与1999年HUBEX观测资料，将TRMM卫星上的星载降雨雷达PR与阜阳的713数字化天气雷达资料在以下3个方面做了比较：(1)强度场的分布；(2)平均反射率廓线；(3)灵敏度。分析、比较结果可知阜阳雷达在探测灵敏度和水平方向的分辨率要高于PR，但由于波束宽度、衰减和资料转化过程计算误差的原因，其远距离处的观测值与真实值有一定偏差。PR波长短，对衰减敏感，尤其是在垂直方向上经过强衰减后，订正值仍偏小。但由于探测方式的不同PR在水平方向上不存在衰减的问题，且其资料分布均匀，因此提出一种用PR     

反射率因子和径向速度共同约束反演多普勒雷达风场

多普勒雷达可以提供降水回波区的风场信息。为了充分运用雷达资料分析中小尺度天气过程,本文假设反射率因子在短时间内的运动满足拉格朗日守恒,提出了采用连续两次的反射率因子回波和径向速度数据共同约束的方法来反演风场,在反演过程中避免了对径向速度的分布进行假设。针对连续两个时次之间反射率回波运动中产生的不同程度的误差做了模拟风场试验,结果表明,当回波运动随机误差不超过40%时,反演结果较为可靠。另外,本文还利用此风场反演方法进行两次中尺度天气过程的实例分析。结果表明,该方法反演的风场与实际风场结构相符,具有一定的应用价值。     

一种反演雷达波束内不均匀反射率场的方法

本文从原始雷达方程出发,提出了一种雷达加密观测方案和退卷积的数据反演方法,以实现雷达天线波束内的反射率不均匀结构信息的提取。反演方法的数值试验表明,此反演方法比常规测量方法在角分辨率方面提高约３倍,对波束内尺度为０．５８－２．０ＨＷ（ＨＷ是天线３ｄＢ波束宽度）的反射率变化有明显的反演能力。观测数据的测量误差对反演质量的影响也进行了计算和分析。本文还讨论了反演方法对雷达观测中的一些技术参数的要求。这些结果对于提高雷达尤其是星载微波主被动遥感器在测量降水量和云中含水量等参数中的精度有重要作用和现实意义。     

Low-angle reflectivity modeling of land clutter

In this letter, a practical mean reflectivity model of radar land clutter (LC) for the complex system design of ground-based radars involved in low-angle targets on some typical terrains is studied using the inductive reasoning method. The functional relationships between the radar parameters and radar surface clutter backscattering are analyzed. Following the recent research work in the area of reflectivity modeling of LC in the literature, the least squares method is employed to estimate the model parameters. The model is validated using reliable practical data and shown to outperform other models in accuracy.     

Comparison of Raindrop Size Distribution Estimates From X-Band and S-Band Polarimetric Observations

This letter evaluates the consistency of rainfall drop size distribution (DSD) parameters that were estimated from attenuation-corrected X-band dual-polarization (XPOL) radar measurements with estimates from a colocated S-band dual-polarization radar (S-Pol). The DSD retrieval technique uses reflectivity and differential reflectivity, and a constrained relationship to estimate the three parameters of a ldquonormalizedrdquo gamma distribution model. The more definitive S-Pol DSD parameter estimates are used as a reference to assess the performance of the corresponding XPOL estimates for different rain-path attenuation values. Results show that XPOL attenuation-corrected profiles can provide rainfall DSD estimations that are consistent to an S-band dual-polarization radar, even in cases of moderate to high rain-path attenuation.     

Comparison of Deep Underwater Measurements and Radar Observations of Rainfall

Deep-water acoustical measurements of rainfall are compared to high-resolution ground radar observations for the first time. The measurements of underwater ambient sound were made from a subsurface mooring near Methoni, Greece, in 2004. The acoustical measurements were at 60-, 200-, 1000-, and 2000-m depths. Simultaneous ground-based polarimetric -band radar observations were made over the acoustic mooring. Comparisons show acoustic detection of rain events and storm structure that are in agreement with the radar observations. Results from a comparison between the underwater sound pressure level at different depths and the observed radar reflectivities are presented.     

Using Multibaseline InSAR to Recover Layovered Terrain Considering Wideband Array Problem

This letter deals with the problems of retrieving height and synthetic aperture radar (SAR) reflectivity of layovered terrain using multibaseline SAR interferometry (InSAR). In particular, we focus on the wideband array problem caused by practically large InSAR arrays and high-resolution SAR images, i.e., the problem of signal envelope misalignment, which is neglected in the work by Gini We propose two methods to eliminate or mitigate the effect of envelope misalignment, one called the aligning method and the other called the joint range cell processing method. In the aligning method, we align each signal envelope for each searched height of the layovered components (i.e., ground resolution cells with different altitudes) during searching procedure. The joint range cell processing method jointly processes the neighboring cells in range to estimate the parameters of layovered components so that the effect of the envelope misalignment can be mitigated. Theoretical analysis and computer simulation results show that both methods have the ability to provide accurate estimation of the heights and radar reflectivities of multiple layovered resolution cells in the presence of large envelope misalignments.      

Target simulator to calibrate wideband radar in measuring the internal layers of the Greenland ice sheet

We developed a hardware target simulator for measuring the system response and testing of an airborne wideband radar that operates over the frequency range 600-900 MHz to map the near-surface internal layers in glacial ice. It uses optical and microwave delay lines for evaluating and optimizing the performance of the wideband radar in the continental United States without expensive field trips to polar regions and can be used to test the radar without interfering with commercial wireless devices and television stations. The target simulator replicates the feed-through signal between the transmit and receive antenna, as well as reflections from the air-firn interface and the internal layers of the ice sheet, which are spaced about 50 cm apart. This is because the measured radar resolution is about 60 cm in free space and can map layers with about 50-cm resolution in firn. We simulated the internal layers by incorporating a feedback loop with a short delay line. The target simulator is being used for testing and evaluation of the radar in the laboratory and for in-flight testing of the radar.     

A neural fuzzy network approach to Radar pulse compression

To make good range resolution and accuracy compatible with a high detection capability while maintaining the low average transmitted power, pulse compression processing giving low-range sidelobes is necessary. The traditional algorithms such as the direct autocorrelation filter (ACF), least squares (LS) inverse filter, and linear programming (LP) filter based on three-element Barker code (B13 code) have been developed. Recently, the neural network algorithms were issued. However, the traditional algorithms cannot achieve the requirements of high signal-to-sidelobe ratio and low integrated sidelobe level (ISL), and the normal neural networks such as the backpropagation (BP) network usually produce the extra problems of low convergence speed and are sensitive to the Doppler frequency shift. To overcome these defects, a new approach using a neural fuzzy network to deal with pulse compression in a radar system is presented. Two different Barker codes are carried out by a six-layer self-constructing neural fuzzy network (SONFIN). Simulation results show that this neural fuzzy network pulse compression (NFNPC) algorithm has significant advantages in noise rejection performance, range resolution ability, and Doppler tolerance, which are superior to the traditional and BP algorithms.     

干涉雷达在DEM测量中的精度影响及处理方法

干涉合成孔径雷达 (INSAR)为数字高程模型 (DEM)测量提供了一种新的手段。它具有同时获取三维信息、测量范围广、空间和高程分辨率高等优点。文中首先扼要介绍了干涉雷达的原理 ,然后重点讨论几个影响测量精度的因素及其处理方法 ,如相关、运动补偿、配准、相位展开等。     

星载寄生式InSAR系统相关性及相对测高精度分析

结合星载寄生式干涉合成孔径雷达(InSAR)的工作原理及高稳定频率源的工作特性,给出了时间和频率同步误差模型。基于该误差模型,分析了时间和频率同步误差对干涉相位的影响,首次推导了同步误差导致的信号去相关。基于星载寄生式InSAR系统的空间几何关系和图像信号模型,详细分析了热噪声、模糊、量化噪声、基线、方位向、配准误差及体散射等去相关效应,给出了考虑上述全部去相关因素时系统的相对测高精度,并仿真分析了以Envisat为主星的寄生式InSAR系统在ASAR雷达7个不同波位下的相对测高性能。理论分析和仿真结果表明,在合理选择轨道和雷达参数的前提下,系统可以达到很好的测高精度。     

Total zero Doppler Steering-a new method for minimizing the Doppler centroid

This letter presents a new method, called total Zero Doppler steering, to perform yaw and pitch steering for spaceborne synthetic aperture radar (SAR) systems. The new method reduces the Doppler centroid to theoretically 0 Hz, independent of the range position of interest. Residual errors are only due to pointing inaccuracy or due to approximations in the implementation of the total zero Doppler steering law. This letter compares the new method with currently applied methods. The attitude angles and the residual Doppler centroid frequencies are calculated and depicted exemplarily for the parameters of TerraSAR-X, for which the new method will be implemented and used. The new method provides a number of advantages. The low residual Doppler centroid and the reduced variation of the Doppler centroid over range allow a more accurate Doppler centroid estimation. Due to the low residual Doppler centroid, the synthetic aperture radar (SAR) processing can be alleviated, since the range cell migration is reduced and the Doppler frequencies are low. This facilitates the use of very efficient processing algorithms, which are based on approximations whose quality is better for low Doppler frequencies. The new method will furthermore optimize the overlap of the azimuth spectra of SAR image pairs for cross-track interferometry. Low Doppler centroids will also reduce the impact of coregistration errors on the interferometric phase. Furthermore, scalloping corrections in the ScanSAR processing are alleviated due to the low variation of the Doppler centroid over range.     

Validation of Envisat Rain Detection and Rain Rate Estimates by Comparing With TRMM Data

This letter provides validation results on both Envisat altimeter rain detection flag and rain rate estimates. This assessment was performed based on 18-month collocated data sets between Envisat with two sensor measurements from the Tropical Rainfall Measuring Mission, i.e., the microwave imager and the precipitation radar. Along with the comparison between closest sensor measurements with strict spatial and temporal criteria, discussion is also provided on environmental context insight when interpreting comparison results based on collocated data.      

桥梁振动监测雷达的设计与实现

针对传统桥梁振动检测方法过程复杂、成本高、精度低、效率低等问题,利用调频连续波技术和干涉测量技术,设计了一个操作方便、精度高、系统小、成本低的桥梁振动监测雷达。利用该雷达对北京地铁某一桥梁在过车时的振动状态进行了监测。试验结果表明该雷达能够准确测量桥梁的强迫振动和自振。所提出的由雷达回波相位反演桥梁形变信息的方法能够有效地测量桥梁振动状态,从而提高桥梁振动状态检测的分辨率及效率。该系统可以应用于桥梁的监测、预警和维护,具有广阔前景。     

综合主动和被动微波数据监测土壤水分变化

微波遥感测量土壤水分的方法主要分主动和被动两种，它们都是基于干燥土壤和水体之间介电常数的巨大差异。估算植被覆盖土壤表面土壤水分必须要考虑地表粗糙度和植被覆盖影响的问题。植被覆盖土壤表面的后向散射包括来自植被的体散射，来自地表的面散射和植被与地表间的交互作用散射项。本研究建立了一个半经验公式模型，用来计算体散射项，综合时间序列的主动和被动微波数据，消除植被覆盖的影响，估算地表土壤水分的变化状况。并应用1997年美国SGP‘97综合实验中的机载800m分辨辐射计ESTAR数据计算表面反射系数，综合Radarsat的SCAN-SAR数据得到体散射项，然后，由NOAA/AVHRR和TM计算得到的NDVI值加权分配50m分辨率的体散射项，最后计算50m分辨率的表面反射系数的变化值，从而得到土壤水分的变化情况，验证数据表明该计算结果与实测值一致。     

合成孔径雷达基带数字信号源的设计与开发

基带数字信号源是合成孔径雷达 (SAR)系统中的关键设备 ,开发基带数字信号源对于满足SAR向多分辨率、多模式的发展具有重要的意义。随着SAR信号处理的完全数字化 ,基带数字信号源已经替代以往使用的声表面波器件 (SAW ) ,成为SAR数字线性调频产生单元的一部分 ,能够为雷达提供所需发射的各种波形信号。同时 ,面对复杂的SAR系统 ,如何测试和验证SAR设备的功能是开发人员所必须面对的重要环节。高速数字信号源的研制 ,对测试雷达视频信号处理设备有着十分重要的意义。该文首先简要介绍了数字信号源在SAR中的作用 ,然后 ,从理论上详细分析了基带数字线性调频信号的产生原理和设计过程 ;讨论并提出了数字信号源作为雷达视频信号处理测试设备的作用 ;给出了按照上述两种功能研制的高速基带数字信号源的性能、组成及其设计方案。     

A SAR conjugate mirror

A radar transponder was constructed, which modifies the received signal such that its complex conjugate is returned to the radar, qualities of the conjugate mirror used in optics and acoustics. For a monostatic synthetic aperture radar (SAR), a perfect conjugate mirror will reflect a signal back to the radar with no phase shift due to the propagation path. The signal received by the transponder is also decorrelated from other targets, enhancing the transponder signal in the SAR image. This letter describes a transponder operated as a SAR conjugate mirror and an experiment with the European Remote Sensing 1 satellite, demonstrating the feasibility and characteristics. The significance for transponder design is addressed and possible applications discussed.