An Evaluation of Two NEXRAD Wind Retrieval Methodologies and Their Use in Atmospheric Dispersion Models

Two entirely different methods for retrieving 3D fields of horizontal winds from Next Generation Weather Radar (NEXRAD) radial velocities have been evaluated using radar wind profiler measurements to determine whether routine wind retrievals would be useful for atmospheric dispersion model applications. The first method uses a physical algorithm based on four-dimensional variational data assimilation, and the second simpler method uses a statistical technique based on an analytic formulation of the background error covariance. Both methods can be run in near–real time, but the simpler method was executed about 2.5 times as fast as the four-dimensional variational method. The observed multiday and diurnal variations in wind speed and direction were reproduced by both methods below 1.5 km above the ground in the vicinity of Oklahoma City, Oklahoma, during July 2003. However, wind retrievals overestimated the strength of the nighttime low-level jet by as much as 65%. The wind speeds and directions obtained from both methods were usually similar when compared with profiler measurements, and neither method outperformed the other statistically. Within a dispersion model framework, the 3D wind fields and transport patterns were often better represented when the wind retrievals were included along with operational data. Despite uncertainties in the wind speed and direction obtained from the wind retrievals that are higher than those from remote sensing radar wind profilers, the inclusion of the wind retrievals is likely to produce more realistic temporal variations in the winds aloft than would be obtained by interpolation using the available radiosondes, especially during rapidly changing synoptic- and mesoscale conditions.- Reference

Maximum Position Alignment Method for Noisy High-Resolution Radar Target Classification

In this paper, the alignment of noisy high-resolution radar signals using the maximum position method is studied. The relationship between the shift estimation and the signal-to-noise ratio is considered. As a result, two analytical expressions are obtained that approximate the root-mean-square error of the difference in the shift estimation with and without noise. These two expressions allow us to improve the understanding of the sensitivity to noise of the Maximum Position alignment method. - Reference

Predicted Detection Performance of MIMO Radar

It has been shown that multiple-input multiple-output (MIMO) radar systems can improve target detection performance significantly by exploiting the spatial diversity gain. We introduce the system model in which the radar target is composed of a finite number of small scatterers and derive the formula to evaluate the theoretical probability of detection for the system having an arbitrary array-target configuration. The results can be used to predict the detection performance of the actual MIMO radar without time-consuming simulations. - Reference

Radar Revisited (review of "Radar Handbook, 3rd ed." by Merrill Skolnik) [Book Reviews]

This is the third edition of an established handbook, edited by one of the most-recognized names in the field of radar technology. The volume is a compilation of 26 chapters, authored by individuals with a thorough command of, and incredible credentials in, the topics of their chapters. Most chapters have a large number of figures (up to several dozen) and extensive bibliographies. Chapters range from fairly quantitative and mathematical ones to cursory and descriptive ones. Some sections of the handbook represent a concise and readable summary of the state-of-the-art of knowledge on their topics; others are a sketchy collection of remarks for which it is difficult to identify the benefits to be derived by the reader. There is little coordination between chapters where similar topics may be discussed, and a lack of any cross-referencing. There are also weaknesses in the index, as well. While the older, classical radar topics receive much attention, the book overlooks newer areas such as coverage of automotive radars. This volume will appeal to the generalists with interest in the conventional radar subjects, and to others as a starting point for locating sources with more detailed information. - Reference

An Orientation-Selective Orthogonal Lapped Transform

A novel critically sampled orientation-selective orthogonal lapped transform called the lapped Hartley transform (LHT) is derived. In a first step, overlapping basis functions are generated by modulating basis functions of a 2-D block Hartley transform by a cosine wave. To achieve invertibility and orthogonality, an iterative filter is applied as prefilter in the analysis and as postfilter in the synthesis operation, respectively. Alternatively, filtering can be restricted to analysis or synthesis, ending up with a biorthogonal transform (LHT-PR, LHT-PO). A statistical analysis based on a 4000-image data base shows that the LHT and LHT-PO have better redundancy removal properties than other block or lapped transforms. Finally, image compression and noise removal examples are given, showing the advantages of the LHT especially in images containing oriented textures. - Reference

Texture Analysis and Classification With Linear Regression Model Based on Wavelet Transform

The wavelet transform as an important multiresolution analysis tool has already been commonly applied to texture analysis and classification. Nevertheless, it ignores the structural information while capturing the spectral information of the texture image at different scales. In this paper, we propose a texture analysis and classification approach with the linear regression model based on the wavelet transform. This method is motivated by the observation that there exists a distinctive correlation between the sample images, belonging to the same kind of texture, at different frequency regions obtained by 2-D wavelet packet transform. Experimentally, it was observed that this correlation varies from texture to texture. The linear regression model is employed to analyze this correlation and extract texture features that characterize the samples. Therefore, our method considers not only the frequency regions but also the correlation between these regions. In contrast, the pyramid-structured wavelet transform (PSWT) and the tree-structured wavelet transform (TSWT) do not consider the correlation between different frequency regions. Experiments show that our method significantly improves the texture classification rate in comparison with the multiresolution methods, including PSWT, TSWT, the Gabor transform, and some recently proposed methods derived from these. - Reference

Orientation Angle Distributions of Drops after an 80-m Fall Using a 2D Video Disdrometer

This note reports on the use of a 2D video disdrometer to estimate the orientation of drops (>2 mm) that were generated artificially and allowed to fall 80 m from a bridge with no obstruction and under calm conditions. This experimental setup enabled a large number of drops to be generated, up to 10 mm in horizontal dimension.

The distribution of the canting angles for all drops >2 mm was found to be nearly symmetric about 0° with standard deviation between 7° and 8°. From the canting angle distributions derived from the two orthogonal camera view planes, the distributions of the polar (θ) and azimuth () angles were deduced; these two angles describe the 2D orientation of the symmetry axis. The azimuthal angle distribution was found to be nearly uniform in the range (0, 2π), whereas the distribution of p_Ω(θ) = p(θ) sinθ was similar in shape to a special form of the Fisher distribution that is valid for describing the statistics on a spherical surface. The standard deviation of p_Ω(θ) showed that larger drops are more stably oriented than smaller ones. This is in agreement with previous radar-based results of standard deviation of the canting angle decreasing with increasing Z_dr.-Reference

Measurements of the Transmission Loss of a Radome at Different Rain Intensities

Results on the transmission loss of a dry and a wet C-band weather radar radome at different rain intensities are presented. Two methods were used in the study, both carried out under laboratory conditions. In the first method, the complex permittivity of a dry radome is measured and the transmission loss calculated. To analyze the transmission loss of a wet radome, the thickness of a continuous water layer on the surface of a radome at different rain intensities and the complex permittivity of water are calculated. In the second method, the transmission loss is measured as a free space transmission measurement with a 1.3-m² piece of a radome panel. The piece is measured as dry and as doused by a rain system designed for the measurements. The measurements are performed with a dirty, cleaned, and waxed radome to examine the effects of maintenance measures with an old radome on the transmission loss. Because the transmission loss as a function of rain intensity is measured with a small piece of radome, a method is developed to scale the free space measurements for a complete 6.7-m-diameter radome with equal dielectric properties. Results of the one-way transmission loss of a dry radome with the permittivity and free space measurements are in a good agreement (0.34 and 0.35 dB, respectively). According to the analysis, a continuous water layer on a radome has a significant influence on the transmission loss. A 3-dB two-way transmission loss caused by a dirty radome is observed at a rain intensity of 15.1 mm h⁻¹. Waxing gives promising results in reducing the wet radome loss because the waxing prevents the formation of a continuous water layer on the surface of the radome.- Reference

A Study on Optimum Tilt Angle for Wind Estimation Using Indian MST Radar

The effect of tilt angle on horizontal wind estimation is studied using Indian mesosphere–stratosphere–troposphere (MST) radar located at Gadanki (13.45°N, 79.18°E). It operates in Doppler beam swinging (DBS) mode with a beamwidth of 3°. Horizontal winds are computed for different tilt angles from 3° to 15° with an increment of 3° from a height range of 3.6–18 km. The effective beam pointing angle (θ_eff) is calculated to determine the effect of aspect sensitivity on the determination of horizontal wind components. For different tilt angles radar-derived winds are compared with simultaneous GPS sonde wind measurements, which were launched from a nearby site. The first method utilizes direct comparison of radar-derived winds with those of GPS sondes using the actual beam pointing angle; the second method uses the effective beam pointing angle derived from the ratios of two oblique beams. For this study a variety of statistics were explored in terms of standard deviation, correlation coefficient, and percentage error. From the results it is observed that in agreement with previous studies, the effective beam pointing angle deviates from the actual beam pointing angle, which results in the underestimation of horizontal wind components, and also when tilt angle is close to zenith and far from zenith, the estimation of horizontal winds is found to be far from true values at different heights. Radar wind estimation has better agreement with GPS sonde measurement when the off-zenith angle is around 10°. It is also found that correction to the actual beam pointing angle provides 3%–6% improved agreement between the radar and GPS wind measurements. -Reference

Multiple Scattering Effects in Pulsed Radar Systems: An Intercomparison Study

In this paper, two different numerical methods capable of computing multiple scattering effects in pulsed-radar systems are compared. Both methods are based on the solution of the time-dependent vectorial form of the radiative transfer equation: one exploits the successive order of scattering approximation, the other a forward Monte Carlo technique.

Different benchmark results are presented (including layers of monodisperse spherical water and ice particles), which are of specific interest for W-band spaceborne cloud radars such as CloudSat’s or EarthCARE’s cloud profiling radars. Results demonstrate a good agreement between the two methods. The pros and cons of the two models are discussed, with a particular focus on the validity of the second order of scattering approximation.- Reference

Differential Reflectivity Calibration for Operational Radars

The conventional technique for calibrating $Z_{dr}$ using natural scatterers is based on vertical-looking observations. In some operational weather radar, this method is not applicable because of mechanical constraints that prohibit vertical measurement or choices in the scanning strategies. A technique for calibrating $Z_{dr}$ based on properties of rain returns is proposed and analyzed. The technique is based on an examination of properties of differential reflectivity measurements collected at increasing elevations. Differential reflectivity observed in rain decreases with increasing elevation due to the increasing view angle. Using the hypothesis of uniform microphysical profiles below the bright band, deviations of the profile of differential reflectivity with elevation with respect to the theoretical profile can be used to detect and quantify the presence of a bias on differential reflectivity. To apply this concept in the presence of a nonuniform microphysical profile, the contribution of vertical changes in microphysics to $Z_{dr}$ variation in height is also accounted for. An error parameter associated with the estimated $Z_{dr}$ bias can be used as a quality indicator of the bias estimation; it allows definition of a criterion based on a threshold of root-mean-square error that permits acceptance or rejection of a $Z_{dr}$ bias estimation obtained with the proposed method. The technique is demonstrated using data collected by an operational weather radar at Arpa Piemonte (Italy) and evaluated using independent disdrometer measurement. Results show that under certain conditions discussed in the paper, this method can provide $Z_{dr}$ calibration within an accuracy of 0.1 dB. - Reference

Simulating Range Oversampled Doppler Radar Profiles of Inhomogeneous Targets

A new technique for generating range oversampled profiles of Doppler radar signals that have been backscattered by distributed targets is presented in this paper. The technique was developed for spaceborne cloud radars, but it can just as well be used for ground-based precipitation or wind-profiling radars. The technique is more versatile than the traditional inverse FFT technique and faster than the individual hydrometeor simulation (Monte Carlo) technique. Doppler radar signals from backscattering hydrometeors are essentially correlated stochastic variables. The technique uses an accurate description of covariances between voltages measured for different pulses and at different positions (range gates) along a profile. A matrix formalism is developed to subsequently transform uncorrelated Gaussian noise into correlated receiver voltages with the appropriate covariances. In particular, the new technique deals with target variability in a physically consistent manner, accounting for the effects of inhomogeneity both within the instantaneous field of view and between subsequent pulses. The new technique is showcased with examples of simulated 95-GHz Doppler radar observations by the Earth Clouds, Aerosols and Radiation Explorer (EarthCARE) space mission. - Reference

A Technique for the Automatic Detection of Insect Clutter in Cloud Radar Returns

The U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program operates 35-GHz millimeter-wavelength cloud radars (MMCRs) in several climatologically distinct regions. The MMCRs, which are centerpiece instruments for the observation of clouds and precipitation, provide continuous, vertically resolved information on all hydrometeors above the ARM Climate Research Facilities (ACRF). However, their ability to observe clouds in the lowest 2–3 km of the atmosphere is often obscured by the presence of strong echoes from insects, especially during the warm months at the continental midlatitude Southern Great Plains (SGP) ACRF. Here, a new automated technique for the detection and elimination of insect-contaminated echoes from the MMCR observations is presented. The technique is based on recorded MMCR Doppler spectra, a feature extractor that conditions insect spectral signatures, and the use of a neural network algorithm for the generation of an insect (clutter) mask. The technique exhibits significant skill in the identification of insect radar returns (more than 92% of insect-induced returns are identified) when the sole input to the classifier is the MMCR Doppler spectrum. The addition of circular polarization observations by the MMCR and ceilometer cloud-base measurements further improve the performance of the technique and form an even more reliable method for the removal of insect radar echoes at the ARM site. Recently, a 94-GHz Doppler polarimetric radar was installed next to the MMCR at the ACRF SGP site. Observations by both radars are used to evaluate the potential of the 94-GHz radar as being insect free and to show that dual wavelength radar reflectivity measurements can be used to identify insect radar returns. - Reference

Potential Role Of Dual- Polarization Radar In The Validation Of Satellite Precipitation Measurements: Rationale and Opportunities

Ground based dual-polarization radars provide physical insight into the development and interpretation of spaceborne precipitation measurements - Reference

Constant false-alarm rate algorithm based on test cell information

A new constant false-alarm rate (CFAR) detection algorithm, designated as switching CFAR (S-CFAR), is proposed and analysed. The S-CFAR algorithm selects CFAR reference samples using the magnitude of the sample in the cell under test, which is an information that has not been exploited in any other existing CFAR detectors. S-CFAR closed-form analysis is presented, and comparisons with other representative CFAR algorithms are given. An S-CFAR detector can be tuned such that it has a small CFAR loss when operating on a homogeneous background while achieving improved robustness in the presence of interfering targets and clutter power transition. The S-CFAR detector is also simple to design and implement since no sample ordering is required. - Reference

Fundamental resolution limits of closely spaced random signals

Fundamental limitations on estimation accuracy are well known and include a variety of lower bounds including the celebrated Cramer-Rao lower bound. However, similar theoretical limitations on resolution have not yet been presented. The authors exploit results from detection theory for deriving fundamental limitations on resolution. In this correspondence the authors discuss the case of zero mean random Gaussian signals with a general and predefined covariance matrix observed with additive white Gaussian noise. The results are general and are not based on any specific resolution technique and therefore hold for any method and for any probability of successful resolution. The resolution limit is a simple expression of the observation interval, the pre-specified resolution success probability and the second derivative of the covariance matrix. As an example, the authors discuss the bearing resolution of two emitters with closely spaced direction of arrival, impinging on an array of sensors. The theoretical limits are compared with empirical performance of the model order selection criteria known as the Akaike information criterion and the minimum description length. - Reference

The Detection of Buried Pipes From Time-of-Flight Radar Data

Ultrawideband radar is commonly used in the frequency range of 50–500 MHz to detect buried pipes at a depth of about 1–2 m depending on the soil characteristics. The typical feature used to locate the pipes is the hyperbolic pattern of the time of flight generated by a linear scan of the antenna above the surface. When the pipes are close together, the hyperbolas overlap, and a straightforward least squares fit is not possible. The Hough transform provides one possible solution. This paper extends the Hough transform by introducing a weighting factor depending on the differentials of the unknown parameters with respect to the experimental errors, namely, the probe position error and the time-of-flight error. This enables optimally placed sets of data pairs to be given greater weight than “ill-conditioned” sets, as for example when all data pairs lie near one end of the arc. The result is a decrease in the background amplitude with respect to the maximum of the peaks in the Hough accumulator space. It is shown that this improvement persists even when many arcs are present. A mathematical analysis with analytical results is given for the case of four unknowns: pipe radius $R$ , pipe center position ($Y$, $Z$), and soil propagation velocity $V$. The results are presented through simulations introducing controlled uncertainties in the probe position, the time of flight, and its bin size. The simulations demonstrate the correlations that occur between the radius, depth, and velocity for given experimental uncertainties. - Reference

Three-Dimensional Motion Estimation of Atmospheric Layers From Image Sequences

In this paper, we address the problem of estimating 3-D motions of a stratified atmosphere from satellite image sequences. The analysis of 3-D atmospheric fluid flows associated with incomplete observation of atmospheric layers due to the sparsity of cloud systems is very difficult. This makes the estimation of dense atmospheric motion field from satellite image sequences very difficult. The recovery of the vertical component of fluid motion from a monocular sequence of image observations is a very challenging problem for which no solution exists in the literature. Based on a physically sound vertical decomposition of the atmosphere into cloud layers of different altitudes, we propose here a dense motion estimator dedicated to the extraction of 3-D wind fields characterizing the dynamics of a layered atmosphere. Wind estimation is performed over the complete 3-D space, using a multilayer model describing a stack of dynamic horizontal layers of evolving thickness, interacting at their boundaries via vertical winds. The efficiency of our approach is demonstrated on synthetic and real sequences. - Reference

A Two-Dimensional Spectrum Model for General Bistatic SAR

This paper derives a 2-D spectrum model for general bistatic synthetic aperture radar (SAR). By introducing some new parameters such as equivalent monostatic parameters, bistatic factor, and weighted-equivalent range, the 2-D spectrum of general bistatic SAR can be expressed in the form of monostatic SAR even when the transmitter and receiver move along unparallel trajectories with different velocities. The result formulates bistatic SAR into an equivalent monostatic SAR model and would be useful for developing efficient bistatic SAR algorithms in frequency-domain or hybrid-domain processing. Simulation results are given to validate the performance of the model. For special bistatic SAR configurations, the model can be simplified. Compared to other similar models, the proposed model is clearer and much more concise. - Reference

Information Theory-Based Approach for Contrast Analysis in Polarimetric and/or Interferometric SAR Images

We propose a new approach for evaluating the contribution of the different channels of polarimetric and interferometric synthetic aperture radar (PolInSAR) images. For that purpose, we demonstrate that the Bhattacharyya distance between the probability density functions of neighboring regions in the image provides an efficient scalar contrast measure. We show that the analysis of this contrast measure allows one to precisely characterize the contribution of each channel for different system configurations, including intensity, polarimetric, and interferometric images. We illustrate this approach using a real synthetic aperture radar image to compare several polarimetric system architectures. Since PolInSAR imaging configurations can correspond to complex and expensive systems, the proposed method can be helpful in system imaging optimization. - Reference

Sequential Along-Track Integration for Early Detection of Moving Targets

This paper concerns the joint multiframe sequential target detection and track estimation in early-warning radar surveillance systems. The rationale for applying sequential procedures in such a scenario is that they promise a sensitivity increase of the sensor or, alternatively, a reduction in the time needed to take a decision. Unlike previous works on sequential radar detection, the attention is not restricted to stationary targets, namely position changes during the illumination period are allowed. Starting from previous sequential rules, different truncated sequential strategies are proposed and assessed: they are aimed at orienting the sensor resources towards either the detection or the track estimation or the position estimation. Bounds on the performances of the proposed procedures in terms of the system parameters are derived and computational complexity is examined. Also, numerical experiments are provided to elicit the interplay between sensor-target parameters and system performances, and to quantify the gain with respect to other fixed-sample-size procedures. - Reference

Signal Synthesis and Receiver Design for MIMO Radar Imaging

Multiple-input–multiple-output (MIMO) radar is an emerging technology that has significant potential for advancing the state-of-the-art of modern radar. When orthogonal waveforms are transmitted, with $M+N$ ($N$ transmit and $M$ receive) antennas, an $MN$-element filled virtual array can be obtained. To successfully utilize such an array for high-resolution MIMO radar imaging, constant-modulus transmit signal synthesis and optimal receive filter design play critical roles. We present in this paper a computationally attractive cyclic optimization algorithm for the synthesis of constant-modulus transmit signals with good auto- and cross-correlation properties. Then we go on to discuss the use of an instrumental variables approach to design receive filters that can be used to minimize the impact of scatterers in nearby range bins on the received signals from the range bin of interest (the so-called range compression problem). Finally, we present a number of numerical examples to demonstrate the effectiveness of the proposed approaches.- Reference

Maximum Position Alignment Method for Noisy High-Resolution Radar Target Classification

In this paper, the alignment of noisy high-resolution radar signals using the maximum position method is studied. The relationship between the shift estimation and the signal-to-noise ratio is considered. As a result, two analytical expressions are obtained that approximate the root-mean-square error of the difference in the shift estimation with and without noise. These two expressions allow us to improve the understanding of the sensitivity to noise of the Maximum Position alignment method. - Reference

Imaging Simulation of Bistatic Synthetic Aperture Radar and Its Polarimetric Analysis

Employing the 3-D mapping and projection algorithm (MPA), an imaging simulation of bistatic synthetic aperture radar (BISAR) observation over a complex scenario is developed. Based on the explicit expression of the point target response of stripmap BISAR imaging, raw data are efficiently generated from the scattering map precalculated by MPA. Some examples of BISAR image simulation are studied. The polarimetric characteristics of a BISAR image are then discussed. It is found that some typical polarimetric parameters such as Cloude's $alpha$, $beta$, and $gamma$ might become unable to describe the scattering mechanism under bistatic observation. A transform of unified bistatic polar bases for a BISAR image is proposed. The parameters $\alpha$, $\beta$ , and $\gamma$ are modified to retain the property of orientation independence in the bistatic circumstance. Analysis of simulated images shows that the redefined $\alpha$, $\beta$ , and $\gamma$ after the unified bistatic polar bases transform well describe different scattering mechanisms in BISAR imaging. It provides a primary tool for BISAR image interpretation and terrain classification. Reference

Whitening Dual-Polarized Weather Radar Signals With a Hermitian Transformation

Oversampling weather radar signals in range and then whitening these signals has been shown to improve the accuracy of spectral moments. For dual-polarized radar, the polarimetric variables depend upon information gleaned from the cross correlation of the different received signals. Theoretical improvements to the polarimetric variables have been provided to date, but experimental evidence of improvements through whitening has been limited. This paper provides an analysis of the effects of whitening on the estimated cross correlation along with experimental results of whitening applied to polarimetric variables. Different whitening transformations based solely on covariance matrix inversions will be shown to affect the copolar correlation of the whitened data. A Hermitian symmetric whitening transformation will be shown to produce better estimates of polarimetric variables obtained from whitened data than the original whitening transformation defined for use with range oversampling.Reference

Holographic Research to Enhance Aircraft Antennas

The US Air Force has funded researchers at HRL Laboratories in Malibu, California to work on innovative new holographic techniques to create antennas that do not detract from an aircrafts aerodynamics.

The team are working on holographic impedance surfaces that will make antennas completely flush with the surface of the aircraft while still maintaining or even enhancing current protruding antenna capabilities.

The team are building the surfaces out of metallic materials on a substrate.

Programme manager at the Air Force Office of Scientific Research, Dr Arje Nachman said that the technology offers several benefits.

"One attractive benefit offered by these interesting surfaces is that if the tail of an aircraft obstructs the beam of an antenna then the tail can be covered by a suitably crafted impedance surface in such a way that the antenna beam flows around the tail as if the tail weren't there," Nachman said.

The HRL team is now trying to extend the capabilities of the electromagnetic impedance and experiment with practical implementations of it.

The researchers plan to create new kinds of unit cells and are also seeking new mapping techniques that allow those cells to be positioned over complex objects.

By Daniel Garrun.