Backscattering Radar Cross Section Research Articles

New Method for High Wind Speed Retrieval Using Altimeters Observations

By comparing the normalized radar backscatter cross sections (NRCS) and significant wave height detected by the Jason-1 altimeter with high wind speeds data, an empirical algorithm valid for wind speeds larger than 20 m s-1 is developed and proposed. The study shows that the Root Mean Square Error (RMSE) and average relative error of the new algorithm are 5.44m s-1 and 16.5%, suggesting increased accuracy by including significant wave height in the retrieval algorithm. Therefore, the proposed algorithm, suitable for retrieving sea surface wind speeds in typhoon and other strong wind conditions, could be treated as supplementary algorithm to the Jason-1 operational algorithm.

Cross‐polarization geophysical model function for C‐band radar backscattering from the ocean surface and wind speed retrieval

AbstractThe wind speed sensitivity of cross‐polarization (cross‐pol) radar backscattering cross section (VH) from the ocean surface increases toward high winds. The signal saturation problem of VH, if it exists, occurs at a much higher wind speed compared to the copolarization (copol: VV or HH) sea returns. These properties make VH a better choice over VV or HH for monitoring severe weather. Combined with high spatial resolution of the synthetic aperture radar (SAR), the development of hurricane wind retrieval using VH is advancing rapidly. This paper describes a cross‐pol C‐band radar backscattering geophysical model function (GMF) with incidence angle dependence for the full wind speed range in the available data sets (up to 56 m/s). The GMF is derived from RADARSAT‐2 (R2) dual‐polarization (dual‐pol) ScanSAR modes with 300 and 500 km swaths. The proposed GMF is compared to other published algorithms. The result shows that the simulated VH cross section and the retrieved wind speed with the proposed GMF is in better agreement with measurements. With careful treatment of noise, the VH‐retrieved wind speeds may extend to mild or moderate conditions. The higher fraction of non‐Bragg contribution in VH can be exploited for analysis of surface wave breaking.

What do triple‐frequency radar signatures reveal about aggregate snowflakes?

AbstractA large data set of volume element models of aggregate snowflakes was created, building the snowflakes from various models of ice crystals found in the atmosphere: dendrites, needles, plates, and bullet rosettes, as well as spheroidal crystals for comparison. Several different sizes for the constituent crystals were also used. The radar backscattering cross sections of the snowflakes were computed from the models using the discrete dipole approximation (DDA) at 13.6 GHz (Ku band), 35.6 GHz (Ka band) and 94.0 GHz (W band), and the effects of the choice of crystal model and size on the Ku/Ka band and Ka/W band dual‐wavelength ratios (DWR) was investigated. It was found that the aggregate DWRs were very similar for all naturally occurring ice crystal types investigated in this study. This implies that the choice of crystal type is at most of secondary importance in the forward model of scattering used for snowfall retrievals but also, conversely, that the identification of the crystal type from triple‐frequency observations is likely to be difficult. In contrast, the size of the constituent ice crystals does have a nonnegligible impact on the triple‐frequency signatures. Additionally, it was found that the triple‐frequency signatures found in some experimental data, resembling those resulting from spheroidal model snowflakes, cannot be reproduced using the aggregates with any of the crystal types that were investigated. This suggests that besides aggregation, other mechanisms of snowflake formation from ice crystals must be considered in snowfall retrieval algorithms.

Design of shared aperture metamaterial and its applications for high gain and low radar cross section antenna

A shared aperture metamaterial (SA-MTM) with partially reflection and absorber characteristics is presented. The SA-MTM is composed of two metallic layers separated by a dielectric spacer; the top absorbing surface (AS) consists of oblique cross metallic pattern loaded with lumped resistances, and the bottom partially reflecting surface (PRS) consists of etched parallel slots in a metallic layer. An SA-MTM with partial reflection and absorption characteristics is fulfilled by making the absorbing surface and partially reflecting surface shared the same aperture in the vertical direction. The SA-MTM is applied to the waveguide slot antenna; the Fabry-Perot resonance cavity constructed by the PRS and the metallic ground layer of the waveguide slot antenna can achieve high gain, while the AS can obtain the low radar cross section (RCS) characteristic antenna by absorbing the incident wave. Simulation and experimental results demonstrate that the antenna with SA-MTM gain is enhanced above 3 dB in the operation frequency range, the backscattering RCS is obviously reduced in a frequency range of 2–9 GHz. This idea can help us design a high gain and low RCS antenna, which overcomes the conflict between scattering and radiation performance of antenna.

A low radar cross-section artificial magnetic conductor reflection screen covering X and Ku band

Based on the properties of the artificial magnetic conductor (AMC), a broadband low radar cross-section (RCS) reflection screen covering X and Ku band is designed and fabricated. The reflection screen is formed by combining two AMC cells, i.e., AMC1 with a dual band Jerusalem cross structure, and AMC2 with a wideband metal square patch structure. By optimizing the structures of these AMC cells, it is achieved that the frequency corresponding to the inversion point of the AMC1 reflection phase curve is equal or close to the frequency corresponding to the null point of the AMC2 reflection phase curve. Therefore, the valid reflection phase difference band is broadened and the RCS is reduced in a wider band. In addition, presented in this paper is a theoretical formula to calculate the reflection energy peak direction. When the incident angle, chessboard unit dimension and observed frequency are fixed, the reflection energy peak direction can be calculated by the formula. The calculation results from the theoretical formula are consistent with the HFSS simulation results, so the theoretical formula is valid. The simulation results indicate that, compared with the same-dimension metal RCS, the backscattering RCS is reduced by more than 10 dB in a frequency range of 7.4-17.0 GHz, except minority frequencies close to 9.8 GHz. The 10 dB-reducing RCS bandwidth covers the entire X band and most of Ku band, and the relative bandwidth is 78.7%. The largest reduction reaches 40.3 dB at 11.6 GHz. The simulations and the measurements are in good agreement. The results validate the broadband low RCS property of the reflection screen.

EFFECTS OF INFLATED CONE ON SATELLITE'S RADAR CROSS SECTIONS IN S-BAND VIA FDTD SIMULATIONS

Satellites are the most important link in today's battlefield, and with the advancement of anti-satellite technologies such as anti-satellite missiles and directed energy weapons, satellites are becoming vulnerable to attack. The vulnerability of a satellite depends greatly on the probability of it being detected and tracked, and optics and radars are the two main means of detection. To avoid detection, several suggestions have been made in the past to deflect ambient light and decrease the RCS (radar cross section). The most notable RF stealth suggestion among them is the proposal of using an inflatable polymer cone to change its shape and reduce the satellite's RCS. In this study we examine the accuracy of a commercial FDTD code with a theoretical Mie scattering RCS, and use it to calculate the RCS of this so-called stealth satellite in the S-band, and analyze its frequency and radar incident angle dependence. Results indicate that this shape is advantageous in boresight monostatic backscatter RCS reduction, but in other directions the RCS increases due to the sheer size effect, which makes it even more vulnerable to bi-static radar tracking. When it is slant illuminated, the RCS of the stealth satellite shows no RCS reduction effects. This inflated device is susceptible to space debris damage and cumbersome to operate, and may interfere with the original mission of the satellite. The best strategy for satellite self-defense is orbit change.

Characterizing the Radar Backscatter-Cross-Section Sensitivities of Ice-Phase Hydrometeor Size Distributions via a Simple Scaling of the Clausius–Mossotti Factor

AbstractOne of the challenges that limit the amount of information that can be inferred from radar measurements of ice and mixed-phase precipitating clouds is the variability in ice mass within hydrometeors. The variable amount of ice mass within particles of a given size drives further variability in single-scattering properties that results in uncertainties of forward-modeled remote sensing quantities. Nonspherical ice-phase hydrometeors are often approximated as spheroids to simplify the calculation of single-scattering properties, yet offline calculations remain necessary to quantify these radiative properties as a function of size in discrete increments. In this paper, a simple scaling of the Clausius–Mossotti factor is used that allows for an approximation of the scattering and extinction cross sections for an arbitrary mass–dimensional power-law relationship of a nonspherical particle given a single T-matrix calculation. Using data collected by the University of Wyoming King Air in snow clouds over the Colorado Park Range, the uncertainty in forward-modeled radar reflectivity to assumptions regarding mass–dimensional relationships is examined. This is accomplished by taking advantage of independently measured condensed mass and particle size distributions to estimate the variability of the prefactor in the mass–dimensional power law. Then, calculating the partial derivative of the radar backscatter cross sections using the scaling relationships, an estimate is made of the statistical uncertainty in forward-modeled radar reflectivity. Uncertainties on the order of 4 dB are found in this term for the dataset considered.

Backscattering from small-scale breaking wave turbulence structure generated by FLUENT

A breaking wave can exert a great influence on the electromagnetic (EM) scattering result from sea surfaces. In this paper, the process of small-scale wave breaking is simulated by the commercial computational fluid dynamics (CFD) software FLUENT, and the backscattering radar cross section (RCS) of the turbulence structure after breaking is calculated with the method of moments. The scattering results can reflect the turbulent intensities of the wave profiles and can indicate high polarization ratios at moderate incident angles, which should be attributed to the incoherent backscatter from surface disturbance of turbulence structure. Compared with the wave profile before breaking, the turbulence structure has no obvious geometrical characteristic of a plunging breaker, and no sea spikes are present at large incident angles either. In summary, the study of EM scattering from turbulence structure can provide a basis to explain the anomalies of EM scattering from sea surfaces and help us understand the scattering mechanism about the breaking wave more completely.

Radar cross section reduction of a cavity in the ground plane: TE polarization

The reduction of backscatter radar cross section(RCS) in TE polarization for a rectangular cavity embedded in the ground plane is investigated in this paper. It is established by placing a thin, multilayered radar absorbing material(RAM) with possibly different permittivities at the bottom of the cavity. A minimization problem with respect to the backscatter RCS is formulated to determine the synthesis of RAM. The underlying scattered field is governed by a generalized Helmholtz equation with transparent boundary condition. The gradient with respect to the material permittivity is derived by the adjoint state method. A fast solver for the Helmholtz equation is presented for the optimization scheme. Numerical examples are presented to show the efficiency of the algorithm for RCS reduction.

Design of double‐sided axe‐shaped ultra‐wideband antenna with reduced radar cross‐section

In this study, a novel axe-shaped ultra-wideband (UWB) antenna with low-backscattering radar cross-section (RCS) is designed. The designed UWB antenna bandwidth is 3.4–15.5 GHz covering the entire band assigned for the UWB applications. The RCS of the designed antenna is reduced by modifying the geometrical shape. This technique is based on the subtraction of the metal areas which have the minimum current distributions on the surface of the printed antenna. Therefore the backscattering RCS of the modified axe-shaped UWB antenna is reduced up to 10 dB in the whole frequency band while maintaining the radiation performance. The size of the antenna is 0.32λ l ×0.39λ l, where λ l is the wavelength at the lower operation frequency and the fractional bandwidth is 128%. The radiation patterns, scattering characteristics and the RCS of both the modified and reference axe-shaped UWB antennas are simulated and experimentally verified. The simulation and measurement results are in a good agreement.

Radar Cross Section Reduction of a Cavity in the Ground Plane

AbstractThis paper investigates the reduction of backscatter radar cross section (RCS) for a rectangular cavity embedded in the ground plane. The bottom of the cavity is coated by a thin, multilayered radar absorbing material (RAM) with possibly different permittivities. The objective is to minimize the backscatter RCS by the incidence of a plane wave over a single or a set of incident angles. By formulating the scattering problem as a Helmholtz equation with artificial boundary condition, the gradient with respect to the material permittivities is determined efficiently by the adjoint state method, which is integrated into a nonlinear optimization scheme. Numerical example shows the RCS may be significantly reduced.

A kind of shared aperture radar absorbing material with absorber and phase cancellation characteristics

A method of designing a kind of shared aperture radar absorbing material (SA-RAM) is presented, in which method the scattering problem of passive metamaterial (MTM) is converted into the radiation problem of active array. Multifunctional SA-RAM is realized by optimizing the position, amplitude, and phase of the MTM subarray composed of finite MTM structures based on the array theory. An SA-RAM with absorber and phase cancellation characteristics is fulfilled by interleaving artificial magnetic conductor (AMC) subarray and perfect metamaterial absorber (PMA) subarray. Simulation and experimental results demonstrate that the backscattering radar cross section (RCS) of SA-RAM is smaller than that of the metal plate in a frequency range of 5.5-8.3 GHz. Especially, the RCS reduction is caused by high absorbance at 5.54 GHz and by phase cancellation between AMC subarray and PMA subarray at 7.0 GHz. The idea can help to design radar absorbing material, which combines frequency stealth with space stealth function.

Modeling radar backscattering from melting snowflakes using spheroids with nonuniform distribution of water

In a number of studies it is reported that at the early stages, melting of aggregate snowflakes is enhanced at lower parts. In this paper, the manifestation of the resulting nonuniform distribution of water is studied for radar backscattering cross sections at C, Ku, Ka and W bands. The melting particles are described as spheroids with a mixture of water and air at the bottom part of the particle and a mixture of ice and air at the upper part. The radar backscattering is modeled using the discrete-dipole approximation in a horizontally pointing geometry. The results are compared to the T-matrix method, Mie theory, and the Rayleigh approximation using the Maxwell Garnett mixing formula. We find that the differential reflectivity and the linear depolarization ratio show systematic differences between the discrete-dipole approximation and the T-matrix method, but that the differences are relatively small. The horizontal cross sections show only small differences between the methods with the aspect ratio and the presence of resonance peaks having a larger effect on it than the nonuniform distribution of water. Overall, the effect of anisotropic distribution of water, reported for early stages of melting, is not significant for radar observations at the studied frequencies.

Analysis of backscattering RCS of targets coated with parabolic distribution and time-varying plasma media

The backscattering radar cross section (RCS) of targets coated with parabolic distribution and time-varying unmagnetized plasma is analyzed by the finite-difference time-domain (FDTD) method based on the Runge–Kutta exponential time differencing (RKETD) technique. The two-dimensional RKETD-FDTD formulations are derived. The effects of the plasma parameters on the RCS are investigated. The numerical results illustrate that plasma cloaking system can successfully reduce the backscattering RCS, the plasma stealth is effective, and reasonable selection of different parameters of plasma can enhance its effectiveness.

Investigation of EM scattering from electrically large sea surface with breaking wave at low grazing angles

According to the high-resolution radar data, backscattering scattering from sea surfaces may give rise to super events of strong scattering and large Doppler offsets for horizontal (HH) polarization at low grazing angles (LGA), which are usually considered to be caused by breaking waves. In this paper, the mesoscale breaking wave model is introduced to account for the contribution of breaking waves, which can be responsible for sea spikes and high Doppler shift. The backscattering radar cross-section (RCS) from a single breaking wave is computed and the scattering from sea surfaces is analyzed by a composite surface scattering model, in which the phase factor of every facet is modified by the capillary waves. By using the slope criterion, the spatial distribution of the breaking waves is obtained. At the same time, the RCS of sea surfaces with breaking waves is calculated and the results agree with measured data well at LGA for HH polarization. Considering the Bragg phase velocity, orbital motion of facets and wind drift, the Doppler spectrum of sea surface with breaking wave is simulated, and when compared with the result without breaking wave, there is a good improvement for HH polarization, and for the vertical polarization, the revision is little, which are agreeable with the results of recent researches.

Estimation of tropical cyclone parameters and wind fields from SAR images

The traditional method of Synthetic Aperture Radar (SAR) wind field retrieval is based on an empirical relation between the near surface winds and the normalized radar backscatter cross section to estimate wind speeds, where this relation is called the geophysical model function (GMF). However, the accuracy rapidly decreases due to the impact of rainfall on the measurement of SAR and the saturation of backscattered intensity under the condition of tropical cyclone. Because of no available instrument synchronously monitoring rain rate on the satellite platform of SAR, we have to derive the precipitation of the SAR observation time from non-simultaneous passive microwave observations of rain in combination with geostationary IR images, and then use the model of rain correction to remove the impact of rain on SAR wind field measurements. For the saturation of radar backscatter cross section in high wind speed conditions, we develop an approach to estimate tropical cyclone parameters and wind fields based on the improved Holland model and the SAR image features of tropical cyclone. To retrieve the low-to-moderate wind speed, the wind direction of tropical cyclone is estimated from the SAR image using wavelet analysis. And then the maximum wind speed and the central pressure of tropical cyclone are calculated by a least square minimization of the difference between the improved Holland model and the low-to-moderate wind speed retrieved from SAR. In addition, wind fields are estimated from the improved Holland model using the above-mentioned parameters of tropical cyclone as input. To evaluate the accuracy of our approach, the SAR images of typhoon Aere, typhoon Khanun, and hurricane Ophelia are used to estimate tropical cyclone parameters and wind fields, which are compared with the best track data and reanalyzed wind fields of the Joint Typhoon Warning Center (JTWC) and the Hurricane Research Division (HRD). The results indicate that the tropical cyclone center, maximum wind speed, and central pressure are generally consistent with the best track data, and wind fields agree well with reanalyzed data from HRD.

A Transmission-Line Model for Wave Excitation of a Porous Conducting Sphere

A wire mesh conformed to a spherical surface forms a spherical polyhedron that has unique resonator and scattering properties for electromagnetic (EM) waves. Excitation of the spherical polyhedron by an incident plane wave at specified resonant frequencies yields large internal electric fields. Narrowband enhancements in the backscatter radar cross section are found at these same frequencies. These effects are explained with a model where the mesh is treated as an inductive frequency-selective surface that is used to match a shorted-line resonator attached to a transmission line. With this transmission-line model, the impedance of the inductive surface is shown to be an addition to the expression for the spherical harmonic series used to describe solid conducting spheres and spherical cavities. The resonant modes for the internal and external electric fields of the porous sphere are found with this formulation. The surface mesh is used in the low-frequency limit, where the EM wavelength is much larger than the polygon's holes in the mesh. The impedance (inductance and resistance) of the mesh are adjusted by varying the radii of the edges of the polygons and the conductivity of the edge material. Applications of the spherical porous conducting resonator (SPCR) include glow plasma discharges, measurements of dielectric constants of gases, and frequency-selective radar targets.

Performance analysis of China dual-F requency airborne precipitation radar

The prototypes of China's Ku- and Ka-band PR have been successfully manufactured, and the first airborne field campaign conducted. Measurement data obtained in the field campaign have been processed to validate the radar performance. Comparisons of the measured radar backscattering cross section between ADPR and TRMM PR over sea and precipitation data between airborne radar and ground-based weather radar/mobile ground-based meteorological radar/TRMM PR have been made to validate ADPR's ability in precipitation detection. Important radar performance indices are evaluated using the field campaign datasets. The overall performance shows the dual-frequency airborne PR is qualified for the development of future dual-frequency spaceborne radar in China. The sensitivities and range resolutions of both Ku-band and Ka-band PRs are satisfying, and the sidelobe in pulse compression mode as well as the dynamic range in short-pulse mode of Ka-band PR can be improved. Future work will focus on furthering the performance analysis and improving the performance indices of the spaceborne PR.

Extraction of Underwater Laver Cultivation Nets by SAR Polarimetric Entropy

This letter describes a technique of extracting and estimating the underwater laver cultivation nets by using the entropy analysis of polarimetric synthetic aperture radar (PolSAR) data. The cultivation nets are placed at 10-20 cm below the sea surface, so that the Bragg waves responsible for L-band radar backscatter do not fully develop in this area of effectively shallow water. Consequently, the surface becomes smooth, and the backscatter radar cross section (RCS) becomes small in comparison with that from deep water without cultivation nets. If RCS from the cultivation area is at the system noise level, the image can be considered as arising from a random process, and the polarimetric entropy should be higher than the open sea area where the radar backscatter is dominated by the single-bounce surface scattering process. We will show that, using the data acquired by Phased Array L-band SAR onboard the Advanced Land Observing Satellite over the Tokyo Bay, Japan, the polarimetric entropy is an effective means of extracting underwater cultivation areas in comparison with the amplitude images. The area of the laver cultivation is then estimated by applying a constant false alarm rate to the entropy images to yield good agreement with the ground-truth data.

Implementation of a Dual-Interrogation-Mode Embroidered RFID-Enabled Strain Sensor

This letter studies a dual-interrogation-mode (hybrid chip-enabled/chipless) technique to detect an embroidered radio frequency identification (RFID)-enabled sensor. We have created a hybrid RFID strain sensor utilizing both chipped and chipless approaches. A comparison between the read-range extraction technique, which relies upon the detection of the threshold power that is required to activate an RFID IC, and a radar-cross-section (RCS)-based technique that does not require any protocol is presented. An embroidered RFID sensor with an electrical length that is directly linked to the applied strain is realized and interrogated using both techniques. The sensitivities for the chipless radar technique and for the chip-enabled read range extraction technique are 0.66% and 0.43% frequency shift, respectively, for 1% strain variation. Simulations and measurements validate the dual interrogation mode and provide the proof of concept that a chip-enabled RFID sensor tag can be detected accurately utilizing backscattering RCS measurements.