Backscattering Radar Cross Section Research Articles

Numerical and Experimental Study on Backscattering Doppler Characteristics From 2-D Nonlinear Sealike Surface at Low Grazing Angle

This article studies both experimentally and numerically the backscattering radar cross section (RCS) and Doppler spectrum characteristics from 2-D linear and nonlinear time-varying sea-like surface at UHF band under low grazing incidence. The small slope approximation and choppy wave model are applied to solve rough surface scattering and generate nonlinear sea waves, respectively. A coherent Doppler radar operating at 340 MHz was deployed at the tip of Huangqi peninsula in the southeast coast of China to measure the echo spectrum from the actual sea surfaces. At the same time, an ocean buoy was placed in the radar coverage to monitor the sea states. The RCS and Doppler spectrum are analyzed and compared comprehensively between radar measurements and numerical predictions. After compensating the influence of wind direction, the responses of radar echo power and numerical predicted RCS to wind speed are basically consistent and are more sensitive in low sea states. The correlation of Doppler spectra between radar measurements and numerical simulations exceed 0.96 during the whole experiment. The intensity of the high-order peaks increases rapidly with the wind speed, while the intensity of the Bragg peak decreases slightly. Doppler spectrum will be shifted by an amount proportional to radial surface current.

A Review of Wave Spectrum Models as Applied to the Problem of Radar Probing of the Sea Surface

AbstractThe most common models of the wind wave spectrum are reviewed, and the compliance of the studied spectra with several fundamental criteria is estimated. These criteria are the ability to simulate diverse wave climate and agreement between model‐based calculations of the mean square slope and experimental data. The spreading function of the spectrum should also correspond to the experimentally measured Doppler spectrum, while the dependence of the radar backscatter cross section should conform to geophysical model functions for various wavelength ranges of incident electromagnetic radiation at moderate incidence angles (20–60°). An analysis has shown that none of the considered spectrum models fully satisfies all the criteria; thus, a new spectrum model for wind waves was developed. Boundary wave numbers for various wavelength ranges of incident electromagnetic radiation within the framework of a two‐scale surface model were determined for the new model. The spectrum model can be used to simulate ripple attenuation in oil slicks and to calculate the radar backscatter cross section inside slick.

Effects of Wind Wave Spectra on Radar Backscatter From Sea Surface at Different Microwave Bands: A Numerical Study

Wind wave spectrum describes the quasi-periodic nature of the ocean surface oscillations and plays an indispensable role in the study of microwave electromagnetic scattering from sea surface. A reliable spectrum model suitable for radar cross section (RCS) predictions at different radar frequencies is desired. This paper evaluated the performances of five common spectrum models (i.e., Fung spectrum, Durden-Vesecky spectrum, Apel spectrum, Elfouhaily spectrum, and the newest version of Hwang spectrum, H18) on the normalized radar backscattering cross section (NRBCS) simulations based on advanced integral equation model (AIEM) at L-, C-, X-, and Ku-bands versus incidence angle, wind direction, and wind speed by comparing with the model and measured data for validation. These results indicate no single wave spectrum of them is satisfying for all the four radar frequencies, e.g., Apel and H18 spectra are better for L- and C-bands, Apel spectrum for X-band, and Elfouhaily and H18 spectra for Ku-band. Given this, three average composite spectrum models are constructed using different spectral models (i.e., all five spectra, Apel + Elfouhaily + H18, and Apel + H18) to simulate NRBCSs, similar to that of the individual spectrum model. It is concluded that the combination of Apel and H18 spectra overall performs best among the individual one and other composited spectra in like-polarized NRBCSs versus incidence angles, wind directions, and wind speeds, for wind speed greater than 30 m/s where the combination of the five spectra work well at Ku-band.

Studies on the Effects of the Plasma Wake Flow Fields of Hypersonic Reentry Blunt Cone on Electromagnetic Wave

The propagation and scattering properties of electromagnetic (EM) waves interacting with the wake flow fields of hypersonic reentry blunt cone are studied by applying the Runge-Kutta exponential time differencing-finite-difference time-domain (FDTD) algorithm in this paper. The parameters of wake flow fields are obtained from the Fluent software. The numerical results show that the reentry plasma has a significant effect on EM wave propagation and scattering of the conductive blunt cone covered with plasma. The attenuation becomes more intense with an increase in the propagation distance due to absorbing and collision, and the reflection coefficients increase with the increase in the Mach number while the transmission coefficients decrease. On the other hand, the backscattering radar cross section (RCS) fluctuates with the varying Mach numbers and the EM wave frequencies at the given wake flow fields. The RCS always decreases with the increase in the plasma thickness of the wake flow fields. In addition, the weak magnetic field has little effect on the backscattering RCS, and the strong magnetic field will lead to the reduction of the backscattering RCS.

Analyses of Electromagnetic Properties of a Hypersonic Object With Plasma Sheath

In this paper, we present a rigorous numerical approach for the electromagnetic (EM) analyses of a hypersonic object with plasma sheath. First, the effective EM parameters of the plasma flow are constructed by solving the Navier–Stokes fluid dynamics equation combined with proper thermochemical models. Then, the EM characteristics of the hypersonic complex are examined by the method of moments based on the volume–surface integral equations. Both the fluid field properties and the associated EM effects are analyzed for a real sphere–cone object that flies at a hypervelocity of 7650 m/s and at an altitude of 61 or 71 km. The impacts of the plasma sheath on the propagation, scattering, and radiation properties are discussed. The plasma sheath may block the transmission and radiation of radio waves for a wide range of frequencies, as the attenuation may exceed 20 dB. Also, the plasma sheath may influence the backscattering radar cross section to an extent of 20 dB. As the frequency increases, the effects tend to weaken and eventually may be ignored beyond 50 GHz. The simulation results are found in good agreement with available experimental data, which indicates that the proposed approach is reliable for predicting the level of radio blackout.

Analysis of Ku- and Ka-Band Sea Surface Backscattering Characteristics at Low-Incidence Angles Based on the GPM Dual-Frequency Precipitation Radar Measurements

The co-located normalized radar backscatter cross section measurements from the Global Precipitation Measurement (GPM) Ku/Ka-band dual-frequency precipitation radar (DPR) and sea surface wind; wave and temperature observations from the National Data Buoy Center (NDBC) moored buoys are used to analyze the dependence and sensitivity of Ku- and Ka-band backscatter on surface conditions at low-incidence angles. Then the potential for inverting wind and wave parameters directly from low-incidence σ0 measurements is discussed. The results show that the KaPR σ0 is more sensitive to surface conditions than the KuPR σ0 overall. Nevertheless; both the KuPR σ0 and KaPR σ0 are strongly correlated with wind speed (U10) and average wave steepness (δa) with the exception of specific transitional incidence angles. Moreover, U10 and δa could be retrieved from pointwise σ0 near nadir and near 18°. Near 18°; wind direction information is needed as the effect of wind direction on σ0 becomes increasingly significant with incidence angle. To improve the performance of U10 retrieval; especially for low U10; auxiliary δa information would be most helpful; and sea surface temperature is better taken into account. Other wave parameters; such as significant wave height; wave period and wave age; are partly correlated with σ0. It is generally more difficult to retrieve those parameters directly from pointwise σ0. For the retrieval of those wave parameters; various auxiliary information is needed. Wind direction and wave direction cannot be retrieved from pointwise σ0.

Low-Frequency Sea Surface Radar Doppler Echo

The sea surface normalized radar backscatter cross-section (NRCS) and Doppler velocity (DV) exhibit energy at low frequencies (LF) below the surface wave peak. These NRCS and DV variations are coherent and thus may produce a bias in the DV averaged over large footprints, which is important for interpretation of Doppler scatterometer measurements. To understand the origin of LF variations, the platform-borne Ka-band radar measurements with well-pronounced LF variations at frequencies below wave peak (0.19 Hz) are analyzed. These data show that the LF NRCS is coherent with wind speed at 21 m height while the LF DV is not. The NRCS-wind correlation is significant only at frequencies below 0.01 Hz indicating either differences between near-surface wind (affecting radar signal) and 21-m height wind (actually measured) or contributions of other mechanisms of LF radar signal variations. It is shown that non-linearity in NRCS-wave slope Modulation Transfer Function (MTF) and inherent averaging within radar footprint account for NRCS and DV LF variance, with the exception of VV NRCS for which almost half of the LF variance is unexplainable by these mechanisms and perhaps attributable to wind fluctuations. Although the distribution of radar DV is quasi-Gaussian, suggesting virtually little impact of non-linearity, the LF DV variations arise due to footprint averaging of correlated local DV and non-linear NRCS. Numerical simulations demonstrate that MTF non-linearity weakly affects traditional linear MTF estimate (less than 10% for typical MTF magnitudes less than 20). Thus the linear MTF is a good approximation to evaluate the DV averaged over large footprints typical of satellite observations.

Study of sea-surface slope distribution and its effect on radar backscatter based on Global Precipitation Measurement Ku-band precipitation radar measurements

The collocated normalized radar backscattering cross-section measurements from the Global Precipitation Measurement (GPM) Ku-band precipitation radar (KuPR) and the winds from the moored buoys are used to study the effect of different sea-surface slope probability density functions (PDFs), including the Gaussian PDF, the Gram–Charlier PDF, and the Liu PDF, on the geometrical optics (GO) model predictions of the radar backscatter at low incidence angles (0 deg to 18 deg) at different sea states. First, the peakedness coefficient in the Liu distribution is determined using the collocations at the normal incidence angle, and the results indicate that the peakedness coefficient is a nonlinear function of the wind speed. Then, the performance of the modified Liu distribution, i.e., Liu distribution using the obtained peakedness coefficient estimate; the Gaussian distribution; and the Gram–Charlier distribution is analyzed. The results show that the GO model predictions with the modified Liu distribution agree best with the KuPR measurements, followed by the predictions with the Gaussian distribution, while the predictions with the Gram–Charlier distribution have larger differences as the total or the slick filtered, not the radar filtered, probability density is included in the distribution. The best-performing distribution changes with incidence angle and changes with wind speed.

The First Results of Monitoring the Formation and Destruction of the Ice Cover in Winter 2014–2015 on Ilmen Lake according to the Measurements of Dual-Frequency Precipitation Radar

The launch of the Dual-frequency Precipitation Radar (DPR) opens up new opportunities for studying and monitoring the land and inland waters. It is the first time radar with a swath (±65°) covering regions with cold climate where waters are covered with ice and land with snow for prolonged periods of time has been used. It is also the first time that the remote sensing is carried out at small incidence angles (less than 19°) at two frequencies (13.6 and 35.5 GHz). The high spatial resolution (4–5 km) significantly increases the number of objects that can be studied using the new radar. Ilmen Lake is chosen as the first test object for the development of complex programs for processing and analyzing data obtained by the DPR. The problem of diagnostics of ice-cover formation and destruction according to DPR data has been considered. It is shown that the dependence of the radar backscatter cross section on the incidence angle for autumn ice is different from that of spring ice, and can be used for classification. A comparison with scattering on the water surface has shown that, at incidence angles exceeding 10°, it is possible to discern all three types of reflecting surfaces: open water, autumn ice, and spring ice, under the condition of making repeated measurements to avoid possible ambiguity caused by wind.

Using snowflake surface-area-to-volume ratio to model and interpret snowfall triple-frequency radar signatures

Abstract. The snowflake microstructure determines the microwave scattering properties of individual snowflakes and has a strong impact on snowfall radar signatures. In this study, individual snowflakes are represented by collections of randomly distributed ice spheres where the size and number of the constituent ice spheres are specified by the snowflake mass and surface-area-to-volume ratio (SAV) and the bounding volume of each ice sphere collection is given by the snowflake maximum dimension. Radar backscatter cross sections for the ice sphere collections are calculated at X-, Ku-, Ka-, and W-band frequencies and then used to model triple-frequency radar signatures for exponential snowflake size distributions (SSDs). Additionally, snowflake complexity values obtained from high-resolution multi-view snowflake images are used as an indicator of snowflake SAV to derive snowfall triple-frequency radar signatures. The modeled snowfall triple-frequency radar signatures cover a wide range of triple-frequency signatures that were previously determined from radar reflectivity measurements and illustrate characteristic differences related to snow type, quantified through snowflake SAV, and snowflake size. The results show high sensitivity to snowflake SAV and SSD maximum size but are generally less affected by uncertainties in the parameterization of snowflake mass, indicating the importance of snowflake SAV for the interpretation of snowfall triple-frequency radar signatures.

Impacts of oil spills on altimeter waveforms and radar backscatter cross section

AbstractOcean surface films can damp short capillary‐gravity waves, reduce the surface mean square slope, and induce “sigma0 blooms” in satellite altimeter data. No study has ascertained the effect of such film on altimeter measurements due to lack of film data. The availability of Environmental Response Management Application (ERMA) oil cover, daily oil spill extent, and thickness data acquired during the Deepwater Horizon (DWH) oil spill accident provides a unique opportunity to evaluate the impact of surface film on altimeter data. In this study, the Jason‐1/2 passes nearest to the DWH platform are analyzed to understand the waveform distortion caused by the spill as well as the variation of σ0 as a function of oil thickness, wind speed, and radar band. Jason‐1/2 Ku‐band σ0 increased by 10 dB at low wind speed (<3 m s−1) in the oil‐covered area. The mean σ0 in Ku and C bands increased by 1.0–3.5 dB for thick oil and 0.9–2.9 dB for thin oil while the waveforms are strongly distorted. As the wind increases up to 6 m s−1, the mean σ0 bloom and waveform distortion in both Ku and C bands weakened for both thick and thin oil. When wind exceeds 6 m s−1, only does the σ0 in Ku band slightly increase by 0.2–0.5 dB for thick oil. The study shows that high‐resolution altimeter data can certainly help better evaluate the thickness of oil spill, particularly at low wind speeds.

Modelling of Electromagnetic Scattering by a Hypersonic Cone-Like Body in Near Space

A numerical procedure for analysis of electromagnetic scattering by a hypersonic cone-like body flying in the near space is presented. First, the fluid dynamics equation is numerically solved to obtain the electron density, colliding frequency, and the air temperature around the body. They are used to calculate the complex relative dielectric constants of the plasma sheath. Then the volume-surface integral equation method is adopted to analyze the scattering properties of the body plus the plasma sheath. The Backscattering Radar Cross-Sections (BRCS) for the body flying at different speeds, attack angles, and elevations are examined. Numerical results show that the BRCS at a frequency higher than 300 MHz is only slightly affected if the speed is smaller than 7 Mach. The BRCS at 1 GHz would be significantly reduced if the speed is greater than 7 Mach and is continuously increased, which can be attributed to the absorption by the lossy plasma sheath. Typically, the BRCS is influenced by 5~10 dBm for a change of attack angle within 0~15 degrees, or for a change of elevation within 30~70 km above the ground.

Self-Adaptive TSM-RT for the Fast Analysis of EM Scattering From 3-D Large-Scale Sea Surface

A self-adaptive two-scale model-ray tracing (TSM-RT) method is proposed to further improve the computational efficiency of TSM-RT method in this letter. For the previous TSM-RT method, the mesh size of large triangles does not change with the incident angle. In the contrast, the mesh size of the large triangles is related to the incident angle as well as the roughness of sea surface in the self-adaptive TSM-RT method, where the number of large triangles is further decreased. Correspondingly, the number of initial rays and intersections between the ray and large triangles is also decreased. Therefore, the computational efficiency is improved. In addition, the backscattering radar cross section of the three-dimensional sea surface is calculated. It demonstrated that the simulation results of self-adaptive TSM-RT method show a great agreement with those of the previous TSM-RT method, and a good speedup ratio is achieved.

Broadband RCS Reduction Based on Spiral-Coded Metasurface

In this letter, an efficient approach for broadband radar cross-section (RCS) reduction is proposed. By coding eight types of linear-phase gradients in a spiral pattern, the reflected wave was uniformly diffused, and thus, the backscatter RCS reduction was achieved within an ultrabroadband. For verification, a spiral-coded metasurface composed of $8\times 8$ subarrays is designed, fabricated, and measured. Numerical and experimental results both show that the thin metasurface enables a 10 dB RCS reduction over an ultrabroadband ranging from 12.2 to 23.4 GHz. Moreover, the good merit of our approach is further demonstrated by comparing the scattering behavior to the steering beam of a regular sequential layout pattern. Our approach features broad band, polarization insensitivity, wide incident angles, and easy design without any optimization, promising great potential in real-world low-scattering stealth applications.

Study of scattering from turbulence structure generated by propeller with FLUENT

In this article, the turbulence structure generated by a propeller is simulated with the computational fluid dynamics (CFD) software FLUENT. With the method of moments, the backscattering radar cross sections (RCS) of the turbulence structure are calculated. The scattering results can reflect the turbulent intensity of the wave profiles. For the wake turbulence with low rotating speed, the scattering intensity of HH polarization is much smaller than VV polarization at large incident angles. When the turbulence becomes stronger with high rotating speed, the scattering intensity of HH polarization also becomes stronger at large incident angles, which is almost the same with VV polarization. And also, the bistatic scattering of the turbulence structure has the similar situation. These scattering results indicate that the turbulence structure can also give rise to an anomaly compared with traditional sea surface. The study of electromagnetic (EM) scattering from turbulence structure generated by the propeller can help in better understanding of the scattering from different kinds of waves and provide more bases to explain the anomalies of EM scattering from sea surfaces.

How thick are Mercury’s polar water ice deposits?

An estimate is made of the thickness of the radar-bright deposits in craters near to Mercury’s north pole. To construct an objective set of craters for this measurement, an automated crater finding algorithm is developed and applied to a digital elevation model based on data from the Mercury Laser Altimeter onboard the MESSENGER spacecraft. This produces a catalogue of 663 craters with diameters exceeding 4 km, northwards of latitude +55∘. A subset of 12 larger, well-sampled and fresh polar craters are selected to search for correlations between topography and radar same-sense backscatter cross-section. It is found that the typical excess height associated with the radar-bright regions within these fresh polar craters is (50 ± 35) m. This puts an approximate upper limit on the total polar water ice deposits on Mercury of ∼ 3 × 1015 kg.

Broadband RCS reduction and gain enhancement microstrip antenna using shared aperture artificial composite material based on quasi‐fractal tree

A shared aperture artificial composite material (SA-ACM) with broadband absorption and partial reflection feature is proposed. The SA-ACM is composed of artificial composite material absorber (ACMA) and partial reflection surface (PRS), where the ACMA and PRS share the same aperture in the vertical dimensionality. The ACMA consists of quasi-three-dimensional fractal tree loaded with lumped resistances and metallic pattern on the bottom side of a dielectric slab, and the PRS consists of etched parallel slots in a metallic plane. The SA-ACM as the role of superstrate is applied to the mircrostrip antenna. The Fabry–Perot resonator cavity constructed by the PRS and the metallic ground of the mircrostrip antenna can achieve high gain, while the ACMA can obtain the low radar cross section (RCS) characteristic by absorbing the incident wave. Simulation and experiment results demonstrate that the gain of the antenna with SA-ACM is enhanced over 3 dB in the operation frequency band, and the backscattering RCS is obviously reduced by 10 dB in a wide frequency range.

Ultra-Broadband Backscatter Radar Cross Section Reduction Based on Polarization-Insensitive Metasurface

A novel polarization-insensitive metasurface with angular stability for ultra-broadband (from 7 to 12 GHz) backscatter radar cross section (RCS) reduction is investigated. The proposed metasurface is composed of carefully arranged unit cells with spatially varied dimension, which can diffuse reflection uniformly and avoid the reflection in the specular direction. A proposed metasurface sample is fabricated and tested to validate RCS reduction behavior predicted by full-wave simulation software ANSYS HFSS. A more than 10 dB RCS reduction within the entire X-band is observed, indicating our metasurface may be potentially applied to future stealth technology.

Effects of impedance wedge diffraction on backscattering from breaking waves

Electromagnetic scattering characteristics change significantly from breaking waves, which is considered to be one reason for sea spike phenomenon（HH polarization scattering intensity close to or even greater than VV polarization scattering intensity). Spiky sea clutter is often treated falsely as targets, which affects radar performance in target detection in the sea surface background. Thus the investigation on the physical mechanism of the sea spike phenomenon can help mitigate false alarms. In this paper, the authors investigate the microwave backscattering from the wedge-shaped breaking waves, which is simulated with the dihedral impedance wedge of finite length. The physical optical field of the breaking waves is calculated with the Kirchhoff approximation. Based on the Maliuzhinets method with using the precise impedance boundary condition, the impedance wedge scattering solution in spectral integral representation is presented. The spectral function is derived by the perturbation method with respect to the oblique incident angle based on the incidence normal to or grazing to the edge. After obtaining the spectral function, the asymptotic theory is used to determine the diffraction field of impedance wedge at an arbitrary skew incidence. The equivalent edge currents are derived from the uniform diffraction of impedance wedge by combining the physical optical coefficients and diffracted coefficients. Backscattering radar cross-sections（RCSs) of the diffracted field from 120 impedance wedge are calculated in both HH and VV polarizations, and the effects of frequency and permittivity on the wedge diffraction are discussed as well. The physical optical field backscattering from 135 impedance wedge is compared with the total field with considering the diffraction effects. Further calculations and analyses for backscattering from the three-dimensional extension breaking waves are presented by using the contribution of edge diffraction field to correct the physical optics field. Numerical results show that the backscattering RCS of impedance diffracted field in HH polarization is greater than that in VV polarization in the Keller cone. Therefore, the diffraction effects will make the backscattering RCS of the total field in HH polarization greater than that in VV polarization when the breaking wave grows to near-collapse stage at a small grazing angle with upwind observation. This indicates that the wedge diffraction is one of the causes of sea spike phenomenon.

Target Above Random Rough Surface Scattering Using a Parallelized IPO Accelerated by MLFMM

The radar cross section (RCS) of targets above random rough surfaces having impedance boundaries is evaluated using a developed parallelized iterative physical optics. Integral calculations are accelerated using the multilevel fast multipole method (MLFMM). For example, the RCS of a boat above a Gaussian random rough surface occupying a $(100/3\lambda)^3$ volume, with $\lambda$ being the wavelength at the operation frequency of 1 GHz for one incidence angle, is computed within about 42 s. The result is verified with method of moment-MLFMM which takes about 9.4 min. The backscatter RCS of a $(9\lambda) ^3$ dihedral above a random rough surface for different root-mean-square (rms) heights, correlation lengths, and incidence angles is examined. It is shown that a rough surface with an rms height of $0.2\lambda$ and a correlation length of $\lambda$ can decrease/increase the RCS of a vertically placed impedance dihedral corner reflector by about 5 dB at an elevation angle of 45°.