The technique of bistatic RCS estimate is proposed for sea and terrain clutter with application of monostatic RCS measured data for small grazing angles that can be applied in centimeter and millimeter bands of radio waves. The models of bistatic scattering are analyzed including the model of small perturbations, facet model and theorem of equivalence; the limitations of these models are discussed. The features of bistatic scattering near the specular points (the forward scattering) are analyzed and estimates are carried out the RCS values for different terrain types.
In directions differing from quasi-specular one, the normalized bistatic RCS dependences on grazing angles and bistatic angle are obtained for different types of terrain and for sea at frequencies of 10.0 and 35.0 GHz. These data are founded at results of RCS measurements carried out by author for monostatic regime. The comparison of calculated results with experimental data available for bistatic radar was carried out. It is shown the good coincidence of these results. Thus, the proposed technique permits to evaluate the bistatic RCS using the data of monostatic measurements for small grazing angles at wide band of frequencies up to 95 GHz.
Land spatial statistical characteristics obtained experimentally and from the topographical maps are analyzed. It is shown that the probability density function of backscattered signal is determined by the temporal fluctuations and the spatial fluctuations because of scattered areas replacement. The experimental spatial statistical characteristics of micro-scale roughness (rms height, the slope distributions, and autocorrelation functions) are presented. It was shown that for mathematical description of surface for different fields it is better to use the fractal process. The spatial statistics of land backscattering for surfaces without vegetation is determined by the distributions of surface local slopes. The roughness height and slope distributions for different terrain types are presented that are obtained from electronic topographical maps. The method of clutter map development is analyzed, it is shown its great dependence on radar and target heights.
The technique of radar clutter map elaboration for land backscattering at the frequency band of 3 - 100 GHz for small grazing angles is proposed taken into consideration the terrain relief and vegetation. The sources and methods for receiving of the initial data for modeling are analyzed and described. A brief description of operation principle and area of application of specially designed neural structure (consolidation tree) in the model is presented.
The main computation relations for determination o MMW radar land clutter statistical characteristics are analyzed. Expressions for normalized RCS determination of different surface types and polarization features of backscattering signals are discussed. Spatial and temporal statistical characteristics of the quadrature components and the amplitudes of scattered signals are analyzed; the influence of spatial characteristics of real land terrain on the quadrature component and amplitude distributions is discussed. It is shown that the amplitude pdf is approximated by the Weibull's law and the distribution of quadrature components is described by the compound Gaussian law. The spatial distributions for different terrain types are presented. As result, the algorithms for radar clutter modeling at millimeter band of radiowaves are obtained taking into consideration the spatial statistics of natural land surface.
A new algorithm for multichannel radar image mutual superimposing (registration) and geometric correction is proposed. It uses ground control points (GCPs) selected manually or automatically for warp and base images and the branch-and-bound technique for initial estimation of geometric transform parameters. We show that due to taking into account the GCP selection errors for both warp and base images, the proposed modification of branch-and-bound algorithm allows to additionally improve the accuracy of warp parameter estimation. To reduce the computational complexity of branch-and-bound algorithm an approach based on optimal selection of center position for warp image coordinate system is proposed. It is shown that the coordinate system shifting towards the center of warp image GCP bounding box permits to accelerate the search process. For additional speeding up the branch-and-bound algorithm, bounded alignment approach is applied. For establishing the correspondence between GCPs and for detection of erroneous points, a novel method of statistical analysis of GCP superimposing errors is put forward. The designed algorithm is tested for both simulated and real data demonstrates high efficiency and robustness.
The land and rain clutter rejection for radar systems of millimeter band of radiowaves is estimated for some types of the radiated signals (the periodic uncoded pulsed sequence, the pulsed-compression signals, the unmodulated continuous signals and the continuous signals with harmonic frequency modulation). It is shown that the use of periodical uncoded pulsed sequence with unambiguous target range determination is preferable. The use of the complex signals with inpulsed modulation permits to increase the clutter rejection if the signal duration after pulsed compression is less than the uncoded pulse duration. Otherwise the clutter accumulation from the large "lighting" area or volume leads to decreasing of the SCR and increases the clutter power from the sidelobes of the ambiguity function. For the continuous harmonical signal the clutter power from the near zone of radar increases and FM of this signal does not permit to provide the clutter rejection to levels obtained for the pulsed sequence.
KEYWORDS: Polarization, Radar, Backscatter, Statistical analysis, Scattering, Foam, Doppler effect, Signal processing, Light sources and illumination, X band
The paper consists of two parts. In the first part the main characteristics of heavy sea are discussed that are essential for radar observation at grazing angles, the sea surface spike characteristics are analyzed in the framework of the statistical theory of random process surges above some boundary, the sea surface shadowing function is considered. In the second part the radar spike characteristics of sea backscattering at frequencies of 10.0, 35.0, 75.0 and 140.0 GHz are analyzed including the polarization and polarization-spectral characteristics of rough sea backscattered signals. During the spikes the depolarization coefficients, spectrum central frequency and spectrum width increasing is marked and the second maximum appearance in the scattered signal spectra is observed. The probable mechanisms are analyzed, the one of these is the backscattering from spray-drop fraction formed by breaking of sea wave.
Transformations of the statistical characteristics of the scattered signals from the low-altitude targets are analyzed, caused by the multipath propagation over sea surface and multiple reflections at path `target-surface- target'. It is shown that the target movement over the diffuse scattering surface leads to change of radar cross- section probability density function in comparison with Swerling's models. The differences of the experimental distributions from models increased with radar wavelength shortening.
The characteristics of snow backscattering in microwave band are considered. It is shown that they depend upon many parameters describing the state of snow cover. The basic dependences obtained from theory of radar backscattering and experimental data are discussed and compared. Real radar remote sensing data for different dates in fall/winter period are presented for Eastern regions of Ukraine. It is demonstrated that the effects predicted theoretically are observed for remote sensing images obtained from side-look radar installed on the satellite “Ocean”. It is also shown that remote sensing means in some conditions permit to get the maps of snow cover thickness but in this case the reference data from several on-land meteorological stations are needed for calibration purposes.
The paper consists of two parts. In the first part the main characteristics of heavy sea are discussed that are essential for radar observation at grazing angles, the sea surface spike characteristics are analyzed in the framework of the statistical theory of random process surges above some boundary, the sea surface shadowing function is considered. In the second part the radar spike characteristics of sea backscattering at frequencies of 10.0, 35.0, 75.0 and 140.0 GHz are analyzed.
Methodology and stages of data processing in multichannel airborne radar imaging systems are considered. It is shown that data fusion in such systems requires special techniques, algorithms, and software for image processing and information retrieval. Some approaches and methods are proposed. The results are demonstrated for simulated and real images.
A novel vector filter called modified vector sigma filter is proposed for processing the multichannel remote sensing radar images. It is demonstrated through simulations and real data examples that the proposed filter is able to provide an excellent combination of properties. It possesses efficient suppression of multiplicative noise and good edge preservation. Moreover, it simultaneously ensures an ability to remove spikes from images and excellent preservation of fine details even if they are characterized by rather low contrasts. These features occur to be useful for further interpretation of multichannel radar images, e.g. for determination of bare soil characteristics like erosion state. For simulated images it is shown that the application of the modified vector sigma filter is preferable in respect with its componentwise counterpart as the former technique provides less misclassification errors.
KEYWORDS: Radar, Signal attenuation, Antennas, Target detection, Extremely high frequency, X band, Wave propagation, Radio propagation, Vegetation, Medium wave
The joint influence of multipath propagation and rain attenuation effects and also the land and rain clutter on the millimeter wave radar operation, in particular, on the maximum detection range and target detectability is considered. The parameters of millimeter wave radar are compared with parameters of analogous radar of X-band and the comparison is carried out for two cases: with frequency change an antenna aperture is constant (1) and with frequency change an antenna gain is constant, i.e. the proportional change of antenna aperture takes the place (2). The results of this analysis are presented for different types of terrain (quasi-smooth, rough, rough with vegetation, etc.) and rains with intensity from 2 to 10 mm/h. It is shown that for constant antenna aperture and joint effects of multipath propagation and rain attenuation the all-weather millimeter wave radar is more effective than analogous X-band radar at ranges less than 2 - 4 km.
The multistage procedure of multichannel image pre- and post- processing is proposed. It includes nonlinear image-to-image and image-to-topology map superimposing with optimization of transform parameters, image separate and vector filtering for remained error reduction and image enhancement. Then the image preliminary recognition is to be performed for determined homogeneous regions, two possible ways of data interpretation based on the use of radiophysical models or supervised learning neural networks are discussed. It is shown that the proposed procedure provides a satisfactory determination of field lots with different erosion and other useful results.
The results of experiments for soil erosion determination with the use of dual-polarization radar set at wavelengths of 3 and 0.8 cm are presented. It is shown that the specific radar cross-section (RCS) is a function of soil erosion at the range of incidence angles from 35 degrees to 60 degrees. The sensitivity of soil backscattering to erosion is more high when the ratio of RCS for HH and VV polarizations is used. The best description of surface roughness spatial spectra is fractal one. The results of image processing obtained by airborne radar remote sensing system of Ku-band with dual- polarization reception are discussed and the comparison of results of soil erosion determination by radar remote sensing techniques with in situ measurements is made.
The possibilities of radar observation of effects occurring during aerodynamic object flights with sonic and supersonic speeds are considered. Three mechanisms resulting in electromagnetic wave backscattering are analyzed: reflections from shock wave taking place for atmosphere object flight with supersonic speed, reflections from sonic field leading to atmosphere parameter modulation, and scattering from atmosphere turbulence and parameters of which are varied by sonic field influence. The results of experimental study of sonic interaction with atmosphere turbulence are presented. It is shown that the most perspective mechanisms for radar observation are the first and the third ones, the directions of further investigations are put forward.
Theoretical background of the multichannel (multifrequency and multipolarization) approach to radar remote sensing soil moisture and erosion state estimation is presented. Possible variants of radar data acquisition and their processing are discussed, they are based on preliminary experimental investigation results. The necessary operations of image-to-image and image-to-topology map superimposing as well as radar data preliminary filtering are considered. The results of remote sensing soil characteristic estimation using a multichannel airborne radar system are compared with in situ measurements, rather good accuracy of remote sensing estimates is proved.
The techniques of soil moisture and erosion state estimation using multichannel airborne radar remote sensing system are considered. The experimental data proving the efficiency of the proposed approach and showing the dependence of scattered signal intensity on soil characteristics are presented. The algorithms of radar image preprocessing are discussed.
The theoretical and experimental research results of radar targets statistical characteristics at X- ad Ka-bands are presented. Data on radar cross-sections (RCS) for different targets including air, land, and marine targets are presented and the comparison of experimental results with theoretical models is done. The probability distributions of the signal instantaneous values, amplitudes, and RCS for different types of targets are obtained and the comparison of experimental distributions with standard Swerling's models is performed. The experimental results of power spectra investigations for different targets are discussed. For small marine targets the correlation between target radar spectra and sea surface spectra is determined.
We present the theoretical and experimental study results of sea and land clutter and explosion scattering obtained in the millimeter wave band and needed for detection and identification analysis of radars operating in frequency band from 10 to 75 GHz. The empirical models for calculation of specific radar cross- section as a function of grazing angle, frequency and wind velocity are elaborated. The models for power spectra calculation of sea and land clutter are obtained. The radar reflection models from explosion are analyzed and it is shown that reflections for a long time are caused by the scattering by turbulent gas products of explosion.
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