RADAR SIGNATURE ASSESSMENT OF REFLECTOR ANTENNA WITH A KNIFE-SHAPED SCREEN

Dihedral corner reflector is an important class of object in radar cross section (RCS) studies. A right angled corner reflector provides a large back scattering over a wide angular range in a plane normal to its wedge. Since complex targets consist of corners, RCS reduction of corner reflector is an important research area. This paper presents the backscattering reduction of a 90 degree dihedral corner reflector by loading metallo-dielectric structures based on Sierpinski carpet arrays. Measurements at X-band using arch method show a reduction of back scattering of around 19 dB at 8.5 GHz. Here, the internal reflecting surfaces of dihedral corner are loaded with Sierpinski carpet array of second iterated fractals. The variation of back scattered power with frequency as well as angle of incidence is measured and presented. Effect of the thickness of the metallo-dielectric structures is also studied and the results are presented. This technique may find application in RCS reduction of complex targets.

This paper reports on the comparison of the radar cross-section (RCS) of a 4-element retrodirective array (RDA) calculated from its measured gain with the predicted RCS of a flat plate, and a 90/spl deg/ dihedral corner reflector. The comparison showed that the boresight RCS of the 4-element RDA is 2.33 dB higher than the boresight RCS of the 90/spl deg/ dihedral passive corner reflector, but 3.66 dB lower than the flat plate at broadside Incidence of the same sizes. In this paper, the different scattering field effects that cause the RCS of the 4-element RDA to be lower that the RCS of the flat plate at angles less than /spl plusmn/10/spl deg/ was explained. In addition, the paper shows for the first time that retrodirective arrays can be made to have better or worse RCS performance than passive reflectors depending on the gain of the antenna elements and the performance of the conjugate circuit.

Trihedral corner reflectors are widely used as calibration targets for imaging radars because of their large radar cross section (RCS) and extremely wide RCS pattern. An important source of uncertainty in the RCS of a trihedral sitting on a ground plane is the coherent interaction of the ground plane with the trihedral. At UHF and low microwave frequencies the large physical size of corner reflectors become a limiting factor in regard to difficulties in field deployment and deviation of their RCS from the expected values. In this paper, a general class of corner reflectors with high-aperture efficiency referred to as self-illuminating corner reflectors, is introduced whose coherent interaction with the surrounding terrain is minimized and their total surface area is two-thirds of that of a triangular corner reflector having the same maximum RCS. Analytical expressions based on geometrical optics and a new numerical solution based on near-field physical optics for the RCS of two simple self-illuminating corner reflectors are presented and compared with backscatter measurements. Also the panel geometry for an optimum corner reflector which has the shortest edge length among polygonal self-illuminating corner reflectors is obtained. High-aperture efficiency is achieved at the expense of azimuth and elevation beamwidth. It is shown that the 1-dB RCS beamwidths of the optimal corner reflectors, both in azimuth and elevation directions, are about 16/spl deg/. RCS measurements of corner reflectors in the presence of a ground plane show that the RCS of self-illuminating corner reflectors are less affected by the coherent ground interaction.

Passive point targets such as trihedral and dihedral corner reflectors are commonly used for synthetic aperture radar calibration because of their large radar cross section and wide radar cross section pattern. These types of passive point targets are not expensive to manufacture comparing to active point targets such as transponder and they are easy to deploy in the field without using power to work. Perforating the corner reflector is an important factor in building the corner reflectors to allow quick drainage from heavy rain, cleaning from dust and reduce the effect of wind as well as reducing the weight of the corner reflectors. In this paper, we analysis the effect of perforating the corner reflector on maximum radar cross section using square trihedral corner reflector with four different hole sizes and three varying hole centre spacings. The results showed that a hole size of one-tenth of the wavelength reduced the maximum radar cross section by less than 1 dB from the theoretical radar cross section and the spacing between the two holes did not have much effect while one-sixth of the wavelength reduced the maximum radar cross section by about 1 dB and the reduction between the spacing is very small about 0.10 dB. Finally, a hole size with one-fourth of the wavelength reduced the maximum radar cross section by more than 2 dB and the reduction between the spacing increased while one-third of the wavelength has a reduction of about 4 dB.

In this paper we use a non-stationary approach and analyze ultra-wideband (UWB) radar data using time-frequency and time-scale transformations. The time-frequency transformations considered are the Short-Time Fourier Transform (STFT), the Wigner-Ville Distribution (WD), the Instantaneous Power Spectrum (IPS), and the ZAM transform. Two discrete implementations of the Wavelet Transform (DWT) are also investigated: the decimated A- trous algorithm proposed by Holschneider et al, which uses non-orthogonal wavelets; and the Mallat algorithm, which employs orthogonal wavelets. The transients under study are UWB radar returns from a boat (with and without corner reflector) in the presence of sea clutter, multipath, and radio frequency interferences (RFI). Results show that all time-frequency and time-scale transforms clearly detect the transient radar returns corresponding to the boat with a corner reflector. However, as the radar cross section of the target decreases (boat without a corner reflector), results change drastically as the RFI component dominates the signal. Simulations show that the Instantaneous Power Spectrum may be better adapted for localizing the transient among the time-frequency techniques studied. The decimated A-trous algorithm has the best time resolution of the techniques studied as the return appears better localized in the scalogram.

Radar cross section (RCS) is not necessarily in direct proportion to object size, particular structure shape that can cause multiple reflection usually has larger RCS. In this article we demonstrate how a small object with a large RCS, like the corner reflector, can be shadowed by a large object with a small bistatic RCS for certain direction, like a fighter engine air inlet, to achieve RF stealth effect in certain direction. Multi-level-fast-multipole-method (MLFMM) simulations are used to calculate the RCS of these two structures in S-band, and compare their RCS when the corner reflector is placed inside the air inlet. Results show RCS reduction up to 17 dBs is possible, but the combined RCS is not as small as the air inlet itself because reflection from the corner reflector can still find its way out to the radar.

Corner reflectors used for calibration of radar stations, despite their apparent simplicity, have a number of disadvantages that manifest themselves both in the design and in their manufacture and operation. One of the main disadvantages is that in order to achieve high values of the radar cross-section (RCS), corner reflectors must have large linear dimensions. In addition, to ensure reliable electrical contact, their designs are usually non-separable. All of the above together complicates their operation and transportation, due to the large dimensions and weight of the products. The aim of the article is to develop and investigate by strict electrodynamic methods a corner reflector of a collapsible lightweight design, as well as to evaluate the influence of design features on RCS. The paper presents the results of modeling corner reflectors of a prefabricated structure by strict electrodynamic methods and investigates the influence of design features on its RCS. In particular, the influence of gaps at the top and between the edges of the reflector has been analyzed, the limits of the permissible sizes of these defects have been determined. Based on the results of the study, a lightweight collapsible layout of a corner reflector with square faces has been developed and manufactured. The correspondence of the RCS of the manufactured corner reflector layout to the calculated values has been experimentally confirmed. The performed studies have shown the practical feasibility of lightweight corner reflectors of collapsible design and confirmed the possibility of their use in radar system testing. Information about the impact of defects in the manufacture of corner reflectors on their RCS enables developers to use simpler and cheaper technological and design solutions in the development of corner reflectors.

Radar cross section (RCS) is one of the stealth signature by which an object/target can be identified at longer ranges. In stealth scenario it is always desirable to reduce the RCS. However sometimes it is also required to increase the RCS of certain targets in order to use them as a practice target for testing onboard weaponry, to detect our own targets/sail boats in the open sea. Corner reflectors are structures formed with two or three metal plates. Corner reflectors are designed to provide large mono static cross section over a wide range of frequencies and aspect angles. The corner reflector is used as an echo enhancement device on sail boats and other targets to improve their detect ability. The present paper deals with the study on the behavior of different types of corner reflectors with respect to different azimuth angles and frequencies. The study is also extended to determine the orthogonal error analysis of varies Corner Reflector that is the work emphasis the reduction in RCS due to orthogonality error between plates of reflectors occurred because of fabricated errors, mechanical stress, flatness. This study will be very much useful in assessing the target capability to cater to missile scenario. In order to do this analysis Matlab coding and Matlab GUI are used.

Corner reflectors form the major scattering centers in the radar signatures of ships, aircraft and vehicles. The radar cross section (RCS) reduction of these flare spots (corner reflectors) plays an important role in the design of these targets with reduced detectability in radar. This paper deals with the RCS reduction of square dihedral and trihedral corner reflectors, coated with radar absorbing material (RAM). The measurements were carried out in an anechoic chamber at Centro Tecnico Aeroespacial (CTA)-Ministerio da Defesa facilities, and the RAM coating was manufactured at Divisao de Materiais of Instituto de Aeronautica e Espaao (IAE)/CTA. The processed RAM applied on a flat plate was evaluated by reflection coefficient measurements by using the RCS self-calibrated technique.

A dihedral corner reflector has a wide-angle dispersion characteristic, and it is a primary scattering position in certain orientations of a metal body. Furthermore, the technology is quite complex for its radar cross section (RCS) to be reduced. The paper first investigates the technological feasibility of reducing its RCS with plasma. Then, the RCS variations of a metal corner reflector when its surfaces are covered with plasma are analyzed by software emulation (the experimental equipment is being developed). The results show that the plasma has strong electromagnetic wave absorption characteristics. In addition, the attenuation of RCS declines with increasing incident wave frequency when the plasma. frequency and electron collision frequency remain steady. The results show that application of a plasma has practical engineering value in terms of reducting the RCS.

A statistical approach to radar backscattering from terrain is taken in this paper. The normalized radar cross section, \sigma_{0} , has been computed for two different terrain models. The value of \sigma_{0} , is obtained for both models as a function of grazing angle, \theta , and radiation wavelength, \lambda . The first model is a distribution of isolated independent scatterers such as corner reflectors. For such surfaces a wavelength dependence for \sigma_{0} is obtained, and, depending upon the density of scatterers and their average size, the theoretical results indicate that the local dependence of \sigma_{0} on \lambda can be as \lambda^{-6}, \lambda^{-4} or \lambda^{-2} . For such surfaces, \sigma_{0} is independent of \theta . Where reflection occurs from specularly reflecting facets on the surface and where the distribution of surface slopes is Gaussian, the \theta dependence turns out to be of the form e^{-k cot^{2}\theta/2s_{0}^{2}} where s_{0} is the standard deviation of the surface-slope distribution. The precise form of \sigma_{0} depends upon the space spectrum of the slopes. Two cases are worked out, one where such a spectrum is flat out to some cutoff, and the other where the space spectrum has a single peak at a particular wave number. In either case, for small enough \lambda,\sigma_{0} varies as \lambda^{-2} . As the wavelength becomes large compared to the facet size, the facet no longer behaves as a specular reflector and instead becomes more like an isotropic scatterer. For any particular wavelength one may expect that the radar return be the result of the addition of two types of backscattering. The large facets will behave as specular-type reflectors, while the smaller facets will act as the isotropic scatterers discussed in the first model.

The investigation of how to control the microwave scattering from strong scattering sources has vital applications in military and civilian areas. As a typical target, a rightangle dihedral corner reflector has been extensively investigated because of its strong electromagnetic scattering over a wide observation angle range. In this paper, a right-angle dihedral corner reflector with three loading conditions of the camouflage grass described in our previous investigation is discussed to investigate the radar cross section (RCS) reduction effect for the typical target. For different RCS reduction effects of the camouflage grass loading conditions, which include paste, cover and shield, the calculated results and the measurement results are presented and compared. The loading of camouflage grass greatly reduces the RCS of the corner reflector over a wide angle range at 10 GHz and a broad band from 4 GHz to 18 GHz. The paste loading condition of camouflage grass achieves the best RCS reduction effect. Then, the difference in the monostatic RCS curves is analyzed and explained by multiple scattering effects. The advantages of employing camouflage grass lie in its ability to clearly reduce the RCS of the dihedral corner reflector without shaping the targets or use absorbing materials.

RCS (Radar Cross Section) reduction by proper shaping is applied to the problem of reducing the RCS levels of a center fed parabolic reflector antenna system such that antenna gain, sidelobe level, and physical size of the reflector are preserved. A detailed numerical - physical investigation of the electromagnetic scattering phenomena associated with parabolic reflector antennas is presented. The electromagnetic scattering phenomenon are identified and categorized according to their contributions to the overall RCS level of the antenna system. The dominant scattering phenomena is used to formulate a low RCS design requirement for the design of low RCS reflector antennas. These antennas have radiation characteristics similar to center fed parabolic antennas, but have greatly reduced RCS levels. A new low RCS reflector antenna system is designed to replace an existing center fed parabolic reflector antenna system. The new antenna is designed by combining a high frequency, ray based, reflector design technique with the low RCS design requirements. The newly designed low RCS reflector antenna system is found to have transmitting characteristics similar to that of the original center fed reflector parabolic antenna but with RCS levels reduced by up to 25 dB.

Most avian radars use the resonance effect within the S-band to estimate the maximum bird detection distance. However, the specified range (2 km) for a standard avian target (SAT) with a 0.5 kg mass and 0.025 m2 radar cross section (RCS) as reported by the US Federal Aviation Administration is too small to cover the airfield clear zone at airports. The research shows that the measured RCS value of a pigeon is 0.25 m2, and furthermore, the authors argue that Ku-band avian radar can detect a pigeon as a SAT at a longer range, 12 km. They attribute this magnification effect on bird RCS due to the corner reflector effect related to wingbeats. During certain flapping motions, the wings and body of birds in flight act as corner reflectors; thereby, when radar echoes fall upon either face will impinge onto the other face, and subsequently reflected towards the illuminator, thus contributing considerably to bird RCS. The measured RCS of a pigeon is consistent with the theoretical RCS of a dihedral corner reflector. An expanded coverage of the avian radar larger than the main area of an airfield clear zone can improve the detection probability and reduce bird strike hazards.

The radar corner reflector stands as a typical passive jamming decoy in electronic countermeasures, which presents distinctive Radar Cross Section (RCS) features in monostatic or bistatic radar owing to its particular geometric structure. In this paper, the monostatic and bistatic RCS characteristics of a single octahedral corner reflector and octahedral corner reflector array are analyzed. The impact of different bistatic base angles on the RCS area of the corner reflector is also studied by contrasting the main lobe width and statistical parameters of the RCS, which provides theoretical analysis and data support for the effective identification of targets and corner reflector decoys.