Radar Cross Section Calibration Research Articles

Highly Accurate Radar Cross-Section and Transfer Function Measurement of a Digital Calibration Transponder without Known Reference—Part I: Measurement and Results

Active Radar Calibrators (ARC), also called calibration transponders, are often used as reference targets for absolute radiometric calibration of radar systems due to their large achievable Radar Cross-Section (RCS). However, before using a transponder as a reference target, the hardware itself has to be calibrated. A novel method, called the three-transponder method, was proposed some years ago and allows for RCS calibration of digital transponders without using any RCS targets as reference. In this paper, this technique is further refined and applied to a setup utilizing only one digital transponder. The accurate measurement design is described and a novel, elaborated data processing scheme is developed to minimize remaining noise and clutter effects in the data. A comprehensive error analysis is presented in the second part of this paper.

Comprehensive Characterization of an Ellipsoidal Cylinder Calibrator for Radar Cross Section Measurement

Currently, a set of scaled squat circular cylinders (SCCs) have been widely used for co-polarimetric radar cross section (RCS) calibration in various test ranges where low RCS metal pylons are used. The major drawback of a circular cylinder (CC) is its high sensitivity to elevation angle, as well as the frequency-dispersive theoretical RCS. In this work, we propose a new RCS calibrator, the metal ellipsoidal cylinder (EC), whose RCS characteristics are similar to a metal sphere, while it can be mounted on a metal pylon with little electromagnetic (EM) interactions as a CC does. A high-frequency RCS expression is derived using geometrical optics (GO) for the new calibrator. The scattering mechanisms are analyzed to establish the accurate parametric representation of the theoretical RCS. Based on surface current analysis and numerical calculation, the calibrator-pylon interaction (CPI), the angular sensitivity, and the bistatic RCS characteristics are discussed, enabling the calibration uncertainty evaluation for applying the proposed calibrator in the ground-plane range (GPR). Numerical and experimental results of a manufactured squat EC (SEC) are presented to demonstrate the excellent performance of the proposed EC calibrator for co-polarimetric RCS calibration.

MIMO Radar Calibration and Imagery for Near-Field Scattering Diagnosis

Multiple-input multiple-output (MIMO) radar is an enabling technique for high-resolution imaging, which is especially useful for near-field electromagnetic scattering diagnosis of complex targets. Among others, high sidelobes and radar cross section (RCS) calibration uncertainty are the major challenges for such applications, due to array nonuniformity, imperfect channels, and antenna pattern tapering. These shortcomings prevent a MIMO radar from obtaining high-quality images with enough dynamic range and RCS accuracy. In this paper, we develop a complete solution for these problems. A novel adaptive weighting technique is proposed, where the complex weights are optimized for exact amplitude and phase error calibration of a MIMO array and for azimuth sidelobe reduction. A MIMO filtered backprojection algorithm is developed for image formation with improved RCS calibration accuracy, where propagation path-loss, antenna pattern tapering, and phase distortion due to the near-field spherical wave front are compensated. Both indoor and outdoor field test results are presented to show the high-quality images obtained using the proposed techniques, demonstrating the applicability of a MIMO radar for diagnostic RCS imaging of complex targets.

Fast and Accurate RCS Calculation for Squat Cylinder Calibrators [Measurements Corner

Squat metal cylinders are commonly used as calibrators in radar cross section (RCS) measurements. In this paper, based on electromagnetic scaling principles and complex exponential (CE) model, a novel procedure for accurate RCS calculation of squat cylinder calibrators is proposed. Using the currently developed technique, calculation of the ultrawideband (UWB) RCS amplitude and phase for a set of squat cylinders with a specified diameter-to-height ratio (DHR) can be implemented in real time, as soon as the CE model has been developed from the UWB data for a single cylinder of any size, which may be calculated using exact numerical solutions such as the method of moment (MoM). Examples demonstrate that, in RCS measurements, the uncertainty of CE model calculation is well within 0.03 dB for ka5, meeting the requirements for almost all RCS calibration applications.

Study of scattering characteristics for metallic spheres at terahertz frequencies

In this paper, the analytical solution of metallic spheres which takes the frequency dispersion characteristic into account is studied at terahertz frequencies. In order to characterizing metals at terahertz frequencies, the classical relaxation effect frequency dispersion model is used and a calculated analytical solution of metallic spheres is given with respect to the large permittivity of metallic spheres. The analytical solution supplies an effective tool for the scattering analysis of metallic spheres at terahertz frequencies. The simulation results show that the main scattering signature of a polished aluminum sphere comes from the specular reflection at terahertz frequencies and a polished aluminum sphere should be characterized by migratory scattering center model. These conclusions may provide an important reference for terahertz radar cross section calibration and scattering source diagnosis.

THE THREE-TRANSPONDER METHOD: A NOVEL METHOD FOR ACCURATE TRANSPONDER RCS CALIBRATION

Transponders (also known as polarimetric active radar calibrators or PARCs) are commonly used for radiometric calibration of synthetic aperture radars (SARs). Currently three methods for the determination of a transponder's frequency-dependent radar cross section (RCS) are used in practice. These require either to measure disassembled transponder components, or a separate radiometric measurement standard (like a flat, metallic plate or a corner reflector), leading to additional uncertainty contributions for the calibration result. In this paper, a novel method is introduced which neither requires disassembly nor an additional radiometric reference. Instead, the measurement results can be directly traced back to a realization of the meter, lowering total measurement uncertainties. The method is similar in approach to the well known three-antenna method, but is based on the radar equation instead of Friis transmission formula. The suitability of the method is demonstrated by a measurement campaign for DLR's three new Kalibri C-band transponders, completed by an uncertainty analysis. The method is not universally applicable for all transponder calibrations because (a) three devices are necessary (instead of only one for the known methods), and (b) the transponders must provide certain additional features. Nevertheless, these features have become standard in modern SAR calibration transponder designs. The novel, potentially more accurate three-transponder method is thus a viable alternative for transponder RCS calibration, ultimately contributing to synthetic aperture radars with a reduced radiometric measurement uncertainty.

Electromagnetic scattering characteristic of aluminous targets in the terahertz and far infrared region

In the terahertz and far infrared region, aluminum is in a state of transition from conductor to dielectric, and the research of the interaction between aluminous target and electromagnetic wave is meaningful for scattering prediction of targets. With the available error criterion model, dielectric function of aluminum is determined by fitting to experimental data in the terahertz and far infrared region. The transmitted parameters in aluminum are deduced by considering different loss mechanisms. Reflection and transmission characteristics on the interface of aluminum are investigated, and the reflection coefficients are given as a function of frequency. Results show that the transmitted parameters in aluminum keep their continuity and coherency from microwave to terahertz frequency. RCS (radar cross-section) results of aluminum plates computed by IBC method demonstrate that the increased wave impedance of aluminous targets has little impact on its backscattering, and the polished aluminous plate or sphere can still be treated as a perfect electrical conductor and used as a reference for RCS calibration.

English

Radar cross-section (RCS) is an important study parameter for defence applications specially dealing with airborne weapon system. The RCS parameter guides the detection range for a target and is therefore studied to understand the effectiveness of a weapon system. It is not only important to understand the RCS characteristics of a target but also to look into the diagnostic mode of study where factors contributing to a particular RCS values are studied. This further opens up subject like RCS suppression and stealth. The paper discusses the RCS principle, control, and need of measurements. Classification of RCS in terms of popular usage is explained with detailed theory of RF imaging and inverse synthetic aperture radar (ISAR). The various types of RCS measurement ranges are explained with brief discussion on outdoor RCS measurement range. The RCS calibration plays a critical role in referencing the measurement to absolute values and has been described.The RCS facility at Reseach Centre Imarat, Hyderabad, is explained with some details of different activities that are carried out including RAM evaluation, scale model testing, and diagnostic imaging.Defence Science Journal, 2010, 60(2), pp.204-212, DOI:http://dx.doi.org/10.14429/dsj.60.341