Li Nanjing

Circular SAR experiment for human body imaging

In this paper, the CSAR (circular synthetic aperture radar) technique is introduced firstly. The principle of CSAR 3D (three dimensional) imaging and the spatial wavenumber support are analyzed and the 3D resolution is discussed secondly. Secondly, Ku band fully-polarimetric CSAR human body imaging experiments are performed, and the experimental data is processed by a fast 3D backprojection algorithm, the corresponding amplitude images are shown. Finally, the future experiments for human body imaging are discussed.

A new algorithm about transforming from near-field to far-field of radar target scattering

In order to meet the approximate plane-wave irradiating condition, adequate large field or compact range system is needed for RCS measurement of large aircraft targets. However, an outside testing field site or a compact range system is very expensive, so a new RCS extrapolation method named Hankel algorithm based on near-distance testing has been presented. According to the fact that objective reflectivity spatial distribution is unchanged with measurement state in ISAR imaging, here the multi-scattering center model is built. Then, the height of stealth target is always short, and the far-field condition can easily be satisfied on vertical plane, so the incident spherical wave is equivalent to cylindrical wave. Once more, the relationship between far-field and near-field is deduced by regarding image of the target as secondary radiating source. Finally, the RCS of target can be obtained with the scattering information of near-field extrapolated at full angular aspect. Taking derivative of the near-field data with respect to frequency in algorithm, the error can be reduced in simplifying formula, and just meet the requirement of stepped-frequency system. The results of simulation and experiments show that the algorithm is precise as well as effective.

The research of near-field INISAR imaging diagnosis

The study of electromagnetic scatter characteristic is significative for reducing the RCS(radar cross section) of radar target and improving the stealth performance of weapon system. For diagnosing the scattering coefficients distribution of radar target exactly, the mechanism of microwave imaging is analyzed in this paper, a novel near-distance high-resolution microwave imaging approach is presented. Rectify the curved wave front according to the structure characteristic of aircraft, the modified spherically back-projecting method is used to reconstruct 2-D image in near-field, then the 3-D INISAR image is derived from two ISAR images at different altitudes by using interferometric image processing. The effectiveness of the approach is demonstrated by the imaging experiment of aircraft scale model in microwave anechoic chamber, and it shows that the novel approach has potential signification in engineering.

The Modeling and Simulation of A near-distance microwave imaging system

For the purpose of diagnosing the scattering points of radar target exactly, a near-distance microwave imaging system is designed. The theoretical equation of imaging can be gained by mathematical model with cooperation object. Imaging equation can be solved by spherical wave filter back projection arithmetic quickly. The simulation result show that good performance 2D image with high resolution can be obtained by utilizing spherical filter back projection arithmetic, which can digest the scattering points exactly and in detail.

SAR Three-Dimensional Imaging Experiments with Microwave Anechoic Chamber SAR Data

In this paper, two 3-D (three-dimensional) imaging techniques, MB-TomoSAR (multi-baseline SAR tomography) and CSAR (circular synthetic aperture radar), are introduced firstly. Secondly, the indoor experimental systems for acquiring SAR data in the microwave anechoic chamber are described, then the experiments of the two 3-D imaging mode for the metallic spheres and human body are performed, respectively. Finally, the 3-D imaging capability of the MB-TomoSAR and CSAR is demonstrated with the experimental data.

Investigation on method of improving precision for Radio Frequency Simulation System

This paper presents an amendment method of system accuracy by calculating target with equivalent phase centre of antenna instead of mechanical position of antenna. At the very beginning, how RF simulation system works as a moving target has been briefly described. Then the error of RF simulation system consisted with spherical antenna array is also analyzed. Considering the total error comes from various errors of whole system, so we can substitute electrical position of antennas for that of target presented. Results of experiment show that most of the errors can be counteracted by using this method, and the simulation accuracy has been improved.

A new technique to measure RCS of large target at near distance

In general, RCS measurement must meet the far-field condition, which need a vast measuring area or a costly compact range. To solve the problem, a new technique by extrapolation correction dealing with near-distance RCS measurement of aircraft targets is set up. The RCS of aircraft target under spherical wave illuminating can be adopted to extrapolate far-field RCS of them. The convolution arithmetic utilizing conductive board as a reference target deduces the phase-correction coefficient g(x) between spherical and plane wave, and in fact the convolution calculation of g(x) can be replaced by Fast Fourier Transform, which is related to the scattering field of the reference target measured in near-distance, thus the calculating procedure could be completed simply and quickly. By theoretical simulation for a simple target, the RCS extrapolated can be obtained correctly. Experimental results of complex aircraft target show that this new technique makes experiment agree to theory precisely, moreover, it permits the measurement distance reduced by 15 percent of the minimum distance of far-field. Also, there is no serious limitation of measurement for target in dimension of aperture and depth.

Investigation on spectrum estimation extrapolation method of antenna measurement

Measurement precision of Low frequency antenna is poor for the apparent reflection from absorbers in anechoic chamber. According time-frequency transform theory, a certain bandwidth signal holds the ability of certain range resolution, so a time-domain antenna measurement model can be built. Based on real frequency bandwidth, extrapolating the frequency band to lower frequency and higher frequency by spectrum estimation, thus the broader frequency bandwidth is gained, higher range resolution can be realized, so multi-path reflection can be isolated and removed, as a result, measuring precision of antenna pattern will be improved apparently. Results from simulation and experiment show that the method is correct and effective.

A New ethod to improve Precision of Target Position in RFS

Radio frequency simulation systems (RFSS) are widely applied in the radio seeker research, but in low band (1-2GHz), the position precision of radio frequency array as a key parameter is hard to insure by hardware ways, and often hard to meet the experimental requirement. One new method by software calibration is put forward. By analyzing the position errors of array systematically, errors are decomposed into systematical error of array element and random error of the system environment. By summarizing the distributing character of systematical error, a beforehand setting calibration arithmetic is deduced, the equation of array corrective transformation can be calculated exactly. Finally by experimental validation, the position precision of array is enhanced by more than four times.