Research On Synthetic Aperture 3-D Imaging Based Radar Cross Section Measurement

In order to meet the increasing application demand of remote sensing, target recognition, reconnaissance and anti-reconnaissance, the measurement of Radar Cross Section(RCS) becomes one of research hotspots. Due to the difficulty of complex and composite targets’ RCS calculation via electromagnetic computing methods, the actual measurement is a more effective and accurate method. However, the conventional far-field measurement and compact-range measurement have disadvantages such as high cost, large environmental impact or target size limitation. As a new type of target scattering coefficient distribution measurement technology, the Synthetic Aperture Radar(SAR) 3-D imaging technique has great advantages in solving above problems. Based on the SAR 3-D imaging technique, for problems such as Near-Field-For-Far-Field(NFFFF) transformation, lack of signal coherence and high computational cost, this dissertation focuses on the synthetic aperture 3-D imaging based RCS measurement technology, the key researchs including the plane-wave synthesis 3-D imaging based RCS measurement method, the steady electromagnetic current 3-D imaging based RCS measurement method and the subaperture approximation fast method. The main works and innovation points are as follow.1. Researches on the basic principle of synthetic aperture 3-D imaging based RCS measurement technology, the main problems and superiorities of this technology. First of all, begin with the microwave 3-D Linear Array SAR principle, the basic geometric structure and signal model are introduced, which are the theoretical basis of novel RCS measurement method. The 3-D Back-Project(BP) imaging algorithm that has high precision and high computational cost, are analyzed. Secondly, the division of scattering region, the definition of RCS and the 3-D resolution of Linear Array SAR are analyzed and used to indicate that high resolution SAR imaging based RCS measurement needed to be set in the near-field, which causes needing the NFFFF transformation. The scattering center model is introduced and used to indicate the problem of lack of signal coherence under large synthetic aperture angle. Finally, the three technologies of 1-D, 2-D and 3-D imaging based RCS measurements are contrastively analyzed, and the 3-D imaging based RCS measurement is the most optimistical method. Owing to its advantages such as 3-D high resolution capability, the technique can spatially distinguish the RCS of different parts from a complex object, or the targets’ RCS from environment, which is available for outdoor measurement and low-cost measurement. Its advantages are verified by real test experiments.2. For the near-field measurement requirement, the plane-wave synthesis 3-D imaging based RCS measurement method is proposed, to calculate the far-field RCS. First of all, based on the digital signal processing method of plane-wave synthesis, the NFFFF transformation of incident wave and scattered wave under the system architecture of near-field Linear Array SAR are obtained, which satisfies the requirement of far-field RCS. Secondly, the BP operator is improved, the improved BP operator based NFFFF transformation method is proposed, and the improved BP operator based imaging modle is established, which can obtain NFFFF transformation and 3-D image at the same time.Thirdly, the plane-wave synthesis 3-D imaging based RCS measurement method is proposed, which can calculat the far-field RCS as a function of angle and frequency from the 3-D microwave image. Its ability of extracting the RCS of different parts and its precision are verified by simulation experiments. Finally, unlike the plane-wave synthesis algorithm that can not synthesize the plane-wave under sparse array, the improved BP operator based NFFFF transformation method is proved available under above condition, and it is verified by the numerical experiments of far-field RCS measurement.3. For the problem of lack of signal coherence, the steady electromagnetic current 3-D imaging based RCS measurement method is proposed, to improve the accuracy of RCS measurement under large synthetic aperture angle. First of all, through the analysis of real test data, the echo phase reversal phenomenon of the targets such as cavity and corner reflector under large synthetic aperture angle is discovered, which can fail the scattering center model and lower the accuracy of RCS measurement. Secondly, for this problem, the solution of fixed transmitter is proposed, and the steady electromagnetic current imaging model is builded. The(complex) values of the 3-D microwave image are proportional to the superposition of the volume electric current density and the volume magnetic current density. Finally, the steady electromagnetic current imaging model is applied to RCS measurement, and the steady electromagnetic current 3-D imaging based RCS measurement method is proposed. Compared to the plane-wave synthesis 3-D imaging based RCS measurement method, it can improve the measuring accuracy under large synthetic aperture angle, but lower the resolution and measurement efficiency. Thus, these two methods are mutually complementary and can be integrated to one system for stronger applicability.4. For the problem of high computational cost, based on the sparsity characteristic of 3-D microwave image, the subaperture approximation fast method is proposed. First of all, based on the 3-D BP method and sparsity characteristic of 3-D microwave image, numerous computational cost of 3-D BP method is unnecessary. Secondly, the method of the sparsity information can be extracted from the lower resolution image which is obtained using the subaperture of the(virtual) array, and be used for high-resolution imaging to reduce the imaging region and reject the unnecessary computational cost, is proposed, which is called the subaperture approximation fast method and verified by real test experiments. Thirldly, the effect of subaperture option, target structure and the sparsity ratio to the computational cost are analyzed. Once decieded the subaperture option, the computational cost is only related to the the target characteristics(the top ological structure and the sparsity ratio). When the sparsity ratio is larger than 97.6%, the computational cost of SA metho d is lower than 10% of the 3-D BP method in average situation. Finally, the subaperture approximation fast methd is applied to the RCS measurement. Reliable results have been achieved and the data processing efficiency of RCS measurement is improved.In a word, this dissertation builds the basic principles of the synthetic aperture 3-D imaging based RCS measurement technology, and obtains a series of valuable research results for NFFFF transformation, measurement model and efficiency optimization. The research results provide an important theoretical guidance and technical support for scattering characteristic measurement technology.