Study On Off-Grid Problems In CS-ISAR Imaging

Due to high resolution, inverse synthetic aperture radar(ISAR) has been widely used in the military and civilian fields. However, ISAR imaging encounters many challenges such as high sampling rate, large data storage and real-time processing. The investigation of these issues has become a hot research topic in the area of ISAR imaging.By using the sparse characteristics of ISAR target, the problems of Off-Grid and parameter estimation in compressed-sensing-based(CS-based) ISAR imaging has been addressed in depth in the thesis. For sparse probing frequency(SPF) signal, improved CS-based ISAR imaging algorithms are proposed. The main works are summarized as follows:1. To address the issue that the traditional ISAR signal don’t fully apply the sparse characteristics to achieve CS-based ISAR imaging, the ISAR imaging model using the SPF signal is established, and corresponding imaging flowchart is given. The feasibility of this model is verified by simulations.2. The BOMP and BLOOMP algorithms based on band exclusion technology and local optimization technology are respectively proposed for solving the Off-Grid problem, which cannot be resolved in the traditional sparse reconstruction algorithms.These two algorithms are detailedly proved in theory, and meanwhile, their computational complexity and reconstruction performance in noise are analyzed.Simulation results show that under the same simulation conditions, compared with BOMP algorithm, BLOOMP algorithm has superior performance in terms of reconstruction success rate and reconstruction error.3. For the problem of real ISAR imaging with unknown target parameters, joint parameter estimation and ISAR imaging reconstruction methods are proposed.(1)When the target point is located on the grid of the base matrix that is depend on unknown rotation rate, a SPF imaging method based on sparse Bayesian learning(SBL)is proposed. Both the rotation rate and a high-resolution ISAR image can be simultaneously retrieved by jointly applying the SBL technique and a gradient-based search algorithm. The effectiveness of the proposed method is validated by experimental results with both simulated data and real data.(2)When the target point isn’t located on the grid of the base matrix, a joint parameters estimation and ISAR imaging method that based on Off-Grid model is proposed. By improved mathematical model, both the rotation rate and high resolution ISAR images can be simultaneously retrieved in Off-Grid circumstances by the use of combined parameter perturbation algorithm and gradient search algorithm. The effectiveness of the proposed method is validated by simulation.