Spaceborne-Groundbase Bistatic ISAR Imaging of Aircraft Target With Large Rotational Angle via Bi-ISAR-PFA Based on Two-Step Motion Parameter Estimation

To enhance the 2-D resolution for spaceborne-groundbase bistatic inverse synthetic aperture radar (Bi-ISAR) imaging of aircraft targets, obtaining a large rotational angle between the target and Bi-ISAR is necessary. However, image distortion and defocusing effects will occur due to range cell migration induced by the large rotational angle, the time-varying bistatic factor, and the effective rotation velocity, which make it challenging to construct high-resolution and well-focused Bi-ISAR images. In order to overcome this issue, a Bi-ISAR polar formatting algorithm (Bi-ISAR-PFA) based on two-step motion parameter estimation is proposed in this article. First, the received signal model for spaceborne-groundbase Bi-ISAR imaging of aircraft targets is established, and the shape of the support region is analyzed in detail. Then, the principle of Bi-ISAR-PFA is introduced, which consists of three steps: 1) coarse estimation of motion parameters; 2) fine estimation by means of the proposed particle-swarm-optimization-based Bi-ISAR image quality optimization algorithm; and 3) construction of Bi-ISAR images in the joint range and cross-range domain through the proposed Bi-ISAR-PFA procedure. A quantitative analysis of the estimation errors and their effects on Bi-ISAR imaging via the proposed Bi-ISAR-PFA is also provided to verify the robustness of the proposed algorithm. Finally, several simulations are carried out to validate the effectiveness of the proposed algorithm.