A novel imaging algorithm is presented in this article for focusing the very high resolution spaceborne synthetic aperture radar (SAR) data of spotlight mode. The long synthetic aperture time in spotlight mode brings new problems, such as Doppler spectrum aliasing and curved orbit. The imaging results will be filled with ambiguity and would suffer from resolution reduction if Doppler spectrum aliasing is not handled. The error introduced by curved orbit will degrade resolution of the target and introduces asymmetric sidelobes in azimuth direction. The start-stop approximation is no longer applicable, since it introduces two effect, e.g., "fast-time" effect and "slow-time" effect, which will cause defocusing and range-dependent azimuth shift, respectively if not handled. The proposed algorithm combines the two-steps processing approach (TSPA) and the backprojection algorithm (BPA). First, the initial step of TSPA is used to get a high azimuth sampling rate which is higher than pulse repetition frequency to avoid the Doppler spectrum aliasing. Then, the "fast-time" effect of start-stop approximation is corrected in the 2-D frequency domain. Finally, the BPA is used to correct the error introduced by the curved orbit and the "slow-time" effect of start-stop approximation. The experimental results of spaceborne SAR data acquired by Gaofen-3 (GF-3) SAR system demonstrate the feasibility of the proposed algorithm. Index Terms-Backprojection algorithm (BPA), spotlight, synthetic aperture radar (SAR), two-steps processing. I. INTRODUCTION S YNTHETIC aperture radar (SAR) is an important remote sensing tool which has been widely used for earth observation operating in all day and all weather conditions [1]. Modern Manuscript
High resolution synthetic aperture radar (SAR) imaging has extensive application value especially in military reconnaissance and disaster monitoring. The motion of the satellite during the transmission and reception of the signal introduces notable errors in the high resolution SAR spotlight mode, which will lead to a defocused SAR image if not handled. To address this problem, an accurate correct echo model based on non-start-stop model is derived to describe the property of the SAR signal in the paper. Then, in the imaging processing, an azimuth-time-varying range frequency modulation rate is used for range compression. The range history and compensation phase are also derived based on the correct echo model. Then, combining the correct echo model and Cartesian factorized backprojection (CFBP) algorithm, a modified CFBP algorithm is proposed for SAR imaging to improve the accuracy and efficiency of processing. Besides, the influence of residual error due to mismatch is analyzed in detail. In the end, the simulation experiment and Gaofen-3 (GF-3) data experiment are carried out to demonstrate the feasibility of the proposed algorithm.
LuTan-1 (LT-1) is an innovative spaceborne radar Earth observation mission including two satellites equipped with synthetic aperture radar (SAR) which will be launched in 2022. Active phased array antennas that can be divided into two physical channels are equipped on each satellite. The signal can be transmitted through the full antenna without broadening and recorded by each channel. Therefore, two methods can be used to preprocess the dual-channel receiving signals, referred to as dual-channel echo reconstruction and dual-channel echo synthesis. The former is inherited from the traditional high-resolution wide-swath mode and the latter is a method that takes coherent superposition as the reference. This paper researches the impacts of the two methods in system performance and imaging quality. Principles and theoretical models are firstly given. Furthermore, the system performance under the “L1A_SM_S” working mode of the LT-1 is simulated to compare the differences between the two methods, which mainly focuses on azimuth ambiguity-to-signal ratio, noise equivalent sigma zero, and the performance of block adaptive quantization. Afterwards, the test data acquired by the ground validation system of the LT-1 are used for the hardware-in-the-loop simulation to demonstrate the imaging quality between the two methods. Finally, a quantitative comparison is given.
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