Multi-frequency tomographic SAR: A novel 3-D imaging configuration for limited acquisitions

Tomographic synthetic aperture radar (TomoSAR) technology, as an extension of interferometric SAR (InSAR), solves the layover problem and realizes three-dimensional (3-D) imaging. Now, it is an important research direction in the field of radar imaging. However, TomoSAR usually requires the SAR sensor to make enough acquisitions at different spatial locations to achieve high-quality 3-D imaging, which is high time-costly and inefficient. To solve this problem, we extend multi-frequency (MF) InSAR and propose a novel SAR 3-D imaging configuration: MF-TomoSAR. MF-TomoSAR utilizes limited acquisitions with enhanced degrees of freedom (DOF) in frequency to accomplish 3-D imaging. It can achieve a similar imaging quality as the traditional TomoSAR while significantly improving 3-D imaging efficiency. The main contributions are summarized as follows: First, inspired by the idea of extending multi-baseline (MB) InSAR to TomoSAR, the single baseline (SB) MF-TomoSAR signal model is proposed. The SBMF-TomoSAR model utilizes interferometric processing to eliminate the effects of scattering changes due to different working frequencies (WFs). In the extreme case of only one fixed baseline, multiple interferograms with different WFs can be considered as samples at different spatial frequencies (SFs) to achieve 3-D imaging through spectral estimation. Then, in order to solve the sampling limitation caused by a fixed baseline, the MF-TomoSAR configuration is generalized to a general case of multiple baselines, and the MBMF-TomoSAR signal model is proposed. The MBMF-TomoSAR model realizes SF sampling through different WFs with multiple baselines to achieve sampling expansion and ensure the 3-D imaging quality. Finally, the MF-TomoSAR processing framework is proposed with the baseline distribution optimization method. The MF-TomoSAR configuration (either SB or MB) does not change the essence of spectral estimation in tomographic processing, and the classical tomographic processing algorithms can be directly applied to MF-TomoSAR processing. The computer simulation and unmanned aerial vehicle (UAV) SAR 3-D imaging experiment verify the effectiveness of the proposed MF-TomoSAR configuration.