A novel image formation method for electromagnetic vortex SAR with orbital-angular-momentum

Electromagnetic (EM) vortex wave carrying orbital angular momentum (OAM) has attracted a lot of attention in radar imaging, due to its potential capability of new degree of freedom for information modulation. Most existing OAM-based radar imaging methods require abundant OAM modes to realize the azimuth resolution. Switching between the OAM modes frequently increases the burden of radar antenna and the complexity of beam steering. In this paper, a novel electromagnetic vortex synthetic aperture radar (EMV-SAR) model with equivalent squint imaging is established. The geometrical model and echo signal model are derived correspondingly. By analyzing the echo signal model, amplitude and phase modulation introduced by the OAM affect the azimuth focusing, and traditional imaging algorithms are no longer applicable. Hence, a novel image formation method based on the traditional Chirp-Scaling (CS) algorithm is proposed for the EMV-SAR. The amplitude weighting function and phase modulation function are derived accurately, and high-precision focusing processing is achieved by modified CS algorithm. Point targets simulation results validate that the image focusing performance can be improved significantly using the proposed algorithm.