Fast-Fourier Time-Domain SAR Reconstruction for Millimeter-Wave FMCW 3-D Imaging

This article presents an accurate phase calibration technique and a fast time-domain reconstruction algorithm for frequency-modulated continuous wave (FMCW) synthetic aperture radar (SAR) imaging in 3-D. The phase calibration is tailored toward millimeter-wave imaging systems that operate over a wide bandwidth. The first step in the calibration is to estimate the phase nonlinearities in the chirp signal. This is done by using a two-point relative distance calibration measurement. Range deskewing is used to perform a range independent nonlinearity correction to the measured intermediate frequency (IF) signals. The second step in the calibration is to bring the phase reference to the phase center of the antenna by using a conducting stencil as a reference target. The calibrated IF signal is shown to follow a time-domain dispersion relation that is conducive to fast near-field imaging. Based on this, a holographic time-domain 3-D image reconstruction algorithm is presented. The reconstruction is composed of two 2-D fast-Fourier transforms (FFTs) and a 1-D nonuniform FFT (NUFFT). The proposed algorithm is two orders of magnitude faster than existing time-domain reconstruction algorithms for FMCW 3-D imaging radars. A W-band (76–84 GHz) FMCW system is used to experimentally verify the proposed calibration and reconstruction. The measured lateral resolution is shown to reach the theoretical diffraction limit, demonstrating near-perfect phase reconstruction. Several experiments of 3-D targets and concealed objects are presented to demonstrate the high quality of 3-D images that can be generated from FMCW SAR data. The experimental datasets and codes are available at: https://github.com/AdityaMuppala.