Signal Characteristics Analysis and Intra-Pulse Motion Compensation for FMCW CSAR

The combination of frequency modulation continuous wave (FMCW) technology and circular synthetic aperture radar (CSAR) would pave the way for the development of the microminiature unmanned aerial vehicle (UAV) earth observation application. Regarding the FMCW CSAR imaging process, the stop-and-go approximation is no longer effective due to the longer pulse duration. In addition, the signal characteristics of FMCW CSAR with higher frequency band and higher resolution need to be further studied. Aiming at millimeter-wave FMCW CSAR imaging, the signal models of FMCW CSAR in the time domain and wavenumber domain are first derived in this work. Then, the signal characteristics including the two-dimensional (2-D) wavenumber spectrum, the spectrum support region, the point spread function, and the resolution are analyzed. Moreover, the effects of intra-pulse movement are analyzed, including the range offset error, the main lobe broadening, and the additional phase error in the range dimension, as well as the extra first-order and second-order phase errors in the 2-D wavenumber domain. Based on the intra-pulse motion analysis, two criterions are provided to determine whether the intra-pulse movement of FMCW CSAR is negligible. An improved back projection (BP) algorithm compensating the intra-pulse motion in the range domain and another algorithm combining the traditional BP and compensation the intra-pulse motion in the wavenumber domain are proposed. Simulation results verify the correctness of the FMCW CSAR signal characteristics analysis and the effectiveness of the intra-pulse movement compensation methods.