A High-Band Phase Synchronization Approach for Distributed UAV SAR

To achieve the multistatic phase synchronization of distributed uncrewed aerial vehicle synthetic aperture radar (UAV SAR), the two-way direct wave links are established between every two radar units and the time-varying phase fluctuations in bistatic echoes can be eliminated utilizing the traditional two-way phase synchronization method. However, the vibration environment of the UAV platform causes the degradation of the frequency source stability. Especially at high-carrier frequencies, the phase fluctuations doubling to RF terminal break the phase unwrapping condition, and the time-varying π-ambiguity is introduced in phase synchronization error estimation value that causes the SAR image defocused. In addition, to satisfy the general baseline design requirements, there may be direct wave link obstruction between stations, resulting in interruption of direct wave reception and subsequent failure of phase synchronization. Even if the direct wave link remains unobstructed, potential multipath effects can degrade synchronization accuracy. To address the above issues, a reliable distributed UAV SAR high-band phase synchronization approach is proposed, in which a coarse-to-fine two-stage phase synchronization is performed based on two-way direct waves and echoes, respectively. In the coarse synchronization stage, the time-varying π-ambiguity after unwrapping is effectively suppressed using two-way multiplication and local unwrapping techniques, and full-link phase synchronization is achieved utilizing the phase triangle under conditions of partially obstructed direct-wave links. In the fine synchronization stage, the two-way echo of the calibration target is utilized instead of the full scene echo to avoid multipath effects in the direct-wave link, thereby achieving higher phase synchronization accuracy. This approach has been applied in the high-band distributed UAV SAR system, and the real measured data shows the effectiveness of the approach.