Multistatic Collaborative Imaging for Shipborne GNSS-S Radar: Method and Experimental Verification

Global navigation satellite system (GNSS) signals have many advantages, such as numerous satellites and global coverage. There will be huge application potential when utilized as electromagnetic illuminators to construct a passive radar. However, because of GNSS signal power limitations, the signal-to-noise ratio (SNR) of single satellite imaging is often low and vulnerable to electromagnetic interference. The image quality will be greatly enhanced if the signal energy of several GNSS satellites can be fully exploited. However, it is a huge challenge to achieve the coherence of multisatellite signals because different satellites have distinct imaging geometries and motion Doppler. In order to suppress interference and produce images with a high SNR, this research presents a multistatic coherent synthetic aperture imaging method. To demodulate and decode the scattered echoes, a phase-locked loop (PLL) is first utilized to track direct signals from several satellites to create reference signals with constant phases. Then, secondary demodulation functions are constructed to remove the residual Doppler frequency. Finally, the back-projection algorithm is used to perform collaborative coherent imaging on multistatic GNSS echoes. A shipboard experiment was conducted to verify the method. A high-quality image of the ship was obtained in a synthetic aperture time of 3 s by collaborative imaging of six BDS satellites.