Navigation satellite signals have the advantages of all-day, all-weather, and global coverage, and the use of navigation signals for the detection of moving targets has significant application prospects. However, the GNSS signal is very weak, and the signal power is greatly attenuated after being scattered by the target. In order to detect the echo signal, a long integration time is required. However, the movement of the target will cause the echo signal to produce unpredictable range migration and a Doppler frequency shift, which will weaken the cumulative effect of long-term integration. This paper proposes a weak signal processing method with amplitude and phase self-correction for moving target detection in the GNSS-S radar. First, the phase consistency of the echoes of a single GNSS satellite is realized by the block expansion compression and phase differential correction method to improve the coherent accumulation gain; then, multi-star joint accumulation is carried out after the signal amplitudes of multiple satellites are corrected by the improved keystone method, so as to obtain a stable echo signal track. This method can effectively improve the integral gain of the scattering signal of the moving target and realize target detection. The simulation results and field tests show that this method can effectively improve the SNR of the GNSS-S signal and can realize the detection of small moving targets such as cars with GNSS-S radar.

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.

The GNSS-S radar utilizes the signals of a global navigation satellite system (GNSS) to carry out target detection. Due to the very low power of GNSS signals, long-term accumulation is needed to improve the gain of the echo signals. However, when it is used for moving object detection, the random movement of the target will cause residual Doppler frequency after the echoes are correlated and compressed through the direct signal. The residual Doppler frequency will cause two problems: on the one hand, the signal coherence will deteriorate, affecting the coherent accumulation gain; on the other hand, the amplitude of the signal after compression will decrease due to the sensitivity of GNSS signals to Doppler frequency. Therefore, how to increase the signal amplitude and eliminate the phase fluctuation caused by the Doppler frequency shift in the GNSS echoes of moving targets is an important issue for GNSS-S radar to detect moving targets. This paper proposes a dual-frequency GNSS echo enhancement method that uses the dual-frequency signals transmitted by the GNSS satellites to enhance and regularize the target echo. First, the phase relationship model of the GNSS dual-frequency echo is constructed, and the phase difference is made to the compressed dual-frequency echo signal to obtain the differential phase without fluctuation; then, the amplitudes of the dual-frequency echo signals are added together; and finally, a new signal with enhanced amplitude and consistent phase is constructed by using the dual-frequency additive amplitude and differential phase, and the long-term coherent accumulation of the signal is carried out, which can improve the processing gain of the weak echo signal of the moving target. The simulation and field experiments show that this method makes full use of the energy of the GNSS dual-frequency signal and eliminates the phase fluctuation in the echo signal of the moving target so that the compressed signal energy remains consistent in the slow-time dimension. After long-term coherent accumulation, the echo SNR was greatly improved, which enabled the detection of two high-speed cars by GNSS-S radar in the experiment.

Based on the bistatic scattering mechanism of time-varying sea scene and the imaging geometry model of shift-invariant mode bistatic synthetic aperture radar (SAR), a fast simulation method of bistatic SAR echo signal in sea scene is proposed. Through two-dimensional Fourier transform of the sea surface model and the scattering function of the radar incident wave sea surface target, the dispersion relationship of the sea surface is applied to modulate the frequency domain scattering function in the frequency domain, and realizes the simulation of the bistatic SAR fast echo signal in the sea scene. Finally, the SAR image of the ocean scene is given, which proves the effectiveness of the method.