# Dual-Frequency Signal Enhancement Method of Moving Target Echoes for GNSS-S Radar
###### Abstract  
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. GNSS-based moving target detection; weak signal; dual-frequency difference; GNSS-S radar 2023  
14 September 202314 September 202 However, the GNSS signal structure is mainly designed for navigation, positioning, and timing [5]. When used for target detection, it faces many problems, such as extremely low signal power and poor resistance to Doppler frequency shift [6; 7; 8]. In recent years, many scientific and technological workers have conducted extensive theoretical research and experimental verification work around the key technical challenges of GNSS-based target detection. The concept of GNSS-based radar detection was proposed in reference [9]. A maritime target detection test was carried out in references [10; 11], which verified the feasibility of using GNSS signals for target detection. In reference [12], the system link budget using GNSS signals for detection was analyzed, and it was pointed out that when limited by the signal power of GNSS, the realization of target detection requires long-term integration. In reference [13], a signal processing architecture for GNSS-based radar was proposed that utilizes correlation operations between direct signals and reflected signals to improve the processing gain of reflected signals. In references [14; 15], the method of multiframe accumulation is used to accumulate GNSS echo signals for a long time, achieving the detection of maritime target echo signals. In references [16; 17; 18], a GNSS-based detection test for ground-level buildings was carried out. Since the target is still, long-term coherent integration is used to obtain the echo signal of the target. In references [19; 20], experiments were conducted using GNSS signals to detect ships, achieving long-term energy accumulation by correcting the distance migration of target motion.