# Dual-Frequency Signal Enhancement Method of Moving Target Echoes for GNSS-S Radar
## 4 Discussion  
In the present research, this article proposes a dual-frequency processing method to address the problem of weak GNSS echo signals and residual Doppler effects on signal accumulation gain when using GNSS-S radar for moving target detection. This method utilizes signals of two frequencies naturally transmitted by GNSS satellites to achieve an increase in echo gain from two aspects: on the one hand, the amplitude of the echo signals at two frequencies is added to make up for the amplitude attenuation of the single-frequency signal caused by residual Doppler after correlation compression; on the other hand, the phase fluctuation of the echo signal caused by the residual Doppler is eliminated through the phase difference in the dual-frequency signal, and the gain of coherent accumulation of the echo signal is improved.  
The GNSS signal itself is very weak (the power reaching the ground is only about \(-130\) dBm), and when used for target detection, the scattered echo power of the target further attenuates. To achieve the detection of target echoes, it is necessary to accumulate the signals for a long time to improve the SNR of the echoes. When used for detecting moving targets, the movement of the target brings additional Doppler frequency, which makes it very difficult to perform long-term coherent accumulation. In existing research, some scholars have combined coherent integration with incoherent integration to further improve SNR. However, due to the square loss of incoherent integration, the improvement effect of SNR is limited. Other scholars have carried out research on time-frequency analysis methods such as fractional Fourier transform and Radon Fourier transform to analyze the frequency characteristics of the echoes. After identifying the Doppler frequency in the echo, they can suppress the fluctuation of the echo phase by compensation to extend the coherent integration time. However, when the target motion is a high-order motion, the time-frequency analysis operation will be very complex and the amount of calculation will be large.  
However, as the GNSS signal is a PRN-encoded signal, it is sensitive to Doppler frequency shifts. The residual Doppler caused by target motion will cause attenuation of signal amplitude when performing correlation compression on the echo signal, which cannot be compensated by later coherent accumulation. Fortunately, the GNSS satellite  
Figure 26: Echo trace diagram of four combined satellite signals.  
system naturally transmits signals at multiple frequencies, and these signals have natural similarities. They are all sent by the same satellite and have the same propagation path. Therefore, if the signals of these multiple frequencies can be fully utilized during target detection, the energy of the echo signal will be significantly increased to make up for the shortcomings of weak signal power.  
In addition, the multiple frequency signals transmitted by GNSS satellites are of the same source and frequency. Using the dual-frequency difference method proposed in this paper, the phase fluctuation caused by target motion can be effectively eliminated, which will improve the coherence of the echo signals, and a longer coherent accumulation time can be used to improve the SNR of the echo signals. This is very helpful for improving the detection ability of weak GNSS echo signals. Moreover, the algorithm is simple and can avoid complex mathematical operations, which will facilitate the realization of real-time processing.  
However, the processing method in this paper only considers signals with two frequencies, while the GNSS satellite system usually transmits signals at multiple frequencies, for example, BDS transmits signals at three frequencies at the same time. If three or more frequency signals can be further considered in the processing process, the gain of the target echo can be increased again.