A Second Trip Echo Suppression Method using Two Different Code Sequences

A new second trip echo suppression method for pulsed Doppler weather radars is proposed. In pulse radars, appearance of the strong scatterers such as storms, mountains, and so on beyond the unambiguous range of radars, second trip echoes sometimes contaminate desired signals i.e. first trip echoes. To suppress the second trip echoes, transmitted signal is often modulated by using phase coding scheme. The phase coding scheme, which separate overlaid echoes in frequency domain, alters the spectra of the overlaid signals and enables one to estimate the spectral moments of each component. However, as mentioned above, there are many cases where the second trip echoes are much stronger than the first trip echoes. In that case, the first trip echoes are buried in the second trip echoes, which lead poor accuracy of detecting the first trip echo. The proposed method utilizes two different code sequences; one is an arbitrary code sequence such as random phase code, while the other sequence is produced by multiplying the previous sequence by positive integer k (k > 1). When phase correction to the second trip echoes is achieved for the two received signals modulated by each code sequence, the second trip echoes become identical, while the first trip echoes are different from each other. So by subtracting the one signal from the other, only the first trip echoes remain, which enables high accuracy of the detection. To demonstrate this scenario, we performed the simulation and confirmed the effectiveness of the proposed method. Figure 1 shows the block diagram of the proposed method. For the first code sequence (code #1), we can use an arbitrary code such as the random phase code, SZ code, and so on. The other code sequence (code #2) is made by the integer multiple of the previous sequence. Both received signals, modulated by code #1 and code #2, are cohered to their second trip echoes as an initial state. As a first step, initial phase and amplitude compensation are achieved. In the next step, we subtract the received signal modulated by code #1 from the received signal modulated by code #2. As a result, the first trip echo is reconstructed. Figure 2 shows the simulation results of the conventional method using random phase code (left) and the proposed method (right). In each graph, horizontal axis indicates Doppler spectrum width of the first trip echo (w1), vertical axis indicates ratio of the power between first trip echo (p1) and the second trip echo (p2). Color of each cell indicates the standard error of estimated Doppler velocity of the first trip echo. These results suggest that the accuracy of estimated Doppler velocity is degraded as the second-trip echo is getting strong in the conventional method. On the other hand, estimated Doppler velocity keeps accurate values regardless of power ratio p1/p2 in the proposed method.