Multiple Targets Detection of Linear Frequency-Modulated Continuous Wave Active Sonar Using Fractional Fourier Transform

Abstract

In order to improve the detection performance of active sonar and make up for the shortcomings of pulsed active sonar (PAS), linear frequency-modulated continuous wave (LFMCW) was applied in continuous active sonar (CAS). The expression of transmitting waveform was presented, and the echo model of underwater moving targets was established. To detect the LFMCW echoes, the received signal was first demodulated by the transmitting signal, resulting in a de-chirped signal. Unlike radar system, the target velocity of active sonar was not negligible, comparing with the sound velocity in water, and the de-chirped signal was also a LFM signal, whose chirp rate and frequency were related with the velocity and distance of the target. Fractional Fourier Transform (FRFT), a time-frequency analysis method which is generally used to detect and estimate LFM signals, was proposed to detect and estimate the de-chirped signal. In multiple targets situation, “CLEAN” algorithm was proposed to solve the problem of the detection interference from strong targets to weak targets. Simulations show that LFMCW active sonar with the de-chirp-FRFT receiver can get much higher processing gain than conventional PAS with a matched filter (MF) receiver. The resolutions of velocity and distance with the de-chirp-FRFT algorithm fully meets the needs of underwater target detection. The simulation result also shows that the proposed CLEAN algorithm can detect LFMCW multi-target echoes effectively.