Linear frequency-modulated continuous wave active sonar signal processing

Linear frequency-modulated continuous wave (LFMCW) is applied in continuous active sonar (CAS). Methods for echo detection and direct path interference (DPI) suppression are studied and evaluated by simulation and sea trial data. To detect the LFMCW echo, the received signal is first demodulated by the transmitting signal, resulting in a de-chirped signal, which is similar as in LFMCW radar. However, in active sonar case the target velocity is no longer negligible comparing with the sound velocity. Thus the de-chirped signal cannot be simplified as a CW signal. Instead, it is a LFM signal, with its sweep rate and center frequency related with the velocity and distance of the target. Time-frequency analysis methods which are generally used to detect and estimate LFM signals can then be used in LFMCW active sonar. In this work, the Fractional Fourier Transform (FRFT) is chosen. Simulations and experiments show that LFMCW active sonar with the de-chirp-FRFT receiver can get much higher processing gain than conventional LFM pulsed active sonar with a matched filter (MF) receiver. To reduce the direct path interference (DPI) from the transmitter, bi-static mode is preferred. DPI is eliminated by filtering the de-chirped signal in the frequency domain. The major drawback of this method is that targets close to the transmitter will be lost. The disadvantage of the de-chirp-FRFT algorithm is that the processing results can be obtained only once every long work cycle, which is not propitious to target tracking. The future work will be focused on the improvement of refresh rate.