Chirp sub-bottom profiler signal processing method based on fractional Fourier transform

Abstract

The traditional signal processing method of chirp sub-bottom profiling is a type of matched filtering and is an optimal detector when there is no Doppler shift. Fractional Fourier transform (FrFT) is a linear transformation whose orthogonal basis is a linear frequency modulation signal, so there is no cross term interference when processing multiple or multipath chirp signals. The central characteristic of FrFT is that it has a similar theoretical basis to pulse compression, which makes it possible to use FrFT in the signal processing of chirp sub-bottom profiling. In this paper, the basic principle and theory of chirp sub-bottom profiler signal processing based on FrFT is introduced in detail. Firstly, orthogonal transform is performed to obtain the corresponding analytical form of the echo signal, then the time domain signal is transformed to a fractional Fourier domain (u domain) by optimal FrFT, and the signal enhancement is done using the u domain shading method. Finally, time dimensional transform is used to get the time domain envelope signal of the enhanced sediment impulse response. Time dimensional transform is realized by sample sequence mapping, time offset compensation, extraction and low pass filtering. This process can transform a u domain signal to the time domain directly without complex calculations. Performance comparison of this method to matched filtering is given too, and recommendations for future work are presented.