Time-frequency representation adapted to chirp signals: application to analysis of sphere scattering

This work deals with the analysis of underwater target echoes by means of time-frequency representation. The processed echoes have been collected during experiments with a wide band, constant beamwidth transducer emitting linear frequency modulation signals (chirp) in the frequency range of 10-150 kHz. Sphere echoes are multicomponent signals because of the superimposition of various propagation modes of the acoustic waves inside and around the sphere. It results that the whole sphere echo is a sum of superimposed chirp signals. We show that the analysis of the target echo by means of the now classical time-frequency representations gives a great deal of difficulties because of the resulting interferences. In this work, we propose a time-frequency representation adapted to chirp signals in which the interference can be considerably attenuated. Therefore the identification of the signal features are greatly simplified