FSAT gallery - sensing and imaging


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Wavenumber Spiral FSAT: sensing and imaging

Imaging approach

The directional properties of the spiral FSAT enable imaging of 2D areas through the data recorded from a single device. The direction-of-arrival (DOA) is indicated by a peak in the signal spectrum centered around the frequency corresponding to the direction of the incoming wave. The traveled distance is instead evaluated through the estimation of time-of-flight (TOF) after dispersion compensation, which can be performed by frequency warping.

The implemented imaging approach thus consists in the following steps:

Remove dispersion by warping the recorded waveform (Fig. 1 (a)). The output signal can be directly plotted as a function of traveled distance (Fig. 1 (b)).
Calculate the spectrogram of the warped signal through a Short-Time Fourier Transform (STFT, Fig. 1 (c)).
Map the frequency axis of the warped spectrogram onto the associated angular information according to the frequency-directivity properties of the FSAT. This provides a polar image of the monitored area.

4. A polar to Cartesian coordinate conversion completes the imaging computation (Fig. 1 (d)).

Fig. 1: (a) Typical FSAT output signal. (b) Warped frequency Transform (WFT) of the signal. (c) Short Time Fourier Transform of (b). (d) Imaging results after frequency-angle and polar to Cartesian remapping.

Pitch-catch simulation

Simulation of the FSAT sensing behavior has been performed in pitch-catch scenarios with single or multiple acoustic sources driven by a broadband pulse excitation. Fig. 2 and Fig. 3 show the source imaging results obtained with the algorithm described above.

Fig. 2: Imaging of a single acoustic source, whose actual location is indicated by the ‘+’ marker.

Fig. 3: Imaging of three acoustic sources, whose actual location is indicated by ‘+’ markers.

Pulse-echo simulation

Combined active-passive behavior of the spiral FSAT has been studied by pulse-echo simulations in which the device is excited with a broadband pulse to illuminate the whole scan area, and scatterers are located by recording the reflected wavefield. Forward and backward propagation of the signal is accounted for by dividing the distance information by 2. Fig. 4 shows imaging results for a single-scatterer configuration.