Ultrasonic Imaging in Air with a Broadband Inverse Synthetic Aperture Sonar

An experimental ultrasound scanning sonar has been developed that uses a phased ultrasonic transducer array to image objects through the air. The sonar generates arbitrary long duration broadband signals and uses pulse compression to achieve a high range resolution and a high signal to noise ratio. This paper considers using the sonar in an inverse synthetic aperture side-scan configuration, where instead of moving a sonar past the objects of interest, they are moved past the fixed sonar, in this case using a conveyor system. A high speed image reconstruction algorithm is presented for the production of diffraction limited imagery, based on recent developments in synthetic aperture radar/sonar spatial frequency domain (wavenumber) reconstruction algorithms, suitable for broadband (low Q) signals. Test results using the sonar are presented and its performance is discussed. Finally, the paper concludes with a discussion of the practical difficulties with imaging in air using ultrasound and suggestions for using reference targets to ameliorate some of the medium induced phase abberations.