Phased array acoustic imaging in ground co-ordinates, with extension to synthetic aperture processing

The authors describe a digital implementation of time delay sonar imaging for linear swept frequency (chirp) sources. The technique, based on phase shifting of match filtered individual transducer echoes is a direct equivalent of analogue `phased array´ processing. While the technique can be used for forward, side, or downward looking sonar, the authors concentrate on side scan imaging in a ground stabilised frame of reference. This allows the sonar record to be generated in the survey coordinate system without rectification or scan conversion, Computing and sampling costs are minimised by sparse population of the receiver aperture, using a separate transmission array with controlled beamwidth to suppress grating lobes. Synthetic aperture processing uses essentially the same computation, and is carried out for a wideband signal without frequency decomposition. When different arrays are used for transmission and reception, azimuthal resolution depends on transmission array length, but maximum movement between pings without aliassing is set by the length of the receiver array, so high resolution is compatible with fast survey. As receiver aperture length increases, the resolution of the physical aperture images which combine to form the synthetic image also increases, and self-registration of these images to reduce positioning requirements becomes interesting. This possibility is discussed briefly