Chris Jones, Senior Engineer         Ocean Acoustics Department         Applied Physics Laboratory         University of Washington      Office of Naval Research	PIMS — Pelagic Imaging Mid-frequency Sonar There are no commercial sonar systems capable of imaging large horizontal areas of the ocean using the waveguide, other than Navy towed line-array systems. Ship mounted swath bathymetry systems, side-scan sonars, and split- or single-beam echosounding systems generally do not have sufficient horizontal array aperature for waveguide imaging. Horizontal aperature provides the necessary array gain and beamforming resolution to form images with sufficient SNR and spatial resolution to resolve weak signals from fish schools at longe range. We developed a prototype, horizontally oriented, multi-beam sonar with sufficient beamforming resolution and flexibility in processing to investigate new methods of waveguide imaging. Figure 1 illustrates the system being deployed. The operating frequency of 12 kHz was chosen to maximize sonar range in a waveguide, while minimizing array size (approximately 4' diameter, 5' height). The array is circular, enabling a circular horizontal images of the ocean to be formed with a single transmission. Towed arrays and sidescan systems requires forward motion and ship maneuvers to image a desired section of the ocean. The circular array can be fixed in space while forming an instantaneous, two-dimensional, horizontal images of a circular region of the ocean. In this capacity, the array is well suited for long-term observation, such as in the context of a cabled obsevatory or moored system. Because the array is relatively small, it can be deployed over the side of a stationary ship, much like a CTD rosette. In this capacity, the system can rapidly image an area of the ocean in responce to traditional down-looking acoustic surveys and/or other remote platforms (i.e., AUVs) and oceanographic measurements. The PIMS (Pelagic Imaging Mid-frequency Sonar; Figure 1) consists of a circular receiving array and a linear transmitting array. The circular receiving array has multiple receiving hydrophones (64) that form steered receiving beams with ~5 degrees of horizontal resolution. The transmit array has 16 elements, creating a projected beam that is narrow ~9 degrees in the vertical dimension and omni-directional (360 degrees) in the horizontal direction. The imaging resolution of the system is defined by the intersection of the receiver and transmitter beams (~ 5x9 degrees beams). Figure 2 illustrates the imaging geometry and two deployment configurations. Figure 1. Pelagic Imaging Mid-frequency Sonar. Initial system testing/calibration was conducted at APL-UW. Prior to at-sea deployments, a significant effort was devoted to developing the beamforming and imaging capabilities for the new system. Imaging fish at long range requires extracting weak signals from noisy data and devising new strategies for collecting and processing data, significantly different than traditional multibeam data processing. Figure 3 illustrates two different approaches to beamforming, showing beam patterns derived from the same calibration data. The less accurate and faster method of phase-shift beamforming (shown in the left panel) is used to form images rapidly in near real-time. The better time-delay method (shown in the right panel) is used in postprocessing to extract weak signals. The right panel also illustrates a typical beam pattern realized with the new circular array, showing very good back and side lobe supression. Note that there is no left/right amiguity with a circular array, as there is with towed line arrays. Figure 2. Imaging geometry and two deployment configurations. A circular, horizontal image is created by transmitting a pulse and recording backscatter as a function of time/range along multiple radials in the waveguide. Multiple transmissions in time form a time series of backscatter images. Multiple rapid images in time at a fixed location can improve image quality by averaging incoherent noise, while enhancing weak coherent signals. Multiple images over longer time periods can be used to observe fish behavior and horizontal spatial structure. Figure 3. Two types of beamforming developed for the PIMS array. Left) a fast beamforming method developed of near real-time imaging that has less resolution and higher side and back lobes. Right) an improved beamforming method that can be used in postprocessing to improve image quality.