Synthetic aperture sonar: An evolving technology

Sea Technology, Jun 1999 by Garrood, Dennis, Lehtomaki, Norm, Luk, Tony, Neudorfer, Mark, Palowitch, Andrew

Radar Technology Used Underwater for Mine Hunting and Unexploded-Ordnance Location

Raytheon Systems Co. (RSC) and Dynamics Technology Inc. (DTI) have demonstrated that the synthetic aperture developed for radar can be used to improve underwater sonar images. Synthetic aperture sonar (SAS) technology uses the forward motion of a small physical array to synthesize a much larger array. The long synthesized array produces a much higher along-track resolution and signal-to-noise ratio (contrast) than that of the physical array alone. Using SAS technology, the alongtrack resolution can be made constantindependent of frequency and range. As a consequence, lower operating frequencies (lower absorption) can be used in a SAS to increase range or penetrate the seafloor without compromising resolution.

RSC and DTI were contracted by Defense Advanced Research Projects Agency (DARPA) at the end of calendar year 1995 to demonstrate 10 centimeters of along track (azimuth) resolution at a range of 1,000 meters. The data for SAS processing was to be gathered under varied multipath conditions. To accomplish these objectives, RSC designed and tested a sonar data-collection system. Most of the hardware was based on existing RSC advanced development systems. The DTI image processing algorithms were modified from code developed by the synthetic aperture radar community to incorporate the physics of underwater acoustics. The contractual arrangement called for RSC to be responsible for the SAS hardware, integration of navigation sensors, and conduct of the field tests and DTI to be responsible for the SAS algorithms.

Separate from the DARPA SAS contract, RSC won a broad agency announcement award from National Defense Center of Excellence for Research in Ocean Sciences (CEROS), an agency of the State of Hawaii to develop SAS for the location of buried objects. The detection of unexploded ordnance (UXO) buried in near-shore waters is of interest to both the Department of Defense (DoD) and the State of Hawaii. RSC developed a purpose-built system that leveraged hardware from the DARPA SAS program together with advanced sonar technologies developed on RSC investment funds.

Key discriminators of these two programs are the emphasis on the SAS algorithm development and an experimental determination of the limits of the SAS. Algorithm development has focussed on extraction of elementposition information from sonar backscatter, computational efficiency, focussing in the subbottom and interferometric SAS (IFSAS). The physical parameters explored include the maximum resolution at long ranges, image formation under multipath conditions, formation of SAS images in the subbottom, and control of synthetic aperture grating lobes caused by yaw errors.

System Description

Two test-bed SAS systems were developed sharing a common keel hydrophone, tow winch, and umbilical. The systems have different projectors housed in the adjustable wings customized to the differing system purpose: mine hunting (DARPA) and unexploded ordnance detection (CEROS). The major features of the systems are:

Operating Frequency (kHz): DARPA 50; CEROS 12.5

Transmit Bandwidth (kHz): 8

Transmit Beamwidth (degrees): DARPA 2V, 20H; CEROS 64V, 84H

Doppler sonar: DARPA, RDI; CEROS, none

Tow capability: DARPA heavy; CEROS heavy & neutral

Real-time logged channels: DARPA 16; CEROS 64

Operational depth (meters): 50

Tow speed (knots): 2 to 10.

The CEROS towfish consists of three acoustic arrays, an inertial measurement unit (IMU), a depth sensor, signal conditioning, and a telemetry system to transport the sonar and sensor data to equipment on the surface. Surface equipment includes sonar and sensor displays as well as data-logging facilities and a GPS system for measuring ship's position. Two tow configurations are supported:

The heavy tow, towed from the vehicle midpoint with submerged weight of 1,500 kilograms

Neutral tow, using a depressor weight and buoyant tether to the front of the towfish.

The heavy tow, which is easier to handle from most vessels, is used in confined, sheltered waters such as Puget Sound (State of Washington). The neutral tow is used in open water where a heavy depressor weight decouples the towfish motion from the tow-vessel heave.

When configured for UXO detection tests, the 5.5-meter towfish (height 1.5 meters) employs wing and keel acoustic transducer arrays to provide along-track and cross-track acoustic aperture to increase the target-to-bottom reverberation ratio and allow for spatial rejection of spurious boundary responses. The wing angle can be adjusted to preset positions between 0 deg and 90 deg depending on the mode of operation.

The projector arrays consist of the upper and lower 5 x 5 sets of elements on each wing. Four transmitters are used to drive subsets of the 25-element array. Time series of arbitrary waveforms can be downloaded to the transmitter(s) control logic. Time delays in the time-series waveforms are used to shape the final transmitted beampattern. The area of the array is designed for shallow-water operation by limiting power to approximately 1 watt/ square centimeter to avoid cavitation at full drive power.