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Improving Sonar Imagery Enhances Naval Preparedness
Kenneth G. Foote,
Woods Hole Oceanographic Institution
The role of sonar in underwater defense is diverse and vital. Mine countermeasures (MCM) is just one application for which the quality of acoustic data is paramount: Consequential decisions are made on their basis.
The number of electronic, signal processing and other operations that enable imaging and measurement is vast. How is order assured in a flood of data requiring rapid, synchronous compounding of multiple signal channels and automatic target recognition (ATR) sufficient for recognizing objects and measuring their physical attributes?
The answer is calibration. This enables the output to be expressed in absolute physical units of backscattering, in turn enabling cross-comparison with measurements performed by the same sonar at different times, as for change detection, and by different sonars on the same object or patch of seafloor or sub-bottom volume.
Sonar calibration has further advantages: verification of performance against specifications or other operating standards, detection of a performance change, and short- and long-term monitoring of the health of the sensor. It also offers a path to improvements in sonar imagery and measurement.
This annual special issue has often, if indirectly, highlighted the importance of calibration. Advances in the detection and classification of mines with synthetic aperture sonar (SAS) were described in the 2012 issue. One year later, reducing high false alarm rates in MCM, and anti-submarine warfare, operations was described. The respective advances are to be achieved by measurement of target strength to augment imaging and inversion for material-property information. Calibration is essential to both of these projected advances. In the case of imaging, as in SAS, it is essential for the balanced weighting of multi-ping channels.
Notwithstanding the complexity of multiple sonars and processing, calibration is straightforward if undertaken by the standard-target method. This has been in worldwide use for the calibration of scientific echosounders used in fisheries research since the early 1980s. The method has since been extended to multibeam sonars that provide the water column signal and to the difference-frequency band, 1 to 6 kilohertz, of a parametric sonar used for sub-bottom profiling and water column detection.
According to the standard-target method, the combined transmit-receive response of the sonar is measured to a tolerance determined partly by the noise. This influence may be minimized by a judicious choice of the target and operating conditions for the calibration, with allowance for performance in situ near to the time and place of operations. The beam pattern can also be determined in situ, exploiting backscattering by the seafloor. Measurement of both the overall system response and the beam pattern is an unbeatable combination for accurate, efficient, rapid, and cost-effective calibration of a sonar as configured for operational use. Measurement results can now be expressed in absolute physical units of backscattering, including echo processing. This will satisfy the sonar engineer and user alike. For those in command of naval assets depending on sonar performance, it will give valuable pre-mission assurance of readiness.
This message may resonate with some. There is another group that may find recourse to history persuasive. This is the centennial of the start of the First World War. It has been said that Winston Churchill, the First Lord of the Admiralty, never fully recovered from the disastrous Dardanelles Campaign, whose outcome was determined by mining of the strait. Anecdotally, an ex-Navy commander was overheard saying that the worst experience of his life, bar none, was realizing that his ship was in a minefield. In both cases, the responsibility was felt and taken personally. These stories are powerful motivators for scientists and engineers, the more so when developing sonar for MCM. Seizing every opportunity to improve naval sonar images and measurements makes for greater safety where it matters most.