Unexploded ordnance (UXO) in coastal areas remains an unwanted barrier to safe passage of commercial sea traffic during peacetime in many parts of the world. Clearing these areas requires sophisticated underwater sensor systems that can detect and identify UXO for subsequent marking and removal. Testing and evaluation of these sensor systems is expensive and would benefit from design tools that help to identify potential problems via simulation of sensor concepts. For sensors based on acoustic technologies (sonar), a natural test bed representative of the many environments possible with UXO and with characteristics that can be controlled for sensor testing and comparison is difficult to find. Bottom characteristics, water depth, and water turbidity can be quite variable between tests and even over the duration of a given test. Test targets can also experience varying levels of burial.
Under these conditions, assessing a given sensor’s performance against another’s or assessing a given sensor’s performance over the range of environmental conditions likely to be encountered can be problematic without extensive field measurements. Given the expense of underwater tests, sensor performance modeling makes a great deal of sense for mitigating the need for extensive field testing and helping to identify potential difficulties. Where sensors have not been tested under common conditions, sensor modeling can be used to extend a sensor’s performance curve past the test conditions; thus, making consistent comparisons and assessments possible.
The objective of this project was to adapt and use the sonar performance prediction capability developed at Naval Surface Warfare Center – Panama City (NSWCPC) to support testing and evaluation of sonar and environment configurations that could address the underwater UXO problem.
This development built upon existing sonar performance prediction software (PCSWAT) developed to aid sonar design for underwater mine-countermeasure purposes at NSWCPC. The wide variety and distribution of UXO required a number of scientific issues to be resolved in order to update PCSWAT appropriately. UXO are expected to be completely buried, sometimes quite deeply. Thus, the sonar response of buried targets needed to be investigated and new target scattering algorithms used to account for burial effects formulated, checked through controlled measurements, and incorporated into PCSWAT. Among the issues that were explored to accomplish this were: what environmental parameters are most important in modeling the response of buried targets, how does one accurately measure these inputs, can the target models currently built into PCSWAT be extended to the shapes and sizes encountered with UXO, and what environmental factors affect acoustic detection of buried targets the most. Modeling and experimental measurements were carried out to answer these questions to the degree possible.
The latest version of PCSWAT (v. 9) now has sonar simulation capability for buried UXO targets with some validation in sandy underwater environments. Five UXO shapes spanning the range of UXO sizes have been built in as representative targets for use in simulations. Algorithms to account for surface roughness effects, which have been shown to significantly enhance shallow-grazing-angle detection of buried targets, have been included. Some concerns remain to be resolved in validation of the buried target predictions with measurements at shallow sonar grazing angles, but the current version of PCSWAT is expected to provide realistic simulations of sonar imagery for sonar operated above the critical grazing angle on targets lying above or buried in sandy sediments. For shallow grazing angles, performance prediction is expected to be good for targets lying above or buried under flat surfaces and to provide acceptable trends for targets buried under rough, rippled sand surfaces.
While ongoing research will work toward improving the fidelity of predictions and extending validation to more diverse environments, the current version of PCSWAT should provide a useful tool for obtaining a first-order assessment of existing sonar against unburied and buried UXO.