TECHNOLOGY AREA(S): Battlespace

ACQUISITION PROGRAM: PMA 290 Maritime Surveillance Aircraft

OBJECTIVE: Improve the detection rate of targets through the understanding and modeling of ocean variability resulting in a robust model that can eventually be incorporated into mission planning software.

DESCRIPTION: Variance in transmission loss (TL) between an acoustic source and a target can have a profound effect on detection performance using low-frequency (50 � 3000 Hz) active acoustics.� This variance has often been observed in real data, but is not well-captured in modeling, simulation, or post-test reconstruction.� While ambient noise or reverberation is a prominent contributor to signal excess in the sonar equation, it cannot be controlled, is easily measured, and has shown reproducibility between like measurements.� However, unlike ambient noise, TL has shown a large variance between like measurements.� This poor repeatability of measurements is especially challenging where detections are made at the threshold (i.e., close to or at 0 signal excess).� If this variability can be well understood in terms of the ocean environment then it would allow for a more accurate prediction that will aid test planning as well as post-test reconstruction.

In underwater acoustics the ocean environment, characterized by the sound velocity, current, and depth profiles, determines the exploitable propagation path to gain detections.� One such path is the surface duct.� This duct is often present in the top 600 feet of the ocean, where sensors and targets can be easily deployed.� Such an environment causes TL variability [Ref 1], a phenomenon known to be frequency dependent.� Basic oceanographic and acoustic research results on surface duct are available in the literature [Refs 2-3].� Several pieces of physics resulting in TL variability have been suggested, including sea surface scattering [Refs 4-5] and sound energy leaking out of the duct [Ref 6].� Models typically treat the surface duct as two-dimensional and static.� In reality, the duct could be three-dimensional and dynamic.� While models exist to deal with such spatial and temporal variability, the ocean is often not sampled enough for these models to be useful (a single sound velocity profile, for example).� The need exists to develop a stand-alone surface duct software model based on ocean physics that takes into account ocean variability and measured TL variance to aid asset placement in test planning and offer detection uncertainty.� This modeling effort would later be expanded to include surface and bottom interactions and allow the model to compute TL variance over the complete water column.

Work produced in Phase II may become classified. Note: The prospective contractor(s) must be U.S. Owned and Operated with no Foreign Influence as defined by DOD 5220.22-M, National Industrial Security Program Operating Manual, unless acceptable mitigating procedures can and have been implemented and approved by the Defense Security Service (DSS).� The selected contractor and/or subcontractor must be able to acquire and maintain a secret level facility and Personnel Security Clearances, in order to perform on advanced phases of this contract as set forth by DSS and NAVAIR in order to gain access to classified information pertaining to the national defense of the United States and its allies; this will be an inherent requirement.� The selected company will be required to safeguard classified material IAW DoD 5220.22-M during the advance phases of this contract.

PHASE I: Analyze a series of Government-furnished acoustic data sets with ducted propagation and provide a preliminary reconstruction of the acoustic environment.� During the Phase I Option, if awarded, develop a concept for a surface duct model that can predict the observed acoustic environment variability.� With the use of highly sampled (many sound speed profiles) data sets and hind cast ocean model data, further develop the model to provide increasingly accurate sensor level outputs.� The Phase I effort will include plans for a Phase II.

PHASE II: Expand the model to include bottom returns.� Bottom loss and bottom reverberation are currently treated as two separate quantities: Bottom Loss as part of TL, and reverberation is fitted by Lambert�s Law with variable Mackenzie's coefficient.� In reality, however, the two quantities are related by geo-acoustics of the bottom and should be treated in a uniform manner.� The goal is to generate sensor-level signals which are from the bottom return.� This allows consistent TL and reverberation treatment, rather than artificially separating them into reflection and scattering. At the end of the phase the developed model can be employed as a sensor-level simulator that may mimic real system performance.

It is probable that the work under this effort will be classified under Phase II (see Description section for details).

PHASE III DUAL USE APPLICATIONS: Verify and validate the model.� Integrate it into an engineering version of a tactical decision aid, such as the Multistatics Planning Acoustics Toolkit (MPAcT).� The developed technology will benefit the oceanographic community to include academic/research and oceanographic mission planners, as well as the oil exploration industry.

REFERENCES:

1. Acoustic transmission in an ocean surface duct, performed by U.S. Navy Electronics Laboratory, San Diego, California, and analyzed by Arthur D Little, Inc., Dept. of the Navy, Naval Ship Systems Command, NO bsr � 93055, Project Serial Number SF 101-03-21, Task 11353, Nov 1966. https://ia600500.us.archive.org/12/items/acoustictransmis00usna/acoustictransmis00usna.pdf

2. Porter, M. B., Piacsek, S., Henderson, L., and Jensen, F. B. �Surface duct propagation and the ocean mixed layer.� Oceanography and Acoustics Prediction and Propagation Models, 1st ed., edited by A. Robinson and D. Lee (AIP, New York, 1993), pp. 50-79. ISBN: 1563962039. http://trove.nla.gov.au/work/11481132?selectedversion=NBD10719975

3. Jensen, F. B., Kuperman, W. A., Porter, M. B., and Schmidt, H. Schmidt. �Computational Ocean Acoustics�, 2nd ed. (Springer, New York, 2011), pp. 494-495. http://www.springer.com/us/book/9781441986771

4. Vadov, R. �Acoustic propagation in the subsurface sound channel.� Acoustical Physics, January 2006, 52, pp, 6�16. https://link.springer.com/article/10.1134/S1063771006010027

5. Mellen, R. & Browning, D. �Attenuation in surface ducts.� The Journal of the Acoustical Society of America. 63, pp, 1624-1626. http://asa.scitation.org/doi/abs/10.1121/1.381859

6. Weston, C. Esmond, and Ferris, A. Ferris. �The duct leakage relation for the surface sound channel.� The Journal of the Acoustical Society of America. 89, pp. 156�164 (1991). http://asa.scitation.org/doi/abs/10.1121/1.400521

KEYWORDS: Ocean Variability; Transmission Loss; Modeling; Mission Planning; Underwater Acoustics; Surface Duct