N101-014 TITLE: High Gain Array of Velocity Sensors

TECHNOLOGY AREAS: Air Platform, Sensors, Battlespace

ACQUISITION PROGRAM: Advanced Extended Echo Ranging (AEER) ACAT IV; PMA-264, Air ASW Systems

RESTRICTION ON PERFORMANCE BY FOREIGN CITIZENS (i.e., those holding non-U.S. Passports): This topic is "ITAR Restricted." The information and materials provided pursuant to or resulting from this topic are restricted under the International Traffic in Arms Regulations (ITAR), 22 CFR Parts 120 - 130, which control the export of defense-related material and services, including the export of sensitive technical data. Foreign Citizens may perform work under an award resulting from this topic only if they hold the "Permanent Resident Card", or are designated as "Protected Individuals" as defined by 8 U.S.C. 1324b(a)(3). If a proposal for this topic contains participation by a foreign citizen who is not in one of the above two categories, the proposal will be rejected.

OBJECTIVE: Develop a concept for a free floating high gain array of velocity sensors that is deployable in an inexpensive A-size sonobuoy and can operate passively in deep and shallow water.

DESCRIPTION: A sonobuoy that could provide large array gain (>24db required) would be an appealing complement to the US Navy�s airborne Anti-Submarine Warfare (ASW) acoustic capability. To provide this much gain in an A-size form factor* it is necessary to make maximum use of velocity sensors. The use of velocity sensors has the potential to reduce the array aperture by a factor of three or four. Both two and three axis velocity sensors have been initially analyzed for a line array of velocity sensors under isotropic noise conditions [1]. To realize maximum gain it is expected that realistic vertical ambient noise profiles will have to be used to determine the array gain from various sites, sensors and depths. The array should be designed for frequencies on the order of 1000 Hz and the sonobuoy bandwidth shall be determined by the aggregate bandwidth which is a function of the acoustic bandwidth of the individual sensors, number of sensors, and the number of bits per sample. The sensor array must be compatible with the A-size sonobuoy form factor and employ the new sonobuoy digital data link. In-buoy signal processing is allowed to alleviate data link issues.

The new data link shall employ Continuous Gaussian Frequency-Shift Keying (CGFSK) with a signal data rate of 320 kbits, a modulation index of 0.75, and a bandwidth time product of 0.3. A total of 288 kbits shall be used for nominal acoustic data transmittal, with 32 kbits reserved for overhead. This Radio Frequency (RF) constraint is noted due to the expected large number of channels generated by two or three axis velocity sensors. Appropriate processing and beamforming shall be implemented to take full advantage of velocity sensor potential.

In addition to the array geometry, and because of the large number of velocity sensors expected to be needed, a major challenge of the effort will be sonobuoy development.

* A-size � refers to the standard U.S. Navy Sonobuoy form factor: right-circular cylinder of diameter d=4.875" and of length L=36"; maximum weight is 39 lbs.

PHASE I: Develop an initial conceptual design for the high gain (>24db) velocity sensor array. Perform modeling and simulate candidate arrays in realistic noise fields at various sites, sensors and depths. Develop innovative packaging concepts for an A-size design. Develop or identify innovative design for small inexpensive velocity sensors.

PHASE II: Develop, construct, and demonstrate the operation of a prototype array through over the side testing utilizing electronically generated broadband and narrowband signals. Validate the over the side prototype meets design goal for array gain. Provide signal processing needed to demonstrate array performance. Complete component design and fabrication of an A-size prototype to illustrate packaging concepts.

PHASE III: Develop a production design of Phase II solution. Demonstrate full operational functionality in Navy-supported test scenarios. Transition the developed technology for fleet use and provide a detailed supportability plan.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Passive detection of acoustic signals from the array has potential applications in marine mammal detection, drug interdiction and terrorist security systems. The Coast Guard will find it useful in coastline and harbor defense.

REFERENCES:
1. Cray, Benjamin A., Nuttal, Albert H., "Directivity Factors for Linear Arrays of Velocity Sensors", J. Acoustic Soc. Am. 110 (1) pg 324-33, July 2001.

2. Silvia, Manual T., "Theoretical and Experimental Investigation of Acoustic Dyadic Sensors", SITTEL Corporation Technical Report TP 4, Jul 25 2001, (DTIC Report No. ADA3433).

3. Urick, R. J., "Principles of Underwater Sound", 3rd Edition, McGraw Hill, 1983.

4. McConnell, J. A., Jensen, S. C., Rudzinsky, J. P., "Forming First-and Second-Order Cardioids Using Multimode Hydrophones", MTS/IEEE Oceans 2006 Conf. Proc., Boston, MA, September 18-21, 2006.

KEYWORDS: passive acoustics; velocity sensors; arrays; array gain; two-axis; three-axis

** TOPIC AUTHOR (TPOC) **

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