OBJECTIVE: Determine the feasibility of developing a Reliable Acoustic Path Vector Sensor Sonobuoy System DESCRIPTION: The Navy is becoming increasingly interested in the prospect of deploying acoustic sensing systems below critical depth in the ocean at extremely deep depths close to or on the ocean bottom in convergent zone type environments . At these depths the ambient noise structure and sound propagation physics are unique  and have the potential to be exploited by future surveillance systems. The concept of reliable acoustic path sensing within the sonobuoy community is not new since a sonobuoy known as the reliable acoustic path sonobuoy was developed in the 1980s, but never went into production . That buoy employed monostatic sonar system architecture and had an operational frequency of approximately 4 kHz. Recent investigation of the ambient noise structure in the deep ocean  suggests that a passive directional sonobuoy system covering the band from 5 to 500 Hz would be of interest. When the sea state is calm, the ambient levels are nominally 40 to 50 dB re 1 1/4Pa2/Hz . A directional sonobuoy comprised of a single triaxial vector sensor having an electronic noise floor that is 15 to 20 dB below the ambient (25db re 1 1/4Pa2/Hz goal) is thought to be well-suited for this application particularly in view of the array gains achievable as a result of the anisotropic noise field. The low electronic noise specification puts extreme demands on the sensor given the A-size sonobuoy form factor coupled with operation at 6 km depths . Another major challenge concerns the use of a hard-wire telemetry link to route the data from the sensor to a surface buoy which in turn has an RF link. The term hard-wire is meant in the general sense of having a physical connection from the sensor to the surface buoy. Given this physical connection, special consideration must be made concerning how the system is packaged, deployed, and mitigates self-noise. In addition to the difficult requirements for the triaxial sensor, innovative sonobuoy engineering concepts are needed to achieve this goal. The lessons learned regarding mitigation of vertical motion and flow-induced noise on the AN/SSQ-53 DIFAR sonobuoy  should be folded into the design, as appropriate. RF link considered shall be the new proposed sonobuoy link which is composed CPGFSK (Continuous Phase Gaussian Frequency Shift Keying) waveform of 320 kbps of which 288 kbps can be acoustic data. Note that A-size refers to the standard U.S. Navy Sonobuoy form factor or a right-circular cylinder having a diameter, length, and maximum weight of D=4.875 inches, L=36 inches, and W=39 pounds. PHASE I: Perform modeling and simulation studies to evaluate prospective sensor, telemetry, packaging, deployment, and self-noise remediation designs within the overall architecture of an A-size sonobuoy PHASE II: Fabricate and test a pre-prototype sonobuoy for an over-the-side test in a convergence zone type area location PHASE III: Develop a production design of the 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: Use of these sensors has potential applications in seismology, marine mammal detection, and terrorist security systems.