Using passive sonar for underwater acoustic source localization in a shallow-water environment is challenging due to the complexities of underwater acoustic propagation. Matched-field processing (MFP) exploits both measured and model-predicted acoustic pressures to localize acoustic sources. However, the ambiguity surface obtained through MFP contains artifacts that limit its ability to reveal the location of the acoustic sources. This work introduces a robust scheme for shallow-water source localization that exploits the inherent sparse structure of the localization problem and the use of a model characterizing the acoustic propagation environment. To this end, the underwater acoustic source-localization problem is cast as a sparsity-inducing stochastic optimization problem that is robust to model mismatch. The resulting source-location map (SLM) yields reduced ambiguities and improved resolution, even at low signal-to-noise ratios, when compared to those obtained via classical MFP approaches. An iterative solver based on block-coordinate descent is developed whose computational complexity per iteration is linear with respect to the number of locations considered for the SLM. Numerical tests illustrate the performance of the algorithm.
Results are reported from field tests of networked acoustic modems used for wireless real-time delivery of oceanographic measurements from a distributed array of subsurface instruments in coastal waters. The network demonstrated consists of sensor nodes, repeater nodes, gateway nodes, and a shore-based control center. Sensors are oceanographic instruments interfaced with acoustic modems, deployed in trawl-resistant bottom frames with azimuthally omnidirectional acoustic signaling needed for flexible network rerouting. Repeaters are individual acoustic modems to relay data so the array covers a larger area; only these relatively low-cost nodes are suited for deployment unprotected from trawlers. Gateways are buoys with acoustic modems interfaced to cellular telephone modems for communication between the underwater network and the shore. The experiment site is the inner continental shelf off Montauk Point, New York, and Block Island, Rhode Island, with a U.S. Coast Guard navigation buoy equipped as a gateway. Conditions span a variety of sound-speed profiles, water depths (ϳ25-50 m), and seasons. Long-term average rates of successful transmissions fall to about 50% at a range of 3-4 km in the typically adverse shallow-water acoustic channel. This is adequate for networked acoustic modems to be cost effective in providing quantities of data typically required for data assimilative modeling of coastal oceanographic processes. Modem range degrades in association with increased winds; numerically modeled rays indicate that direct paths between nodes commonly do not exist. Networking functions demonstrated include handshaking protocols, receive-all gateway mode, and rerouting of data pathways from shore in response to a repeater node that is trawled out.
Performance of networked undersea acoustic communications is improved through implementation of handshaking between each pair of adjacent modems along the network route. The handshake begins with the sending modem transmitting a short Request To Send (RTS) packet to the receiving modem. On successful receipt of the RTS, the receiving modem replies with a short Clear To Send (CTS) packet. In the event of failure to complete the handshake, a timer in the transmitting modem triggers additional RTS transmissions. On successful completion of the RTSKTS handshake, the sending modem transmits the data packet. Large data packets can require acoustic transmission times on the order of tens of seconds. During these long transmissions, there is increased potential for dropped packets as a result of unrecoverable bit errors. The Automatic Repeat Request (ARQ) is a means of accomplishing a successful, error-free data transfer in the event of such dropped data. The receiving modem, upon receipt of a corrupted data packet, issues a short ARQ packet to the sending modem that acts as a request to resend the data packet or portions thereof. Statistics from an actual undersea acoustic network demonstrate the advantages of using RTSKTS handshaking and ARQ retransmissions.
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