C.L. Fletcher

Navy's Space and Naval Warfare Systems Command, San Diego, California, United States

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Publications (9)0 Total impact

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    ABSTRACT: Underwater passive acoustic directional sensors and Seaweb through-water networked acoustic communications are implemented in the Intracoastal Waterway at Morehead City, North Carolina on the U.S. eastern seaboard. The objective is to demonstrate capability for first-alert protection of a high-value port facility against asymmetric threats that intelligence sources indicate are arriving via watercraft. Battery-powered acoustic sensors are rapidly deployed at widely separated chokepoint locations in shallow 5-10 meter water. These sensors autonomously detect the passage of a maritime vessel and generate a contact report indicating time, location and heading of the target. Seaweb through-water acoustic communications delivers the contact report via a scalable wide-area underwater network including multiple acoustic repeater nodes and a radio/acoustic communications (Racom) gateway buoy. The Racom gateway telemeters the contact report via Iridium satellite communications to an ashore command center with low latency. The in situ acoustic detection is corroborated using shore-based video surveillance to classify the contact as friendly or actionable.
    Waterside Security Conference (WSS), 2010 International; 12/2010
  • C.L. Fletcher, J.A. Rice, R.K. Creber
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    ABSTRACT: The Seaweb server is a suite of software applications for managing the network-layer operations of underwater networks employing acoustic modems. It resides at manned command centers (ashore, afloat, submerged, aloft, or afar) and handles application-layer telemetry with mobile and stationary underwater nodes. The server communicates with the undersea network through gateway nodes, such as moored buoys, surface vehicles, or submarine sonars.
    OCEANS 2003. Proceedings; 02/2003
  • R.K. Creber, J.A. Rice, P.A. Baxley, C.L. Fletcher
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    ABSTRACT: The 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 RTS/CTS 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 re-send the data packet or portions thereof. Statistics from an actual undersea acoustic network demonstrate the advantages of using RTS/CTS handshaking and ARQ retransmissions
    OCEANS, 2001. MTS/IEEE Conference and Exhibition; 02/2001
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    ABSTRACT: An experiment in the Adriatic Sea investigated the potential practicality of an inexpensive drifting hydrophone array, preliminarily for use in controlled passive acoustic experiments. Some of the technical challenges that needed to be overcome to make such a system practical included: (1) hydrodynamic stability, (2) dynamic acoustic element localization, (3) in-buoy signal processing to decrease communications bandwidth, and (4) data transmission via low power radio frequency communications from a small buoy. We addressed some of the practical engineering problems inherent in the design of an air-deployed drifting array that is envisioned to eventually be deployable from a sonobuoy launcher, assume a short-term stable configuration while freely drifting in the ocean, and relay compressed or processed data via radio. New developments in electronics (e.g., single-conductor, time-division, digital multiplexing), sensor technology, digital signal processing, and communications were applied to the design problem. In this paper we describe the results from a sea test conducted in the Adriatic Sea during October, 2000. We conclude that: (1) deployment of such an apparatus from a small vessel was realistic and can be extended to autonomous deployment from a variety of platforms; (2) once deployed, the apparatus could be expected to assume a steady-state physical configuration that was stable over representative ADLA deployment time scales (hours to days), given varying oceanographic and meteorological conditions. It is shown that, over a range of conditions, the array behaved in a predictable and measurable manner sufficient to yield data usable as input to standard beamforming algorithms
    OCEANS, 2001. MTS/IEEE Conference and Exhibition; 02/2001
  • Source
    C.L. Fletcher, J.A. Rice, R.K. Creber, D.L. Codiga
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    ABSTRACT: A "seaweb server" manages the network-layer operation of underwater networks employing telesonar modems. The server is a suite of software routines that operates at manned command centers ashore, afloat, submerged, or aloft. It provides an operator with an interface to the undersea network via gateway nodes. It handles data packets and utility packets and archives the incoming and outgoing data into a database. Through a Web browser, clients access incoming data archived by the server and submit outgoing data to the server for prioritized delivery to the undersea network
    OCEANS, 2001. MTS/IEEE Conference and Exhibition; 02/2001
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    ABSTRACT: Seaweb '98, Seaweb '99, and Seaweb 2000 begin a series of annual experiments incrementally advancing telesonar underwater acoustic signaling and ranging technology for undersea wireless networks. The constraints imposed by acoustic transmission through shallow water channels have yielded channel-tolerant signaling methods, hybrid multi-user access strategies, novel network topologies, half-duplex handshake protocols, and iterative power-control techniques. Seawebs '98 and '99 respectively included ten and fifteen battery-powered, anchored telesonar nodes organized as noncentralized hi-directional networks. These tests demonstrated the feasibility of battery-powered, wide-area undersea networks linked via radio gateway buoy to the terrestrial internet. Testing involved delivery of remotely sensed data from the sea and remote control from manned command centers ashore and afloat. Seaweb 2000 introduces new telesonar modem hardware and a compact protocol for advanced network development
    OCEANS 2000 MTS/IEEE Conference and Exhibition; 02/2000
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    ABSTRACT: The problem of robust gain control in autonomous sonar systems involves balancing efficient use of the information channel against the possible loss of data. If not carefully designed and executed, automatic gain control (AGC) systems can be oscillatory, so gain change criteria must be conservatively implemented to ensure a stable system that tends to remain at a particular gain setting in the presence of short transient signals. This paper describes the design and implementation of an automatic gain control technique for a passive line array of hydrophones. The method allows an inexpensive, low-power, 12-bit analog-to-digital converter to be used to digitize hydrophone outputs in an acoustic environment wherein the input signal amplitude may vary by as much as roughly 100 dB. The algorithm runs on a low power (less than 12 mW) programmable logic device (PLD). In real time, the system autonomously examines a 944 kbps digital data stream, makes decisions, and sends gain commands to individual sensor elements in an array. For power efficiency, no microprocessor intelligence is utilized to implement the AGC algorithm, which is simple enough to be implemented in digital hardware with counters, timers, and comparators in a single PLD that also controls the array signal multiplexing. We describe the performance of the technique as implemented in a 96-element seafloor planar array deployed in the Timor Sea as part of the November 1998 Rapidly Deployable Systems Experiment
    OCEANS '99 MTS/IEEE. Riding the Crest into the 21st Century; 02/1999
  • V.K. McDonald, J.A. Rice, C.L. Fletcher
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    ABSTRACT: The US Navy is developing undersea acoustic telemetry and ranging technology (telesonar) for use in shallow water. This paper describes an autonomous, high-fidelity, testbed developed to aid data acquisition, signal design, and modem development
    OCEANS '98 Conference Proceedings; 01/1998
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    ABSTRACT: Seaweb networks use digital signal processor (DSP)-based telesonar underwater acoustic modems to interconnect fixed and mobile nodes. Backbone nodes are autonomous, stationary sensors and telesonar repeaters. Peripheral nodes include unmanned undersea vehicles (UUVs) and specialized devices such as low-frequency sonar projectors. Gateway nodes pro-vide interfaces with command centers afloat, submerged, ashore, and aloft, including access to ter-restrial, airborne, and space-based networks. Seaweb command, control, communications, and navigation (C 3 N) technology coordinates deployable assets for accomplishing given missions in littoral ocean environments. A series of annual experiments drives seaweb technology devel-opment by implementing increas-ingly sophisticated wide-area networks of deployable autonomous undersea sensors.