Wireless underwater acoustic beamforming using chip-scale atomic clock timers

Abstract

Obtaining the directionality of the sound field is typically performed using a cabled array of hydrophones, connected to a central data collection device. The cable introduces vulnerability wherein an electrical break in the line or dragging by currents or anchors may render the array inoperable. An underwater wireless beamforming system, which transmits time and recorded low-frequency sounds using directional high-frequency acoustic communications between elements, is presented here. Each element is contained in a standalone housing that contains batteries, amplifiers, signal conditioning hardware and a field-programmable gate array (FPGA)-based signal processing computer. Low-frequency sounds are recorded at a sampling rate of 2 kHz, then transmitted using binary phase-shift-key (BPSK) encoding at a center frequency of 750 kHz. Inter-element spacing estimates are facilitated by the time-stamped high-frequency signals. System timing is controlled by a Symmetricom® SA.45s chip-scale atomic clock (CSAC) embedded in each unit, connected to the FPGA. In addition, a CSAC output waveform is divided and modified to form the high-frequency multiplier waveform for the BPSK signal. The elements were deployed in shallow water offshore of Panama City, FL, in August 2015. [This work was supported by the Office of Naval Research.]