The many paths to underwater acoustic communication

Because most electromagnetic waves are strongly attenuated underwater, communications—between a ship and remote sensors or unmanned autonomous vehicles, for example—typically rely on sound waves. But transmitting information acoustically through the noisy ocean environment presents its own challenges: Repeated scattering and distortions cause an acoustic signal to reach the receiver via multiple paths, arriving from varying directions with varying delays. (See the articles by Tom Sanford, Kathie Kelly, and David Farmer, Physics Today, February 2011, page 24 , and by Bill Kuperman and Jim Lynch, October 2004, page 55 .) For extracting the information, so-called passive time reversal is an increasingly common approach. The transmitter precedes each message with a short pulse. Using an array of sensors, the receiver documents the pulse’s spread in space and time; the figure shows the multiple paths experimentally recorded for one such pulse. By reversing the measurements in time and mathematically correlating them with the message that follows, one can effectively aim the sensor array so that it focuses on the transmitter. (For more on time-reversed acoustics, see the article by Mathias Fink, Physics Today, March 1997, page 34 .) Surface waves and relative motion between the sender and receiver introduce variable, path-dependent Doppler shifts. Compensation techniques typically apply an overall Doppler correction. But Sérgio Jesus, Salman Siddiqui, and António Silva at the University of Algarve in Portugal demonstrate a method to calculate and incorporate frequency corrections separately for each arriving wavefront. Testing the method with real data, the team achieved a nearly 5-dB reduction in the transmission’s mean square error. (S. M. Jesus, S. I. Siddiqui, A. Silva, J. Acoust. Soc. Am. 137, EL300, 2015, doi:10.1121/1.4915005 .)