Results from a field test of an autonomous drifting line array

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.