Implementation of an acoustic data bus

Advancement in the field of signal processing in the past several decades has resulted in noise ranging being recognized as an important condition-monitoring requirement for naval vessels. It can provide valuable information pertinent to its maintenance and combat-worthiness. In its most basic form, an underwater noise range (UWNR) consists of a number of fixed seabed hydrophones arranged to detect the sound radiated from a passing surface ship of submarine. The received signal is transmitted along a communication medium (underwater cables/microwave link/etc.) to a suitable portable or land-based analysis and recording equipment. At the shore end, the signal is subjected to a preliminary on-line and a detailed off-line exploratory analysis. The purpose of the first is to determine the "good-ness" of the run and to decide on modification of the "run-schedules". The purpose of the latter is to perform a detailed examination of the signals and compare it with similar observations collected earlier from the same vessel or different vessels. These analysis results may be considered as the information output of the system - the recorded signals being the data outputs. Noise ranging is an activity requiring a detailed planning, many months in advance. The available time of the vessel on the range has to be utilized in the most optimum fashion. This necessitates a detailed online analysis of the acoustic signal originating from various parts of the ship over a wide frequency range. An acoustic data bus (ADB) over which the analysis machines could be connected is an ideal architecture to facilitate such an analysis. This work presents the structure, design and implementation aspects of such a system based on a set of DSP boards interconnected in a copper TAXI set-up. In the system under implementation, the acoustic signals are digitized close to the sensor. This reduces further noise contamination, thus allowing an SNR of 186 dB. The digitized signals from various sensors are sent to an underwater junction box where they are multiplexed and converted in the optical domain. This multiplexed stream is transported over FO cables to a data handling system (DHS) at shore. DHS performs the optical to electrical conversion and presents the signal stream to a pair of DSPs. The DSPs act as the hub of the ADB. It demultiplexes the signals into various channels and attaches headers to it. The header contains environmental parameters like air and water pressure and DGPS tracking information, needed to determine the extent of the various corrections (range, Doppler, etc.) to the signal spectrum. The DSPs at DHS are connected to a set of acoustic analysis computers (AAC) using individual copper TAXI lines. This arrangement permits a sustained data rate of 20 MBPS and distances of a few hundreds of meters between DHS and AACs. Each AAC contains a complementary DSP board that receives the signal from the DHS and performs various signal processing functions. The AAC can select any channel for analysis. The suite consists of narrow band, third octave, Lofar and Demon analysis. This work presents the general structure of the noise range proposed. The system is designed for high reliability at the underwater segments and high performance at the land segment.