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Publications

The acoustic oceanographic buoy. A light acoustic data acquisition system

Publication . Soares, C.; Zabel, F.; Martins, C.; Silva, A.; Jesus, S. M.

The Acoustic Oceanographic Buoy (AOB) is a light acoustic receiving device that incorporates acoustic and non-acoustic signals received in various channels along a vertical line array that provide oceanographic and environment measurements all of which are uniquely GPS time referenced. The physical characteristics of the AOB, in terms of size, weight and autonomy, will tend to those of a standard sonobuoy with, however, the capabilities: of local data storage, dedicated signal-processing, GPS self localizing, real-time monitoring and online data transmission.

2007Conference object

Open Access

Passive time reversal probe-signal capture optimization for underwater communications

Publication . Silva, A.; Jesus, S. M.; Gomes, João

Passive Time Reversal (pTR) is an emerging technique for underwater communications where a channel probe is sent ahead of the data packet. The channel probe must be captured in a time window for post crosscorrelation with the data. The Signal to Noise Ratio (SNR) of pTR, and hence the detector error rate, will be dependent on the starting point and duration of this time window. Typically the beginning and length of the time window should depend on the time dispersion of the acoustic channel which, in turn, depends on the environment properties and on the experimental geometry. Heuristic reasoning would suggest that if a short time window fails to include all significant multipath it will result in imperfect focusing, while an overly long window will reduce the efficiency of the communication system by introducing additional noise in the pTR system. That problem calls for an optimization that has previously been addressed only heuristically. In order to bring the pTR capabilities to a practical modem the engineering problem of time window automatic optimisation must be solved, and that is the main purpose of our paper. An expression for the optimal pTR SNR time window length is presented and its validity confirmed with simulations and real data from the INTIFANTE’00 sea trial.

2007Book part

Open Access

Acoustic maritime rapid environmental assessment 2004 during the MREA'04 sea trial

Publication . Jesus, S. M.; Soares, C.; Felisberto, P.; Silva, A.; Farinha, L.; Martins, C.

Environmental inversion of acoustic signals for bottom and water column properties is being proposed in the literature as an interesting concept for complementing direct hydrographic and oceanographic measurements for Rapid Environmental Assessment (REA). The acoustic contribution to REA can be cast as the result of the inversion of ocean acoustic properties to be assimilated into ocean circulation models specifically tailored and calibrated to the scale of the area under observation. Traditional ocean tomography systems and methods for their requirements of long and well populated receiving arrays and precise knowledge of the source/receiver geometries are not well adapted to operational Acoustic REA (AREA). The Acoustic Oceanographic Buoy (AOB) was proposed as an innovative concept that responds to the operational requirements of AREA. That concept includes the development of water column and geo-acoustic inversion methods being able to retrieve environmental true properties from signals received on a drifting network of acoustic-oceanographic sensors - the AOBs. An AOB prototype and a preliminary version of the inversion code, was tested at sea during the Maritime Rapid Environmental Assessment 2003 (MREA’03) sea trial and was reported in [1]. On a separate register it should be noted that the characterization of the environment between the source and the receiver also contributes to the identification of the acoustic channel response and therefore provides a basis for fulfilling the objectives of project NUACE1. The present report describes the data sets and results gathered during the MREA’04 sea trial that took place from 29 March to 19 April 2004 off the west coast of Portugal, south of Lisboa (Portugal), with the objectives of testing an improved version of the individual AOB and its functionality in a simple network. The acoustic part of the experiment lasted for four days between April 7 and April 10, 2004 and involved the transmission and reception of pre-coded signals along range-dependent and range-independent acoustic tracks.

2005Report

Open Access

AOB - an easily deployable, reconfigurable and multifunctional acoustic-oceanographic system

Publication . Jesus, S. M.; Soares, C.; Silva, A.; Hermand, J.-P.; Coelho, E.

The concept of an easy to use and easy to deploy ocean acoustic tomographic OAT system is presented. The system is composed of a network of buoys and a data inversion online processor. This study concerns the individual node of that network—the acoustic-oceanographic buoy AOB —the data inversion technique and the testing of the system at sea. The AOB is a lightweight surface buoy with a vertical array of acoustic and temperature sensors to be hand deployed in a free-drifting configuration from a small boat. The data are locally stored and transmitted online to a remote station for processing and monitoring. Data inversion is based on a broadband matched-field tomography technique where known and unknown parameters are simultaneously searched for focalization . In situ recorded temperature data serve for algorithm initialization and calibration. The AOB was successfully deployed in several consecutive days during two rapid environmental assessment sea trials in 2003 Mediterranean and 2004 Atlantic . Data collected at sea also show that the AOB can be reconfigured as a receiving array for underwater coherent communications in the band up to 15 kHz.

2004Conference object

Open Access

The acoustic oceanographic buoy telemetry system: an advanced sonobuoy that meets acoustic rapid environmental assessment requirements

Publication . Silva, A.; Zabel, F.; Martins, C.

In the past few years Rapid Environmental Assessment (REA), applied to shallow waters, has become one of the most challenging topics in ocean acoustics. The REA concept evolved after the cold war when the outset of regional conflicts shifted the potential operational areas from open ocean towards littoral areas, and has been identified by NATO as a new warfare requirement.

2007Journal article

Open Access