A. Pescetto

Spread Spectrum Modulation for Acoustic Communication in Shallow Water Channel

A wireless underwater acoustic communication system based on the combination of chirp modulation and direct-sequence spread-spectrum signaling is presented. The use of chirp signals takes advantage of the low Doppler sensitivity in the matched filter operation whereas the choice of pseudo noise (PN) sequences allows one to reduce narrowband interference arising from other users and self-interference due to multipath propagation. The system here described is made of a transmitter encoding the bits with linear chirps multiplied by PN sequences, and a rake receiver that allows one to positively exploit the energy present in the most significant propagation paths. Moreover a tracking procedure that allows an adaptation to the instantaneous Doppler shift has been devised and tested. Results, obtained on real data over a long-range shallow-water channel with a moving transmitting platform, are presented and discussed.

Experimental validation of a chirp-based underwater acoustic communication method

A wireless underwater acoustic communication algorithm based on the combination of chirp modulation and direct-sequence spread-spectrum signalling is presented. In this paper, the processing chain design is proposed, discussed, and demonstrated using real data. The communication algorithm is made of a transmission block encoding the bits with adequate linear chirps multiplied by pseudo noise (PN) sequences, and a rake receiver that allows one to positively exploit the energy present in the most significant propagation paths. The use of chirp signals takes advantage of the low Doppler sensitivity in the matched filter operation whereas the choice of PN sequences allows one to reduce narrowband interference arising from other users and self-interference due to multipath propagation. Moreover a tracking procedure that allows an adaptation to the instantaneous Doppler shift has been devised and tested. Some experiments have been carried out changing the distance and the speed between transmitter and receiver. Results show that the developed communication method is able to handle the multipath phenomenon and the Doppler effect, allowing one to achieve a bit error rate less than 10^(-3) for long ranges for bit rates of about 15 and 230 bit/s.

Data extraction system for underwater particle holography

Pulsed laser holography in an extremely powerful technique for the study of particle fields as it allows instantaneous, non-invasive high- resolution recording of substantial volumes. By relaying the real image one can obtain the size, shape, position and - if multiple exposures are made - velocity of every object in the recorded field. Manual analysis of large volumes containing thousands of particles is, however, an enormous and time-consuming task, with operator fatigue an unpredictable source of errors. Clearly the value of holographic measurements also depends crucially on the quality of the reconstructed image: not only will poor resolution degrade the size and shape measurements, but aberrations such as coma and astigmatism can change the perceived centroid of a particle, affecting position and velocity measurements. For large-scale applications of particle field holography, specifically the in situ recording of marine plankton with Holocam, we have developed an automated data extraction system that can be readily switched between the in-line and off-axis geometries and provides optimised reconstruction from holograms recorded underwater. As a videocamera is automatically stepped through the 200 by 200 by 1000mm sample volume, image processing and object tracking routines locate and extract particle images for further classification by a separate software module.

Feature tracking and depth estimation in front-scan sonar image sequences

This paper describes a set of methods that make it possible to estimate the position of a feature inside a three-dimensional (3D) space by starting from a sequence of two-dimensional (2D) acoustic images of the seafloor acquired with a sonar system. The front-scan sonar devoted to generate a 2D image of the seafloor to sail over, and allows one to collect a sequence of images showing a specific feature during the approach of the ship. This fact seems to make it possible to recover the 3D position of a feature by comparing the feature position along the sequence of images acquired from different (known) ship positions. A feature extraction and analysis, a Kalman filter for robust feature tracking, and some ad hoc equations for depth estimation are proposed. Simulated image sequences demonstrated the great potential of the developed system, even though real sequences pointed out some inaccuracies due to errors concerning the knowledge of the ship position and the discrete image nature.

A camera to record underwater particles using simultaneous in-line and off-axis hologrammetry and its associated replay facility

Summary form only given. The HOLOMAR collaboration have designed and built an underwater holographic camera specifically designed to record marine plankton to a maximum depth of 100 m below sea level. The camera records holograms using both the in-line and the off-axis techniques simultaneously. The laser is a specially designed frequency doubled Nd-YAG laser, using a passive Q-switch and LBO doubler. It produces a single pulse with an energy of 700 mJ and duration of less than 10 ns. The oscillator-amplifier design results in a single longitudinal mode with a measured coherence length of over 2 m. Considerable care was taken in the optical design to minimise the effects of distortion arising from the change in refractive index. The computer controlled, fully automated replay machine and image processing and analysis systems allow large amounts of data to be extracted from the holograms without manual intervention.