EFFECT OF ENVIRONMENTAL VARIABILITY ON MODEL-BASED SIGNAL PROCESSING: REVIEW OF EXPERIMENTAL RESULTS IN THE MEDITERRANEAN JEAN-PIERRE HERMAND Dept of Optics and Acoustics, Universite´ Libre de Bruxelles, ave F.-D. Roosevelt 50, CP194/5, B-1050 Brussels, Belgium E-mail:
[email protected] Over the last decade experiments were conducted in the Mediterranean for localising controlled sound sources and for deducing bottom geo-acoustic properties from the measurement of the acoustic-channel impulse response over a broad frequency range. Inversion of the large time-bandwidth-product signals transmitted to probe the medium was performed using a coherent receiver, the model-based matched filter. The reference channels incorporate Green’s function models for partially known or hypothesised environmental conditions and source parameters (range, depth and Doppler). The search terminated when most of the time-spread energy on single or multiple elements of the receive array was recombined coherently into a single peak. Although performance limitations were imposed by environmental and modelling uncertainties the model-based processor was always seen as intrinsically robust to the acoustic-signal variability encountered in our experiments. Substantial processing gains were obtained in most situations depending on the time dispersion, spatial diversity, predictability and coherence of the specific acoustic channel. Most importantly, correct and stable inversion results were possible even from a sparse but representative set of hydrologic data.
1 Introduction Ocean-duct and shallow-water transmission channels are characterised by time dispersion and frequency-selective fading of the acoustic signal. The performance of a conventional matched filter (MF) receiver can be drastically improved if the reference channel compensates for the amplitude and phase distortion occurred in the time dispersive medium >dH. Source-location and environmental parameters were determined by varying the reference channel of the model-based matched filter (MBMF) receiver >1, nH. It was shown that the use of large time-bandwidth(TW)-product signals for probing the medium was important not only to improve resolution performance but also to reduce propagation fading. This paper focuses on the effects of environmental variability and other modelling uncertainties upon MBMF inversion results and the role of the transmit frequency bandwidth and spatial receive aperture in limiting these effects. In Sect. 2 a theory is proposed to explain the robustness of a multichannel MBMF receiver to environmental and geometric variability, observed in various experiments. Section 3 reviews source localisation results achieved with a towed source and horizontal receive array (HRA). Section 4 reviews geo-acoustic inversion results obtained with a fixed source and single and multiple elements of a vertical receive array (VRA). Section 5 concludes the paper. 155 N.G. Pace and F.B. Jensen (eds.), Impact of Littoral Environmental Variability on Acoustic Predictions and Sonar Performance, 155-162. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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Uncertainty in model-based matched filter processing
Let us define frequency integrals of the Green’s function G(ω, r) of a stationary medium, ! !2 ! ! " " ! 1 ! 1 ! ! α(r) ! ! G(ω, r)dω! and β(r) ! |G(ω, r)|2 dω (1) ∆ω ! ∆ω ! Π
Π
where r is the radius vector between the source and a receiver and Π is the frequency range of radiation. For a signal with centre frequency ω0 and large time-bandwidth product ∆t∆ω such that ∆t(∆ω)2 /ω0 ! 2π, the coefficients α(r) and β(r) determine to the output peak signal-to-noise ratio (SNR) ρ(r) of the MF and optimum receivers, ρmf (r) = 2E/N0 α(r)
