Mostrando recursos 1 - 3 de 3

  1. Classification of three-dimensional ocean features using three-dimensional empirical orthogonal functions

    Martins, N.; Calado, L.; Paula, A. C. de; Jesus, S. M.
    Acoustic tomography is now a well known method for remote estimation of water column properties. The problem is ill-conditioned and computationally intensive, if each spatial point varies freely in the inversion. Empirical orhogonal functions (EOFs) efficiently regularize the inversion, leading to a few (2, 3) coefficients to be estimated, giving a coherent estimate of the field. At small scales, EOFs are typically depth-dependent basis functions. The extension of the concept to larger-scale anisotropic fields requires horizontal discretization into cells, with corresponding coefficients. This becomes unstable and computationally intensive, having been overcome by two-dimensional depth-range EOFs, in the past. The present work...
    - 09-jun-2018

  2. From oceanographic to acoustic forecasting: acoustic model calibration using in situ acoustic measures

    Martins, N.; Jesus, S. M.
    Sonar performance prediction relies heavily on acoustic propagation models and environmental representations of the oceanic area in which the sonar is to operate. The performance estimate is derived from a predicted acoustic eld, which is the output of a propagation model. Though well developed nowadays, acoustic propagation modeling is limited in practice by simpli cations in the numerical methods, in the environmental structure to consider (for computational reasons), and even in the knowledge of some environmental properties. This is complicated by the fact that, in sonar performance prediction, the environmental properties need to be predicted for a far future, in the order...
    - 18-may-2018

  3. UAN: underwater acoustic network

    Caiti, A.; Felisberto, P.; Husoy, T.; Jesus, S. M.; Karasalo, I.; Massimelli, R.; Reinen, Tor Arne; Silva, A.
    Acoustic networks are for underwater what wifi is for terrestrial networks. The ocean is a nearly perfect media for acoustic waves in which regards long range propagation but poses a number of challenges in terms of available bandwidth, Doppler spread and channel fading. These limitations originate in the physical properties of the ocean, namely its anisotropy and boundary interaction which are particularly relevant in coastal waters where acoustic propagation becomes predominantly de- pendent on seafloor and sea surface properties. The acoustic communication channel is therefore multipath dominated and time and Doppler spread variable. The problem is aggravated when involving moving...
    - 31-may-2018

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