
PHITOM  Probabilistic HighFrequency Ocean Tomography for Underwater Communications and NavigationProject PTDC/EEATEL/71263/2006, sponsored by the portuguese national science foundation (FCT) Partners: ISR/IST, ISR/CINTALUniversity of Algarve Status: Ongoing, started December 2007 PI: João Gomes AbstractEnvironmental surveying and military applications such as submarine detection have driven much of the research effort on underwater acoustics over the last decades. Although absorption effectively limits highfrequency (HF) underwater acoustics to relatively shortrange scenarios, it has enjoyed a continued interest for special applications. Short range applications based on HF benefit from high spatial resolution, wide bandwidth, small size and low cost. Applications of current interest include underwater communications with autonomous underwater vehicles, wireless underwater sampling and monitoring networks, mine hunting, torpedo homing, and seafloor mapping. In particular, current underwater modems employ generic architectures originally developed for wireless terrestrial communications, with small modifications that account for specificities of underwater propagation. While this approach works well in many cases of practical interest, it is not uncommon for an acoustic link to fail by poorly understood reasons (usually explained a posteriori by hand waving arguments). Acoustic modeling at low frequencies (LF) up to about 2 kHz, say, and surveying technologies have become sufficiently precise to currently enable applications such as modelbased source tracking using matchedfield processing and tomography/focalization to determine environmental parameters. HF acoustic modeling, which requires that fundamental uncertainties about the environment be accounted for using appropriate statistical tools, is less developed than at LF. This project aims to develop and apply techniques of ocean acoustic tomography (OAT) to the HF signals used in digital communications, incorporating into the receiver some awareness of the environment and the spatial configuration of the acoustic link that is almost totally lacking in current underwater modems. Having this capability built into the receiver is very appealing for a number of reasons.
In a communications context, and assuming integration with navigation systems at both channel endpoints, it seems reasonable to admit that basic side information such as receiver and transmitter velocities and coarse position estimates may be available. These may be obtained at the receiver by interrogating its local navigation system, and by decoding a short dedicated message (possibly transmitted using a lowspeed but robust modulation) containing similar data generated at the transmitter. Feeding back refined estimates to the local navigation system would establish a feedback loop integrating navigation and communications that has often been envisaged in the technical literature on underwater robotics, but which has not been clearly accomplished so far. Realistically, the proposed estimation will require some side information regarding water depth, velocity profile and the type of seafloor. Given the low penetration of HF sound in sediments, the parameter space to be searched will possibly have lower dimensionality than in LF OAT. Moreover, the proposed environment matching criterion will be based on ray travel times, for which the velocity profile may be discretized fairly roughly. The inverse problem is cast in a Bayesian inference framework that naturally integrates a priori information about environment parameters and their uncertainties, and lends itself to time recursive estimation by sequential Bayesian methods. The methods and algorithms to be developed will be tested with data collected at sea, and integrated to form a coherent tool for HF OAT and communications. Still, given the large computational load associated with most algorithms in (LF) OAT and simulationbased Bayesian inference, the goal at this stage is not to attain realtime operation but rather to demonstrate the concept that useful data can be extracted from a communications system besides the actual digital messages. In our vision for the future, this type of system would play the role of a master environment monitor, supplying local realtime communication systems with physical side information. Receiver design techniques for incorporating this information are beyond the scope of this project. ObjectivesThis project has the following objectives:
An important strategic objective is to take advantage of the complementary competences of the two teams involved in the project (ISRIST and ISR/CINTALUniversity of Algarve), leading to relevant contributions in both wireless telecommunications and oceanography, and stronger skills in the areas of statistical inference and stochastic modeling. TasksT1  Estimation of Channel ResponsesExpected resultsThis task is expected to produce algorithms (and matlab code) for sequential estimation of scattering functions with moderate computational complexity. The work will also produce guidelines for computing the misfit between scattering functions.T2  Probabilistic Forward ModelingExpected resultsThis task is expected to produce algorithms and code to compute the conditional pdf of observations (the expected structure of scattering functions) given a set of model parameters. The sensitivity analysis will produce guidelines for choosing a coarse parameter discretization grid that is appropriate to the nominal environment and the bandwidth of digital communications waveforms, which influences the temporal and spatial resolution. Simulation code will be produced to generate realizations of acoustic data and scattering functions in a fluctuating environment with mobility at the source and receiver. This simulator will be built around the Bellhop ray propagation code. T3  Inverse Problem SolvingExpected resultsThis task is expected to produce algorithms and software for computing marginal posterior densities for both navigation and oceanographic parameters that effectively solve the inverse problem. Algorithms will also be implemented for sequential updating of estimates when long records of acoustic data (lasting several minutes) are processed. MAP estimates and dispersion metrics will be calculated for the parameters of interest. T4  Data Collection, Testing and IntegrationExpected resultsTesting of the developed algorithms with experimental data previously collected at sea is expected to illustrate the practical feasibility of extracting environmental and navigation data as a byproduct of acoustic communications. If existing data is deemed unsuitable or insufficient for the purposes of this project, additional missions will be conducted to collect more acoustic and environmental data in the HF band of interest. A (nonreal time) software implementation integrating estimation, modeling and inversion algorithms will be developed. From a set of acoustic data files and configuration data describing the nominal environment and associated uncertainties, the system is expected to output a series of snapshots with (i) source/receiver positions and velocities, (ii) refined environmental parameters, and (iii) expected scattering functions. 