Underwater acoustic communications. From point-to-point to networks

This is a review presentation that addresses recent developments in underwater acoustic telemetry as a tool for ocean observation, monitoring and protection. Distributed sensing is a paradigm with important reflections in oceanic technology where bottom installed structures can not always be connected to a central hub through cabled networks. Moreover, recent developments in ocean robotics lead to the off-the-shelf availability of autonomous underwater vehicles that rely on wireless communications for sending information and receiving commands. Unlike its aerial counterpart wireless underwater communications, are strongly affected and limited by the propagation media: the low speed of propagation, highly limited bandwidth, spatial and time variability of environmental properties and randomness, all together contribute to the slow adoption of standardized and reliable underwater communications networks. This paper addresses the main issues regarding and characterizing the underwater acoustic communication channel as well as the proposed techniques to overcome those issues for, in a first stage, point-to-point (P2P) communications and then for the set up of full underwater networks comprising both fixed and mobile nodes. The presentation is illustrated by real data based examples drawn from experiments carried out at sea by the Signal Processing Laboratory, University of Algarve, Portugal (SiPLAB, www.siplab.fct.ualg.pt) in numerous national and European research projects in the last 15 years.Work supported by FCT (Portugal), ONR (USA), European Commission (EU), CMRE (former NURC, NATO) and numerous other agencies and institutions that will be properly acknowledged and cited in the presentation

Localização de fontes acústicas em águas pouco profundas

O Matched-Field Processing(MFP), sob a sua forma convencional, designa uma técnica de processamento intensivo que tem como objectivo localizar uma fonte acústica através da correlação do sinal recebido com o sinal predicto por um modelo de propagação. Esta foi a ideia proposta originalmente por Hinich (1973) e Bucker (1976). Este trabalho faz uma breve revisão histórica da evolução do conceito de MFP nos últimos vinte anos apresentando os diferentes tipos de instrumentãção (antenas horizontais, verticais, randomicas, captor único, etc...) e métodos utilizados. Em seguida serão apresentados os resultados obtidos pelo autor nos dados recolhidos durante a campanha INTIMATE'96 levada a cabo ao largo da Nazaré em Junho de 1996. Os resultados obtidos, utilizando um modelo de raios (ray-tracing) e um único captor, evidenciam o efeito da mar e na localização da fonte e uma grande estabilidade do meio de propagação ao longo de várias horas

Active acoustic time-reversal for underwater acoustic barriers

This work addresses the possibility of using successive transmissions of time delayed channel probe pulses between two closely spaced acoustic sensor arrays for forming an acoustic barrier for target detection in shallow water. One array is a transmit-receive array (TRA) while the other is a receive only vertical line array (VLA). The two arrays are connected via cable or wireless. Time reversed replicas of the acoustic channel response to the probe signals are retransmitted into the ocean propagation plane to form focus peaks at each VLA element. It is shown both theoretically and with simulated data that an optimum disturbance detector can be build from the data received at the VLA. This detector becomes sub optimal due to usual time reversal drawbacks such as ocean non stationarity and spatial sampling limitations. Real data tests are foreseen to take place during summer 2007 to answer questions such as allowable ranges and frequencies of operation.FC

Is the transition redshift a new cosmological number?

Observations from Supernovae Type Ia (SNe Ia) provided strong evidence for an expanding accelerating Universe at intermediate redshifts. This means that the Universe underwent a transition from deceleration to acceleration phases at a transition redshift $z_t$ of the order unity whose value in principle depends on the cosmology as well as on the assumed gravitational theory. Since cosmological accelerating models endowed with a transition redshift are extremely degenerated, in principle, it is interesting to know whether the value of $z_t$ itself can be observationally used as a new cosmic discriminator. After a brief discussion of the potential dynamic role played by the transition redshift, it is argued that future observations combining SNe Ia, the line-of-sight (or "radial") baryon acoustic oscillations, the differential age of galaxies, as well as the redshift drift of the spectral lines may tightly constrain $z_t$, thereby helping to narrow the parameter space for the most realistic models describing the accelerating Universe.Comment: 12 pages, 5 figures. Some discussions about how to estimate the transition redshift have been added. New data by Planck and H(z) data have been mentioned. New references have been adde

Physical limitations of travel time based shallow water tomography

Travel-time-based tomography is a classical method for inverting sound-speed perturbations in an arbitrary environment. A linearization procedure enables relating travel-time perturbations to sound-speed perturbations through a kernel matrix. Thus travel-time-based tomography essentially relies on the inversion of the kernel matrix and is commonly called ‘linear inversion. In practice, its spatial resolution is limited by the number of resolved and independent arrivals, which is a basic linear algebra requirement for linear inversion performance. Physically, arrival independency is much more difficult to determine since it is closely related to the sound propagating channel characteristics. This paper presents a brief review of linear inversion and shows that, in deep water, the number of resolved arrivals is equal to the number of independent arrivals, while in shallow water the number of independent arrivals can be much smaller than the number of resolved arrivals. This implies that in shallow water there are physical limitations to the number of independent travel times. Furthermore, those limitations are explained through the analysis of an equivalent environment with a constant sound speed. The results of this paper are of central importance for the understanding of travel-time-based shallow water tomography

Why Do Firms Engage in Selective Hedging?

Surveys of corporate risk management document that selective hedging, where managers incorporate their market views into firms’ hedging programs, is widespread in the U.S. and other countries. Stulz (1996) argues that selective hedging could enhance the value of firms that possess an information advantage relative to the market and have the financial strength to withstand the additional risk from market timing. We study the practice of selective hedging in a 10-year sample of North American gold mining firms and find that selective hedging is most prevalent among firms that are least likely to meet these valuemaximizing criteria -- (a) smaller firms, i.e., firms that are least likely to have private information about future gold prices; and (b) firms that are closest to financial distress. The latter finding provides support for the alternative possibility suggested by Stulz that selective hedging may also be driven by asset substitution motives. We detect weak relationships between selective hedging and some corporate governance measures, especially board size, but find no evidence of a link between selective hedging and managerial compensation.Corporate risk management, selective hedging, speculation, financial distress, corporate governance, managerial compensation

From governmental accounting into national accounts: adjustments diversity and materiality with evidence from the Iberian countries central governments

In a context where Governments around the world acknowledge a need for more informative governmental financial reporting to improve financial sustainability, the European Council is proposing EU member-States to adopt IPSASs-based standards to all subsectors of the General Government Sector (GGS), which are recognised as also allowing improving government finance statistics reliability. Consequently, the Governmental Accounting (GA) role of running and reporting on Governments’ budgets for purposes of decision-making and accountability is changing to include being a part in the EU budgetary and monetary policy, namely within the Euro zone. Such being considered, this paper objective is twofold. First, it aims at starting a debate in the literature concerning the ability of GA as it is across Europe to meet the European System of National and Regional Accounts (ESA) requirements concerning GGS data. This assumes particular relevance in a context where the two systems have to coexist, but given that budgetary reporting (GA) is the main input to ESA reporting (NA), reconciliation between the two systems is required. The second objective is of a more technical nature, empirically demonstrating diversity and materiality of the main adjustments to be made when converting GGS data from GA into NA. This assessment of adjustments diversity and materiality is done by using evidence for Portugal and Spain, focusing on Central Government data for the period of 2006-2009 and measuring their quantitative impact on the public (budgetary) deficit. The paper concludes that GA systems as they are across EU do not meet ESA requirements, so further alignment is needed to reduce adjustments when translating data from GA into NA as much as possible. Additionally, from the Iberian countries cases, main findings show the adjustments from GA into NA, both for Portugal and Spain, present great diversity. As to materiality, their impact is higher in Spain, but still significant in Portugal. Therefore, both reliability and comparability of final budgetary balances reported by EU member-States within the Excessive Deficit Procedures (EDP) requirements may be questionable

Bayesian acoustic prediction assimilating oceanographic and acoustically inverted data

The prediction of the transmission loss evolution on a day to week frame, in a given oceanic area, is an important issue in modeling the sonar performance. It relies primarily on acoustic propagation models, which convert water column and geometric/ geoacoustic parameters to ‘instantaneous’ acoustic field estimates. In practice, to model the acoustic field, even the most accurate acoustic models have to be fed with simplified environmental descriptions, due to computational issues and to a limited knowledge of the environment. This is a limitation, for example, in acoustic inversion methods, in which, by maximizing the proximity between measured and modeled acoustic signals, the estimated environmental parameters are deviated from reality, forming what is normally called an ‘acoustically equivalent environment’. This problem arises also in standard acoustic prediction, in which, the oceanographic forecasts and bottom data (typically from archives) are fed directly to an acoustic model. The claim in the present work is that, by converting the oceanographic prediction and the bottom properties to ‘acoustically equivalent’ counterparts, the acoustic prediction can be obtained in an optimal way, adapted to the environmental model at hand. Here, acoustic prediction is formulated as a Bayesian estimation problem, in which, the observables are oceanographic forecasts, a set of known bottom parameters, a set of acoustic data, and a set of water column data. The predictive posterior PDF of the future acoustic signal is written as a function of elementary PDF functions relating these observables and ‘acoustically equivalent’ environmental parameters. The latter are obtained by inversion of acoustic data. The concept is tested on simulated data based on water column measurements and forecasts for the MREA’03 sea trial.We thank the partial funding of Funda¸c˜ao para a Ciˆencia e Tecnologia - FCT under POSI, POCTI and POCI programs, and scholarship no. SFRH/BD/9032/2002. Acknowledgements are addressed also to Emanuel Coelho, for conducing the MREA’03 sea trial, and to Peter Gerstoft, for prompt help and improvements of the SAGA inversion package