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

Underwater communications using virtual time reversal in a variable geometry channel

Field experiments using Time-Reversal Mirror( TRM) techniques have shown that the underwater acoustic channel presents a longer stability than it was previously anticipated. Applying such techniques to underwater communications requires that, either the emitted signals are previously ltered by time-reversed replicas of the channel impulse response or that a probe-signal should be transmitted ahead of the data-signal for post channel ltering. In the former case, the time-reversed ltered message is expected to be undone by the actual acoustic channel between the array and the receiver, while in the later, the undoing of the multipath is performed electronically inside the computer and is therefore termed as virtual Time Reversal Mirror(vTRM). The main issues being addressed in recent literature deal with channel stability, focus width and the required array aperture for obtaining reasonable temporal and/or spatial focusing. This paper focus essentially in two practical aspects, generally not addressed: one is to demonstrate the potential application of the vTRM approach to undo the severe intersymbol interference in a real variable geometry channel scenario and, the other, is the importance of optimization of the probe-signal time window length in a real application.This work was supported under projects INFANTE and INTIMATE of FCT, Lisboa, Portugal and project LOCAPASS(Ministry of Defense - FUP)

A post-detection maximum ratio combiner. Experimental assessment on high diversity underwater channels

Maximum Ratio Combiner (MRC) is a diversity combining technique applicable to underwater communications when the data transmitted by a single projector is received by more than one antenna/node. Post Detection MRC (PD-MRC) performs a weightedsum over initial detected outputs of multiple nodes. PDMRC can be applied to noise-only or ISI-only channels with the weights computed from the signal to noise ratio or from the channel impulse response of each node, respectively. In this paper it is shown that in the presence of noisy-ISI channels the weighs can be computed from the detector output constellation of each node. A performance of the PD-MRC is evaluated using real data collected during RADAR’07 experiment. Results show that a gain is always attained using two nodes in the PDMRC as compared to the node having the best MSE. Moreover, the PD-MRC gain is higher when both nodes present a similar MSE

Acoustic estimation of seafloor parameters: a Radial Basis Functions approach

A novel approach to the estimation of seafloor geoacoustic parameters from the measurement of the acoustic field in the water column is introduced. The approach is based on the idea of approximating the inverse function that links the geoacoustic parameters with the measured field through a series expansion of radial basis functions. In particular, Gaussian basis functions are used in order to ensure continuity and smoothness of the approximated inverse. The main advantage of the proposed approach relies on the fact that the series expansion can be computed off-line from simulated data as soon as the experimental configuration is known. Data inversion can then be performed in true real time as soon as the data are acquired. Simulation results are presented in order to show the advantages and limitations of the method. Finally, some inversion results from horizontal towed array data are reported, and are compared with independent estimates of geoacoustic bottom properties

Time reversal and spatial diversity: issues in a time varying geometry test

Underwater acoustic communications in waveguides is known to be prone to severe multipath, which strongly limits practical transmission rates with actual channel equalization techniques. The time reversal principle uses the ocean waveguide response to a basic pulse shape to matched filter the received data sequence. Assuming the ocean response to be a version of the actual pulse shape ocean response corrupted by additive noise, the matched filter output remains a sum of four terms from which only one has the required data sequence in usable form. This paper analysis the ability of such a peculiar matched filter to reject the unwanted terms both with fixed and timevarying source-receiver geometries. One particular parameter with practical interest is the sensitivity of the matched filter performance to a change of the source-receiver range during the processing that induces a mismatch limiting factor. Simulation examples in realistic situations and results obtained with real data, collected during the INTIFANTE’00 sea trial, illustrate the theoretical assertions.NATO Undersea Research Centre; Instituto Hidrográfico; FUP/Ministry of Defence; AOB - REA Joint Research Project; HFi - High Frequency Initiativ

Using normal mode channel structure for narrow band underwater communications in shallow water

Multipath and high temporal and spatial variability of the propagation environment causes severe signal degradation in shallow water acoustic digital communications. Among the many solutions that have been proposed the most known is adaptive equalisation where cyclic training signals are used to adapt the equaliser to the variability of the acoustic channel. When the channel is rapidly changing, equaliser coefficients are frequently adapting and the effective transmitting rate rapidly decreases. Another approach consists in using a priori information obtained from acoustic propagation models. These models can give a deterministic estimate of the true channel impulse response that can be used to detect the transmitted signals. In practice, the use of deterministic acoustic models is mainly dependent of the accuracy of the input environmental parameters. As a first step, this paper presents an exhaustive study of the signal detection sensitivity to model parameters mismatch. The scenario used is composed of a 100 m depth water column with range dependent characteristics. The water column is located over a 10 m thick sediment layer with variable properties. Source-receiver communication is made over a variable distance between 500 and 600 m with the source near the bottom and the receiver near the surface. The communication signals are narrow band (1.5 kHz) pulse amplitude modulated with a carrier frequency of 15 kHz, and the detector is based on the maximum-likelihood sequence detector (MLSD

From governmental accounting to national accounting: implications on the Portuguese central government deficit

Partindo das diferenças mais relevantes entre a Contabilidade Pública (CP) – perspectiva microeconómica – e a Contabilidade Nacional (CN) – perspectiva macroeconómica –, este artigo analisa os principais ajustamentos realizados em Portugal, na conversão dos dados do sector das Administrações Públicas, das Contas Públicas para as Contas Nacionais. Adicionalmente, avalia o impacto desses ajustamentos no défice da Administração Central, a maior parcela do défice público português. Seguindo uma metodologia essencialmente qualitativa, o estudo empírico baseia-se em entrevistas realizadas junto dos responsáveis pela preparação das Contas Nacionais e em diversas fontes documentais. O objectivo é validar os ajustamentos identificados como mais significativos na passagem da CP para a CN e, ao mesmo tempo, avaliar o respectivo impacto nos dados notificados no âmbito do Procedimento relativo aos Défices Excessivos. Usaram-se dados da notificação de Abril de 2008, cobrindo o período compreendido entre 2004 e 2007. Os principais resultados indicam que as diferenças de base contabilística são as mais relevantes e que os ajustamentos subsequentes têm um impacto significativo no défice da Administração Central portuguesa. Assim, esta investigação aponta para a necessidade de maior convergência entre os dois sistemas (CP e CN), nomeadamente quanto aos critérios de reconhecimento das transacções, por forma a ser adoptada uma base contabilística comum, e para um sistema informativo completo e coerente na CP.info:eu-repo/semantics/publishedVersio

Compensating for source depth change and observing surface waves using underwater communication signals

Underwater communications impose great challenges due to the unpredictable changes in the environment. In order to accommodate for these changes, equalizers are used to track the Impulse Responses (IRs) and compensate the intersymbol interference (ISI). Source and array depth shifts are one of the major contributing factors to continuous amplitude and phase changes in IR. These changes magnify the problem of data processing in which correlation between successive received signals is involved, e.g. passive Time Reversal (pTR) where a probe signal is sent ahead of the data for post cross-correlation. In this paper, an environment based algorithm is used for pTR equalization, where an appropriate frequency shift of the estimated IR compensates for the geometric changes such as source and array depth shifts. We have applied this Frequency Shift pTR equalizer (FSpTR) on real data collected from Underwater Acoustic Barrier 2007 (UAB’07) sea trial having 1000 baud BPSK signaling at carrier frequency of 6.25 KHz with a sudden source depth change of 0.5 m at various known instants of time. The results illustrate that a considerable gain can be attained using the environment based equalizer in the presence of source and/or array depth changes. Moreover by close analysis of the FSpTR results we have detected surface wave motion through the frequency shifts caused by the array depth variations

Arrival-based equalizer for underwater communication systems

One of the challenges in the present underwater acoustic communication systems is to combat the underwater channel effects which results in time and frequency spreading of the transmitted signal. The time spreading is caused by the multipath effect while the frequency spreading is due to the time variability of the underwater channel. The passive Time Reversal (pTR) equalizer has been used in underwater communications because of its time focusing property which minimizes the time spreading effect of the underwater channel. In order to compensate for the frequency spreading effect, an improved version of pTR was proposed, called Frequency shift passive time reversal (FSpTR). FSpTR tries to compensate for the frequency spreading by applying a frequency shift in the estimated channel impulse response (IR). In the multipath environment, multiple replicas of the transmitted signal reaches the receiver through different paths where each path is affected differently by environmental variations. In such cases, a single frequency shift fails to compensate for the environmental variations on each path, resulting in degradation in the performance. In this paper, an arrival-based equalizer is proposed to compensate for the environmental variations on each path. The concept of beamforming is integrated with FSpTR equalizer, in this paper, to compensate each arrival separately for the environmental variations. The proposed equalizer is tested with the real data and the results showed that the proposed approach outperforms pTR and FSpTR equalizers and provides a mean MSE gain of 4.9 dB and 4.2 dB respectively.This work is supported by Portuguese Foundation for Science Technology under PHITOM (PTDC/EEATEL/71263/2006) and COGNAT (PTDC/MAR/112446/2009) projects. This work was also supported by European Community’s Sixth Framework Program through the grant to the budget of the Integrated Infrastructure Initiative HYDRALAB III within the Transnational Access Activities, Contract no. 022441

Potential of vertical line array beamforming in underwater acoustic communications

Um dos principais tópicos atuais de pesquisa é ́ a concepçãoo de técnicas de processamento de sinal eficazes para comunicações submarinas. Este interesse é reforç ̧ado pelos desafios inerentes às condições incontroláveis, como a batimetria, a temperatura, e as variações temporais (geométricas) das profundidades da fonte e da antena de receptores. A obtenção de taxas de transmissão modestas é ainda um desafio, devido a estas variações. O presente trabalho aborda o uso potencial do formador de feixes (beamformer) no sistema de comunicações submarinas. O propósito de usar um beamformer neste trabalho é o de separar os diferentes caminhos que chegam do transmissor para o recetor, o que é motivado pelo fato de que, num ambiente real, não é possível distinguir todos os caminhos de propagação, no domínio do tempo. Neste trabalho, é usado o beamformer de atraso-e-soma, que aplica atrasos diferentes para cada hidrofone da antena vertical, e adiciona as saídas resultantes. Cada um dos atrasos é função do ângulo de chegada e da profundidade do hidrofone. O objetivo do beamformer é aplicar os atrasos opostos a cada elemento da antena vertical, para garantir a soma coerente da frente de onda numa direção particular. Ambos resultados com dados simulados e dados reais são apresentados neste trabalho, para mostrar o desempenho do beamformer. Os resultados do beamformer são comparados com os resultados da análise Doppler, para caracterizar chegadas diferentes e fazer corresponder essas chegadas às variações Doppler, mostrando que cada chegada é afetada pelas variações ambientais de uma maneira diferente, resultando numa quantidade diferente de Doppler