732 research outputs found
Low probability of detection underwater acoustic communications
Low probability of detection (LPD) underwater acoustic communications are an essential
requirement for command and control of autonomous underwater vehicles (AUV) or
submarines performing covert missions, avoiding their detection while communicating.
Based on low power signals, these covert communications may also extend the autonomy
of battery-operated AUVs, and contribute to reducing the impacts of the environmental
noise level on marine life. The present thesis aims to develop LPD communications
based on modeled and real data from three shallow water experiments. Thus, a superimposed
training method has been proposed. A bitstream is created superimposing a
long probe to the message before transmission. Computationally simple, the algorithm
explores temporal diversity to increase the processing gain and uses a Wiener filter for
equalization. Experimental results presented bit-error rates (BER) < 10−2 for signal-tonoise
ratios (SNR) < −8 dB. To understand the effects of coastal upwelling phenomena
over low SNR communications, a study compares the acoustic propagation for different
sound speed profiles using a propagation model and analyzes data from the BioCom’19
experiment, performed off Cabo Frio Island (Brazil). Temporal and spatial coherence of
low power signals propagating in this harsh environment are estimated, and both a criterion
for multichannel combining and a double Wiener filter to improve equalization are
presented. Passive time-reversal (pTR) techniques have been widely employed for communications.
To address the pTR channel mismatch due to the environmental variability
between the probe and the data transmissions, this work proposes a superimposed training
pTR (STpTR) approach for single and multichannel systems. Despite the high noise
levels, varying from -5 to +6 dB, the STpTR combined with a Wiener filter achieved
BER < 10−2, for bit rates up to 220 bps. To improve covert communications for AUVs,
this work also presents a study about vector sensor multichannel combining. Using the
STpTR approach, results from an experiment on the coast of Algarve/Portugal indicate
that combining the pressure and particle velocity channels of a vector sensor may provide
an average SNR and mean squared-error gain of up to 9.4 and 3.1 dB, respectively,
compared to the pressure channel. Therefore, a better understanding of the environment
combined with the superimposed training pTR using a vector sensor may improve the
LPD communication system’s performance and robustness while keeping covertness.No ´últimos anos, os trabalhos de investigação sobre comunicações acústicas submarinas
com baixa probabilidade de deteção (BPD) tem sido incentivados pela indústria, pelos
governos, e pela própria academia em razão de suas m´múltiplas aplicações. Na ´área militar, as
comunicações BPD permitem que submarinos e veículos autónomos possam se comunicar sem
serem detectados. Na ´área civil, permitem a economia de energia de sensores alimentados
por baterias, aumentando o tempo de funcionamento, bem como contribui para reduzir os
impactos sobre a vida marinha causados pelos altos n´ıveis de ru´ıdo submarino, entre outras
aplica¸c˜oes. Neste contexto, esta tese pretende desenvolver comunica¸c˜oes BPD utilizando um
modelo de propaga¸c˜ao ac´ustica e dados obtidos a partir de trˆes experimentos em ´aguas rasas.
Este trabalho apresenta um m´etodo de treinamento superposto para comunica¸c˜oes submarinas
em um ambiente com baixa rela¸c˜ao sinal/ru´ıdo, e demonstra sua aplica¸c˜ao para
comunica¸c˜oes BPD. Computacionalmente simples, o m´etodo foi desenvolvido para funcionar
com um ´único projetor acústico, transmitindo com baixa potência, e um hidrofone, sem o
ganho de um arranjo de sensores distribuídos no espaço. Antes da transmissão, uma longa
sequência de comprimento m´máximo ´e somada `a mensagem para efeitos de equalização e sincronismo.
Os dois sinais são binários, modulados em fase e possuem 2047 bits. Porém,
possuem amplitudes diferentes. Na realidade, a amplitude do sinal de treinamento ´e ligeiramente
superior `a da mensagem. Em um ambiente com baixa rela¸c˜ao sinal ru´ıdo, um sinal de
treinamento mais forte permite ocultar a mensagem a ser transmitida, bem como melhorar
o ganho para a estima¸c˜ao da resposta impulsiva e para a sincroniza¸c˜ao do sistema. A mensagem
´e composta por 3 bits nulos e 4 sequˆencias de 511 bits. Delimitados por uma curta
sequˆencia de comprimento m´aximo de 31 bits, para dupla sincroniza¸c˜ao, os pacotes de dados
possuem 480 bits e transportam o seguinte pangrama: (The Quick Brown Fox Jumps Over
the Lazy Dog 0123456789!@#$). O m´etodo explora a diversidade temporal do canal, transmitindo
a mesma sequˆencia diversas vezes para aumentar o ganho de processamento do sinal
e implementar a corre¸c˜ao de erros atrav´es da soma coerente dos sinais. A resposta impulsiva
do canal ´e estimada pela transformada r´apida de Hadamard, e a equaliza¸c˜ao do sinal ´e feita
por um filtro de Wiener. A remo¸c˜ao da interferˆencia causada pelo sinal de treinamento ´e
realizada pelo m´etodo “hyperslice cancellation by coordinate zeroing (HCC0)”, e a seguir a
mensagem ´e decodificada. Resultados obtidos a partir de um experimento em ´aguas rasas,
utilizando uma fonte e um ´único hidrofone, apresentaram taxas de erro de bit menores que
10−2, para relações sinal/ruído inferiores a −8 dB.
A ressurgência costeira ´e um fenómeno oceanográfico dinâmico que modifica, profundamente,
a estratificação de temperatura do oceano, influenciando diretamente na propagação
acústica. Por outro lado, os crescentes n´níveis de ruído antropogénico não apenas reduzem
o desempenho dos sistema de comunicação, corrompendo a informação transmitida, mas tamb´em afetam a vida marinha. Para compreender os efeitos da ressurgˆencia costeira sobre
as comunica¸c˜oes com baixa rela¸c˜ao sinal/ru´ıdo, um estudo analisa os dados do experimento
BioCom’19, realizado nas proximidades da Ilha do Cabo Frio, Rio de Janeiro (Brasil). As
respostas impulsivas do canal e a propaga¸c˜ao ac´ustica, para diferentes perfis de velocidade do
som, foram estimadas usando o modelo de propaga¸c˜ao ac´ustica “Monterey-Miami Parabolic
Equation model (MMPE)”. Al´em disso, o desempenho do sistema de comunica¸c˜oes foi correlacionado
com os perfis de temperatura durante o experimento. Os resultados obtidos
indicam uma significativa redu¸c˜ao da energia ac´ustica nos receptores durante a ressurgˆencia,
degradando o desempenho do sistema.
A coerˆencia temporal e espacial dos sinais de baixa potˆencia transmitidos no experimento
BioCom’19 foi estimada, e um crit´erio para combina¸c˜ao dos sinais, provenientes dos m´ultiplos
hidrofones, foi proposto. Utilizando dados de um arranjo piramidal e um arranjo vertical
linear, de 4 hidrofones cada, a coerˆencia foi estimada antes e depois do filtro de Wiener
para mostrar o impacto do multicaminhamento sobre a taxa de erro de bit. Os resultados
mostram que a coˆerencia temporal pode ser utilizada como crit´erio para combinar sinais
consecutivos em um mesmo canal, enquanto a diversidade espacial permite a combina¸c˜ao de
m´ultiplos canais do arranjo de sensores. Sequˆencias cuja coerˆencia temporal esteja acima
de um limite pr´e-definido s˜ao somadas. A coerˆencia espacial entre canais foi estimada e
comparada em termos da taxa de erro de bit. Para diferentes taxas de transmiss˜ao, as taxas
de erro de bit est˜ao em concordˆancia com a evoluc˜ao da coerˆencia espacial. Quanto mais
elevada a coerˆencia, melhor o desempenho e menor a taxa de erro de bit.
Um duplo filtro de Wiener para melhorar a equaliza¸c˜ao dos sinais de baixa potˆencia,
durante a ressurgˆencia, tambem foi proposto. Utilizando dados dos 4 hidrofones de um
arranjo piramidal, as respostas impulsivas foram estimadas para observar a variabilidade
das condi¸c˜oes de propaga¸c˜ao. Em uma condi¸c˜ao de perfil de temperatura isot´ermico, as
respostas impulsivas apresentaram multicaminhamento curto, com chegadas mais fortes nos
receptores. `A medida que a ressurgˆencia ocorria, foram observadas quedas abruptas de
temperatura superiores a 10◦C, na posi¸c˜ao dos hidrofones, acarretando uma forte refra¸c˜ao da
onda sonora para o fundo marinho. Em consequˆencia, sinais mais fracos foram observados
nos hidrofones. Os resultados obtidos com dados do BioCom’19 mostram que, para uma
rela¸c˜ao sinal/ru´ıdo variando entre −3.9 e 7.3 dB, o duplo filtro de Wiener forneceu um
ganho do erro m´edio quadr´atico de at´e 2.8 dB, comparado com o filtro de Wiener simples.
As t´ecnicas de tempo reverso passivo (TRP) tem sido amplamente empregadas nas
comunica¸c˜oes submarinas. Por´em, as r´apidas altera¸c˜oes das condi¸c˜oes de propaga¸c˜ao em
canais submarinos variantes no tempo, durante as transmiss˜oes da sequˆencia de treinamento
e da mensagem, degradam o desempenho das t´ecnicas TRP de equaliza¸c˜ao. Assim, esse
trabalho prop˜oe um m´etodo de TRP utilizando treinamento superposto, para sistemas com
um ou m´ultiplos sensores. O m´etodo proposto utiliza uma sequˆencia de treinamento, com
o efeito Doppler corrigido, para estimar o canal e realizar o TRP. O m´etodo compara 3
differentes estrat´egias para melhorar a performance do sistema de comunica¸c˜ao: a diversidade
temporal devido aos sinais idˆenticos transmitidos continuamente, a diversidade espacial
devido aos 2 arranjos de hidrofones, piramidal e linear, com 4 hidrofones cada um, al´em da
combina¸c˜ao dos 2 arranjos (8 hidrofones). Neste m´etodo, a t´ecnica de TRP com treinamento
superposto minimiza o multicaminhamento e realiza a corre¸c˜ao de erros atrav´es da soma
coerente dos diferentes canais. A interferˆencia intersimb´olica residual ´e removida pelo filtro
de Wiener. Resultados obtidos com dados do experimento BioCom’19 mostram que o m´etodo
proposto pode fornecer um ganho do erro médio quadrático de até 1.62 dB para canais independentes, e 3.13 dB, para canais combinados, comparativamente ao m´etodo sem o TRP,
usando apenas o filtro de Wiener. Neste contexto, o m´etodo de TRP utilizando treinamento
superposto alcan¸cou taxas de erro de bit < 10−2 para uma rela¸c˜ao sinal ru´ıdo, na banda de
transmiss˜ao, variando entre −5 a +6 dB.
Focado em comunica¸c˜oes com baixa probabilidade de detec¸c˜ao para pequenos ve´ıculos
submarinos, este trabalho tamb´em apresenta um estudo sobre a combina¸c˜ao dos canais
de press˜ao e velocidade de part´ıcula dos sensores vetoriais. Os sensores vetoriais possuem
pequenas dimens˜oes, adequadas `a utilizac˜ao em ve´ıculos autˆonomos, e permitem obter um
ganho de diversidade para as comunica¸c˜oes BPD. Para testar o m´etodo de treinamento
superposto com o tempo reverso passivo utilizando sensores vetoriais, um experimento foi
realizado, em ´aguas rasas, na costa do Algarve/Portugal. Para reduzir a rela¸c˜ao sinal/ru´ıdo
para uma faixa de 0 a −10 dB, foi adicionado ru´ıdo gravado no experimento. Os resultados
experimentais indicam que a combina¸c˜ao dos canais podem fornecer um ganho da rela¸c˜ao
sinal/ru´ıdo e do erro m´edio quadr´atico de at´e 9.4 e 3.1 dB, respectivamente, comparados
com os resultados do sensor de press˜ao.
Portanto, as principais contribui¸c˜oes dessa tese s˜ao (i) a proposta do m´etodo de treinamento
superposto para comunica¸c˜oes com baixa probabilidade de detec¸c˜ao, (ii) a compreens
˜ao dos efeitos da ressurgˆencia costeira sobre as comunica¸c˜oes, e seus impactos sobre a
coerˆencia temporal e espacial, (iii) o m´etodo de treinamento superposto em conjunto com o
tempo reverso passivo para lidar com a varia¸c˜ao do canal entre o tempo de recep¸c˜ao do sinal
de treinamento e da mensagem, e (iv) a combina¸c˜ao dos m´ultiplos canais dos sensores vetoriais
para comunica¸c˜oes BPD. Dessa forma, uma melhor compreens˜ao do canal submarino
e a utiliza¸c˜ao dos m´etodos propostos combinados com sensores do estado da arte, como os
sensores vetoriais, se configura como um avan¸co neste campo do conhecimento, permitindo
aumentar a robustez do sistema BPD, bem como reduzir a probabilidade de detec¸c˜ao, mantendo
a ocultação das comunicações.I am also grateful for the sponsorship provided by the Brazilian Navy through the Postgraduate
Study Abroad Program, Grant No. Port.227/MB/2019
Superimposed training low probability of detection underwater communications
This paper proposes a superimposed training method for low probability of detection underwater acoustic communications. A long pilot sequence was superimposed to the message for equalization and synchronization purposes. A fast Hadamard transform (FHT) estimated the channel impulse response and compressed the pilot energy. A Wiener filter performed equalization. The interference signal was removed using hyperslice cancellation by coordinate zeroing. An inverse FHT decompressed the remaining sequence energy and the message was retrieved. Results from a shallow water experiment presented bit error rates <10-2 for signal-to-noise ratios <-8 dB.info:eu-repo/semantics/publishedVersio
Acoustic noise removal by combining wiener and wavelet filtering techniques
This thesis investigates the application of Wiener filtering and wavelet techniques for the removal of noise from underwater acoustic signals. Both FIR and IIR Wiener filters are applied in separate methods which involve the filtering of wavelet coefficients which have been produced through a discrete wavelet decomposition of the acoustic signal. The effectiveness of the noise removal methods is evaluated by applying them to simulated data. The combined Wiener wavelet filtering methods are compared to traditional denoising techniques which include Wiener filtering and wavelet thresholding methodshttp://www.archive.org/details/acousticnoiserem00fornLieutenant Commander, United States NavyApproved for public release; distribution is unlimited
Improved time-frequency de-noising of acoustic signals for underwater detection system
The capability to communicate and perform target localization efficiently in underwater environment is important in many applications. Sound waves are more suitable for underwater communication and target localization because attenuation in water is high for electromagnetic waves. Sound waves are subjected to underwater acoustic noise (UWAN), which is either man-made or natural. Optimum signal detection in UWAN can be achieved with the knowledge of noise statistics. The assumption of Additive White Gaussian noise (AWGN) allows the use of linear correlation (LC) detector. However, the non-Gaussian nature of UWAN results in the poor performance of such detector. This research presents an empirical model of the characteristics of UWAN in shallow waters. Data was measured in Tanjung Balau, Johor, Malaysia on 5 November 2013 and the analysis results showed that the UWAN has a non-Gaussian distribution with characteristics similar to 1/f noise. A complete detection system based on the noise models consisting of a broadband hydrophone, time-frequency distribution, de-noising method, and detection is proposed. In this research, S-transform and wavelet transform were used to generate the time-frequency representation before soft thresholding with modified universal threshold estimation was applied. A Gaussian noise injection detector (GNID) was used to overcome the problem of non-Gaussianity of the UWAN, and its performance was compared with other nonlinear detectors, such as locally optimal (LO) detector, sign correlation (SC) detector, and more conventionally matched filter (MF) detector. This system was evaluated on two types of signals, namely fixed-frequency and linear frequency modulated signals. For de-noising purposes, the S-transform outperformed the wavelet transform in terms of signal-to-noise ratio and root-mean-square error at 4 dB and 3 dB, respectively. The performance of the detectors was evaluated based on the energy-to-noise ratio (ENR) to achieve detection probability of 90% and a false alarm probability of 0.01. Thus, the ENR of the GNID using S-transform denoising, LO detector, SC detector, and MF detector were 8.89 dB, 10.66 dB, 12.7dB, and 12.5 dB, respectively, for the time-varying signal. Among the four detectors, the proposed GNID achieved the best performance, whereas the LC detector showed the weakest performance in the presence of UWAN
An Architecture for High Data Rate Very Low Frequency Communication
Very low frequency (VLF) communication is used for long range shore-to-ship broadcasting applications. This paper proposes an architecture for high data rate VLF communication using Gaussian minimum shift keying (GMSK) modulation and low delay parity check (LDPC) channel coding. Non-data aided techniques are designed and used for carrier phase synchronization, symbol timing recovery, and LDPC code frame synchronization. These require the estimation of the operative Eb/N0 for which a kurtosis based algorithm is used. Also, a method for modeling the probability density function of the received signal under the bit condition is presented in this regard. The modeling of atmospheric radio noise (ARN) that corrupts VLF signals is described and an algorithm for signal enhancement in the presence of ARN in given. The BER performance of the communication system is evaluated for bit rates of 400 bps, 600 bps, and 800 bps for communication bandwidth of ~200 Hz.Defence Science Journal, 2013, 63(1), pp.25-33, DOI:http://dx.doi.org/10.14429/dsj.63.376
Hybrid state estimators for the control of remotely operated underwater vehicles
Submitted in partial fulfillment of the requirements for the degree of Ocean Engineer at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1988This paper explores the use of 'hybrid' state estimators to increase the accuracy and flexibility
of acoustic position measurement systems used in the control of underwater vehicles.
Two different approaches to extend the range of acoustic position measurement systems are
explored. The first approach is the use of an inexpensive Strapdown Inertial Measurement
System (SIMS) to augment the acoustic with inertial position information. This approach is
based on the experience gained using an attitude and inertial measurement package fielded
on the JASON JUNIOR Remotely Operated Vehicle (ROV). The second approach is the use
of a mobile, platform-mounted, acoustic net in conjunction with a platform tracking system.
This second investigation used the JASON ROV as the basis for the simulation work. As motivation,
some of the theoretical and practical difficulties encountered when range is extended
using an unaugmented system are explored. Simulation results are used to demonstrate the
effects of these difficulties on position estimation accuracy and on the performance in closed
loop control of the vehicle. Using measured sensor noise characteristics, a hybrid Kalman
filter is developed for each approach to take the greatest advantage of the available information.
Formulation of the Kalman filter is different for each case. In the second case, the
geographic position of the ROV is the sum of the acoustic net's geographic position, measured
at a different interval by an RF positioning system, and the position of the ROV relative to
the net, as measured acoustically. Closed loop vehicle performance evaluations are made for
representative noise levels and update rates with and without the augmentation discussed in
the first approach. Finally, conclusions are drawn about the benefits and applications of the
hybrid Kalman filter to the control of Remotely Operated Vehicles
Amplitude and phase sonar calibration and the use of target phase for enhanced acoustic target characterisation
This thesis investigates the incorporation of target phase into sonar signal processing, for enhanced information in the context of acoustical oceanography. A sonar system phase calibration method, which includes both the amplitude and phase response is proposed. The technique is an extension of the widespread standard-target sonar calibration method, based on the use of metallic spheres as standard targets. Frequency domain data processing is used, with target phase measured as a phase angle difference between two frequency components. This approach minimizes the impact of range uncertainties in the calibration process. Calibration accuracy is examined by comparison to theoretical full-wave modal solutions. The system complex response is obtained for an operating frequency of 50 to 150 kHz, and sources of ambiguity are examined. The calibrated broadband sonar system is then used to study the complex scattering of objects important for the modelling of marine organism echoes, such as elastic spheres, fluid-filled shells, cylinders and prolate spheroids. Underlying echo formation mechanisms and their interaction are explored. Phase-sensitive sonar systems could be important for the acquisition of increased levels of information, crucial for the development of automated species identification. Studies of sonar system phase calibration and complex scattering from fundamental shapes are necessary in order to incorporate this type of fully-coherent processing into scientific acoustic instruments
Investigation of methods for data communication and power delivery through metals
PhD ThesisThe retrieval of data from a sensor, enclosed by a metallic structure,
such as a naval vessel, pipeline or nuclear flask is often very challenging.
To maintain structural integrity it is not desirable to penetrate the wall
of the structure, preventing any hard-wired solution. Furthermore, the
conductive nature of the structure prevents the use of radio communications.
Applications involving sealed containers also have a requirement
for power delivery, as the periodic changing of batteries is not possible.
Ultrasound has previously been identified as an attractive approach but
there are two key challenges: efficient/reliable ultrasonic transduction
and a method of overcoming the inherent multipath distortion resulting
from boundary reflections. Previous studies have utilised piezoelectric
contact transducers, however, they are impractical due to their reliance
on coupling, i.e. the bond between the transducer and the metal surface,
which leads to concerns over long term reliability. A non-contact
transducer overcomes this key drawback, thus highlighting the electromagnetic
acoustic transducer (EMAT) as a favourable alternative. This
thesis presents the design and testing of an EMAT with appropriate
characteristics for through-metal data communications.
A low cost, low power data transmission scheme is presented for overcoming
acoustic multipath based on pulse position modulation (PPM).
Due to the necessary guard time, the data rate is limited to 50kbps.
A second solution is presented employing continuous wave, Quadrature
phase shift keying (QPSK) modulation, allowing data rates in excess of
1Mbps to be achieved. Equalisation is required to avoid intersymbol interference
(ISI) and a decision feedback equaliser (DFE) is shown to be
adept at mitigating this effect.
The relatively low efficiency of an EMAT makes it unsuitable for power
delivery, consequently, an alternative non-contact approach, utilising inductive
coupling, is explored. Power transfer efficiency of ≈ 4% is shown
to be achievable through 20mm thick stainless steel.ICS department of BAE Systems Submarine Solutions, EPSR
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