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Madhu N.V.,National Institute of Oceanography of India | Jyothibabu R.,National Institute of Oceanography of India | Maheswaran P.A.,Naval Physical Oceanographic Laboratory | Jayaraj K.A.,National Institute of Oceanography of India | Achuthankutty C.T.,National Institute of Oceanography of India
Marine Biology Research | Year: 2012

The surface waters of the northeastern Arabian Sea sustained relatively high chlorophyll a (average 0.81±0.80 mg m -3) and primary production (average 29.5±23.6 mgC m -3 d -1) during the early spring intermonsoon 2000. This was caused primarily by a thick algal bloom spread over a vast area between 17-21°N and 66-70°E. Satellite images showed exceptionally high concentration of chlorophyll a in the bloom area, representing the annually occurring 'spring blooms' during February-March. The causative organism of the bloom was the dinoflagellate, Noctiluca scintillans (Dinophyceae: Noctilucidea), symbiotically associated with an autotrophic prasinophyte Pedinomonas noctilucae. The symbiosis between N. scintillans and P. noctilucae is most likely responsible for their explosive growth (average 3 million cells l -1) over an extensive area, making the northeastern Arabian Sea highly productive (average 607±338 mgC m -2 d -1) even during an oligotrophic period such as spring intermonsoon. © 2012 Copyright Taylor and Francis Group, LLC.

Radhakrishnan S.,Naval Physical & Oceanographic Laboratory | Padmanabham M.,Naval Science & Technological Laboratory
Indian Journal of Geo-Marine Sciences | Year: 2015

Results and analysis of bottom reflection loss measurements made at a shallow water site in the Arabian Sea off Kochi are presented as a function of frequency (2-12 kHz) and seabed grazing angle (110–340). Measurements are interpreted as estimates of the modulus of the plane wave reflection coefficient. First, the bottom reflection loss derived from a homogeneous fluid half-space model is compared with measured results. It is observed that the half-space model fails to provide accurate results for higher grazing angles especially at higher frequencies. Fine-scale sediment layering is considered to be the cause of frequency and grazing angle dependence and for improved modeling, a sediment layer overlying a half-space is used. The best-fit geoacoustic model is obtained by fitting the model to bottom loss data in the least-square sense. Bottom loss based on the best-fit geoacoustic model agrees well with the measured data over the entire frequency range. © 2015, National Institute of Science Communication and Information Resources (NISCAIR). All rights reserved.

Varghese S.,Model Engineering College | Sinchu P.,Naval Physical Oceanographic Laboratory | Subhadra Bhai D.,Naval Physical Oceanographic Laboratory
Procedia Computer Science | Year: 2016

This paper presents an effective solution to the problems of multi-target tracking in passive sonar. Since bearing information alone is used, target crossing problems arises in passive sonar. Nearest Neighbor Joint Probabilistic Data Association (NNJPDA) is employed for the information processing. Further a prediction mechanism is used, where the track bearing depends only on the predicted value not on the measurements. This leads to the correct assignment of measurements with the track in the clutter and thereby avoids the track loss. The state estimation is then refined by Kalman filtering based on the corrected measurements from the NNJPDA technique. Thus accurate and continuous track is maintained. © 2016 The Authors. Published by Elsevier B.V.

Nair B.M.,Naval Physical Oceanographic Laboratory
Indian Journal of Geo-Marine Sciences | Year: 2015

Data processing methodologies used to analyse broadband reverberation signals from an explosive sound source are presented in this paper. Reverberation data was collected at deep (2070 m) and shallow water (57 m) sites in the Bay of Bengal. In these experiments, TNT charges (0.450 kg) were used as sound sources. The received signals were recorded using two hydrophones deployed from the ship. The sonic surface duct thickness at the shallow water site is around 32 m with a limiting ray angle of 3.16º and a lower cut off frequency at 416 Hz. The deep water location had deeper duct (63 m) with a limiting ray angle of about 4.47º and a lower cut off frequency of 177 Hz. One third octave band analysis using multirate filters and time frequency analysis were used to study the reverberation characteristics. Comparison of deep water and shallow water reverberation characteristic across a broad band of frequencies are presented along with analysis based on ray modelling. © 2015, National Institute of Science Communication and Information Resources (NISCAIR). All rights reserved.

Nair B.M.,Naval Physical Oceanographic Laboratory | Arunkumar K.P.,Naval Physical Oceanographic Laboratory
Indian Journal of Geo-Marine Sciences | Year: 2015

Time reversal mirror (TRM) refocuses the received signal back to the original source location regardless of the complexity of the medium of propagation. When the medium contains several reflectors, the time reversal process can be used to focus on the desired target. Two arbitrary scatterers are positioned in the oceanic waveguide at different ranges and depths. Assuming that range of the focused scatterer is known approximately, the converged transmission vector through TRM may be matched against the computed eigenvector corresponding to the largest eigenvalue of the modeled channel transfer matrix between the transmitter and a test scatterer (at test depth) in order to determine the depth of the focused scatterer. Note that this approach of matching with the focused transmission vector leads to an improved depth estimation performance due to better SNR gain over a conventional matched field processor without TRM. © 2015, National Institute of Science Communication and Information Resources (NISCAIR). All rights reserved.

Naidu C.C.,Naval Science and Technological Laboratory | Stalin E.S.,Naval Physical Oceanographic Laboratory
International Conference on Microelectronics, Computing and Communication, MicroCom 2016 | Year: 2016

Establishing an underwater wireless acoustic communication link for practical applications is still a difficult task because of the severe channel conditions. Doppler and range spread are extremely high for under water channels (UWC) compared to ordinary radio channels. Moreover, the ever varying nature of the UWC increases the uncertainty in predicting the worst case values of doppler spread and range spread. In recent years, orthogonal frequency division multiplexing (OFDM) has emerged as the single solution to most of the problems in wireless communication systems. OFDM has dominated over single carrier modulation techniques mainly because of the simple, robust and low complexity receiver design with high data rate and high band width efficiency. But, it is only very recently that people have started exploring the usage of OFDM for UWC. In this paper, the design, modeling and testing of coded OFDM based underwater acoustic communication system for high data rate applications is explained. © 2016 IEEE.

Arunkumar K.P.,Naval Physical Oceanographic Laboratory | Murthy C.R.,Indian Institute of Science | Elango V.,University of California at San Diego
IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC | Year: 2016

We present a scheme for joint sparse-channel recovery and data detection in cyclic-prefix orthogonal frequency division multiplex (CP-OFDM) communication over doubly-spread underwater acoustic channels. Inter-carrier interference (ICI), caused by path-dependent Doppler, results in a non-diagonal channel mixing matrix that makes recovery difficult. To combat the effect of ICI, we consider the sequence of observations from partial interval demodulators, and using a path-based channel model, cast them into a data model amenable for sparse channel recovery. We then propose a two-stage algorithm for joint channel estimation and data detection. In the first stage, we recover the channel from pilot only observations and estimate the unknown data symbols from post-combined partial interval demodulator outputs. In the second stage, we use the data symbols estimated in the first stage to reconstruct the dictionary matrix corresponding to a full interval demodulator, re-estimate the channel using the entire observations including data subcarriers, and use it to detect the unknown data symbols. We also iterate between channel estimation and data detection in each stage, refining the dictionary in every iteration, to further reduce the detection error. Our simulation studies show that initial sparse channel recovery from the outputs of partial-interval demodulators considerably improves data detection performance, in terms of bit error rate, over that from a traditional full length demodulator output, in highly Doppler distorted scenarios. © 2016 IEEE.

Manoj G.,Naval Physical Oceanographic Laboratory | Sreedevi K.,Naval Physical Oceanographic Laboratory | Gopal V.,Naval Physical Oceanographic Laboratory
Procedia Computer Science | Year: 2016

High frequencies of the order of hundreds of kHz to MHz are employed for underwater imaging applications. Signal to noise ratios encountered for the front end receiver sections are seen to be very much reduced for these applications. Hence a low noise preamplifier and filter stage is very essential prior to the conventional antialiasing and digitization functionalities of the front end receivers. In this paper, an attempt is made to design and develop a low noise pre amplifier and filter to achieve a better SNR for the system. The pre amplifier stage designed is interfaced with hydrophones directly for gain and impedance matching purposes. The noise reduction is further achieved by means of a filter following the preamplifier stage which rejects the out of the band noise to an optimum level. Thorough study and analysis is carried out in this paper for the selection of low noise opamps, associated passive components and configuration of the preamplifier sections. Acoustic measurements are carried out for a sensor array of 150 kHz for evaluating the preamplifier and filter performance in the real scenario. © 2016 The Authors. Published by Elsevier B.V.

Manoj G.,Naval Physical Oceanographic Laboratory | Jacob E.,Naval Physical Oceanographic Laboratory | Kundukulam S.O.,Naval Physical Oceanographic Laboratory
Procedia Computer Science | Year: 2016

This paper deals with the design and simulation of mixing scheme for remote system operation connected through long coaxial cable. The remote system requires mixing of DC, high power AC and high speed data, which enables a cost effective interconnections through a single coaxial cable. Two schemes are proposed for the mixing, one with a relay based switching, in which the high power AC is in short pulsed form and other with passive filters. Simulation study using Multisim and Matlab was carried out for both the schemes. A comparison of these two schemes is also discussed. © 2016 The Authors. Published by Elsevier B.V.

Nair N.R.,Naval Physical Oceanographic Laboratory | Jacob R.,Naval Physical Oceanographic Laboratory
International Symposium on Ocean Electronics, SYMPOL | Year: 2013

Boundaries of oceans viz. surface and bottom form a channel for the propagation of underwater sound. Sound propagation in this channel is subj ected to random fluctuations due to the presence of in-homogeneities in water column and interaction with rough sea surface and sea bed. The inhomogeneities in the water column are caused by density microstructures and dynamics of the ocean. As a result of multi-path propagation, underwater sound experiences dispersion, Doppler and amplitude fluctuations. In this scenario, conventional signal detection algorithms do not perform well. This leads to the development of new signal processing algorithms to take care of fluctuations in acoustic signals. The current trend is to use channel factors caused by the ocean environmental variability to derive optimum performance of systems. One of the factors is the Doppler spread. In this paper, estimation of Doppler spread for a CW signal is attempted. © 2013 CUST.

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