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Neema C.P.,Indian Institute of Science | Hareeshkumar P.V.,Naval Physical and Oceanographic Laboratory
Climate Dynamics | Year: 2012

Arabian Sea Mini Warm Pool (ASMWP) is a part of the Indian Ocean Warm Pool and formed in the eastern Arabian Sea prior to the onset of the summer monsoon season. This warm pool attained its maximum intensity during the pre-monsoon season and dissipated with the commencement of summer monsoon. The main focus of the present work was on the triggering of the dissipation of this warm pool and its relation to the onset of summer monsoon over Kerala. This phenomenon was studied utilizing NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric and Research) re-analysis data, TRMM Micro wave Imager (TMI) and observational data. To define the ASMWP, sea surface temperature exceeding 30. 25°C was taken as the criteria. The warm pool attained its maximum dimension and intensity nearly 2 weeks prior to the onset of summer monsoon over Kerala. Interestingly, the warm pool started its dissipation immediately after attaining its maximum core temperature. This information can be included in the present numerical models to enhance the prediction capability. It was also found that the extent and intensity of the ASMWP varied depending on the type of monsoon i. e., excess, normal, and deficient monsoon. Maximum core temperature and wide coverage of the warm pool observed during the excess monsoon years compared to normal and deficient monsoon years. The study also revealed a strong relationship between the salinity in the eastern Arabian Sea and the nature of the monsoon. © 2011 Springer-Verlag.


Ambat S.K.,Indian Institute of Science | Ambat S.K.,Naval Physical and Oceanographic Laboratory | Chatterjee S.,KTH Royal Institute of Technology | Hari K.V.S.,Indian Institute of Science
IEEE Transactions on Signal Processing | Year: 2013

For compressed sensing (CS), we develop a new scheme inspired by data fusion principles. In the proposed fusion based scheme, several CS reconstruction algorithms participate and they are executed in parallel, independently. The final estimate of the underlying sparse signal is derived by fusing the estimates obtained from the participating algorithms. We theoretically analyze this fusion based scheme and derive sufficient conditions for achieving a better reconstruction performance than any participating algorithm. Through simulations, we show that the proposed scheme has two specific advantages: 1) it provides good performance in a low dimensional measurement regime, and 2) it can deal with different statistical natures of the underlying sparse signals. The experimental results on real ECG signals shows that the proposed scheme demands fewer CS measurements for an approximate sparse signal reconstruction. © 1991-2012 IEEE.


Sudha A.K.,Cochin University of Science and Technology | Rao C.V.K.P.,Naval Physical and Oceanographic Laboratory
Remote Sensing Letters | Year: 2013

Oceansat-2 scatterometer (OSCAT)-derived winds were compared with in situ observations made by Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) and Triangle Trans-Ocean Buoy Network (TRITON) buoys in the Indian Ocean and the Pacific Ocean, respectively. Root mean square deviation (RMSD) for wind speed is 1.71 m s-1 for the Indian Ocean and 1.92 m s-1 for the Pacific Ocean. The wind speed accuracies are within the mission requirement (<2 m s-1), but the wind direction errors are higher than the mission requirement (>20°). The RMSD values of wind direction are 51.38? and 44.61° for the Indian Ocean and the Pacific Ocean, respectively. OSCAT winds tend to underestimate the buoy value when the wind speeds were low (<4 m s-1) and overestimate the buoy value when the wind speeds were high (>4 m s0). The wind speed residuals show an increasing trend with the increase of wind speeds. Interestingly, the wind direction residuals decrease with increasing wind speed and deviations of wind directions are conspicuously large for lower wind speeds than for higher wind speeds. OSCAT wind speeds were relatively more accurate for the Indian Ocean than those for the Pacific Ocean. © 2012 Taylor & Francis.


Nishamol P.A.,Naval Physical and Oceanographic Laboratory | Ebenezer D.D.,Naval Physical and Oceanographic Laboratory
Journal of the Acoustical Society of America | Year: 2014

An exact series method is presented to analyze classical Langevin transducers with arbitrary boundary conditions. The transducers consist of an axially polarized piezoelectric solid cylinder sandwiched between two elastic solid cylinders. All three cylinders are of the same diameter. The length to diameter ratio is arbitrary. Complex piezoelectric and elastic coefficients are used to model internal losses. Solutions to the exact linearized governing equations for each cylinder include four series. Each term in each series is an exact solution to the governing equations. Bessel and trigonometric functions that form complete and orthogonal sets in the radial and axial directions, respectively, are used in the series. Asymmetric transducers and boundary conditions are modeled by using axially symmetric and anti-symmetric sets of functions. All interface and boundary conditions are satisfied in a weighted-average sense. The computed input electrical admittance, displacement, and stress in transducers are presented in tables and figures, and are in very good agreement with those obtained using ATILA-a finite element package for the analysis of sonar transducers. For all the transducers considered in the analysis, the maximum difference between the first three resonance frequencies calculated using the present method and ATILA is less than 0.03%. © 2014 Acoustical Society of America.


The fabrication of reproducible low resistance ohmic contacts on electrochemically etched porous silicon is an existing challenge due to the irregular and nanoscale sized Si crystallites and the voids in the structure of the porous silicon. In this study, the effect of the peak firing treatment by a fire-through process at different temperatures (700-825 °C) and electroless Ag deposition steps on the electrical properties of the Ag electrodes have been investigated. The cross-sectional view of the Ag metal/doped Si region by SEM shows that Ag metal firmly coalesces with the doped Si surface. The sintering of the metal contact at a higher peak firing temperature enables the firing-through of the Ag metal paste onto the high resistivity porous silicon that leads to the interaction with the doped Si region. As a result, a good ohmic contact between the Ag metal and doped Si emitter is formed. The specific contact resistance, ρc (Ω.cm2) measurement of the Ag electrode by the three-point probe method shows that the ohmic properties of the just sintered Ag electrodes are very good, with ρc ≅ 1.025 × 10-4 Ω.cm2. An electroless Ag deposition step using AgNO3 solution under constant light illumination was applied to improve the electrical properties of the sintered Ag contacts. It is found that the ρc of the Ag electrodes was improved to about 3.25 × 10-5 Ω.cm2 after applying the electroless Ag deposition on the sintered Ag contacts. The deposition of the pure silver metal on the Ag contacts fills up the micropores and voids and improves the electrical properties of the contact structure. This journal is © 2013 The Royal Society of Chemistry.


In the present study, electrical and microstructural properties of electroplated screen-printed Ag metal contacts in crystalline silicon solar cells have been investigated. The metal contacts were formed by a fire-through process at different firing temperatures (700-850°C) in ambient air. This is followed by a low temperature annealing step at 450°C in ambient nitrogen. Subsequently, an electroless Ag deposition step using AgNO3 solution under light illumination was performed to improve the electrical properties of the Ag contacts. The specific contact resistance, ρc (Ω-cm2) of the Ag contacts was measured by applying the three-point probe method. It shows that a ρc = 2.16 × 10-5 Ω cm2 was measured for the fire-through Ag contacts. It is found that the specific contact resistance of the fire-through Ag contacts was improved to about 2.35 × 10-6 Ω cm 2 after applying an electroless Ag deposition step on the Ag contacts. The cross-sectional view of the Ag metal/doped Si contact region by SEM measurement shows that the Ag metal firmly coalesces with the doped Si region. However, the microstructure of the Ag metal has found a large number of micropores and voids. These may be formed due to the burn off of the volatile organic constituents of the Ag metal paste during the sintering process. The deposition of the pure silver (Ag) metal during the electroless Ag deposition step fills up the micropores and voids in the Ag contacts. As a result, finger conductivity and the electrical properties of the fire-through processed Ag contacts are improved. © 2013 The Royal Society of Chemistry.


Vinod P.N.,Naval Physical and Oceanographic Laboratory
Journal of Materials Science: Materials in Electronics | Year: 2010

Porous silicon has been considered as a promising optoelectronic material for developing a variety of optoelectronic devices and sensors. In the present study, the electrical properties and metallurgical process of the screen-printed Ag metallization formed on the porous silicon surface of the silicon solar cell have been investigated. The contact structure consists of thick-film Ag metal contact patterned on the top of the porous silicon surface. The sintering process consists of a rapid firing step at 750-825 °C in air ambient. It results in the formation of a nearly perfect contact structure between the Ag metal and porous silicon/p-Si structure that forms the top metalization for the screen-printed silicon solar cells. The SEM picture shows that Ag metal firmly coalesces with the silicon surface with a relatively smooth interfacial morphology. This implies that high temperature fire-through step has not introduced any signs of adverse effect of junction puncture or excessive Ag indiffusion, etc. The three-point probe (TPP) method was applied to estimate the specific contact resistance, ρ c (Ω-cm 2) of the contact structure. The TPP measurement shows that contact structure has excellent ohmic properties with ρ c = 1.2 × 10 -6 Ω-cm 2 when the metal contact sintered at 825 °C. This value of the specific contact resistance is almost three orders of magnitude lower than the corresponding value of the ρ c = 7.35 × 10 -3 Ω-cm 2 obtained for the contact structure sintered at 750 °C. This improvement in the specific contact resistance indicates that with increase in the sintering temperature, the barrier properties of the contact structure at the interface of the Ag metal and porous silicon structure improved which in turn results a lower specific contact resistance of the contact structure. © Springer Science+Business Media, LLC 2009.


Vinod P.N.,Naval Physical and Oceanographic Laboratory
Journal of Materials Science: Materials in Electronics | Year: 2011

The specific contact resistance of the screenprinted Ag contacts in the silicon solar cells has been investigated by applying two independent test methodologies such as three-point probe (TPP) and well-known transfer length model (TLM) test structure respectively. This paper presents some comparative results obtained with these two measurement techniques for the screenprinted Ag contacts formed on the porous silicon antireflection coating (ARC) in the crystalline silicon solar cells. The contact structure consists of thick-film Ag metal contacts patterned on the top of the etched porous silicon surface. Five different contact formation temperatures ranging from 725 to 825 °C for few minutes in air ambient followed by a short time annealing step at about 450 °C in nitrogen ambient was applied to the test samples in order to study the specific contact resistance of the screen-printed Ag metal contact structure. The specific contact resistance of the Ag metal contacts extracted based on the TPP as well as TLM test methodologies has been compared and verified. It shows that the extraction procedure based on the TPP method results in specific contact resistance, ρ c = 2.15 × 10 -6 Ω-cm 2 indicating that screen-printed Ag contacts has excellent ohmic properties whereas in the case of TLM method, the best value of the specific contact resistance was found to be about ρ c = 8.34 9 10-5 Ω-cm2. These results indicate that the ρ c value extracted for the screen-printed Ag contacts by TPP method is one order of magnitude lower than that of the corresponding value of the ρ c extracted by TLM method. The advantages and limitations of each of these techniques for quantitatively evaluating the specific contact resistance of the screenprinted Ag contacts are also discussed. ©Springer Science+Business Media, LLC 2011.


George J.,Naval Physical and Oceanographic Laboratory
The Journal of the Acoustical Society of America | Year: 2010

A method is presented to determine the response of a spherical acoustic transducer that consists of a fluid-filled piezoelectric sphere with an elastic coating embedded in infinite fluid to electrical and plane-wave acoustic excitations. The exact spherically symmetric, linear, differential, governing equations are used for the interior and exterior fluids, and elastic and piezoelectric materials. Under acoustic excitation and open circuit boundary condition, the equation governing the piezoelectric sphere is homogeneous and the solution is expressed in terms of Bessel functions. Under electrical excitation, the equation governing the piezoelectric sphere is inhomogeneous and the complementary solution is expressed in terms of Bessel functions and the particular integral is expressed in terms of a power series. Numerical results are presented to illustrate the effect of dimensions of the piezoelectric sphere, fluid loading, elastic coating and internal material losses on the open-circuit receiving sensitivity and transmitting voltage response of the transducer.


Shyni T.N.,Naval Physical and Oceanographic Laboratory | Hareesh Kumar P.V.,Naval Physical and Oceanographic Laboratory
International Journal of Remote Sensing | Year: 2014

Satellite altimetry in combination with ground-truth measurements and the Okubo-Weiss parameter-based eddy-tracking algorithm are used to study eddies in the southeastern Arabian Sea (SEAS) during the summer and winter of 2007 and 2008. In the SEAS, only the cyclonic eddy is present in summer whereas both cyclones and anticyclones are present in winter. These eddies, with dimensions of 60-120 km, propagate westward with slight north-south deflection at a speed of 5-23 cm s-1 (mean 11.8 cm s-1). The lifespan of eddies varies from two to six weeks. Exceptions are a cyclonic eddy in 2007 and an anticyclonic eddy in 2008 that persisted for 6 and 11 weeks, respectively. During summer and the early half of winter, wind-stress curl plays a significant role in the genesis of eddies in the SEAS. However, the propagation of these eddies is not influenced by wind-stress curl. Observations reveal that the thermal structure and currents are modified by these eddies. In winter, the signature of the cyclonic eddy is not prominent on the surface, as the water column is homogeneous up to 100 m. In the summer monsoon season, the signature of the eddy is evident up to the surface. During this period, the southward West India Coastal Current is modified locally by the cyclonic eddy formed in the SEAS. © 2014 Taylor & Francis.

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