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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. Source


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. Source


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. Source


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. Source


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. Source

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