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Search Results: 1 - 10 of 494545 matches for " B. M. Vyas "
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Night-time F-region and daytime E-region ionospheric drifts measured at Udaipur during solar flares
B. M. Vyas,R. Pandey
Annales Geophysicae (ANGEO) , 2003,
Abstract: Ionospheric drifts measured at a low latitude station, Udaipur (Geomag. Lat. 14.5° N), in the night-time F-region and daytime E-region during solar flares have been studied. The night-time observations, which correspond to the F-region drifts, were carried out on five different nights. The daytime observation corresponding to the E-region drifts is only for one day. It is found that the apparent drift during the solar flare period is reduced considerably, in the daytime E-region as well as in the night-time F-region. The East-West and North-South components of the apparent drift speed are also similarly affected. For the daytime E-region drifts during a flare, increased ionization and subsequent reduction of reflection height is proposed to be the cause of reduced drift speeds. For the night-time F-region drifts, a reduced electric field at the F-region heights resulting from coupling of sunlit and dark hemispheres has been proposed to be the possible cause. Key words. Ionosphere (electric fields and currents; ionospheric disturbances)
Studies of Atmospheric Aerosol’s Parameters during Pre-Diwali to Post–Diwali festival period over Indian Semi Arid Station i.e., Udaipur
B. M. Vyas,Vimal Saraswat
Applied Physics Research , 2012, DOI: 10.5539/apr.v4n2p40
Abstract: The paper describes day to day changes observed in atmospheric aerosol’s parameters namely AI300, AOD550, a, TWC, b and MC from their background reference day value over Udaipur during Pre to Post-Diwali period from 2002 to 2007. Before Diwali to Diwali, daily variations in such aerosol’s parameters values show the substantial increase order of 30- 60% from their respective background. Afterward Diwali, such change in AI300 shows the decreasing behavior, in sharp contrast to this, daily variations in AOD550, b and MC exhibit either lower value or attain the same peak value depending upon the corresponding level of TWC and rainfall activity of that particular year. These variations are not instantly but with certain delay in the same period. A considerable amount of implication in aerosols loading is observed in terms of reduction in SHWRF from 20-30% during the enhancement period of aerosol’s parameters. However, DSWRF does not show any appreciable change. The main possible causes of increase in aerosol's loadings within above period are discussed in views of inter mixed effect of local emission activity and long range transport of aerosols from heavy polluted IGP site and in reduction of PBL height and wind speed.
Improvement of Position Accuracy with GAGAN and the Impact of Scintillation on GNSS  [PDF]
Surendra Sunda, B. M. Vyas, S. V. Satish, K. S. Parikh
Positioning (POS) , 2013, DOI: 10.4236/pos.2013.44028
Abstract:

GAGAN is an Indian implementation of Satellite Based Augmentation System (SBAS), developed jointly by Airports Authority of India (AAI) and Indian Space Research Organisation (ISRO). It is in final operational phase with all the required ground and space segments ready. With the availability of GAGAN signal-in-space (SIS), the improvement in position solution is investigated using the two collocated dual frequency GPS receivers. One of the receivers was configured as SBAS receiver and the other was kept as GPS stand-alone receiver. It is found that accuracy in position improved significantly in SBAS receiver due to GAGAN correction. The impact of scintillation on GNSS was also investigated in terms of position degradation and loss of lock of the satellite signals. The manyfold effects of scintillation on GPS and SBAS are discussed in detail. The results indicate ~15% reduction in number of measurements due to loss of lock during severe scintillation.

Performance analysis of Fourier and Vector Matrix Multiply methods for phase reconstruction from slope measurements
M B Roopashree,Akondi Vyas,B Raghavendra Prasad
Physics , 2009,
Abstract: The accuracy of wavefront reconstruction from discrete slope measurements depends on the sampling geometry, coherence length of the incoming wavefronts, wavefront sensor specifications and the accuracy of the reconstruction algorithm. Monte Carlo simulations were performed and a comparison of Fourier and Vector Matrix Multiply reconstruction methods was made with respect to these experimental and computational parameters. It was observed that although Fourier reconstruction gave consistent accuracy when coherence length of wavefronts is larger than the corresponding pitch on the wavefront sensor, VMM method gives even better accuracy when the coherence length closely matches with the wavefront sensor pitch.
Effective coherence length estimation of optical wavefronts
Akondi Vyas,M. B. Roopashree,B. Raghavendra Prasad
Physics , 2009,
Abstract: In adaptive optics, the measurement of spatial coherence length helps in deciding the optimum design parameters of a Shack Hartmann Sensor (SHS). Two methods of estimating the spatial coherence length of optical wavefronts are presented. The first method is based on counting the number of Hough peaks in the wavefront. The second method is based on a simple data mining technique applied on the wavefronts. Optical wavefronts with different properties are simulated and used for statistical analysis. A comparison of the performance of the two methods is presented using Monte Carlo simulations. It is shown that both these methods can become efficient tools in estimating the effective coherence length of optical wavefronts.
Prediction of wavefronts in adaptive optics to reduce servo lag errors using data mining
Akondi Vyas,M B Roopashree,B Raghavendra Prasad
Physics , 2009,
Abstract: Servo lag errors in adaptive optics lead to inaccurate compensation of wavefront distortions. An attempt has been made to predict future wavefronts using data mining on wavefronts of the immediate past to reduce these errors. Monte Carlo simulations were performed on experimentally obtained data that closely follows Kolmogorov phase characteristics. An improvement of 6% in wavefront correction is reported after data mining is performed. Data mining is performed in three steps (a) Data cube Segmentation (b) Polynomial Interpolation and (c) Wavefront Estimation. It is important to optimize the segment size that gives best prediction results. Optimization of the best predictable future helps in selecting a suitable exposure time.
Digital long focal length lenslet array using spatial light modulator
Akondi Vyas,M B Roopashree,B Raghavendra Prasad
Physics , 2009,
Abstract: Under a thin lens and paraxial approximation, the phase transformation function of a lens was simulated on a Liquid Crystal (LC) based Spatial Light Modulator (SLM). The properties of an array of such lenses simulated on transmitting type and reflecting type SLMs were investigated and the limits of its operation in wavefront sensing applications are discussed.
Optimization of Existing Centroiding Algorithms for Shack Hartmann Sensor
Akondi Vyas,M B Roopashree,B R Prasad
Physics , 2009,
Abstract: Three centroiding techniques to estimate the position of the spots in a Shack Hartmann sensor: Normalized Centre of Gravity (CoG), Iteratively Weighted Centre of Gravity (IWCoG) and Intensity Weighted (IWC) centroiding are studied in comparison. The spot pattern at the focal plane of a Shack Hartmann sensor was simulated by including the effect of a background noise. We present the results of optimization of the performance of each of the centroiding techniques as a function of Signal to Noise Ratio (SNR) at different experimental conditions.
Extrapolating Zernike Moments to Predict Future Optical Wave-fronts in Adaptive Optics Using Real Time Data Mining
Akondi Vyas,M. B. Roopashree,B. Raghavendra Prasad
Physics , 2010,
Abstract: We present the details of predicting atmospheric turbulence by mining Zernike moment data obtained from simulations as well as experiments. Temporally correlated optical wave-fronts were simulated such that they followed Kolmogorov phase statistics. The wave-fronts reconstructed either by modal or zonal methods can be represented in terms of Zernike moments. The servo lag error in adaptive optics is minimized by predicting Zernike moments in the near future by using the data from the immediate past. It is shown statistically that the prediction accuracy depends on the number of past phase screens used for prediction and servo lag time scales. The algorithm is optimized in terms of these parameters for real time and efficient operation of the adaptive optics system. On an average, we report more than 3% improvement in the wave-front compensation after prediction. This analysis helps in optimizing the design parameters for sensing and correction in closed loop adaptive optics systems.
Performance of centroiding algorithms at low light level conditions in adaptive optics
Akondi Vyas,M. B. Roopashree,B. Raghavendra Prasad
Physics , 2010, DOI: 10.1109/ARTCom.2009.30
Abstract: The performance metrics of different centroiding algorithms at low light level conditions were optimized in the case of a Shack Hartmann Sensor (SHS) for efficient performance of the adaptive optics system. For short exposures and low photon flux, the Hartmann spot does not have a Gaussian shape due to the photon noise which follows Poissonian statistics. The centroiding estimation error was calculated at different photon levels in the case of changing spot size and shift in the spot using Monte Carlo simulations. This analysis also proves to be helpful in optimizing the SHS specifications at low light levels.
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