Rapid Fluctuations In Amplitude Research Articles

Analysis of Amplitude Scintillation and Positioning Error of IRNSS/GPS/SBAS Receiver for Heavy Rainy Days

Indian Regional Navigation Satellite System (IRNSS) was developed for providing positioning, velocity and timing information to Indian users within the error of 10 m. Amplitude scintillation is the rapid fluctuations in amplitude of navigation signal due to small scale irregularities in electron density presents in ionosphere. Strong scintillation can degrade the accuracy of navigation system and generate the errors in precise positioning services. S4 index is the statistical measurement of amplitude scintillation which reflects scintillation event. The characteristics of amplitude scintillation on IRNSS/SBAS/GPS receiver with all seven satellites during heavy rainy days at the low altitude station IITRAM, Ahmedabad are investigated. The raw data are collected from the IRNSS/GPS/SBAS receiver with L1, L5 and S band at the Ahmedabad city with average altitude of 53 m in Gujarat. Here, real time monitoring system for amplitude scintillation and positioning error is designed for all three bands. S4 values for three bands are calculated and analyzed to define the scintillation event for the period of 14 days. The position errors of heavy rainy days are also found. This study also focuses on time duration over which the carrier to noise ratio acquires zero value, effect of elevation angle of satellite on carrier to noise ratio and ionospheric delay. We found that L1 band suffers more from scintillation compare to L5 and S band. Day to day investigation of amplitude scintillation is carried out here. The results of this investigation would be utilized for understanding of amplitude scintillation effect during rainy weather condition in low altitude station.

Challenges to Equatorial Plasma Bubble and Ionospheric Scintillation Short-Term Forecasting and Future Aspects in East and Southeast Asia

Equatorial plasma bubbles (EPBs) can cause rapid fluctuations in amplitude and phase of radio signals traversing the ionosphere and in turn produce serious ionospheric scintillations and disrupt satellite-based communication links. Whereas numerous studies on the generation and evolution of EPBs have been performed, the prediction of EPB and ionospheric scintillation occurrences still remains unresolved. The generalized Rayleigh–Taylor (R–T) instability has been widely accepted as the physical mechanism responsible for the generation of EPBs. But how the factors, which seed the development of R–T instability and control the dynamics of EPBs and resultant ionospheric scintillations, change on a short-term basis are not clear. In the East and Southeast Asia, there exist significant differences in the generation rates of EPBs at closely located stations, for example, Kototabang (0.2°S, 100.3°E) and Sanya (18.3°N, 109.6°E), indicating that the decorrelation distance of EPB generation is small (hundreds of kilometers) in longitude. In contrast, after the initial generation of EPBs at one longitude, they can drift zonally more than 2000 km and extend from the magnetic equator to middle latitudes of 40° or higher under some conditions. These features make it difficult to identify the possible seeding sources for the EPBs and to accurately predict their occurrence, especially when the onset locations of EPBs are far outside the observation sector. This paper presents a review on the current knowledge of EPBs and ionospheric scintillations in the East and Southeast Asia, including their generation mechanism and occurrence morphology, and discusses some unresolved issues related to their short-term forecasting, including (1) what factors control the generation of EPBs, its day-to-day variability and storm-time behavior, (2) what factors control the evolution and lifetime of EPBs, and (3) how to accurately determine ionospheric scintillation from EPB measurements. Special focus is given to the whole process of the EPB generation, development and disruption. The current observing capabilities, future new facilities and campaign observations in the East and Southeast Asia in helping to better understand the short-term variability of EPBs and ionospheric scintillations are outlined.

Solar Scintillation Effects on the Deep Space Communication Performance for Radio Wave Propagation Through Non-Kolmogorov Turbulence

Solar scintillation refers to the rapid amplitude fluctuations of the radio signal passing through large-scale coronal irregularities. In this letter, the scintillation index model is first derived to characterize radio wave propagation in coronal turbulence during superior solar conjunction. The proposed model takes into account the non-Kolmogorov turbulence model with the Rytov approximation method. After that, the bit error rate (BER) performance is analyzed based on the proposed scintillation index model, which has instructional significance for deep space communication but still remains largely unexplored. Simulation results demonstrate that the coronal parameters have extensive effects on the scintillation index, and further affect the link BER performance. Our proposed scintillation index model also achieves better agreement with the observation data.

Developmental deprivation-induced perceptual and cortical processing deficits in awake-behaving animals.

Sensory deprivation during development induces lifelong changes to central nervous system function that are associated with perceptual impairments. However, the relationship between neural and behavioral deficits is uncertain due to a lack of simultaneous measurements during task performance. Therefore, we telemetrically recorded from auditory cortex neurons in gerbils reared with developmental conductive hearing loss as they performed an auditory task in which rapid fluctuations in amplitude are detected. These data were compared to a measure of auditory brainstem temporal processing from each animal. We found that developmental HL diminished behavioral performance, but did not alter brainstem temporal processing. However, the simultaneous assessment of neural and behavioral processing revealed that perceptual deficits were associated with a degraded cortical population code that could be explained by greater trial-to-trial response variability. Our findings suggest that the perceptual limitations that attend early hearing loss are best explained by an encoding deficit in auditory cortex.

Detection of artifacts from high energy bursts in neonatal EEG

Detection of non-cerebral activities or artifacts, intermixed within the background EEG, is essential to discard them from subsequent pattern analysis. The problem is much harder in neonatal EEG, where the background EEG contains spikes, waves, and rapid fluctuations in amplitude and frequency. Existing artifact detection methods are mostly limited to detect only a subset of artifacts such as ocular, muscle or power line artifacts. Few methods integrate different modules, each for detection of one specific category of artifact. Furthermore, most of the reference approaches are implemented and tested on adult EEG recordings. Direct application of those methods on neonatal EEG causes performance deterioration, due to greater pattern variation and inherent complexity. A method for detection of a wide range of artifact categories in neonatal EEG is thus required. At the same time, the method should be specific enough to preserve the background EEG information. The current study describes a feature based classification approach to detect both repetitive (generated from ECG, EMG, pulse, respiration, etc.) and transient (generated from eye blinking, eye movement, patient movement, etc.) artifacts. It focuses on artifact detection within high energy burst patterns, instead of detecting artifacts within the complete background EEG with wide pattern variation. The objective is to find true burst patterns, which can later be used to identify the Burst-Suppression (BS) pattern, which is commonly observed during newborn seizure. Such selective artifact detection is proven to be more sensitive to artifacts and specific to bursts, compared to the existing artifact detection approaches applied on the complete background EEG. Several time domain, frequency domain, statistical features, and features generated by wavelet decomposition are analyzed to model the proposed bi-classification between burst and artifact segments. A feature selection method is also applied to select the feature subset producing highest classification accuracy. The suggested feature based classification method is executed using our recorded neonatal EEG dataset, consisting of burst and artifact segments. We obtain 78% sensitivity and 72% specificity as the accuracy measures. The accuracy obtained using the proposed method is found to be about 20% higher than that of the reference approaches. Joint use of the proposed method with our previous work on burst detection outperforms reference methods on simultaneous burst and artifact detection. As the proposed method supports detection of a wide range of artifact patterns, it can be improved to incorporate the detection of artifacts within other seizure patterns and background EEG information as well.

Ultrasonic doppler detection of laser-tissue interaction

Laser therapy for the safe and effective coagulation and ablation of tissues requires precise control of the amount of energy delivered to and absorbed by the volume of tissue of interest. We propose that an ultrasonic transducer pointing in the same direction as the laser fiber could be used to monitor the changes in ultrasonic properties caused by the absorption of light by the tissue. A modified 20 MHz pulsed Doppler was used to evaluate the ultrasonic effects of 35 exposures of beef liver and muscle to a high power diode laser in vitro in real time. We found two distinct levels of acoustic activity in the tissue. Type 1 activity consisted of slow variations in the phase and small changes in the amplitude of the echoes, while type 2 activity consisted of large and rapid fluctuations in amplitude and phase. We found that the residual increase in echogenicity of the tissue and the delay to the onset of type 2 activity were functions of laser power and tissue type and were correlated to lesion severity. We hypothesize that type 1 activity corresponds to motion (thermal expansion and contraction) of the tissue, and that type 2 activity corresponds to the creation of gas bubbles (vaporization) in the tissue. We conclude that the absorption of energy changes the acoustic properties of tissue during and after exposure and that Doppler signal processing can be used to differentiate various levels of laser-tissue interaction in real time.

Nonlinear generation of missing modes on a vibrating string

The nonlinear transfer of energy among modes of different frequencies on a vibrating string is investigated both theoretically and experimentally. The nonlinearity is associated with the well-known variation of string tension caused by the vibration modes, but it is essential that at least one of the end supports has finite mechanical admittance if there is to be any mode coupling. If the nonrigid bridge support has zero admittance in a direction parallel to the string, the coupling is of third order in the mode amplitudes. For a more realistic model in which the string changes direction as it passes over a bridge of finite admittance there are additional coupling terms of second order. The first mechanism gives driving terms of frequency 2ωn±ωm where ωn and ωm are, respectively, the angular frequencies of the nth and mth modes present on the string, while the second mechanism gives driving terms of angular frequencies 2ωn and 2ωm. Analysis shows that modes absent from the initial excitation of the string can be driven to appreciable amplitude by these mechanisms, reaching their maximum amplitude after a time typically of order 0.1 s. Modes that are in nearly harmonic frequency relationship behave simply but coupling of modes that are appreciably inharmonic may give rise to rapid amplitude fluctuations. A simple experiment with a wire deflected by a bridge of elastic cord and plucked so as to eliminate a particular mode from the initial excitation provided general semiquantitative confirmation of the theoretical predictions.

Encoding of rapid amplitude fluctuations by Cochlear-nerve fibres in the guinea-pig.

Responses of cochlear nerve fibres in the guinea-pig were measured to sinusoidally amplitude-modulated tones at fibre characteristic frequency. The modulation depth was +/- 3 dB and the modulation rate was varied between 6.25 and 6400 Hz keeping the mean level of the tone constant. The resultant period histogram (locked to the modulation cycle) was used to determine the depth of modulated neural discharge. The functions showing the variation of discharge modulation with modulation frequency were, in general, low-pass. The cut-off of these functions appears to be primarily determined by the filtering properties of the fibres.

Higher-mode Rayleigh waves as an aid to seismic discrimination

abstract Long-period records of surface waves from earthquakes in central Asia are often complex, with rapid amplitude fluctuations and apparent multiple arrivals of energy of the same frequency. Comparison of records from closely spaced earthquakes shows that the complexity is not caused by multipath propagation, but is associated with the seismic source geometry. The multiple arrivals are higher-mode Rayleigh waves traveling with group velocities faster than the fundamental mode. A relatively high ratio of higher-mode to fundamental-mode energy can be produced when an unusual combination of source depth and focal mechanism leads to poor excitation of the fundamental-mode Rayleigh wave, in which case, the amplitude of the higher mode yields a more reliable estimate of the size of the event than the amplitude of the fundamental mode. Several anomalous events in central Asia, which are characterized by unusually low Ms : mb ratios and hence could be suspected to be nuclear explosions, are reclassified as earthquakes when the new, higher-mode scale, Mhs, is employed. The focal depth and mechanism of these anomalous events apparently causes poor excitation of the fundamental-mode Rayleigh wave without significantly affecting the amplitude of the higher mode. Elimination of these source-related factors from traditional Ms : mb discriminants is an essential tool for reliable discrimination between earthquakes and nuclear underground tests.

Rapid Amplitude Fluctuation of Magnetoelastic Waves in Axially Magnetized [110] Oriented YIG Rods

Rapid amplitude fluctuations of magnetoelastic waves have been observed in axially magnetized, [110]-oriented YIG rods as the magnetic field, and hence delay time, are varied. The fluctuations are characterized by an 8 to 14 dB amplitude fluctuation of the entire delayed pulse with a period varying from 0.025 to 0.08 μsec. Analysis based on the interference of two pure shear elastic modes of the YIG sample coupled to the magnetic-spin system predicts an amplitude fluctuation of 11.8 dB with a period of 0.04 μsec, which compares well with the observed results. A description of the observed amplitude fluctuation, and its analysis, is presented. Factors necessary for the observation of this amplitude fluctuation and means by which it can be eliminated are discussed.