Browsing by Author "Deshpande, Kshitija Bharat"
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- Contemporary Ionospheric Scintillation Studies: Statistics, 2D Analytical and 3D Numerical InversionConroy, James Patrick (Virginia Tech, 2022-08-31)The propagation of radiowaves through ionospheric irregularities can lead to random amplitude and phase fluctuations of the signal, otherwise known as scintillation, which can severely impact the performance of Global Navigation Satellite System (GNSS) and communication systems. Research into high latitude scintillation, through statistical analysis and inverse modeling, was completed to provide insight into the temporal and spatial distribution, and irregularity parameters, which can ultimately support the development of impact mitigation techniques, and deepen our understanding of the underlying physics. The work in this dissertation focused on the statistical analysis of Global Positioning System (GPS) scintillation data, data inversion, two-dimensional (2D) and three-dimensional (3D) scintillation modeling. The statistical analysis revealed distinct trends in the distribution of scintillation, while demonstrating that for GPS signals, phase scintillation occurs most frequently and can be treated as stochastic Total Electron Content (TEC); findings which have significant implications for impact mitigation. For the first of two inversion studies, scintillation data associated with a series of Polar Cap Patches (PCPs), which are common large-scale high latitude structures, was inverted to gain insight into the composition of the underlying irregularities. The results of this study suggest that the irregularities can be modeled as rods interbedded with sheets, which is knowledge that is crucial for the anchoring of models used to develop system mitigation techniques. The final study presents the results of modeling and inversion work to identify the conditions under which a 2D analytic version of the 3D numerical Satellite-beacon Ionospheric-scintillation global model of the upper atmosphere (SIGMA) model can be used to perform modeling in high latitude regions. During the study, it was found that the analytic model tends to diverge for electron density variance times irregularity layer thickness values exceeding 2, matched reasonably well for correlation length to thickness ratios up to 0.2, and was incompatible when ratios approached 0.35. An elevation angle limitation was also identified for the 2D model, and inflated values for the electron density variance were observed overall, which are thought to result from the weak scatter limits of the analytic model. These inflated values were particularly acute in the auroral zone during elevated conditions and suggest that the analytic model used in the study is not well suited for modeling the highly elongated irregularities associated with auroral precipitation.
- A Dedicated Search for Low Frequency Radio Transient Astrophysical Events using ETADeshpande, Kshitija Bharat (Virginia Tech, 2009-11-09)Astrophysical phenomena such as self-annihilation of primordial black holes (PBHs), gamma ray bursts (GRBs), and supernovae are expected to produce single dispersed pulses detectable in the low end of the radio spectrum. Analysis of these pulses could provide valuable information about the sources, and the surrounding and intervening medium. The Eight-meter-wavelength Transient Array (ETA) is a radio telescope dedicated to the search for these pulses in an 18 MHz bandwidth centered at 38 MHz. ETA consists of 10 dual-polarized active dipoles providing an all-sky field of view. This thesis describes the results of a search campaign using ETA, namely, a Crab giant pulse (CGP) search. CGPs are already known to exist, and thus provide an excellent diagnostic for system performance. We found 11 CGP candidates in 14 hours of data. Although there has not been a single compelling detection (signal-to-noise ratio > 6), our analysis shows that at least a few of these candidates may be CGPs. We also explain the analysis preparation for PBH and GRB searches. Additionally, we describe the instrument and a software "toolchain" developed for the analysis of data that includes calibration, radio frequency interference (RFI) mitigation, and incoherent dedispersion. A dispersed pulse simulation code was developed and used to test the toolchain. Finally, improvements are suggested.
- Investigation of High Latitude Ionospheric Irregularities utilizing Modeling and GPS ObservationsDeshpande, Kshitija Bharat (Virginia Tech, 2014-07-10)Complex magnetosphere-ionosphere coupling mechanisms result in high latitude irregularities that are difficult to characterize. Until recently, the polar and auroral irregularities remained largely unexplored. Inadequate infrastructures to deploy and maintain advanced dual frequency Global Navigation Satellite System (GNSS) receivers at high latitudes, especially in the Southern hemisphere, makes such an investigation a formidable task. Additionally, the complicated geometry of the magnetic field lines in these regions pose challenges in designing global scintillation models. This dissertation takes some steps towards bridging these gaps while advancing the state-of-the-art high latitude irregularity studies. In the first part of this dissertation, we briefly describe the Autonomous Adaptive Low-Power Instrument Platforms (AAL-PIP) experimental setup. These space science instrument platforms are being deployed in remote locations in Antarctica, improving the coverage of GNSS data availability. We explain in detail the method developed for analyzing high rate (typically 50 Hz) data from a novel dual-frequency Global Positioning System (GPS) receiver called Connected Autonomous Space Environment Sensor (CASES). We also report first observations from CASES at high latitudes. From this study, we established that CASES can be reliably used as a science grade GPS scintillation monitor. Following this, a novel three dimensional (3D) electromagnetic (EM) wave propagation model called "Satellite-beacon Ionospheric-scintillation Global Model of the upper Atmosphere" (SIGMA) was developed to simulate GNSS scintillations on ground. GPS scintillation simulations of significantly high fidelity are now possible with this model. While the model is global, it is the first such model which accounts for the complicated geometry of magnetic field lines at high latitudes. Using SIGMA, a sensitivity study is presented to understand the effect of geographical, propagation and irregularity parameters on the phase scintillations. This allows us to reduce the dimensionality of the design space while solving the inverse problem described next. In the final part, we utilize the tools developed for GPS measurement analysis and SIGMA to characterize the high latitude irregularities. We propose an inverse modeling technique to derive irregularity parameters by comparing the high rate (50 Hz) GNSS observations to the modeled outputs. We consider interhemispheric high latitude datasets for this investigation. We also implement SIGMA for analyzing a substorm event observed by AAL-PIP stations. One of the unique contributions of this research is to demonstrate that such an inverse modeling technique can form a basis in the investigation of the ionospheric irregularities. Moreover, availability of ample auxiliary data from multi-instrument observations can assist in this quest of understanding the physics of high latitude irregularities and their generation mechanisms.