19 research outputs found
Incoherent scatter modeling of Jicamarca radar spectra
No full textThis thesis describes a project on the modeling of spectral characteristics of electron
density irregularities of the topside equatorial ionosphere probed by the Jicamarca Incoherent
Scatter Radar (ISR) located near Lima, Peru. The topside equatorial ionosphere
is a multi-ion plasma and the spectrum of its electron density irregularities can
be modeled by extending the single-ion spectral model developed by Kudeki and Milla
(2011) for a collisional equatorial F-region ionosphere. This single-ion model of Kudeki
and Milla captures the essential physics of the equatorial F-region ionosphere where random
displacements of the dominant oxygen ions are characterized as a Brownian-motion
process, while electron displacements are non-Brownian and described in terms of a numerical
library (constructed using a Monte-Carlo approach) of single-electron ACFs (autocorrelation
functions) parametrized by five state parameters of the F-region consisting of
ionospheric electron density, geomagnetic flux density, electron and ion temperatures, and
the deviation angle of the radar boresight direction from the plane perpendicular to the
geomagnetic field, the so-called magnetic aspect angle. While the extension of the model
to the multi-ion case is straightforward, the discrete nature of the numerical electron ACF
library defined over a grid of input parameters precludes the evaluation of the extended
model with arbitrary and continuously varying input parameters. To overcome this difficulty
a machine learning (ML) based interpolation procedure is developed. The thesis
describes the ML algorithm, the associated training and testing steps, and finally presents
a suite of examples of multi-ion IS spectra obtained with the extended model.Item withdrawn by Laura Spradlin ([email protected]) on 2013-12-10T15:32:49Z
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Mess is More: Iterating Dwelling Design Processes For The Urban Poor In New Delhi, India
No full textThe architectural discipline has always maintained a safe distance from the ‘informal’ settlements by positioning itself as objective outsiders. However, in the case of India’s capital city of Delhi with 73% of its population living in these settlements, the discipline has been forced to re-position itself. Forming outside the claims of regulation and planning, the sheer existence of these settlements challenges the city’s aspiration to become a ‘world city’. Resultant, is a love-hate dynamic where a high-modernist design propaganda seeks to evict the urban poor’s position in the city to a ‘safe’ distance where they are out of sight but in the appropriate range to aid the city function.I base my study in ‘Anand Parbat’,a deplorable resettlement transit neighbourhood developed 4 miles from the city core to re-house slum dwellers evicted from inner city areas. Envisioned as a transition camp, it is home to more than 2000 families. The design of the ‘camp’, involved a process of ‘formalizing’ the informal and invited disciplinarian aid, which by the virtue of ‘always being outside the system’ was exceedingly limited in the understanding and translational of the emergent qualities of a user-generated informal dwelling. With rigid dwelling typologies, unregulated open/shared space structure and an incoherent application of ‘incrementality’, Anand Parbat is an architectural mess.This thesis attempts an iteration to existing approaches by offering an alternative housing scheme. It learns from the contextual informality and rationalises the learnings for a disciplinarian application. It uses infrastructure as invisible layers as the backbone of the dwelling layout and reimagines domestic space for rendering progressive and high-spirited families. By the means of economics, proactive policies and careful design decisions it lays out an itertive and nuanced masterplan scheme that beneath its surface offers various spatial characteristics of an informal settlement from Delhi. The project peaks at building and surface material innovation where to make the dwellings cost effective a brick is modelled out of kitchen waste found commonly in Indian urban poor kitchens. A large variety of colors and sizes are innovated to give the settlement an aesthetical character. Various cost effective techniques are used to involve inhabitants into small-scale buidling processes.The project is rationalised for a widespread application in New Delhi, fully capable to absorb and embody contextual characters - geographic, cultural and economical. The project is also visualised for projective iterations and scenarios that would be cast by its inhabitants to highlight the performance of the design decisions in time.Architecture, Urbanism and Building Sciences | Explorela
A primer on twistronics: A massless Dirac fermion's journey to moir\'{e} patterns and flat bands in twisted bilayer graphene
No full textThe recent discovery of superconductivity in magic-angle twisted bilayer
graphene has sparked a renewed interest in the strongly-correlated physics of
carbons, in stark contrast to preliminary investigations which were
dominated by the one-body physics of the massless Dirac fermions. We thus
provide a self-contained, theoretical perspective of the journey of graphene
from its single-particle physics-dominated regime to the strongly-correlated
physics of the flat bands. Beginning from the origin of the Dirac points in
condensed matter systems, we discuss the effect of the superlattice on the
Fermi velocity and Van Hove singularities in graphene and how it leads
naturally to investigations of the moir\'{e} pattern in van der Waals
heterostructures exemplified by graphene-hexagonal boron-nitride and twisted
bilayer graphene. Subsequently, we illuminate the origin of flat bands in
twisted bilayer graphene at the magic angles by elaborating on a broad range of
prominent theoretical works in a pedagogical way while linking them to
available experimental support, where appropriate. We conclude by providing a
list of topics in the study of the electronic properties of twisted bilayer
graphene not covered by this review but may readily be approached with the help
of this primer.Comment: To be published as topical review in Journal of Physics: Condensed
Mater ( Accepted Manuscript is available online
Moir\'e fractals in twisted graphene layers
No full textTwisted bilayer graphene (TBLG) subject to a sequence of commensurate
external periodic potentials reveals the formation of moir\'{e} fractals (MF)
that share striking similarities with the central place theory (CPT) of
economic geography, thus uncovering a remarkable connection between twistronics
and the geometry of economic zones. MFs arise from the self-similarity of the
emergent hierarchy of Brillouin zones (BZ), forming a nested subband structure
within the bandwidth of the original moir\'{e} bands. We derive the fractal
generators (FG) for TBLG under these external potentials and explore their
impact on the hierarchy of the BZ edges and the wavefunctions at the Dirac
point. By examining realistic super-moir\'{e} structures (SMS) and
demonstrating their equivalence to MFs with periodic perturbations under
specific conditions, we establish MFs as a general description for such
systems. Furthermore, we uncover parallels between the modification of the BZ
hierarchy and magnetic BZ formation in Hofstadter's butterfly (HB), allowing us
to construct an incommensurability measure for MFs \textit{vs.} twist angle.
The resulting bandstructure hierarchy bolsters correlation effects, pushing
more bands within the same energy window for both commensurate and
incommensurate TBLG.Comment: revised main text latex 17 pages including bibliography and
appendices. Accepted for publication in Physical Review
Miniaturized Rectangular Slotted Nameplate Antenna Design for Satellite and Radio Determination Applications
No full textA slotted rectangular nameplate antenna design with a patch bearing the name of the first author is presented. A 6.8 mm × 26 mm × 1.6 mm substrate of FR‐4 epoxy material having a relative permittivity of 4.4 and a dielectric loss tangent of 0.02 is used. Additionally, the feeding technique used is a coaxial mechanism. The standard antenna design parameters, including the reflection coefficient, bandwidth, radiation pattern, gain, directivity, and voltage standing wave radio (VSWR) for the proposed prototype are analyzed using a high‐frequency structure simulator (HFSS) v‐15, and are compared to the measured results. The designed structure may be considered for different satellite‐ and radio‐determination applications at the respective resonant frequencies
Numerical Simulation of Winter Precipitation over the Western Himalayas Using a Weather Research and Forecasting Model during 2001–2016
No full textIn the present study, dynamically downscaled Weather Research and Forecasting (WRF) model simulations of winter (DJF) seasonal precipitation were evaluated over the Western Himalayas (WH) at grey zone configurations (at horizontal resolutions of 15 km (D01) and 5 km (D02)) and further validated using satellite-based (IMERG; 0.1°), observational (IMD; 0.25°), and reanalysis (ERA5; 0.25° and IMDAA; 0.108°) gridded datasets during 2001–2016. The findings demonstrate that both model resolutions (D01 and D02) are effective at representing precipitation characteristics over the Himalayan foothills. Precipitation features over the region, on the other hand, are much clearer and more detailed, with a significant improvement in D02, emphasizing the advantages of higher model grid resolution. Strong correlations and the lowest biases and root mean square errors indicate a closer agreement between model simulations and reanalyses IMDAA and ERA5. Vertical structures of various dynamical and thermodynamical features further confirm the improved and more realistic in WRF simulations with D02. Moreover, the seasonal patterns of upper tropospheric circulation, vertically integrated moisture transport, surface temperature and cloud cover show more realistic simulation in D02 compared to coarser domain D01. The categorical statistics reveal the efficiency of both D01 and D02 in simulating moderate and heavy precipitation events. Overall, our study emphasizes the significance of high-resolution data for simulating precipitation features specifically over complex terrains like WH
SmartSat Constellation - A Deep Reinforcement Learning Approach for Decentralized Coordination
No full textWith rapid advancements in satellite technology, the amount of low earth orbit satellites has grown significantly which are primarily deployed for weather monitoring, earth observation or military purposes. Due to this reason, there has been an increased interest in enhancing the level of autonomy and cognition, onboard satellites to achieve optimal data collection. Optimal data is said to be collected when the satellites in a small sat constellation work together to collect information. This means that even if one of the satellites has missed out on some important information, the others can still collect them. A satellite constellation can be considered as a multi-agent reinforcement learning system. Having these agents coordinate with one another, can reduce the amount of time required to perform a task. The state-of-the-art satellite constellations follow a centralized coordination mechanism in which one primary satellite controls the rest of the satellites. This process is computationally more expensive and requires substantial communication between the satellites.It has a single point of failure and communication might be affected if the primary satellite fails. On the other hand, decentralized coordination allows agents to control their behavior themselves without the command of a supervised master. In this case, there is less inter-satellite communication which reduces the requirement for specialized onboard computational hardware. The proposal constitutes leveraging the Multi-Agent Deep Deterministic Policy Gradient [2] (MADDPG) algorithm to train the agents (satellites) to achieve optimal data collection. There are multiple use cases for the proposed solution such as illegal maritime activity tracking, natural disaster detection and assessing building damage after a natural disaster. The proposed solution focuses on tracking of ships in an extensively simulated environment for which a custom ship environment was created by leveraging OpenAI Gym [12]. By providing on-board autonomy, we aim to reduce frequent Earth Station (ES) communication significantly and enhance data collection capability
Fractional Deep Neural Network via Constrained Optimization
No full textFunded by Naval Postgraduate SchoolThis paper introduces a novel algorithmic framework for a deep neural network (DNN), which in a mathematically rigorous manner, allows us to incorporate history (or memory) into the network – it ensures all layers are connected to one another. This DNN, called Fractional-DNN, can be viewed as a time-discretization of a fractional in time nonlinear ordinary differential equation (ODE). The learning problem then is a minimization problem subject to that fractional ODE as constraints. We emphasize that an analogy between the existing DNN and ODEs, with standard time derivative, is well-known by now. The focus of our work is the Fractional-DNN. Using the Lagrangian approach, we provide a derivation of the backward propagation and the design equations. We test our network on several datasets for classification problems. Fractional-DNN offers various advantages over the existing DNN. The key benefits are a significant improvement to the vanishing gradient issue due to the memory effect, and better handling of nonsmooth data due to the network’s ability to approximate non-smooth functions.This work is partially supported by Department of Navy, Naval Postgraduate School - N00244-20-1-0005, Air Force Office of Scientific Research (AFOSR) under Award NO: FA9550-19-1-0036, and National Science Foundation grants DMS-1818772 and DMS-1913004. The second author is also partially supported by a Provost award at George Mason University under the Industrial Immersion Program.This work is partially supported by Department of Navy, Naval Postgraduate School - N00244-20-1-0005, Air Force Office of Scientific Research (AFOSR) under Award NO: FA9550-19-1-0036, and National Science Foundation grants DMS-1818772 and DMS-1913004. The second author is also partially supported by a Provost award at George Mason University under the Industrial Immersion Program
