Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item IMPROVING IRRIGATION MANAGEMENT AND ROW COVER USE IN STRAWBERRY PRODUCTION IN THE MID-ATLANTIC REGION(2020) Belayneh, Bruk Eshetu; Lea-Cox, John D; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Irrigation strategies that reduce water application and improve water use efficiency could be useful in strawberry production, to save water and reduce the environmental impact of nutrient leaching. Therefore, the effect of moisture availability on the physiology, growth, yield and fruit quality of strawberry (Fragaria X ananassa) was studied under field and greenhouse conditions by implementing deficit irrigation at decreasing matric potentials. Incremental drought stress significantly affected crop physiology, growth and yield, but not fruit quality. The results revealed both physiological and morphological adaptations of strawberries to incremental drought stress that are typical of isohydric plants. Since reduced irrigation applications led to proportional yield losses, there was no significant improvement in the irrigation water use efficiency/water productivity of the crop. Economic analysis showed that the loss of revenue as a result of reduced yields was of a much higher magnitude than the savings associated with reduced irrigation application, making adoption of reduced irrigation strategies such as deficit irrigation unlikely. Nevertheless, results revealed that soil moisture measurement-based irrigation management can be used to improve current (excess irrigation) grower practices, without impacting revenue. The effect of row covers on canopy and soil temperature, was studied in plasticulture strawberry production to more quantify their effects on crop phenology and frost mitigation. Row cover use increased the average temperature measured in the canopy and soil by 6.9 and 9.8%, respectively. Although this seems relatively insignificant, these temperature increases translated to an 84 and 122% increase in growing degree-day accumulation at the canopy and in the soil during a fall study period. In addition, increases in soil temperature were positively correlated with soil moisture. These results indicate the advantages that row covers can provide to growers, as a tool to enhance plant growth and for freeze and frost protection of plants. However, growers need to monitor environmental conditions at canopy level under row covers and in the ambient air in order to gain these benefits without negative consequences for yield.Item Security of Wireless Sensor Networks in the Presence of Captured Nodes(2008-11-24) Bahari, Seyed Farshad; Gligor, Virgil D; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wireless sensor networks (WSNs) pose unique security challenges due to the fact that their nodes operate in an unattended manner in potentially hostile environments. A particularly difficult problem not addressed to date is the handling of node capture by an adversary. A key goal for solving this problem is that of limiting the damage caused by captured nodes. This is important since node capture cannot be prevented: by definition, there is no practical physical mechanism that could keep an adversary from physically accessing a sensor node discovered in an unattended area. Hence, the presence of the adversary within a WSN must be detected, and of course, the earlier the better. Adversary detection is predicated on the fact that access to a captured node's internal state, which includes secrets such as cryptographic keys, incurs a nonzero time delay. This suggests that adversary detection be divided into two phases: (i) in-capture detection, namely detection before the adversary completes the capture process and gets a chance to access a node's internal state and do any network damage, and (ii) post-capture detection, namely detection after the adversary already accessed and possibly used a node's internal state and secrets. Since the adversary is already active in the network in the latter case, it is important to determine the overall network resiliency; i.e., the ability of the network to operate in the presence of an active adversary. In this work we focus on the former case in which we try to identify the presence of the adversary prior to completion of a node capture. To address the problem of in-capture adversary detection, we propose two probabilistic schemes called the pairwise pinging scheme and quorum pinging scheme, whereby the network continuously monitors itself in a distributed and self-organizing manner. We investigate the trade-offs between the network cost-performance and security of these schemes via a Markov Chain model, and present analytical solutions which allow us to choose appropriate performance parameters, such as the expected residual time-to-false-alarm, and security, such as the probability of a missed detection. We show that the quorum pinging is superior to pairwise pinging in terms of both cost-performance and security. Furthermore, we will show that both schemes are scalable with network size and their complexities are linearly proportional to the average node degree of the network. We also analyze the optimum strategy for an adversary to deploy its agents over a sensor network; i.e., the strategy that enables the adversary to achieve the maximum capture ratio with fixed number of agents. The order of node capture, distribution, and location of agents are investigated and an analytical model is provided that describes the optimum path for deploying of agents to target nodes. Numerical data are presented to compare different scenarios for deploying agents and the corresponding performance of each deployment strategy. The proposed optimum strategy validates the physical interpretation under practical scenarios and demonstrates the feasibility of our capture strategy in practice. Finally, the resiliency of the underlying quorum pinging scheme for detecting adversary agents is investigated despite collusion among agents via optimum capture strategy.Item Packet based Inference and Control(2006-09-19) Rabi, Maben; Baras, John S; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Communication constraints in Networked Control systems are frequently limits on data packet rates. To efficiently use the available packet rate budgets, we have to resort to event-triggered packet transmission. We have to sample signal waveforms and transmit packets not at deterministic times but at random times adapted to the signals measured. This thesis poses and solves some basic design problems we face in reaching for the extra efficiency. We start with an estimation problem involving a single sensor. A sensor makes observations of a diffusion process, the state signal, and has to transmit samples of this process to a supervisor which maintains an estimate of the state. The objective of the sensor is to transmit samples strategically to the supervisor to minimize the distortion of the supervisor's estimate while respecting sampling rate constraints. We solve this problem over both finite and infinite horizons when the state is a scalar linear system. We describe the relative performances of the optimal sampling scheme, the best deterministic scheme and of the suboptimal but simple to implement level-triggered sampling scheme. Apart from the utility of finding the optimal sampling strategies and their performances, we also learnt of some interesting properties of the level-triggered sampling scheme. The control problem is harder to solve for the same setting with a single sensor. In the estimation problem for the linear state signal, the estimation error is also a linear diffusion and is reset to zero at sampling times. In the control problem, there is no equivalent to the error signal. We pay attention to an infinite horizon average cost problem where, the sampling strategy is chosen to be level-triggered. We design piece-wise constant controls by translating the problem to one for discrete-time Markov chain formed by the sampled state. Using results on the average cost control of Markov chains, we are able to derive optimality equations and iteratively compute solutions. The last chapter tackles a binary sequential hypothesis testing problem with two sensors. The special feature of the problem is the ability of each sensor to hear the transmissions of the other towards the supervisor. Each sensor is afforded on transmission of a sample of its likelihood ratio process. We restrict attention to level-triggered sampling. Although we are unable to demonstrate overall optimality of the asynchronous scheme we pursue, we are able to describe its advantages over other level-triggered schemes and of course the deterministic one. The chief merits of this thesis are the formulation and solution of some basic problems in multi-agent estimation and control. In the problems we have attacked, we have been able to deal with the differences in information patterns at sensors and supervisors. The main demerits are the ignoring of packet losses and of variable delays in packet transmissions. The situation of packet losses can however be handled at the expense of additional computations. To summarize, this thesis provides valuable generalizations of the works of Astrom and Bernhardsson and of Kushner on timing of Control actions and of Sampling observations respectively.