Robust Control Design for Large Space Structures

The control design problem for the class of future spacecraft referred to as large space structures (LSS) is by now well known. The issue is the reduced order control of a very high order, lightly damped system with uncertain system parameters, particularly in the high frequency modes. A design methodology which incorporates robustness considerations as part of the design process is presented. Combining pertinent results from multivariable systems theory and optimal control and estimation, LQG eigenstructure assignment and LQG frequency shaping, were used to improve singular value robustness measures in the presence of control and observation spillover

An all monolithic MOS A/D converter - Low power clocks, multiplexers, registers, and A/D converter Final report

Research and developments of monolithic, MOS, ten bit, analog to digital converte

From the Deep Woods to Civilization Chapters in the Autobiography of an Indian

https://commons.und.edu/settler-literature/1009/thumbnail.jp

Frayed connections: How long-term nitrogen additions disrupt plant-soil interactions and the carbon cycle of a temperate forest

Forests are expected to mitigate some of the negative effects of climate change by sequestering anthropogenic carbon (C) from the atmosphere, but the degree to which they drawn down C will depend on the availability of key nutrients, such as nitrogen (N). There is a fair amount of uncertainty in the future of the forest C sink, mostly owing to the fate of soil organic matter (SOM) and soil heterotrophic respiration to future conditions. In N limited systems, plants allocate a significant amount of their photosynthate belowground for the acquisition of nutrients, but under conditions of chronic N deposition, plants may shift their allocation and nutrient acquisition strategies to favor aboveground production. In turn, this shift in C allocated belowground can cause a chain reaction of response in the soil, influencing the soil C stocks and persistence of soil C under future global changes. In this dissertation, I explore how the tightly coupled C and N cycles influence one another and the C storage potential of a temperate deciduous forest under conditions of elevated N deposition. I employ three diverse methodologies to determine how N availability controls C cycling and storage: a long-term, whole-watershed N addition experiment at the Fernow Experimental Forest; a short-term, targeted experiment of litter decomposition and SOM characterization; and a soil biogeochemical model comparison. These three methodologies allowed me to answer three broad questions: (1) How do potential changes in nutrient acquisition strategies due to chronic N additions impact the forest C sink? (2) What effects does over 25 years of N additions have on the decomposition and formation of SOM? (3) To what extent does soil biogeochemical model structure (first-order decay dynamics versus microbially explicit) impact model representation of C cycle responses to N additions? For question 1, I constructed C and N budgets for the fertilized and a reference watershed in the long-term N addition experiment. I found that over 25 years of N additions led to a shift in C allocation to favor woody biomass production over belowground C flux and increased the soil C stock and C:N ratio of SOM. For question 2, I measured leaf litter decomposition rates for two years in the fertilized and reference watershed, as well as assessed the composition of the SOM. Leaf litter decay rates were slower in the fertilized watershed, especially for low-quality litter (high C:N and lignin:N ratios). Also, there was an accumulation of particulate organic matter, or undecomposed plant-like SOM, in the fertilized watershed, which was positively related to the bulk soil C:N ratio. Finally, for questions 3, I performed a N perturbation experiments using two structurally distinct soil models and compared these results to data from the Fernow Experimental Forest N addition experiment. This comparison allowed us to identify key mechanisms that models do not include, such as enzyme inhibition and shifting vegetation allocation with N additions, which led the models to miss some key observed responses, especially the reduction in soil respiration. Altogether, this dissertation highlights the importance of plant-soil interactions in the cycling of C and N in forest ecosystems, and how elevated N inputs can cause some disconnects between plant and soil processes that control the storage and sequestration of C

Bolometric light curves of supernovae and post-explosion magnetic fields

The various effects leading to diversity in the bolometric light curves of supernovae are examined: nucleosynthesis, kinematic differences, ejected mass, degree of mixing, and configuration and intensity of the magnetic field are discussed. In Type Ia supernovae, a departure in the bolometric light curve from the full-trapping decline of $^{56}$Co can occur within the two and a half years after the explosion, depending on the evolutionary path followed by the WD during the accretion phase. If convection has developed in the WD core during the presupernova evolution, starting several thousand years before the explosion, a tangled magnetic field close to the equipartition value should have grown in the WD. Such an intense magnetic field would confine positrons where they originate from the $^{56}$Co decays, and preclude a strong departure from the full-trapping decline, as the supernova expands. This situation is expected to occur in C+O Chandrasekhar WDs as opposed to edge-lit detonated sub-Chandrasekhar WDs. If the pre-explosion magnetic field of the WD is less intense than 10$^{5-8}$G, a lack of confinement of the positrons emitted in the $^{56}$Co decay and a departure from full-trapping decline would occur. The time at which it takes place can provide estimates of the original magnetic field of the WD, its configuration, and also of the mass of the supernova ejecta. In SN 1991bg, the bolometric light curve suggests absence of a significant tangled magnetic field (intensity lower than $10^{3}$ G). Chandrasekhar-mass models do not reproduce the bolometric light curve of this supernova. For SN 1972E, on the contrary, there is evidence for a tangled configuration of the magnetic field and its light curve is well reproduced by a Chandrasekhar WD explosion.Comment: 54 pages, including 8 figures. To appear in Ap

Analytic Approximations for Transit Light Curve Observables, Uncertainties, and Covariances

The light curve of an exoplanetary transit can be used to estimate the planetary radius and other parameters of interest. Because accurate parameter estimation is a non-analytic and computationally intensive problem, it is often useful to have analytic approximations for the parameters as well as their uncertainties and covariances. Here we give such formulas, for the case of an exoplanet transiting a star with a uniform brightness distribution. We also assess the advantages of some relatively uncorrelated parameter sets for fitting actual data. When limb darkening is significant, our parameter sets are still useful, although our analytic formulas underpredict the covariances and uncertainties.Comment: 33 pages, 14 figure