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
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Item Phase Tracking Methods for X-ray Pulsar-Based Spacecraft Navigation(2021) Anderson, Kevin; Pines, Darryll J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)X-ray pulsars are potential aids to spacecraft navigation due to the periodicity, uniqueness, and stability of their signals. As the load on the deep space network increases in the future, techniques to navigate with less frequent communication will become desirable. Improved methods of x-ray pulsar-based spacecraft navigation (XNAV) are developed, analyzed, and confirmed over multiple simulated scenarios. A phase-tracking algorithm modeled at the level of individual photon arrivals provides improvements over the current state of the art, and a novel phase maximum likelihood estimator (MLE) is proposed. Relaxing the constant signal frequency assumption with a second-order Taylor polynomial phase model and feedback of frequency and frequency derivative from a third-order digital phase-locked loop is shown to overcome previous phase tracking difficulties due to low flux with millisecond period pulsars (MSPs), which have the best navigation characteristics. Empirical MLE tests are performed to determine threshold observation times for convergence to the Cramer-Rao Bound. A lower limit is identified due to Poisson statistics and an upper limit due to orbit dynamic stress. For a 1 m^2 detector, one second for the Crab pulsar and 4000 seconds for the lowest flux MSPs are required. An analytical method is presented to predict the necessary threshold observation times for signals with pulse widths under 0.15 cycles. Simulations are performed for dynamic stress conditions including two heliocentric trajectories, a cislunar trajectory, and three Earth orbits. The Crab pulsar and four MSPs: B1821-24, B1937+21, J0218+4232, and J0437-4715 are investigated. Position errors of 2 to 7 km are shown for most of the MSPs along the interplanetary and cislunar trajectories. B1821-24 tracks on the Earth orbits with 1 – 2 m^2 detectors with 2.5 – 3.5 km error. B1937+21 and J0218+4232 require larger detector areas. An extended Kalman filter combines multiple pulsar phase tracking range measurements for various observation schedules. Scenarios with one and three detectors are considered. Position error under 3 km is demonstrated for an interplanetary trajectory. Phase tracking shows great promise for deep space navigation and more limited potential in scenarios with greater orbital dynamics.Item Morphotropic Phase Boundaries in Tb1-xDyxFe2 Alloys(2013) Bergstrom Jr., Richard Eaton; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetostrictive alloys, materials that change in dimension under an applied magnetic field, are desired candidates for transducers. Unfortunately, common magnetostrictive metals, alloys, and oxides produce such small strains that they are not a viable option. In the early 1960's rare earths were found to possess extraordinary magnetostriction values at cryogenic temperatures. When alloyed with traditional transition metals they form a Laves phase compound of the form AB2. These Laves phase compounds have shown large magnetostriction values, up to 2500με in TbFe2. A major drawback to using these materials as transducers is their huge magnetocrystalline anisotropy constants, K1 and K2. However, it was found that TbFe2 and DyFe2 have opposing signs of K1 and K2. A pseudo-binary alloy, Tb1-xDyxFe2 (Terfenol-D) TDFx, was formed to decrease the total magnetocrystalline anisotropy. The anisotropy reached a room temperature minimum for TDF73. It is suspected that this minimum of the anisotropy is accompanied by a morphotropic phase boundary (MPB) at which the crystal structure changes from tetragonal to rhombohedral. Unraveling the nature of the temperature and composition dependence of the magnetic and crystalline properties along this MPB is the primary focus of this thesis. The structure of the TDF alloys was probed through macroscopic and microscopic techniques. The maximum in the DC magnetization at the transition temperature from tetragonal to rhombohedral broadens as the transition temperature is increased. This is attributed to decreasing anisotropy at increased temperature. Synchrotron and neutron powder diffraction are utilized to elucidate the microscopic changes in the structure and magnetism. Neutron powder diffraction results were somewhat inconclusive but were sufficient to produce magnetic moments that were invariant, within experimental error, across the transition region. Synchrotron powder diffraction was used to probe the structure at temperatures across the MPB. Reitveld refinement of the structure in TDF65 reveals that large strain gradients exist across the MPBs. This was supplemented by temperature dependent scans of various TDF alloys showing a broadening of the phase transition with increasing temperature which we attribute a widening of the meta-stable [100] and [111] easy directions.