Theses and Dissertations from UMD

Permanent URI for this communityhttp://hdl.handle.net/1903/2

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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    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.
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    Application of X-ray Pulsar Navigation: A Characterization of the Earth Orbit Trade Space
    (2015) Yu, Wayne; Healy, Liam; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The potential for pulsars as a navigation source has been studied since their discovery in 1967. X-ray pulsar navigation (XNAV) is a celestial navigation system that uses the consistent timing nature of X-ray photons from milli-second pulsars (MSP) to perform space navigation. Much of the challenge of XNAV comes from the faint signal, availability, and distant nature of pulsars. This thesis is the study of pulsar XNAV measurements for extended Kalman filter (EKF) tracking performance within a wide trade space of bounded Earth orbits, using a simulation of existing X-ray detector space hardware. An example of an X-ray detector for XNAV is the NASA Station Explorer for X-ray Timing and Navigation (SEXTANT) mission, a technology demonstration of XNAV set to perform on the International Space Station (ISS) in 2016. The study shows that the closed Earth orbit for XNAV performance is reliant on the orbit semi-major axis and eccentricity as well as orbit inclination. These parameters are the primary drivers of pulsar measurement availability and significantly influence the natural spacecraft orbit dynamics. Sensitivity to initial orbit determination error growth due to the scarcity of XNAV measurements within an orbital period require appropriate timing of initial XNAV measurements. The orbit angles of argument of perigee and right ascension of the ascending node, alongside the other orbit parameters, complete the initial cadence of XNAV measurements. The performance of initial XNAV measurements then propagates throughout the experimental period.