Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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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 Human Gait Based Relative Foot Sensing for Personal Navigation(2010) Spiridonov, Timofey N.; Pines, Darryll J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Human gait dynamics were studied to aid the design of a robust personal navigation and tracking system for First Responders traversing a variety of GPS-denied environments. IMU packages comprised of accelerometers, gyroscopes, and magnetometer are positioned on each ankle. Difficulties in eliminating drift over time make inertial systems inaccurate. A novel concept for measuring relative foot distance via a network of RF Phase Modulation sensors is introduced to augment the accuracy of inertial systems. The relative foot sensor should be capable of accurately measuring distances between each node, allowing for the geometric derivation of a drift-free heading and distance. A simulation to design and verify the algorithms was developed for five subjects in different gait modes using gait data from a VICON motion capture system as input. These algorithms were used to predict the distance traveled up to 75 feet, with resulting errors on the order of one percent.Item Gravity Gradiometer Aided Inertial Navigation Within Non-GNSS Environments(2008-01-25) Richeson, Justin Arthur; Pines, Darryll J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gravity gradiometer aiding of a strapdown inertial navigation system (INS) in the event of Global Navigation Satellite System (GNSS) signal loss, or as a complement to an INS/GNSS system, is proposed. Gravity gradiometry is ideal for covert military applications where a self contained, passive, spoof-free aid is desirable, and for space navigation near planetary bodies and moons where GNSS is unavailable. This dissertation provides the first comprehensive discussion on gravity gradiometry fundamentals, map modeling, and regional and altitude effects on the gravitational gradient signal for use as a navigation aid. A thorough methodology to implement strapdown and stabilized gravity gradiometer instruments (GGIs) into an autonomous extended Kalman filter is also presented in the open literature for the first time. Lastly, a brief discussion on extraterrestrial navigation using gravity gradiometry is given. To quantify the potential performance for future gravity gradiometer instruments as an INS aid, extensive Monte Carlo simulations of a hypersonic scramjet cruise missile were performed. The results for the 1000 km range mission indicate that GGI updates significantly improve the navigation accuracy of the autonomous INS. The sensitivities of the system to variations in inertial measurement unit (IMU) quality, gravity field variation, GGI noise, update rate, and type are also investigated along with a baseline INS/Global Positioning System (GPS). Given emerging technologies that have the potential to drastically decrease gradiometer noise levels, a hypothetical future grade gravity gradiometer aided INS is shown to bound root-mean-square (RMS) position errors at 0.336 m, velocity errors at 0.0069 m/s, and attitude errors at 0.00977 degrees, which is comparable to the nominal INS/GPS system with 10 sec updates. The performance of two subsonic cases is also investigated and produced impressive passive navigation accuracy. A commercial aircraft simulation using a future grade GGI provided RMS errors of 0.288 m in position, 0.0050 m/s in velocity, and 0.0135 degrees in attitude. A low altitude and velocity gravity gradiometer based survey simulation similarly showed sub-meter RMS position errors of 0.539 m, velocity errors of 0.0094 m/s, and attitude errors of 0.0198 degrees.