VTechWorks Repository :: Browsing by Author "Schroeder, Kevin Kent"

Browsing by Author "Schroeder, Kevin Kent"

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A* Node Search and Nonlinear Optimization for Satellite Relative Motion Path Planning
Connerney, Ian Edward (Virginia Tech, 2021-11-03)
The capability to perform rendezvous and proximity operations about space objects is central to the next generation of space situational awareness. The ability to diagnose and respond to spacecraft anomalies is often hampered by the lack of capability to perform inspection or testing on the target vehicle in flight. While some limited ability to perform inspection can be provided by an extensible boom, such as the robotic arms deployed on the space shuttle and space station, a free-flying companion vehicle provides maximum flexibility of movement about the target. Safe and efficient utilization of a companion vehicle requires trajectories capable of minimizing spacecraft resources, e.g., time or fuel, while adhering to complex path and state constraints. This paper develops an efficient solution method capable of handling complex constraints based on a grid search A* algorithm and compares solution results against a state-of-the-art nonlinear optimization method. Trajectories are investigated that include nonlinear constraints, such as complex keep-out-regions and thruster plume impingement, that may be required for inspection of a specific target area in a complex environment. This work is widely applicable and can be expanded to apply to a variety of satellite relative motion trajectory planning problems.
Adaptive Controller Development and Evaluation for a 6DOF Controllable Multirotor
Furgiuele, Theresa Chung Wai (Virginia Tech, 2022-10-03)
The omnicopter is a small unmanned aerial vehicle capable of executing decoupled translational and rotational motion (six degree of freedom, 6DOF, motion). The development of controllers for various 6DOF controllable multirotors has been much more limited than development for quadrotors, which makes selecting a controller for a 6DOF multirotor difficult. The omnicopter is subject to various uncertainties and disturbances from hardware changes, structural dynamics, and airflow, making adaptive controllers particularly interesting to investigate. The goal of this research is to design and evaluate the performance of various position and attitude controller combinations for the omnicopter, specifically focusing on adaptive controllers. Simulations are first used to compare combinations of three position controllers, PID, model reference adaptive control, augmented model reference adaptive control (aMRAC), and four attitude controllers, PI/feedback linearization (PIFL), augmented model reference adaptive control, backstepping, and adaptive backstepping (aBack). For the simulations, the omnicopter is commanded to point at and track a stationary aim point as it travels along a $C^0$ continuous trajectory and a trajectory that is $C^1$ continuous. The controllers are stressed by random disturbances and the addition of an unaccounted for suspended mass. The augmented model reference adaptive controller for position control paired with the adaptive backstepping controller for attitude control is shown to be the best controller combination for tracking various trajectories while subject to disturbances. Based on the simulation results, the PID/PIFL and aMRAC/aBack controllers are selected to be compared during three different flight tests. The first flight test is on a $C^1$ continuous trajectory while the omnicopter is commanded to point at and track a stationary aim point. The second flight test is a hover with an unmodeled added weight, and the third is a circular trajectory with a broken blade. As with the simulation results, the adaptive controller is shown to yield better performance than the nonadaptive controller for all scenarios, particularly for position tracking. With an added weight or a broken propeller, the adaptive attitude controller struggles to return to level flight, but is capable of maintaining steady flight when the nonadaptive controller tends to fail. Finally, while model reference adaptive controllers are shown to be effective, their nonlinearity can make them difficult to tune and certify via standard certification methods, such as gain and phase margin. A method for using time delay margin estimates, a potential certification metric, to tune the adaptive parameter tuning gain matrix is shown to be useful when applied to an augmented MRAC controller for a quadrotor.
Analysis of Low-Energy Lunar Transfers in a High-Fidelity Dynamics Model
Torchia, Patrick Jason (Virginia Tech, 2023-07-03)
Renewed interest in returning to the Moon, emboldened by recent directives and missions by NASA, has necessitated the establishment of lunar infrastructure to support continuous human presence. With that, the objective of making this return more cost effective has gained significant importance. Low energy lunar transfers are more efficient ways to reach the Moon than the traditional Hohmann-type transfer. These trajectories leverage the multi-body gravitational effects to reduce overall delta-v requirements, in some cases removing the capture delta-v completely. While the time of flight for these transfers can be much longer than a Hohmann-type transfer, the chaotic design space of these transfers can enable large changes in arrival conditions at the Moon for small changes in initial conditions. Many investigations of these transfers take place in simplified dynamical models, such as the Planar Circular Restricted Three Body Problem, with very few higher-fidelity models being implemented. This approach is good to understand the dynamics of these trajectories as well as provide initial guesses for higher-fidelity models; but approximating the dynamics heavily make these models less applicable to mission design. This thesis aims to investigate the application of a higher-order model to simulate these trajectories. STK Astrogator was used to recreate the NASA GRAIL trajectory; and from the recreated trajectory, a nominal trajectory absent of mid-course corrections was established. This nominal trajectory was used to perform parametric and variational studies of departure and arrival conditions as well as compare to a nominal trajectory in a reduced-fidelity model. An investigation into the post launch correction burn requirements following launch vehicle under-performance was completed. Utilizing low energy transfers proved beneficial to adjusting arrival conditions for low delta-v requirements. All arrival inclinations are reasonably achievable for around 255 m/s. Using 255 m/s as a baseline, right ascension of the ascending node could be reached in a 40 degree range and argument of periapsis in a 50 degree range. Lunar insertion arrival can be varied by 7 hours on either side for less than 80 m/s. Trans-lunar injection epoch can be varied by 7 hours on either side of nominal departure for less than 4 m/s. Orbit radius and initial velocity are the most expensive errors to correct. These trajectories can be tuned to reduce the overall mid-course correction delta-v requirement for differing arrival inclinations if other orbital elements are relaxed. A relationship between placement of post-launch correction maneuver for velocity or radius errors was found. Comparing the trajectory in STK to the Inclined Bi-Elliptic Restricted Four Body problem, revealed that timing of the trajectory is variable while keeping the same arrival and departure conditions. However, solar radiation pressure cannot be ignored for more accurate simulation of these trajectories. This investigation has shown that low energy lunar transfers are a viable method to reach the Moon and their chaotic nature can be leveraged to relax restrictions in the design space.
Analysis of Transfer Trajectories Utilizing Sequential Saturn-Titan Aerocaptures
Payne, Isaac Lee (Virginia Tech, 2023-07-03)
This thesis aims to investigate the potential of a transfer orbit using successive aerocaptures at Saturn and Titan to establish a science orbit around Titan. Titan is an Earth-like moon with a dense atmosphere and organic compounds present. It has many similarities with Earth that are useful to study such as superrotation. Superrotation is when the atmosphere rotates faster than the body it surrounds. In order to study Titan, we need to establish an orbit around it. The Saturn system is distant from Earth, 8.5 Astronomical Units (AU) which makes it difficult to reach from a time and velocity point of view. We propose to use an aerocapture at Saturn to intercept Titan with lower relative velocity in order to perform an aerocapture at Titan. The analysis was performed in primarily MATLAB to simulate the orbits. The results of this showed that we can aerocapture a spacecraft at Saturn and arrive at Titan within roughly 4 to 8 km/s relative velocity regardless of the incoming hyperbolic excess velocity at the Saturn system. This can be improve upon by using intermediate transfer orbits, such as bi-elliptics, to arrive with even lower relative velocities to Titan of as low as 1 km/s. The drag acceleration experienced during the Saturn aerocapture had peak values of between 0.2 and 1.4 g's and acceleration over 50% of the peak is experienced between 6.8 and 8 minutes. This capture method has the potential to make Titan more easily accessible and allow for scientific study of a clear target for improving our understanding of Earth-like processes on other bodies in our solar system.
An Assessment of 3D Tracking Systems and Lidar Data for RPO Simulation
Meland, Tallak Edward (Virginia Tech, 2023-08-30)
This thesis aimed to develop a rendezvous and proximity operation simulation to be tested with physical sensors and hardware, in order to assess the fidelity and performance of low-cost off-the-shelf systems for a hardware-in-the-loop testbed. With the push towards complex autonomous rendezvous missions, a low barrier to entry spacecraft simulator platform allows researchers to test and validate robotics systems, sensors, and algorithms for space applications, without investing in multimillion dollar equipment. This thesis conducted drone flights that followed a representative rendezvous trajectory while collecting lidar data of a target spacecraft model with a lidar sensor affixed to the drone. A relative orbital motion simulation tool was developed to create trajectories of varying orbits and initial conditions, and a representative trajectory was selected for use in drone flights. Two 3D tracking systems, OptiTrack and Vive, were assessed during these flights. OptiTrack is a high-cost state-of-the-art motion capture system that performs pose estimation by tracking reflective markers on a target in the tracking area. Vive is a lower-cost tracking system whose base stations emit lasers for its tracker to detect. Data collection by two lidar types was also assessed during these flights: real lidar data from a physical sensor, and virtual lidar data from a virtual sensor in a virtual environment. Drone flights were therefore performed in these four configurations of tracking system and lidar type, to directly compare the performance of higher-cost configurations with lower-cost configurations. The errors between the tracked drone position time history and the target position time history were analyzed, and the low-cost Vive and real lidar configuration was demonstrated to provide comparable error to the OptiTrack and real lidar configuration because of the dominance of the drone controller error over the tracking system error. In addition, lidar data of a target satellite model was collected by real and virtual lidar sensors during these flights, and point clouds were successfully generated. The resulting point clouds were compared by visualizing the data and noting the characteristics of real lidar data and its error, and how it compared to idealized virtual lidar data of a virtual target satellite model. The resulting real-world data characteristics were found to be modellable which can then be used for more robust simulation development within virtual reality. These results demonstrated that low-cost and open-source hardware and software provide satisfactory results for simulating this kind of spacecraft mission and capturing useful and usable data.
Automated Detection and Analysis of Low Latitude Nightside Equatorial Plasma Bubbles
Adkins, Vincent James (Virginia Tech, 2024-06-21)
Equatorial plasma bubbles (EPBs) are large structures consisting of depleted plasma that generally form on the nightside of Earth's ionosphere along magnetic field lines in the upper thermosphere/ionosphere. While referred to as `bubbles', EPBs tend to be longer along magnetic latitudes and narrower along magnetic longitudes which are on the order of thousands and hundreds of kilometers, respectively. EPBs are a well documented occurrence with observations spanning many decades. As such, much is known about their general behavior, seasonal variation of occurrences, increasing/decreasing occurrences with increasing/decreasing solar activity, and their ability to interact and interfere with radio waves such as GPS. This dissertation expands on this understanding by focusing on the detection and tracking of EPBs in the upper thermosphere/ionosphere along equatorial to low latitudes. To do this, far ultraviolet (FUV) emission observations of the recombination of O$^+$ with electrons via the Global-Scale Observations of the Limb and Disk (GOLD) mission are analyzed. GOLD provides consistent data from geostationary orbit with the eastern region of the Americas, Atlantic, and western Africa. The optical data can be used to pick out gradients in brightness along the 135.6 nm wavelength which correlate with the location of EPBs in the nightside ionosphere. The dissertation provides a novel method to look at and analyze 2-dimensional data with inconsistent time-steps for EPB detection and tracking. During development, preprocessing of large scale (multiple years) data proved to be the largest time sync. To that end, this dissertation tests the possibility of using convolution neural networks for detection of EPBs with the end goal of reducing the amount of preprocessing necessary. Further, data from the Ionospheric Connection Explorer's (ICON's) ion velocity meter (IVM) are compared to EPBs detected via GOLD to understand how the ambient plasma around the EPBs behave. Along with the ambient plasma, zonal and meridional thermospheric winds observed by ICON's Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument are analyzed in conjunction with the same EPBs to understand how winds coincident with EPBs behave. An analysis of winds before EPBs form is also done to observe the potential for both zonal and meridional winds' ability to suppress and amplify EPB formation.
Collocation Method and Model Predictive Control for Accurate Landing of a Mars EDL vehicle
Srinivas, Neeraj (Virginia Tech, 2021-02-02)
This thesis aims at investigating numerical methods through which the accuracy in landing of a Mars entry-descent-landing (EDL) vehicle can be improved. The methods investigated include the collocation method and model predictive control (MPC). The primary control variable utilized in this study is the bank angle of the spacecraft, which is the angle between the lift vector and the vertical direction. Modulating this vector affects the equations of system of equations and the seven state variables, namely altitude, velocity, latitude, longitude, flight path angle, heading angle and total time taken. An optimizer is implemented which utilizes the collocation method, through which the optimal bank angle is found at every discretized state along the trajectory which are equally separated through a definite timestep, which is a function of the end time state. A 3-sigma wind disturbance model is introduced to the system, as a function of the altitude, which introduces uncertainties to the system, resulting in a final state deviating from the targeted location. The trajectory is split into two parts, for better control of the vehicle during the end stages of flight. The MPC aims at reducing the end state deviation, through the implementation of a predictor-corrector algorithm that propagates the trajectory for a certain number of timesteps, followed by running the optimizer from the current disturbed state to the desired target location. At the end of this analysis, a new set of optimal bank angle are found, which account for the wind disturbances and navigates the EDL vehicle to the desired location.
A Comprehensive Entry, Descent, Landing, and Locomotion (EDLL) Vehicle for Planetary Exploration
Schroeder, Kevin Kent (Virginia Tech, 2017-08-26)
The 2012 Decadal Survey has stated that there is a critical role for a Venus In-situ Explore (VISE) missions to a variety of important sites, specifically the Tessera terrain. This work aims to answer the Decadal Survey's call by developing a new comprehensive Entry, Descent, Landing, and Locomotion (EDLL) vehicle for in-situ exploration of Venus, especially in the Tessera regions. TANDEM, the Tension Adjustable Network for Deploying Entry Membrane, is a new planetary probe concept in which all of EDLL is achieved by a single multifunctional tensegrity structure. The concept uses same fundamental concept as the ADEPT (Adaptable Deployable Entry and Placement Technology) deployable heat shield but replaces the standard internal structure with the structure from the tensegrity-actuated rover to provide a combined aeroshell and rover design. The tensegrity system implemented by TANDEM reduces the mass of the overall system while enabling surface locomotion and mitigating risk associated with landing in the rough terrain of Venus's Tessera regions, which is otherwise nearly inaccessible to surface missions. TANDEM was compared to other state-of-the-art lander designs for an in-situ mission to Venus. It was shown that TANDEM provides the same scientific experimentation capabilities that were proposed for the VITaL mission, with a combined mass reduction for the aeroshell and lander of 52% (1445 kg), while eliminating the identified risks associated with entry loads and very rough terrain. Additionally, TANDEM provides locomotion when on the surface as well as a host of other maneuvers during entry and descent, which was not present in the VITaL design. Based on its unique multifunctional infrastructure and excellent crashworthiness for impact on rough surfaces, TANDEM presents a robust system to address some of the Decadal Survey's most pressing questions about Venus.
Design and thermal analysis for a novel EMCCD camera payload in a 1U CubeSat form factor
Angle, Nicholas Blake (Virginia Tech, 2024-06-24)
Nüvü Camēras, a Canadian company that designs a range of CCD and EMCCD cameras and controllers, recently began development on a miniaturized EMCCD controller for a CubeSat form factor. The detector for this payload requires near-cryogenic temperatures, approximately 188K, for performance operation. A temperature requirement of that magnitude for a CubeSat form factor is challenging given the low thermal mass, volume, surface area, and power availability for heat storage, dissipation and control systems that would typically be available for larger form factor spacecraft. The goal of this project is to design and per- form thermal analysis for the Nüvü Camēras CubeSat EMCCD Controller that allows for cold-biased active temperature control of both the controller electronics and detector. The EMCCD controller had an operational temperature range of −35◦C to +60◦C while the detector had a performance range of −110◦C to −85◦C with a desire to maintain a resolu- tion of ±0.25◦C. To meet these requirements, a system was designed within 3D modeling software Autodesk Inventor and imported into Thermal Desktop for thermal analysis and iteration. Models were updated based on thermal analysis results, adjusted by hand, and then tested again until a passive cooling and active heating system that met the require- ments was achieved. The final control system was shown to be capable of cooling from 20◦C (293.15K) to −85◦C (188.15K) and beyond given a Sun Synchronous orbit at 600km with attitude control and operational requirements. It was also shown to be capable of heating up, using resistive heaters on key components, beyond the thermal inertia of the system and environment, indicating viable control on orbit. In the future a PID control method can be implemented, and its use is being investigated by Nüvü Camēras for achieving the desired resolution of ±0.25◦C in the future.
Designing Transfers Between Earth-Moon Halo Orbits Using Manifolds and Optimization
Brown, Gavin Miles (Virginia Tech, 2020-09-03)
Being able to identify fuel efficient transfers between orbits is critical to planning and executing missions involving spacecraft. With a renewed focus on missions in cislunar space, identifying efficient transfers in the dynamical environment characterized by the Circular Restricted Three-Body Problem (CR3BP) will be especially important, both now and in the immediate future. The focus of this thesis is to develop a methodology that can be used to identify a valid low-cost transfer between a variety of orbits in the CR3BP. The approach consists of two distinct parts. First, tools related to dynamical systems theory and manifolds are used to create an initial set of possible transfers. An optimization scheme is then applied to the initial transfers to obtain an optimized set of transfers. Code was developed in MATLAB to implement and test this approach. The methodology and its implementation were evaluated by using the code to identify a low-cost transfer in three different transfer cases. For each transfer case, the best transfers from each set were compared, and important characteristics of the transfers in the first and final sets were examined. The results from those transfer cases were analyzed to determine the overall efficacy of the approach and effectiveness of the implementation code. In all three cases, in terms of cost and continuity characteristics, the best optimized transfers were noticeably different compared to the best manifold transfers. In terms of the transfer path identified, the best optimized and best manifold transfers were noticeably different for two of the three cases. Suggestions for improvements and other possible applications for the developed methodology were then identified and presented.
Encoding the Sensor Allocation Problem for Reinforcement Learning
Penn, Dylan R. (Virginia Tech, 2024-05-16)
Traditionally, space situational awareness (SSA) sensor networks have relied on dynamic programming theory to generate tasking plans which govern how sensors are allocated to observe resident space objects. Deep reinforcement learning (DRL) techniques, with their ability to be trained on simulated environments, which are readily available for the SSA sensor allocation problem, and demonstrated performance in other fields, have potential to exceed performance of deterministic methods. The research presented in this dissertation develops techniques for encoding an SSA environment model to apply DRL to the sensor allocation problem. This dissertation is the compilation of two separate but related studies. The first study compares two alternative invalid action handling techniques, penalization and masking. The second study examines the performance of policies that have forecast state knowledge incorporated in the observation space.
Lunar Laser Ranging for Autonomous Cislunar Spacecraft Navigation
Zaffram, Matthew (Virginia Tech, 2023-08-15)
The number of objects occupying orbital regimes beyond Geosynchronous Earth Orbit and cislunar space are expected to grow in the coming years; Especially with the Moon reemerging as latest frontier in the race for space exploration and technological superiority. In order to support this growth, new methods of autonomously navigating in cislunar space are necessary to reduce demand and reliance on ground based tracking infrastructure. Periodic orbits about the first libration point offer favorable vantage points for scientific or military spacecraft missions involving the Earth or Moon. This thesis develops a new autonomous spacecraft navigation method for cislunar space and analyzes its performance applied to Lyapunov and halo orbits around $L_1$. This method uses existing lunar ranging retroreflectors (LRRR) installed on the Moon's surface in the 1960s and 1970s. A spacecraft can make laser ranging measurements to the LRRR to estimate its orbit states. A simulation platform was created to test this concept in the circular restricted three body problem and evaluate its performance. This navigation method was found to be successful for a subset of Lyapunov and halo orbits when cycling the five measurement targets. Simulation data showed that sub-kilometer position estimation and sub 2 centimeter per second velocity accuracies are achievable without receiving any state updates from external sources.
Mission-driven Sensor Network Design for Space Domain Awareness
Harris, Cameron Douglas (Virginia Tech, 2024-12-09)
This research presents a novel framework for optimizing sensor networks to enhance Space Domain Awareness in the face of a burgeoning resident space object population. By employing advanced metaheuristic optimization techniques and high-fidelity modeling and simulation, this research investigates the intricate interplay between sensor characteristics, network topology, and state estimation performance. The research aims to develop actionable recommendations for optimizing sensor network design, considering factors such as viewing geometry, sensor phenomenology, and background noise. Through rigorous simulations and analysis, this work seeks to contribute significantly to the advancement of Space Domain Awareness. A key product of this research is the development of a novel lattice-based genetic algorithm tailored for constrained metaheuristic optimization that converges in 15% fewer generations than traditional methods. This algorithm demonstrates its effectiveness in producing practical sensor network designs that can enhance space object tracking and surveillance capabilities. Results will show network designs that fill current coverage gaps over the Atlantic and Pacific oceans, remain consistent with geographical and geopolitical boundaries, and exploit regions with favorable environmental conditions. The outcome is a set of actionable solutions that triple observation capacity and reduce catalog observation gap times by up to 50%.
Multiple Gravity Assists for Low Energy Transport in the Planar Circular Restricted 3-Body Problem
Werner, Matthew Allan (Virginia Tech, 2022-06-23)
Much effort in recent times has been devoted to the study of low energy transport in multibody gravitational systems. Despite continuing advancements in computational abilities, such studies can often be demanding or time consuming in the three-body and four-body settings. In this work, the Hamiltonian describing the planar circular restricted three-body problem is rewritten for systems having small mass parameters, resulting in a 2D symplectic twist map describing the evolution of a particle's Keplerian motion following successive close approaches with the secondary. This map, like the true dynamics, admits resonances and other invariant structures in its phase space to be analyzed. Particularly, the map contains rotational invariant circles reminiscent of McGehee's invariant tori blocking transport in the true phase space, adding a new quantitative description to existing chaotic zone estimates about the secondary. Used in a patched three-body setting, the map also serves as a tool for investigating transfer trajectories connecting loose captures about one secondary to the other without any propulsion systems. Any identified initial conditions resulting in such a transfer could then serve as initial guesses to be iterated upon in the continuous system. In this work, the projection of the McGehee torus within the interior realm is identified and quantified, and a transfer from Earth to Venus is exemplified.
Optimization of Disaggregated Space Systems Using the Disaggregated Integral Systems Concept Optimization Technology Methodology
Wagner, Katherine Mott (Virginia Tech, 2020-07-10)
This research describes the development and application of the Disaggregated Integral Systems Concept Optimization Technology (DISCO-Tech) methodology. DISCO-Tech is a modular space system design tool that focuses on the optimization of disaggregated and non-traditional space systems. It uses a variable-length genetic algorithm to simultaneously optimize orbital parameters, payload parameters, and payload distribution for space systems. The solutions produced by the genetic algorithm are evaluated using cost estimation, coverage analysis, and spacecraft sizing modules. A set of validation cases are presented. DISCO-Tech is then applied to three representative space mission design problems. The first problem is the design of a resilient rideshare-manifested fire detection system. This analysis uses a novel framework for evaluating constellation resilience to threats using mixed integer linear programming. A solution is identified where revisit times of under four hours are achievable for $10.5 million, one quarter of the cost of a system manifested using dedicated launches. The second problem applies the same resilience techniques to the design of an expanded GPS monitor station network. Nine additional monitor stations are identified that allow the network to continuously monitor the GPS satellites even when five of the monitor stations are inoperable. The third problem is the design of a formation of satellites for performing sea surface height detection using interferometric synthetic aperture radar techniques. A solution is chosen that meets the performance requirements of an upcoming monolithic system at 70% of the cost of the monolithic system.
The Patch Integral Method (PIM), a New Heat Transfer Analysis Tool for Hypersonic Wind Tunnel Facilities at NASA Langley
Cheatwood, Jonathan Steven (Virginia Tech, 2023-08-22)
The NASA Langley Research Center hypersonic wind tunnels serve a vital role in the field of hypersonics in both helping validate CFD predictions and producing experimental results. These tunnels have been heavily utilized for decades by numerous planetary missions, such as MSL and Orion, commercial and academic partnerships, such as Sierra Space and the University of Maryland, and flight projects such as Artemis and LOFTID. The data acquisition method used in these tunnels is thermography, primarily phosphor and infrared. Image data are not collected during model injection, resulting in a data gap in the time-history of temperature. Historically, an approximate method has been used to obtain heating data with the data gap, but a new, higher-fidelity method has been developed that patches the data gap and performs integral heat transfer analysis on the temperature data, directly solving the heat equation and avoiding unnecessary assumptions. This method has been shown to model the surface heating much more accurately, agree with computational predictions better than the current method, and be an overall more robust method that collapses to a constant film coefficient value much more quickly. The culmination of these aspects results in a method that is a significant improvement over the approximate method and increases the fidelity of the heating results obtained from the NASA Langley Research Center hypersonic wind tunnels.
Preliminary Design of a Titan-Orbiting Stellar Occultation Mission
Wagner, Nathan John (Virginia Tech, 2022-06-09)
This thesis serves to provide a conceptual mission design for a Titan-orbiting stellar occultation mission. Titan has a significant atmosphere much like Earth's. An improved understanding of Titan's atmosphere could provide valuable information about the evolution of Earth's climate. Titan's atmosphere is known to be in a state of superrotation, wherein the atmosphere rotates significantly faster than the surface beneath. The details of the creation and sustainment of this extreme state on Titan in terms of angular momentum exchange remain unknown despite current theories and models. These unknowns, alongside inconsistencies between current models with observations from the Cassini mission, call for an urgent need for Titan atmospheric observations able to resolve atmospheric waves. The science objectives driving the mission design include maximizing the number of measurements, the latitude versus longitude coverage, the latitude versus local solar time coverage, and the mission duration. These measurement needs can be met by a Titan orbiter utilizing a refractive stellar occultation technique. Refractive stellar occultation observes starlight bending through an atmosphere as stars set behind a body. The observed bending profile can be inverted to infer density, temperature, and pressure profiles. This research uses Systems Tool Kit (STK) as a simulation tool to predict measurement coverage for various orbits. The orbital radius was determined to be the driving independent variable which set all other design variables, including the orbital plane which was uniquely selected for a given orbital radius to maximize the number of occultations. The results of this study show that a lower orbital radius is desired as this produces the best combination of measurement number and distribution. This orbital plane should be closely aligned with the Milky Way galactic plane to see the most stars occult. For the lowest sustainable orbital altitude, Low Titan Orbit (LTO) at 1200 km, the orbital plane should be nearly polar to maximize the number of occultations and latitude coverage. The optimal orbit selection (defined by orbital elements a = 3775 km, e = 0, i = 85 degrees, Ω = 87 degrees, ω = 0 degrees, and ν = 0 degrees) for a single satellite can produce nearly 400 stellar occultation opportunities per orbit and provide full latitude versus longitude coverage. A single satellite shows gaps in latitude versus local solar time coverage at mid-latitudes normal to the satellite ground track which may inhibit the diagnosis of the angular momentum flux associated with thermal tides. If necessary, a second satellite in an orbit orthogonal to the first is suggested to close coverage gaps to provide full local time coverage over a Titan day. The optimal orbit selection of this second satellite (defined by orbital elements a = 3775 km, e = 0, i = 5.3 degrees, Ω = 5.9 degrees, ω = 0 degrees, and ν = 0 degrees) provides an additional 343 occultation opportunities per orbit and increases latitude versus local solar time coverage by a factor of 1.5. The understanding of Titan's Earth-like atmosphere could provide insight into climate evolution here on Earth. This concept proposes a novel approach to improving this understanding.
Reactive, Autonomous, Markovian Sensor Tasking in Communication Starved Environments
Kadan, Jonathan Evan (Virginia Tech, 2024-01-02)
The current Space Traffic Management (STM) community was not prepared for the exponential increase in the resident space object (RSO) population that has taken place over the last several years. The combination of poor communication infrastructure and long scheduling lead times of the Space Surveillance Network (SSN) prevent any type of reactive sensor tasking, which is required in event of anomaly detection. This dissertation was designed to survey extensions to the classical notions of covariance based sensor tasking strategies and develop a methodology for evaluating these techniques. A suboptimal partially observable Markov decision process (POMDP) was used as the simulation framework to test various reward functions and decision algorithms while enabling autonomous, reactive sensor tasking. The goal of this work was used the developed evaluation methodology to perform statistical analyses to determine which metrics were most reliable and efficient for Space Traffic Management (STM) of the geosynchronous Earth orbit (GEO) resident space object (RSO) catalog. Hypotheses were tested against simulations of 873 resident space object (RSO) in geosynchronous Earth orbit (GEO) being tracked by 18 heterogeneous, geographically disperse ground-based electro-optical (EO) sensors. This dissertation evaluates the ability of various sensor tasking metrics to produce rewards that maximize geosynchronous Earth orbit (GEO) catalog coverage capability of a sensor network under realistic communication restrictions.
Reconfigurable Resonant Cubic HF Phased Array for In-Space Assembly Operation
Kent, Peter Josiah (Virginia Tech, 2023-02-01)
Conventional two-dimensional phased arrays face two major shortcomings: the presence of ambiguities in direction of arrival measurements and beam broadening endfire effects. The literature provides methods for addressing and minimizing these problems on conventional planar phased array structures, but there has been no investigation into solving these issues with three-dimensional geometries. In this thesis, the design and performance of a cubic phased array that can eliminate endfire effects and dramatically improve direction of arrival ambiguity resolution is investigated. Both beamforming and direction of arrival simulations are performed in MATLAB and 4nec2 simulation environments for cubic phased arrays of various sizes and at different frequencies and demonstrate that the endfire effects are eliminated and direction of arrival ambiguity resolution is dramatically improved. These findings are expected to lead to new designs of high fidelity three-dimensional phased arrays.
Satellite Constellation Optimization for In-Situ Sampling and Reconstruction of Tides in the Thermospheric Gap
Lane, Kayton Anne (Virginia Tech, 2024-01-04)
Earth's atmosphere is a dynamic region with a complex interplay of energetic inputs, outputs, and transport mechanisms. A complete understanding of the atmosphere and how various fields within it interact is essential for predicting atmospheric shifts relevant for spaceflight, the evolution of Earth's climate, radio communications, and other practical applications. In-situ observations of a critical altitude region within Earth's atmosphere from 100-200 km in altitude, a subset of a larger 90 – 400 km altitude region deemed the "Thermospheric Gap", are required for constraining atmospheric models of wind, temperature, and density perturbations caused by atmospheric tides. Observations within this region that are sufficient to fully reconstruct and understand the evolution of tides therein are nonexistent. Certain missions have sought to fill portions of this observation gap, including Daedalus which was selected as a candidate for the Earth Explorer program by the European Space Agency in 2018. This study focuses on the design and optimization of a two-satellite, highly elliptical satellite constellation to perform in-situ observations and reconstruction of tidal features in the 100-200 km region. The model atmosphere for retrieving sample data is composed of DE3 and DE2 tidal features from the Climatological Model of the Thermosphere (CTMT) and background winds from the Thermosphere-Ionosphere-Electrodynamic General Circulation Model (TIEGCM). BoTorch, a Bayesian Optimization package for Python, is integrated with the Ansys Systems Tool Kit (STK) to model the constellation's propagation and simulated atmospheric sampling. A least squares fitting algorithm is utilized to fit the sampled data to a known tidal function form. Key results include 14 Pareto optimal solutions for the satellite constellation based on a set of 7 objective functions, 3 constellation input parameters, and a sample set of n = 86. Four of these solutions are discussed in more detail. The first two are the best and second-best options on the Pareto front for sampling and reconstruction of the input tidal fields. The third is the best solution for latitudinal tidal fitting coverage. The fourth is a compromise solution that nearly minimizes delta-v expenditure, while sacrificing some quality in tidal fitting and fitting coverage.

Browsing by Author "Schroeder, Kevin Kent"

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