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Christian,
John A.
Christian,
John A.
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ItemLOST in Space: Optimal Triangulation for Celestial Localization(Georgia Institute of Technology, 2022-10) Henry, Sébastien ; Christian, John A.Optical measurements are a key part of modern interplanetary navigation. The statistically optimal Linear Optimal Sine Triangulation (LOST) algorithm is applied to the context of celestial navigation. In addition to optimal triangulation methods, celestial navigation requires the consideration or target ephemeris errors, light aberration, and light time-of-flight. In most cases, only light aberration and light time-of-flight change the expected direction of the measured line-of-sight (LOS). These effects are found to be non-negligible at typical observer velocities (for light aberration) and planet velocities (for light time-of-flight). The effects of the position uncertainty of planets are only important when the observer is close to them. The LOST framework provides a mechanism to conveniently consider all of these effects.
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ItemCrater Projection in Linear Pushbroom Camera Images(Georgia Institute of Technology, 2022-10) Mancini, Michela ; DeVries, Carl ; Thrasher, Ava C. ; Christian, John A.Science images of the Moon and Mars are often captured with pushbroom cameras. Craters with elliptical rims are common objects of interest within the the images produced by such sensors. This work provides a framework to analyze the appearance of crater rims in pushbroom images. With knowledge of only common ellipse parameters describing the crater rim, explicit formulations are developed and shown to be convenient for drawing the apparent crater in pushbroom images. Implicit forms are also developed and indicate the orbital conditions under which craters form conics in images. Several numerical examples are provided which demonstrate how different ellipse formulations can be interpreted and used in practice.
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ItemMethods for Navigation in the Nearby Interstellar Medium(Georgia Institute of Technology, 2022-09) Christian, John A.Recent years have seen an increased interest in sending dedicated spacecraft to explore the nearby interstellar medium (NISM). Such a mission would be instrumented to study the so-called heliosphere on the outskirts of our Solar System, where the solar wind and helopspheric magnetic field interact with interstellar environment (e.g., cosmic radiation). While the scientific value of such a mission is clear, the design and operation of a spacecraft to accomplish this mission is difficult. Indeed, due to the immense distances involved, navigation is expected to be amongst the most challenging tasks. This work explores a variety of navigation observables and frameworks that one might use to navigate a mission within the NISM. Detailed models are presented for all of the major sources of navigation information, including Earth-based radiometric tracking, visible-spectrum star sightings, X-ray pulsar navigation (XNAV), StarNAV, and others. The utility of these observables is then studied within an orbit determination framework, along with consideration of the quality of state knowledge most likely required to operate in the NISM. Issues related to time-keeping are also discussed. Numerical results are presented as a way to illustrate the efficacy of various approaches.
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ItemThe CuSPED Mission: CubeSat for GNSS Sounding of the Ionosphere-Plasmasphere Electron Density(Georgia Institute of Technology, 2016-01) Gross, Jason N. ; Kessee, Amy M. ; Christian, John A. ; Gu, Yu ; Scime, Earl ; Komjathy, Attila ; Lightsey, E. Glenn ; Pollock, Craig J.The CubeSat for GNSS Sounding of Ionosphere-Plasmasphere Electron Density (CuSPED) is a 3U CubeSat mission concept that has been developed in response to the NASA Heliophysics program's decadal science goal of the determining of the dynamics and coupling of the Earth's magnetosphere, ionosphere, and atmosphere and their response to solar and terrestrial inputs. The mission was formulated through a collaboration between West Virginia University, Georgia Tech, NASA GSFC and NASA JPL, and features a 3U CubeSat that hosts both a miniaturized space capable Global Navigation Satellite System (GNSS) receiver for topside atmospheric sounding, along with a Thermal Electron Capped Hemi- spherical Spectrometer (TECHS) for the purpose of in situ electron precipitation measurements. These two complimentary measurement techniques will provide data for the purpose of constraining ionosphere-magnetosphere coupling models and will also enable studies of the local plasma environment and spacecraft charging; a phenomenon which is known to lead to significant errors in the measurement of low-energy, charged species from instruments aboard spacecraft traversing the ionosphere. The CUSPED mission has been proposed, and this paper provides an overview of the concept including its science motivation and implementation.
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ItemSpatio-Temporal Scale Space Analysis of Photometric Signals with Tracking Error(Georgia Institute of Technology, 2015-09) Flewelling, Brien R. ; Murphy, Timothy S. ; Rhodes, Andrew P. ; Holzinger, Marcus J. ; Christian, John A.This paper will investigate the application of Scale-Space Theory, specifically Curvature Scale Space, to 1-Dimensional light curve signals generated by reducing imagery sequences taken from simulated telescopes tasked in various modes. As an observed object with a variable light curve is viewed from a sensor achieving a perfect rate track mode, there is a trade between the time fidelity of the reconstructed signal and integration time required to make accurate detections. As the tracking error increases, a sensor in a step-stare con-ops for example may trade spatial samples for intensity information as a function of time. This is commonly seen in streak observations of tumbling resident space objects. The method presented here will demonstrate how consistent light curves with maximum time resolution can be generated from observation sequences with variable tracking error, and sensor integration times. Additionally, the sparse representation of these signals using Curvature Scale-Space feature images will be investigated as a means for rapid correlation of light-curves against a large database. The proposed rapid correlations could be used to identify variable operating modes of a known object, or to identify an object as a member of a database using a method dependent on the order of the number of salient features as opposed to the number of observations.