| Summary: | The 2017 Great American Eclipse provided an excellent opportunity for scientists and engineers to study the ionosphere. The dynamics of the ionosphere are affected by the amount of solar radiation it receives and a total solar eclipse produces a short perturbation to the incoming solar radiation. Analyzing how the ionosphere reacts to this type perturbation could lead to new levels of understanding of it. This study develops a nonlinear filter that estimates the state of the ionosphere's 3-D electron density profile given total electron content (TEC) measurements from dual-frequency GPS receivers located on the ground and on low-Earth-orbiting spacecraft. The electron density profile is parameterized by a bi-quintic latitude/longitude spline of Chapman Profile parameters that define the vertical electron density profile. These Chapman parameters and various latitude and longitude partial derivatives are defined at a set of latitude/longitude spline grid points. Bi-quintic interpolation between the points defines the parameters' values and the corresponding Chapman profiles at all latitude/longitude points. The Chapman parameter values and their partial derivatives at the latitude/longitude spline nodes constitute the unknowns that the nonlinear filter estimates. The filter is tested with non-eclipse datasets to determine its reliability. It performs well but does not estimate the biases of the receivers as precisely as desired. Many attempts to improve the filter's bias estimation ability are presented and tried. Eclipse datasets are input to the filter and analyzed. The filter produced results that suggest that the altitude of peak electron density increased significantly near and within the eclipse path and that the vertical TEC (VTEC) was drastically decreased near and within the eclipse path. The changes in VTEC and altitudes of peak electron density caused by the eclipse leave a lasting effect that alters the density profile for anywhere from 15 minutes to several hours. === MS === The 2017 Great American Eclipse garnered much attention in the media and scientific community. Solar eclipses provide unique opportunities to observe the ionosphere’s behavior as a result of irregular solar radiation patterns. Many devices are used to measure this behavior, including GPS receivers. Typically, GPS receivers are used to navigate by extracting and combining carrier phase and pseudorange data from signals of at least four GPS satellites. When the position of a GPS receiver is well-known, information about the portion of the ionosphere that the signal traveled through can be estimated from the GPS signals. This estimation procedure has been done with ground-based and orbiting GPS receivers. However, fusing the two data sources has never been done and will be a primary focus of this study. After demonstrating the performance of the estimation algorithm, it is used to estimate the state of the ionosphere as it was perturbed by the 2017 Great American Eclipse.
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