@phdthesis{,
    author = {Jacob, Robert},
    title = {The Gravitational Field of Comet Churyumov-Gerasimenko from Radio Science and Optical Data Combined Orbit Determination},
    editor = {},
    booktitle = {},
    series = {},
    journal = {},
    address = {},
    publisher = {},
    edition = {},
    year = {2022},
    isbn = {},
    volume = {},
    number =  {},
    pages = {},
    url = {},
    doi = {},
    keywords = {Celestial mechanics, comets: individual: 67P/Churyumov–Gerasimenko, small bodies, Student's t-distribution, SIFT, gravitational field determination, precise orbit determination, optical navigation, sparse bundle adjustment, spherical harmonics, radio science},
    abstract = {ESA's Rosetta mission was launched from Kourou in 2004 to enter the orbit of the Jupiter-family-comet Churyumov-Gerasimenko after a ten-year journey through the solar system. The orbiter accompanied the comet on its way through the solar system for more than two years. The aim of the work "The Gravitational Field of Comet Churyumov-Gerasimenko from Radio Science and Optical Data Combined Orbit Determination" is the precise determination of the comet’s gravitational field parameters. The main topic is the processing of optical data from the navigation camera and the camera system OSIRIS, which were on board the spacecraft Rosetta. The techniques developed and combined allow the accurate estimation of the camera position at the time of image acquisition. First, for the search of corresponding image points, the Scale Invariant Feature Transform (SIFT) is combined with Polynomial Least Squares Matching and a correction for the change in illumination conditions. Bundle Adjustment is used to estimate the position of the camera as well as the triangulated points on the surface of the comet, and the noise in the image data is modeled according to the t-distribution. This makes the method particularly robust against false measurements, so-called outliers. The positions of the camera at the time of the image acquisition provide a decisive contribution to the determination of the orbit of the spacecraft relative to the comet nucleus. Already established methods of Radio Science Investigations (RSI) for orbit determination from frequency and time-of-flight measurements can thus be improved in their accuracy. The uncertainties in the spacecraft’s orbit are reduced, which at the same time leads to a more accurate estimate of the comet’s gravitational field parameters. In addition, the developed methods can be used to gain insights into the rotational properties of the comet. With the developed methods, 1.93 × 10^6 points on the comet’s surface were automatically detected in 10975 images and their position was measured 36.88 × 10^6 times with subpixel precision. All images used originate from orbits within 35 kilometers of the comet’s nucleus. The reprojection error of the triangulated points results in 0.6 pixel RMS, achieving subpixel accuracy in the bundle adjustment. The average uncertainty of the resulting camera positions is 7 meters across all images and 3.7 meters in the close orbits at the end of the mission in 2016. Before perihelion, an increase in the rotation period from 12.4040 to 12.4060 ± 0.0001h can be measured from September 2014 to February 2015 due to comet outgassing. After perihelion, a decreasing rotation period from 12.0607 to 12.0567 ± 0.0001h is observed from February to September 2016. Additional periods in a rotation mode beyond the main inertial axes could not be confirmed. From orbital integration of the spacecraft, a gravity field in spherical harmonics up to degree and order 4 could be determined. Within the uncertainties all gravity field coefficients are in the range of a homogeneous density distribution within the nucleus. With GM = 665.71 ± 0.43 m^3 s^−2 in August 2016, the total mass of the comet could be determined based on optical data. When combined with the established methods from Doppler measurements, this results in a reduction of the uncertainties of the gravity field parameters to 53% of the initial values.},
    note = {},
    
    school = {Universität der Bundeswehr München},
    
}