An increasing number of GPS applications require a high level of accuracy. To reduce the error contributed by the GPS ephemerides, an accurate modeling of the forces acting on GPS satellites is necessary. These forces can be categorized into gravitational and non-gravitational forces. The non-gravitational forces are a significant contribution to the total force on a GPS satellite but they are still not fully understood whereas the gravitational forces are well modeled. This study focuses on two non-gravitational forces: Solar Radiation Pressure (SRP) and the y-bias force. Different SRP models are available in the University of Texas Multi-Satellite Orbit Determination Program (MSODP). The recently developed University College London model was implemented for the purpose of this study. Several techniques to compute parameters associated with SRP models and the y-bias force during an orbit prediction were examined. Using the International GNSS Service (IGS) precise ephemerides as a reference, five different models were compared in the study. Satellite Laser Ranging (SLR) residuals were also studied to validate the approach. Results showed that the analytical UCL model performed as well as a purely empirical model such as the Extended CODE model. This is important since analytical models attempt to represent the physical phenomena and thus might be better suited to separate SRP from other forces. The y-bias force was then shown to have a once per revolution effect. The time evolution of the y-bias was found to be dependent on the SRP model used, the satellite Block type, the orbital plane, and the attitude of the satellite which suggests that estimates of y-bias contain errors from other sources, particularly the SRP models. The dependency of the y-bias evolution on the orbital plane suggests that the orientation of the plane towards the Sun is important. / text

<p>Comets are the most primordial and least evolved bodies in the Solar System. As such, they are unique sources of information regarding the early history of the Solar System. However, little is known about cometary nuclei since they are very difficult to observe due to the obscuring coma. Indirect methods are therefore often used to extract knowledge about nucleus parameters such as size, shape, density, material strength, and rotational properties. For example, tidal and non-tidal splitting of cometary nuclei can provide important information about nuclear densities and material strengths, but only if the criteria for mechanical stability are well known. Masses and densities of cometary nuclei can also be obtained by studying orbital modifications due to non-gravitational forces, but only if the thermophysics of comets can be modelled accurately. </p><p>A detailed investigation is made regarding the mechanical stability of small Solar System bodies. New expressions for the Roche distance are derived, as functions of the size, shape, density, material strength, rotational period, and spin axis orientation of a body. The critical rotational period for centrifugal breakup in free space is also considered, and the resulting formulae are applied to comets for which the size, shape and rotational period have been estimated observationally, in order to place constraints on their densities and material strengths. </p><p>A new thermophysical model of cometary nuclei is developed, focusing on two rarely studied features - layer absorption of solar energy, and parallel modelling of the nucleus and innermost coma. Sophisticated modelling of radiative transfer processes and the kinetics of gas in thermodynamic non-equilibrium form the basis for this work. The new model is applied to Comet 19P/Borrelly, and its density is estimated by reproducing the non-gravitational changes of its orbit.</p>

Comets are the most primordial and least evolved bodies in the Solar System. As such, they are unique sources of information regarding the early history of the Solar System. However, little is known about cometary nuclei since they are very difficult to observe due to the obscuring coma. Indirect methods are therefore often used to extract knowledge about nucleus parameters such as size, shape, density, material strength, and rotational properties. For example, tidal and non-tidal splitting of cometary nuclei can provide important information about nuclear densities and material strengths, but only if the criteria for mechanical stability are well known. Masses and densities of cometary nuclei can also be obtained by studying orbital modifications due to non-gravitational forces, but only if the thermophysics of comets can be modelled accurately. A detailed investigation is made regarding the mechanical stability of small Solar System bodies. New expressions for the Roche distance are derived, as functions of the size, shape, density, material strength, rotational period, and spin axis orientation of a body. The critical rotational period for centrifugal breakup in free space is also considered, and the resulting formulae are applied to comets for which the size, shape and rotational period have been estimated observationally, in order to place constraints on their densities and material strengths. A new thermophysical model of cometary nuclei is developed, focusing on two rarely studied features - layer absorption of solar energy, and parallel modelling of the nucleus and innermost coma. Sophisticated modelling of radiative transfer processes and the kinetics of gas in thermodynamic non-equilibrium form the basis for this work. The new model is applied to Comet 19P/Borrelly, and its density is estimated by reproducing the non-gravitational changes of its orbit.