| Popis: |
Over the coming decades, as the volume of space missions continues to grow and iversify, so too does the demand for methods to achieve improved position, navigation, and timing service performance in geostationary orbit and beyond. For high-altitude spacecraft to utilize GNSS navigation techniques, it is required that they must utilize signals from GNSS space vehicles from across the limb of the Earth, which severely limits the quality and quantity of measurements that can be made. The performance potential for such a spacecraft can be improved, however, by having the ability to utilize any combination of satellites visible at a given time, across all GNSS constellations. Interoperability of this kind depends primarily on the ability to resolve the differences in each system’s specific time scale. This thesis proposes and develops methods to provide high-accuracy GNSS measurements from a GNSS receiver in low Earth orbit to facilitate the estimation of GNSS inter-constellation timing offsets. The Bobcat-1 CubeSat was developed to support the collection of this data, and this thesis describes the capabilities and measurement accuracy achieved by the CubeSat as well as the post-processing performed to produce precise inter- constellation timing offsets. Furthermore, a study was conducted using a simulation that was developed to evaluate the performance impacts that GNSS system time offsets impose on a user in geostationary orbit. This study produces quality of service benchmarks that are used to provide performance targets for the accuracy of the inter-constellation time offset estimates enabled by data from Bobcat-1. This thesis compares the inter-constellation timing offset estimates achieved by these methods against the Galileo-to-GPS time offset that is produced by the Galileo Control Segment and included within the navigation messages of Galileo space vehicles. |
