GeoSAR Mission: Orbit Determination Techniques for a Ground-based Interferometer System

The aim of Geostationary Synthetic Aperture Radar (GeoSAR) missions is to obtain daily images of events that take place over the Earth's surface. Some of them are critical to monitor (e.g. land stability control, natural risks prevention or accurate numerical weather prediction models from water vapour atmospheric mapping) since their fast evolution is not observable with current Low Earth Orbit (LEO) based systems. Thus, the fixed location of a geostationary satellite with respect to the Earth's surface may be used to permanent monitor the evolution of this kind of events. The term fixed is in quotation marks due to the geostationary satellite does not remain still. Some perturbations such as the forces exerted by the Earth's equatorial bulge, third bodies (e.g. the Moon or the Sun), etc., degrade the geostationary Earth orbit (GEO) enabling the satellite to describe an elliptical movement. Such path can consequently be used to form the Synthetic Aperture needed to obtain the Synthetic Aperture Radar (SAR) images. The recent analysis of GeoSAR missions shows that the range history of every point of the scene during echo acquisitions must be known precisely in order to obtain focused images of the Earth's surface. Therefore, the orbit of the geostationary satellite must be known in an unprecedented precision (cm), which is far from typical repositioning of satellites in GEO orbits. In order to determine the satellite orbit precisely, two systems are presently being under research in GeoSAR missions: a group of Active Radar Calibrators (ARCs), and a ground-based interferometer system. This project will focus on the latter. The ground-based interferometer system makes use of the transmissions of present communication satellites located in GEO orbits to provide interferometric phase observations. Such observations can be used to calculate the satellite orbit with a certain precision. This project will discuss the precision outcome that two differential correction techniques ideally suited for orbit determination offer when using the interferometric phase observations. Such techniques are the Least Squares (LS) technique, and the extended Kalman filters (EKF). The performance of both techniques will be evaluated in different situations in order to obtain a complete overview of their precision outcome. Then, the error results of both techniques will be compared in order to determine the best technique to be used for orbit determination when a ground-based interferometer system is considered. Finally, this project will also address the absolute phase ambiguity resolution. Such issue is of special importance when dealing with interferometer systems since the LS and EKF techniques do not achieve convergence unless the absolute phase cycles of the transmitted signals are taken into account. Thus, the last chapter of this project will be devoted to explain a method to overcome the absolute phase ambiguity.