Towards Opportunistic Navigation with LEO Satellites: Adaptive Clock Estimation and Closed-Loop Ephemeris Tracking

Low Earth orbit (LEO) satellites inherently possess desirable attributes for navigation: (i) abundance, (ii) geometric and spectral diversity, and (iii) high received powers. However, the first prerequisite to satellite navigation is to know the satellites’ ephemeris (i.e., position and velocity over time) and clock error states. Unlike global navigation satellite systems (GNSS), specifically designed for navigation, with satellites in medium Earth orbit (MEO) that constantly transmit ephemeris and clock corrections to users in their signals, LEO satellites, mainly operated by private companies, generally do not openly send such information in their proprietary signals. The quality of oscillators on-board LEO satellites' as well as their clock error states are completely unknown. Moreover, the most accurate publicly available information on LEO satellites' ephemerides is in the form of two-line element (TLE) files, which yield ephemerides with errors of a few kilometers in position and a few meters per second in velocity. Consequently, LEO satellites' states are completely unknown (clock errors) or uncertain at best (ephemeris). This thesis addresses the aforementioned challenges by performing the opportunistic estimation of LEO satellites' states. An adaptive estimator is developed to estimate in real-time the clock quality on-board LEO satellites and a complete framework is discussed to opportunistically refine the TLE-generated LEO satellites' ephemeris. The findings of this thesis are demonstrated experimentally with Orbcomm LEO satellites and are shown to improve opportunistic receiver positioning with LEO satellites' signals