Enhancing satellite navigation for low earth and geostationary orbit missions
Keywords: Aeronautics and Astronautics.

The use of Global Positioning System (GPS) and upcoming Global Navigation Satellite System (GNSS) signals for Geostationary Orbit (GEO) and Highly Elliptical Orbit (HEO) space missions has special design challenges. Such missions are at an altitude above the altitude of the GNSS constellations. Consequently, the signals reaching an onboard receiver originate from GNSS satellites on the opposite side of Earth. The received signals are 10 to 100 times weaker with limited satellite spatial diversity. GNSS signal reception at GEO and beyond is dependent on accurately modelling the side lobes of the GNSS satellite transmit antenna array. Starting with the GPS Block III satellites, the GPS Interface Control Document (ICD) provides specifications on the gain characteristics of the main lobe of the transmit antenna array. There is no information in the literature that describes the side lobes of the transmit antenna pattern. Pictures of antennas onboard the Galileo Full Operational Capability (FOC) satellites indicates a transmit array of 28 patch antennas. No information can be found in the literature that characterizes the gain pattern for the Galileo FOC transmit antenna array. In this dissertation, GPS Block III and Galileo FOC transmit array main and side lobe gain patterns have been reversed engineered using computational electromagnetics. Using the reverse engineered transmit antenna gain patterns, satellite visibility and accuracy is evaluated onboard a GEO satellite using a combined GPS plus Galileo satellite constellation. Traditional ground-based satellite laser ranging has accuracy in the kilometer class. Leveraging both the main lobe and side lobes of a combined GPS plus Galileo constellation can result in at least two orders of improvement compared to ground-based approaches. Persistent autonomous RMS 3-D positioning accuracies of 9 -- 15 m can be achieved at GEO. Specular multipath resulting from the body structure and solar arrays is the dominant error source onboard Low Earth Orbit (LEO) satellites. In particular, solar panel induced specular reflections onboard the International Space Station (ISS) can cause up to 50m in GPS positioning error. Conventional multipath mitigation strategies are insufficient overcome this problem. In this work, a novel implementation of adaptive digital beamforming and predictive antenna nulling is demonstrated to overcome multipath. Using live sky data, a 4x decrease in positioning errors is achieved using a simple four element antenna array. A combined GPS + Galileo system is chosen to leverage the common L1 signal which will be transmitted by both constellations. Given the rather weak signal reception at GEO and beyond, custom signal acquisition algorithms are required. Such implementation cannot be found in commercial GNSS receivers. A real-time L1 C/A receiver with adaptive digital beamforming has been developed. The receiver has been implemented on the Xilinx Virtex-5QV rad-hard FPGA. To overcome the need for an external coprocessor, a dual core LEON3 processor has also been implemented within the same FPGA. Receiver performance and design methodologies adopted in its implementation are discussed in this thesis.