Genesis orbit and geodetic parameter estimation based on GNSS: Impact of transmit antenna phase pattern errors

Kurzfassung

The ESA Genesis mission, which obtained green light at ESA's Council Meeting at Ministerial Level in November 2022 and which is expected to be launched in 2028, aims to significantly enhance the accuracy and stability of the Terrestrial Reference Frame (TRF). This shall be achieved by equipping one satellite at approximately 6000 km altitude with well-calibrated instruments for all four space-geodetic techniques contributing to TRF realizations, i.e., Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and by exploiting the such realized very precise space co-locations. The Genesis satellite is foreseen to carry a zenith- and nadir-pointing GNSS antenna to track (at least) GPS and Galileo signals. Because of its very high altitude, Genesis will cover much larger nadir angles as seen from the GNSS transmitting antennas, compared to receivers at ground or in low Earth orbit. This will partly result in GNSS observations with lower signal-to-noise ratios. Furthermore, to date, only little reliable information is available on the GNSS transmit antenna gain and carrier phase patterns at very large nadir angles. These problems lead to questions with respect to the best possible exploitation of GNSS data by Genesis, e.g., whether phase pattern calibrations will need to be performed by means of tracked GNSS data (which would weaken the GNSS contribution to Genesis). This study focuses on uncertainties of GNSS transmit antenna phase patterns at larger nadir angles and aims to assess their impact on GNSS-based POD of Genesis, as well as GNSS-based global network solutions including data from ground stations and Genesis. In particular, the effect is studied for the estimation of GNSS orbits and clock corrections, Earth rotation and geocenter parameters and station coordinates. To accomplish this, we employ simulated GNSS pseudo-range and carrier phase data for Genesis and an extended set of IGS ground stations, which have been generated based on detailed link-budget computations and a comprehensive set of transmit antenna gain patterns. The data are used for closed-loop simulation investigations. We construct realistic errors for the GNSS transmit antenna phase patterns and apply these in the reconstruction step. Comparison of the reconstructed orbit and geodetic parameter solutions to the simulation truth offers a quantification of the impact of these errors on the estimated parameters. We study 37 days in 2023, covering an entire cycle of Sun-orbital plane geometries of the considered Genesis orbit, and present the impact of the uncertainties on the recovered solution for the entire time span.