Simulation of the ionospheric contribution on Genesis observables

The ESA Genesis mission, which will be launched in 2029, will provide for the first time in space the colocation of all four space geodesy techniques on the same platform: Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) and Very Long Baseline Interferometry (VLBI). Genesis aims at contributing to the ITRF accuracy requirement, being of 1 mm with a long-term stability of 0.1 mm/year, to provide a coordinate system for the most rigorous navigation applications on our planet. Because Genesis spacecraft will use radio signals that travel into the Earth’s ionosphere (including plasmasphere), its contribution must be accurately assessed and removed to ensure precise and reliable measurements that will be used to compute the ITRF.
In this work, we investigate the magnitude and the variability of the ionospheric contribution on all Genesis observables using radio wavelengths: GNSS, DORIS and VLBI. If the ionospheric-free combination is currently implemented to correct for the first-order effect, the higher order ionospheric terms (HOI), including the bending terms, have an amplitude than can reach several centimeters under adverse conditions, for instance low elevation, solar maximum, equatorial region, etc. In addition, note that future algorithms and applications would use undifferenced measurements, which requires to estimate all error sources, including ionosphere, down to the millimeter. We address the latest developments of our RayTrIO (Tray-Tracing for Ionosphere) algorithm, which computes, based on 3D ionospheric modeling and a ray-tracing module, the ionospheric error for the various geometrical configurations that will be encountered during the Genesis mission: GNSS ground or space receiver to GNSS satellite, GNSS radio-occultation, DORIS observations, VLBI ground-to-Genesis link. Using actual GNSS observations and Genesis orbit simulations, we investigate the ionospheric error budget from UHF measurements of the DORIS system to Ku-band observed by VLBI.