Integration of space-based co-locations for enhanced reference frames: Investigating the Potential of ESA’s Genesis Mission

The Federal Agency for Cartography and Geodesy (BKG) is currently taking part in the research project GENESIS-D (a consortium of the main German geodetic institutes). The goal of this project and our studies therein is to be able to consistently process, combine and validate observation data of ESA’s upcoming Genesis mission in the future. Genesis will allow, for the first time, the co-location in space of all four main space geodetic techniques, namely VLBI, SLR, GNSS, and DORIS. The orbit combination will enable a quantification of inter-technique systematic discrepancies, and will increase the inter-technique consistency during the determination of the international terrestrial reference frame (ITRF). Already now, several Low-Earth-Orbiting (LEO) satellites represent a co-location in-space for the three satellite-based space-geodetic techniques, i.e., GNSS, SLR and DORIS. Using such LEOs allows to study potential hurdles in harvesting the full potential of Genesis and preparing the analysis and combination software to fully exploit satellite co-locations.

In this work, we have chosen the altimetry satellite Sentinel-6A Michael Freilich (S6A) as a proxy for Genesis in order to conduct research and software development regarding the combination of SLR and GNSS. For now, SLR data to S6A and to the SLR specific satellites LAGEOS-1 and LAGEOS-2 have been analyzed.

For the two cannonball-shaped LAGEOS satellites we estimate weekly arcs, whereas for S6A daily arcs are set up, due to the lower altitude and more complicated radiation-pressure modelling. By accumulating the daily arc S6A normal equations (NEQs) into weekly NEQs and stacking them with the LAGEOS-based NEQs, we obtain weekly solutions that include satellite orbits, station coordinates, Earth rotation parameters, geocenter coordinates, and SLR range biases. We compare all parameters estimated in the framework of the three solution types, that are (i) LAGEOS-only; (ii) S6A-only; and (iii) LAGEOS+S6A.

We find that the station coordinate repeatability for (i) LAGEOS is better than for (ii) S6A, which is expected given the use of a single-satellite LEO solution. Generally, the single-satellite solution of (ii) S6A yields worse results compared to the other two solutions as these are multi-satellite solutions. On the other hand, the advantage of S6A shows up, e.g., in the six-fold increase in low-elevation observations to S6A compared to LAGEOS, which facilitates the decorrelation between range bias and station height. The analysis of the impact of S6A data on the global LAGEOS solution offers insight into the potential impact that Genesis will have on current SLR products. It also ensures the early identification and resolution of software issues, allowing Genesis data to be evaluated from the outset of the mission. This study will be expanded in the future by a global GNSS solution as well as GNSS analysis of S6A data, as well as the subsequent combination with SLR.