Geodetic Technique Combinations at the Observation Level: Advancements in Terrestrial Reference Frame Recovery

JPL has developed a terrestrial reference frame (TRF) based on combining space geodetic techniques at the observation level. The current version includes observations from the Global Positioning System (GPS), Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI); and it has been shown to be competitive with the international standard (ITRF2020) in terms of fundamental frame parameters (origin and scale) and their temporal evolution, both linear and seasonal. In this work, we build on the current processing infrastructure and add Doppler Orbitography by Radiopositioning Integrated on Satellite (DORIS) observations to the combination. The four-technique solution strategy uses space ties in low-Earth orbit to connect SLR, GPS and DORIS and ground ties to connect VLBI to GPS. This solution can be viewed as a pathfinder for the upcoming GENESIS mission which will provide a space tie for all 4 techniques. Additionally, we revise parametrization of the SLR range biases to improve retrieval of the scale. Finally, we discuss future enhancements to the TRF realization process. These enhancements include recovering the time-variable gravity field by jointly using observations from the frame realization and GRACE / GRACE-Follow On missions.