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The objective of the future Global Geodetic Observing System (GGOS) to realize the terrestrial reference system (TRS) with 1 mm accuracy and 0.1 mm/yr long-term stability remains challenging when the four space geodetic techniques – GNSS, SLR, VLBI, and DORIS – are processed independently in the traditional, technique-specific manner. Key challenges arise from the sparse and highly inhomogeneous global distribution of co-location sites used to tie the individual solutions together, as well as from the treatment of technique-specific calibration parameters, such as GNSS antenna phase center offsets and SLR range biases. In this presentation, we present work carried out by the ESA/ESOC Navigation Support Office on the joint processing of GNSS and SLR at the observation level. Our approach – fittingly referred to as COOL (“COmbination at the Observation Level”) – incorporates the primary geodetic ILRS targets LAGEOS-1, LAGEOS-2, LARES-2, Etalon-1, and Etalon-2, and makes use of space ties provided by the Sentinel and Galileo satellites to directly link the two geodetic techniques. Particular attention is given to the Galileo transmit antenna z-offsets and the numerous SLR range biases, which require careful treatment, as they are known to directly influence the scale of the reference frame solution. The primary motivation for this work is to ensure full readiness for the future ESA GENESIS mission, which aims to establish a highly accurate, next-generation International Terrestrial Reference Frame (ITRF) through the use of all four space geodetic techniques, including DORIS and VLBI, on a single platform and the exploitation of the space ties between them.