The optimal combination of the different space geodetic techniques, and accounting for the individual strengths and weaknesses thereof, allows the determination of global terrestrial reference frames (TRFs) with the highest accuracy, stability and consistency. Global TRFs are the indispensable basis upon which Earth system modeling takes place since they provide the absolute reference for the relative space geodetic measurements. To reliably monitor and predict phenomena pertinent to climate change such as the global sea level rise, TRFs need to be accurate to 1 mm and long-term stable to 1 mm/decade. However, state-of-the-art TRFs cannot fulfill these requirements by almost one order of magnitude, thus failing to realize the goals of the Global Geodetic Observing System. This presentation will cover recent combination studies of the different space geodetic techniques, including extensive simulations to improve current and future TRFs.