The IAG’s Global Geodetic Observing System (GGOS) provides the fundamental ground- and space-based geodetic observations needed to improve our understanding of the Earth’s variable shape, rotation, and gravity. Modernizing existing ground-based systems and developing future space-based systems will enable greater understanding of the causes and consequences of these variations. Simulations are needed to help plan the development and deployment of the future systems and to help study the impact of the systems on geodetic data and products including the terrestrial reference frame. This session is a forum for discussing past, present, and future geodetic observing systems, the use of simulations in developing and deploying the systems, and the use of the systems for improving our understanding of the dynamic Earth.