Geodesy is the science of measuring the Earth and other celestial bodies. The "three pillars" of geodesy are the time-dependent shape, gravity field and rotation of the Earth, which today are observed with a wide variety of geodetic techniques. With these techniques, geodesy has the potential to determine with utmost precision the geometric shape of land, ice, and ocean surfaces as a global function of space and time. The geometric methods, when combined with time-variable gravity and the static geoid, allow the inference of mass anomalies, mass transport phenomena and mass exchange in the Earth's system, and in particular the water cycle. Variations in Earth rotation reflect mass transport in the Earth system and the exchange of angular momentum among its components, which is related to the system's dynamics. The observations of the variable shape, gravity field, and rotation of the Earth allow the realization of the reference systems that are required in order to assign coordinates to points and objects, and to describe the motion of the Earth in space. The dense web of microwave radiation used for geodetic positioning passes through the atmosphere, its interaction with the atmosphere yields important weather parameter information.
The global geodetic observing system (ggos) comprises all geodetic infrastructure, data collection streams, data centers, processing and analysis centers involved in the determination of the Earth's shape, gravity field and rotation on all scales from global to regional to local. ggos is a huge system with many shareholders developing, deploying and maintain infrastructur from GNSS, satellite missions, to ground-based tracking stations and analysis centers. Some of the componets are well coordinated while others are only loosely connected to the other parts of ggos. However, in the end, all individual parts together provide an increaingly more detailed picture of Earth's shape, gravity field and rotation and their changes in time.
The Global Geodetic Observing System (GGOS) is the Observing System of the International Association of Geodesy (IAG). The vision of this organizational component of IAG is to empower with geodesy the advancement of society. GGOS is IAG's flagship component that advances the use of geodetic observing methods for Earth system and planetary science and applications. GGOS accomplishes its mission by defining the geodetic infrastructure that is needed to meet scientific and societal requirements, by advocating for the establishment and maintenance of this geodetic infrastructure, by coordinating interaction between the IAG Services, Commissions, and stakeholders, by improving the quality of and accessibility to geodetic observations and products, and by educating the scientific community about the benefits of geodetic research and the public about the fundamental role that geodesy plays in society. The infrastructure GGOS is built on is part of the larger ggos entity, and in many cases, GGOS depends for its success on other parts of ggos.