The ground-based geodetic infrastructure consists of all terrestrial networks of geodetic stations contributing to the geodetic reference frames or to Earth system monitoring:
- The global network of GGOS ground sites, coordinated with the GGOS-BNO, includes:
- Core sites with all of the major observation space techniques: VLBI, SLR, GNSS, DORIS (where available);
- Co-location sites with two or more of the major observation space techniques, but less than the full core site complement;
- Newly added requirement for core and co-location sites are absolute and superconducting gravimetry; and
- A variety of additional sensors and instruments (e.g. meteorological sensors, water vapour radiometers, etc.).
- The co-location of the different space geodetic techniques allows not only the integration of individual technique-specific networks into a unique terrestrial reference frame, but also validation of the results. The combination allows us to exploit the strengths of each technique and mitigate some of the weaknesses of others.
- The integration of the individual technique specific networks is coordinated by the International Earth Rotation and Reference Systems Service (IERS) and the International Gravity Field Service (IGFS).GGOS Core Sites
- The global network of radio telescopes is used for the microwave observation of extragalactic objects (quasars) by VLBI. The operation of the VLBI global network is coordinated by the International VLBI Service for Geodesy and Astrometry.
- The global network of satellite and lunar laser ranging stations is coordinated by the International Laser Ranging Service.
- The global network of GNSS stations is coordinated by the International GNSS Service, providing precise GNSS satellite orbits, satellite clock synchronisation and reference products for the optimal application of GNSS positioning, navigation and time transfer.
- The global network of absolute gravity stations recorded by the International Gravimetric Bureau (BGI) and the global network of superconducting gravimeters, tiltmeters, strainmeters and other geodynamic sensors are inventoried by the International Geodynamics and Earth Tide Service (IGETS).
- The global network of tide gauge records are managed by the Permanent Service for Mean Sea Level (PSMSL).
The global networks, in particular the GNSS and gravity station networks, are extended by regional densifications, which allow the accessibility to the reference frames at local levels. These networks are coordinated by the regional sub-commissions of the International Association of Geodesy for reference frames and gravity field modelling. The regional networks are further extended by national reference networks, including levelling networks, which are determined and maintained by the national bodies responsible for the geodetic infrastructure.