The GGOS Core Sites co-locate the major observation techniques and a variety of additional sensors and instruments (e.g. meteorological sensors, water vapour radiometers, etc.). The co-location of the different techniques allows not only the integration of individual technique-specific networks into a unique terrestrial reference frame but also to the assessment of the observation quality and accuracy as well as the mutual validation of the results. The integration of the individual technique-specific networks is coordinated by the IERS.