The GGOS Core Sites co-locate the major observation techniques to integrate the individual technique-specific networks into a unique terrestrial reference frame. These sites enable the exploitation of the strengths of each technique by linking them through the ground local ties, the carefully measured positions of the reference points of each technique relative to each other.

Multi-technique sites also enable the evaluation of the quality and accuracy of the observations, the mutual validation of the results and the identification of possible systematic errors. The integration of the geodetic products from the individual technique-specific networks is coordinated by the IERS.

Other than the specific systems related to each space geodesy technique, a variety of additional sensors and instruments are in place to perform auxiliary measurements and monitoring (e.g. meteorological sensors, water vapour radiometers, etc.). The definition and requirements for GGOS Core sites are detailed in the document GGOS Requirements for Core Sites. This document is currently under review by the BNO, with the goal to publish an update to reflect the latest technological developments and include the requirements for gravity and physical heights.