The permanent monitoring of the Earth's rotation requires the coordinated use of VLBI and satellite techniques. VLBI provides the absolute reference for the determination of universal time, precession and nutation. The satellite techniques ( GPS, SLR, DORIS) provide the daily interpolation and the short-term prediction of universal time in the highly accurate but sparser VLBI reference values; they also provide daily values of polar motion.

The maintenance of the ICRS requires the VLBI monitoring of extragalactic objects by regional or global networks. Its link with the Solar System reference frame requires LLR observations.

The maintenance of the ITRF includes monitoring the motion of the reference sites providing access to the reference frame, in particular via densification programs. It requires the maintenance of global networks observed with VLBI, LLR, GPS, SLR and DORIS, sampling the major tectonic plates. Some stations of each technique are colocated with stations of the other ones, ensuring a balanced distribution. IERS sites have to be included in densification campaigns in order to ensure an accurate link.

The maintenance of the orbits in the satellite techniques requires permanent observation by core networks with a global coverage, operated with LLR, GPS, SLR, DORIS.