ITRF and co-location sites

The main focus of this position paper is to review the current status of the co-location sites, necessary for ITRF combination. The definition of a co-location site is given in terms of distance between geodetic markers or instrument reference points of different techniques and accuracy of the local ties connecting them. Distribution and quality of the current co-location site are summarized. Given the precision increase of the measurement of the space geodesy techniques and modelling, 1 mm precision or better should be the goal of all new local tie surveys. Moreover, SINEX files with full variance-covariance information is now mandatory.