Report of Informal Working Group on Measuring and Monitoring Site Ties

Precise measurement and maintenance of the Terrestrial Reference Frame (TRF) is essential to long-term monitoring of global change. The TRF is realized through the integration of high precision GNSS, SLR, VLBI and DORIS networks. Each of these techniques has unique strengths and weaknesses. Station positions and velocities from the different techniques are combined to obtain the most accurate and robust TRF. An essential ingredient in this combination is the local eccentricity vectors at collocation sites. Hence accurate measurement of this vector is very important. At a few sites different techniques give different results for site position evolution. Determining the cause of this requires re-measuring the eccentricity vectors. The problem is further complicated because the "invariant point" for most techniques cannot be measured directly, and must be inferred. In September 2008 an informal working group with representatives from most of the space geodesy techniques looked at the application of new surveying and measuring technologies to improve the measurement and the monitoring of local ties. We looked at factors such as ease of use, ability to automate, and cost. We also studied issues such as site layout and monumentation. We report on our conclusions from this workshop and plans for future work. One goal is to perform a proof of concept experiment in the near future. We welcome input from the global geodetic community.