ITRF2008G Terrestrial Frame Computed with GIPSY Software

We have constructed a terrestrial reference frame using 40 stations from each of the four techniques DORIS, GPS, SLR, and VLBI. We started with the sinex files that each technique submitted for ITRF2008 and performed our analysis with JPL's GIPSY software. All but 40 sites were removed from each technique's sinex files. These sites were selected to provide good global coverage, linear behavior with no breaks, and sufficient ties to link all techniques. After editing, the files from each technique were combined separately. Then all techniques and tie measurements were combined to obtain ITRF2008G. Datum definitions were chosen to mimic those for ITRF2008. The origin was defined by SLR, the scale was defined by VLBI minus 0.52 ppb, and the orientation was defined by GPS. The GIPSY frame ITRF2008G was then compared to ITRF2008. The two frames agree remarkably well, with frame offsets less than 1 mm and frame drifts less than 0.1 mm/yr. This result shows that the core reference frame can be established with 40 good sites from each technique. Various densification techniques such as point positioning can then be used to extend this core frame to thousands of sites.