Global coordinates with centimeter accuracy in the International Terrestrial Reference Frame using GPS

Using 21 days of Global Positioning System (GPS) data from 21 globally distributed receivers operating during early 1991, we solve for a 7-parameter transformation between a GPS free-network solution and coordinates of 12 stations listed in the International Terrestrial Reference Frame (ITRF). Standard errors of GPS coordinates are derived by applying an orthogonal projection operator to the free-network covariance. The weighted RMS difference between 33 transformed GPS and ITRF coordinates is 12 mm in the northern hemisphere. Best results are obtained by mapping ITRF coordinates to the epoch of this experiment assuming no vertical site motions. Fixing selected sites in the GPS solution to ITRF'90 does not improve the agreement. We conclude the use of fiducial constraints is unnecessary for global networks.