Perhaps the most visible development in the 1990s has been the rapid emergence of permanent, continuously operating GPS networks. Through much of the 1980s the guiding concept for GPS geodesy was to deploy a relatively few permanent fiducial sites to provide reference control for episodic regional campaigns. Many factors have since combined to overturn that model: the cost of mounting campaigns; the declining cost of receivers; the expansion of the Internet; the lure of catching a large earthquake in the act (accomplished at Landers and Northridge in southern California); and the precipitous drop in the cost of data analysis.
GPS geodesy in the 1980s was notable for the grinding toil required to analyze the data. By 1990, improved receivers and analysis techniques [ Blewitt, 1993] had set the stage for a revolution. Four events in 1991 set it in motion: the 3-week GIG'91 campaign [ Melbourne et al, 1991], which deployed more than 100 receivers around the world; installation of a 5-site permanent GPS geodetic array (PGGA) in southern California [ Bock, 1991; Lindqwister et al, 1991]; operation of a permanent global GPS network following GIG'91; and the advent of high-speed workstations. By mid-1991, teams in the U.S. and Europe were laboring to mold existing analysis tools into engines of high-volume production. Meanwhile, the International GPS Service for Geodynamics, or IGS [ Mueller and Beutler, 1992] and the International Earth Rotation Service (IERS) were planning a 3-month test campaign for an operational service to provide data, continuous GPS orbits, and daily global geodetic parameters. When the test campaign began in June 1992 with more than 25 permanent global sites, seven centers using six different analysis systems were able to handle the huge data flow [ Beutler and Brockmann, 1993].
Today, seven centers produce daily global solutions for the IGS. The permanent global network numbers over 70 sites; the PGGA serves as the framework for a dense Southern California Integrated GPS Network (SCIGN) numbering nearly 30 sites, with a plan to expand to 250 within three years; Japan is installing two networks totaling more than 100 sites, while Australia has put in 15 new sites including 5 in Antarctica.