The United States Department of Commerce's National Geodetic Survey (NGS) has developed a nationwide network of continuously operating reference stations (CORS) designed to support the broad spectrum of post-processed, relative Global Positioning System (GPS) techniques and applications. This network now comprises more than 900 permanent, geodetic-quality GPS receivers, in various installation configurations. The network was established through an exemplary cooperative effort, involving nearly

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... 00 organizations from various government levels, academia, and the private sector. CORS GPS observational data are freely provided to the user community via the Internet and are capable of supporting high-accuracy positioning requirements. In addition to enhancing geospatial positioning, applications of CORS data include the following: a critical role in defining the nation's geodetic reference system; the ability to characterize the free electron content of the ionosphere; and an important source of precipitable water vapor input to meteorological forecasts. Associated with the CORS network, the Online Positioning User Service (OPUS) is a free, automated Web-based utility that processes submitted GPS observation data with respect to the CORS network, providing corresponding positional coordinates, via email, usually in minutes. This powerful tool has proven to be very popular with the geospatial community and is changing the way NGS is able to provide access to the National Spatial Reference System.