Continuously Operating Reference Station (CORS): History, Applications, and Future Enhancements

The National Oceanic and Atmospheric Administration’s National Geodetic Survey !NGS" manages the National Continuously Operating Reference Station !CORS" system that comprises a network of over 1,350 sites, each containing a geodetic quality Global Navigation Satellite System receiver. This network is currently growing at a rate of about 15 sites per month. NGS collects, processes, and distributes data from these sites in support of high-accuracy three-dimensional positioning activities throughout the United States, its territories, and a few foreign countries. CORS data are also used by geophysicists, meteorologists, atmospheric and ionospheric scientists, and others in support of a wide variety of applications. This paper addresses the history of the CORS network, some of its applications, and plans for enhancing it within the next few years. DOI: 10.1061/!ASCE"0733-9453!2008"134:4!95" CE Database subject headings: Satellites; Geodetic surveys; History. Historical Introduction The history of the Continuously Operating Reference Stations !CORS" system is intimately connected to the National Oceanic and Atmospheric Administration’s !NOAA’s" National Geodetic Survey !NGS" and this agency’s mission to define, maintain, and provide access to the U.S. National Spatial Reference System !NSRS". The NSRS constitutes the official system of the civilian government for enabling a user to determine geodetic latitude, longitude and height, plus orthometric height, geopotential, acceleration of gravity, and deflection of the vertical at any point within the United States and its territories. The NSRS contains information about its orientation and scale relative to international reference frames, as well as the precise orbits of all satellites used in defining or accessing the NSRS. Last, the NSRS also contains all necessary information to describe how all of these quantities change over time. The NSRS is crucial for meeting our nation’s economic, social, and environmental needs. NGS recognized the potential contributions of the Global Positioning System !GPS" for enhancing the NSRS in the early stages of GPS development. Hence, in the late 1980s, this agency embarked on applying GPS instrumentation and field techniques to improve the NSRS. NGS quickly converted its traditional horizontal field operations !which applied line-of-sight instruments" to three-dimensional !3D" field operations using GPS instrumentation. Snay !1989" reported that traditional line-of-sight techniques provided positional coordinates with relative accuracies of approximately 1:250,000 among the primary horizontal reference stations in the NSRS. GPS, on the other hand, easily yields relative accuracies exceeding 1:1,000,000. In addition, because of the line-of-sight requirement, many of the older reference stations had been installed in locations, like mountain tops, which are difficult to access. NGS first applied GPS to determine positional coordinates for the brass disks and other monuments that served as traditional reference stations. Starting with Tennessee in 1987, NGS collaborated with various state and federal agencies and others to establish a high accuracy reference network !HARN"—also called a high precision geodetic network—in each of the 50 states. For each HARN survey, many new reference marks were positioned so that, as compared to the preexisting reference marks, the new ones would be located in more accessible places !e.g., near public roads" and/or they would provide a relatively less obstructed view of the sky. These statewide HARNs were embedded into a more accurate sparse nationwide network whose points were also positioned using GPS techniques, first in 1987 and again in 1990 !Soler et al. 1992". Once a HARN survey was completed in a particular state, NGS performed a statewide readjustment of the HARN data, together with all archived classical geodetic surveys and local GPS projects performed in that state, to compute consistent positional coordinates for the associated ground marks. Anticipating the need to perform accurate HARN surveys, NGS introduced, in the fall of 1986, the Cooperative International GPS Network !CIGNET" !Chin et al. 1987", the forerunner of the CORS network. Each CIGNET site was equipped with a highquality dual frequency GPS receiver that continuously recorded signals from GPS satellites. The primary intention was to make dependable tracking data available from a network of ground stations to compute precise ephemeredes !orbits" for the GPS satellites. In 1989, CIGNET contained three stations in the United States !MOJA in Mojave, Calif.; RICH in Richmond, Fla.; and WEST in Westford, Mass.". These early CORS were equipped with Mini-Mac 2816-AT dual-frequency codeless receivers !Aero Service Division, Western Geophysical Company of America, Houston". In 1990, CIGNET expanded into the southern hemiChief, Spatial Reference System Division, National Geodetic Survey, NOS, National Oceanic and Atmospheric Administration, 1315 East-West Hwy., Silver Spring, MD 20910. E-mail: Richard.Snay@noaa.gov Chief Technical Officer, Spatial Reference System Division, National Geodetic Survey, NOS, National Oceanic and Atmospheric Administration, 1315 East-West Hwy., Silver Spring, MD 20910. E-mail: Tom.Soler@noaa.gov Note. Discussion open until April 1, 2009. Separate discussions must be submitted for individual papers. The manuscript for this paper was submitted for review and possible publication on May 18, 2007; approved on April 21, 2008. This paper is part of the Journal of Surveying Engineering, Vol. 134, No. 4, November 1, 2008. ©ASCE, ISSN 0733-9453/ 2008/4-95–104/$25.00. JOURNAL OF SURVEYING ENGINEERING © ASCE / NOVEMBER 2008 / 95 sphere. By the end of 1991, CIGNET comprised a total of 21 sites spanning all continents except Antarctica. As is now the case, all tracking data were collected by several partners and made freely available to GPS investigators through NGS archives !Schenewerk et al. 1990". Gradually, NGS augmented the CIGNET network, generating the core of the first public global GPS network that, unknowingly at the time, evolved into the current International Global Navigation Satellite System !GNSS" Service !IGS" network under the auspices of the International Association of Geodesy. The concept of covering the entire United States with a network of CORS to enhance the NSRS was first postulated by Strange !1994". Soon after, Strange and Weston !1995" published a preliminary description of the CORS system. Around this same time, several other federal agencies were also starting to establish networks of continuously operating GPS base stations, but for different reasons. The U.S. Coast Guard !USCG" wished to supplement its LORAN radionavigation service by offering the differential GPS !DGPS" service to support safe marine navigation in U.S. coastal waters. Similarly, the U.S. Army Corps of Engineers !USACE" sought a cost efficient navigation system to support their inland waterway operations !dredging, hydrographic surveys, etc.". They collaborated with the USCG to extend the DGPS service inland along several of the major rivers. Finally, the Federal Aviation Administration !FAA" wanted to use some type of CORS to support safe air navigation. The FAA developed their Wide Area Augmentation System !WAAS". Other federal agencies like NASA’s Jet Propulsion Laboratory !JPL" and the U.S. Geological Survey were already heavily invested in using CORS sites to determine satellite orbits and study crustal motion. Because of the similarities between these projects, the U.S. General Accounting Office directed these agencies to work together and to coordinate activities and equipment procurements to reduce the expense to the federal government and the U.S. taxpayer. NGS found itself in an advisory role helping to define the GPS equipment specifications needed to support the missions of all these agencies !Spofford and Weston 1998". Since the late 1980s, data from both CIGNET and JPL sites were used to support global GPS orbit computations !Schutz et al. 1990". In 1994, NGS officially began building the CORS network by installing a GPS receiver on the campus of the National Institutes of Standards and Technology, formerly called the National Bureau of Standards, in Gaithersburg, Md. Six months later, NGS installed a GPS receiver near Boulder, Colo. and with time incorporated into the CORS network a number of continuously operating GPS fiducial stations that originally were part of CIGNET. Data from all these sites were made available via the Internet and, progressively, NGS added selected U.S. permanent GPS base stations to the CORS network. The USCG and USACE began installing their DGPS sites and FAA their WAAS sites in 1995. NGS worked with these agencies to incorporate both the DGPS and WAAS sites into the CORS network. The initial phase of the USCG network was largely completed by January 1996, although more sites have since been added. Other federal, state, and locally sponsored continuously operating receivers were identified and gradually included into the CORS network from 1995 onwards. By 1995 NGS obtained access to more than 50 geodetic quality GPS receivers, most of them deployed by USCG and other participating agencies without the need by NGS to install, maintain, or operate any of the sites. The Texas Department of Transportation was the first state agency to join the CORS system with the inclusion of their ten-station Regional Reference Point network that provided significant coverage in Texas. By 1996 the number of CORS sites had increased to 85. By making contact with interested agencies and arranging to exchange data, NGS expanded the network to 108 sites by December 1997. The 200-site milestone was surpassed in 2000, and since then the CORS network has grown to its current size of approximately 1,350 sites, and it continues to grow in importance as the primary way for the geodetic-surveying community