precise GPS orbital data within 24 hours of the last observation, and the inclusion of precipitable water vapor measurement capability at many reference stations.

Additional papers related to the IGS network described the flow and archiving of data and the distribution of data products that can be accessed via the Internet. Every day, data flows automatically from each reference station in the IGS network through global data centers to the service's data analysis centers. The data analysis centers produce IGS data products, which include daily precise GPS satellite orbits, satellite clock corrections, earth rotation parameters, and reference station positions. Public access to these products through the World Wide Web is enabled by the Central Bureau Information System. Although the papers discussed so far have focused on networks that are used for either navigation or geoscience applications, most GPS reference station networks have the potential to provide both real-time differential corrections and post-processed data to a wide variety of users. Within the United States, the NOAA-National Geodetic Survey's (NGS) Continuously Operating Reference System (CORS) program has been established to make this dual use possible. The CORS network, described in a paper presented by William Strange, integrates and collects data from reference stations operated by NASA, NOAA, the FAA, the USCG, the Army Corps of Engineers, and in some cases, even the IGS. The post-processed data generated from these different sources is integrated at the CORS Central Data Facility, and the resulting data products are made available to users on CD ROM and via the Internet.

WORKING GROUP DISCUSSIONS

The working group on networks, data sources, and static positioning applications brought key network operators and users together to discuss options for improving coordination and shared use of the nation's GPS reference station infrastructure. The group noted that the growth rate of GPS networks will require closer cooperation in the United States, similar to the cooperation at the international level that exists in the IGS. Frequently, a group of researchers or a government agency develops plans to install a permanent GPS reference station in a given location, not knowing that a station is already operating a short distance away that could meet their requirements with only minor modifications

Although some scientific researchers may require autonomous control of a GPS reference station and its ancillary equipment, many users only need to know how to obtain data in a timely manner, and if necessary, how to request a configuration change, such as a higher sampling rate, from the operators of existing GPS networks. However, this requires sufficient knowledge of network operations on the part of users to prevent unreasonable requests from interfering with the primary function of a network or reference station. It also requires better dissemination of information on the part of network operators and controlling agencies. In the opinion of the working group, the creation of a coordinated “catalog” of GPS networks and their technical characteristics, as well as instructions for accessing related data and information systems, may be a useful way to disseminate information to potential users.

Table 2-1, which includes information on each of the GPS networks represented by the working group, is an example of the type of information that could be included in a comprehensive catalogue. Posting a catalog on the Internet, along with links to each network's controlling organization, could also allow users in the scientific community to send feedback, suggestions, and appropriate requests directly to network operators.



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