requires strong support for gravity satellite missions and a revitalized U.S. terrestrial (ground and airborne) gravity program. Such a program also would support the multiple scientific and civil applications that call for monitoring changes in the gravity field over regional and global scales.

NATIONAL HIGH-PRECISION, REAL-TIME GNSS/GPS NETWORKS

Chapters 2 and 3 of this report discuss various scientific applications that require high-precision, real-time GNSS/GPS networks. The report also identifies new and future applications of societal importance that call for a rapidly sampled (at least one hertz), real-time GNSS/GPS data stream. These include systems that enable autonomous navigation for land, sea, and air vehicles and robotic equipment; precision tracking of aircraft for laser and radar imaging; monitoring of space weather in the ionosphere; early warning for such natural hazards as earthquakes and tsunamis; improved forecasting of extreme weather events; measurement of ground displacement in landslides; and monitoring of critical structures after a natural disaster to inform emergency response efforts.

The Plate Boundary Observatory (PBO) GPS network, a major component of the NSF-led EarthScope program, could serve as the backbone for a national high-precision, real-time GNSS/GPS network. This 1,100-station network, built with uniform high-quality equipment, standards, and monuments, represents a large capital investment. The potential to transition this infrastructure from research to operations at the completion of the EarthScope project presents a unique opportunity for the nation. With long-term maintenance, densification, and upgrades to facilitate tracking of other navigation satellite constellations, the PBO network would serve the dual purposes of providing both a national backbone for high-precision applications and local reference stations for surveyors and local commercial and governmental service providers.

While there is an overlap between the PBO and other geodetic networks (for example, CORS and state networks), most such networks were not built specifically to support precise geodesy, and the lack of deep anchors tying the GNSS/GPS receiver to the ground in these other systems may introduce uncontrolled movements of the receiver relative to the reference system as a result of shallow deformations or temperature effects (see Agnew, 2007). Any GNSS/GPS network built to scientific standards, however, could be joined with the PBO network. One example is the approximately 200-station GPS network operated by the U.S. Geological Survey to monitor seismic and volcanic hazards in the western United States. Present-generation high-quality GNSS/GPS receivers in this and other networks are capable of high-rate sampling and streaming data over the Internet; consequently, many of these sites either already operate in real-time or could be upgraded to do so.

The strategies for densification of a national high-precision, real-time GNSS/GPS network could be responsive to the needs of specific applications. For example, early warning systems for earthquakes and tsunamis would require a more dense station spacing (approximately 20 kilometers apart) along the west coast of the United States. Weather prediction, on the other hand, might only require 50-kilometer spacing but would require expansion offshore to help predict the strength and tracks of hurricanes. In addition to such applications, a national high-precision, real-time network also would meet the needs of scientists conducting long-term research studies. Shared use of a single network with common transmission of data and data archiving would yield significant cost savings.

Recommendation: The United States should establish and maintain a high-precision GNSS/GPS national network constructed to scientific specifications, capable of streaming high-rate data in real-time. All GNSS/GPS data from this network should be available in real-time without restrictions (and at no cost or a cost not exceeding the marginal cost of distribution), as well as in archived data files.



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