Global Positioning Finds Applications in Geosciences Research

Continuous global positioning satellite tracking networks are being established by a variety of national and international organizations to provide basic infrastructure for space-age navigation, surveying, science, engineering, and atmospheric sensing. The emerging networks present opportunities for a variety of geoscience applications. To make the most of these opportunities, the geoscience community should clarify and communicate its research goals to organizations establishing global positioning networks.

Global Positioning Satellites and Networks

Soon after the space age began, it was recognized that the orbits of satellites provided a valuable reference frame for global navigation systems. Today, the 24-satellite Global Positioning System (GPS) operated by the Department of Defense is the most widely used system, followed by a smaller group of Russian satellites called GLONASS. These systems may be augmented by future commercial low-Earth orbit communication and positioning satellite services.

The reference frames of satellite orbits and the Earth's crust are linked using ground-based tracking networks. The first global GPS tracking network was established more than 15 years ago by the U.S. Department of Defense. This network included five tracking stations and was used to determine orbital positions with an accuracy of about 10 m. More recently, the International Association of Geodesy (IAG) established the International GPS Service for Geodynamics (IGS) comprising more than 50 international organizations, to support geodetic and geophysical GPS applications [e.g., Zumberge et al., 1994]. The IGS network includes more than 50 globally distributed tracking stations that provide high-accuracy GPS orbits (~10 cm), allowing users to place their GPS stations in the International Terrestrial Reference Frame with centimeter accuracy. IGS orbits are available within 2 to 3 weeks via anonymous ftp (igscb.jpl.nasa.gov) or World Wide Web (http://igscb.jpl.nasa.gov).

Regional continuous GPS networks designed mainly to monitor strain for earthquake research and forecasting have operated in Japan and California for several years. A similar network in Sweden, designed to measure postglacial deformation, has operated for more than a year. Similar systems are being established across Europe, Australia, and other regions. In addition, networks designed to improve GPS accuracy are being established for aircraft and coastal navigation, to support surveying applications, for dam and dike monitoring, and for atmospheric measurements.

Applications

Navigation. Increasingly, land, sea, and air vehicles are making increasing use of the capability of GPS to determine point positions with an accuracy of 100 m. Positioning with so-called Differential GPS (DGPS), which uses radio links between mobile and fixed GPS receivers, provides relative positions in real time with an accuracy of 10 cm to 1 m for civil aviation and coastal navigation. An example of the use of DGPS is the coordination of highway and railroad traffic.

Science. High-accuracy GPS positioning has been improved by 2 orders of magnitude during the past decade, and now millimeter accuracies are being routinely achieved. This high accuracy makes GPS useful for tectonic, earthquake, volcano, Earth rotation, sea level, satellite altimetry, glaciology, meteorology, global climate, ionosphere, hydrology and ecology studies.

Engineering, Surveying, and Resource Management. GPS is also being used for surveying, monitoring dams, dikes, landslides and subsidence, positioning civil infrastructure for use in geographic information systems, time transfer for telecommunications systems, automated construction and facilities management, precision farming, forest and resource management, and natural resource exploration.

Weather Forecasting. By exploiting delays in the GPS signals introduced by water vapor and other components of the atmosphere, GPS networks can provide valuable data for assimilation into numerical weather prediction models.

Multidisciplinary Use of GPS Networks

Table 1. Estimated Number of Sites Operating and Proposed in Various Tracking Networks

 
 
_____________________________________________ 
 
 
 
Networks       Current Sites   Proposed Sites 
 
_____________________________________________ 
 
 
 
Regional            70             360 
 
National           250             875 
 
International       75             125 
 
Subtotal           395            1360 
 
                   _____________________ 
 
 
 
Total Current           1755 
 
 and Proposed 
 
_____________________________________________ 
 

Regional networks address civil and scientific applications in localized areas; National networks address aviation and coastal navigation; and International networks contribute to the global tracking objectives. Applications may overlap, and estimation of the number of sites is appproximate.

Other multidisciplinary geoscience applications are also emerging [e.g., UNAVCO Report, 1994]. To succeed in these applications, the geoscience community must ensure that organizations establishing and operating GPS networks understand the value of multipurpose installations and consider the requirements of other users in making equipment, data management and other application decisions. The authors are working with the National Research Council and several agencies to convene a workshop for this purpose. Also contributing to this activity are M. Bevis, Y. Bock, R. Chadwick, J. Davis, R. Neilan, W. Prescott, and W. Strange.

References

Zumberge, J., R. Neilan, G. Beutler, and J. Kouba, The International GPS service for geodynamics - Benefits to users, in Proceedings of Institute of Navigation, 7th Technical Meeting, September 1994.

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