Global Positioning System Studies at NBMG

Fall 1990

Engineering geology and neotectonic studies at the Nevada Bureau of Mines and Geology (NBMG) received a significant boost with the recent acquisition of two Trimble 4000ST dual-frequency Global Positioning System (GPS) receivers. These instruments use satellites to make precise geodetic measurements, and will be used for studies of tectonic deformation and ground subsidence.

The GPS system was developed by the U.S. Department of Defense for tracking troop movements and missile guidance. It provides highly accurate positions using a constellation of satellites launched specifically for this purpose. Currently, 13 satellites allow use of the system for about 7 hours per day, but within a couple of years, the full constellation of 24 GPS satellites will provide 24-hour per day, worldwide coverage.

GPS can support an infinite number users and has numerous potential applications because it is a passive system (i.e., it is based on radio signals that are received from satellites; the ground unit do not need to send a radio signal) -- Potential civilian uses include surveying mapping, photogrammetric control, geographical and land information systems (GIS/LIS) registration, geodetics, marine navigation, air-traffic control, rail-line control, emergency response, delivery services, backcountry navigation, and so on. Combined with digitized road maps it will even provide computer displays current positions, routes, and destination for passenger vehicles. Getting lost in unfamiliar areas will be a thing of the past. The system's uses, all based on knowing where someone or something located, are so varied that it is sometime referred to as a new utility, similar to electrical or phone service.

The GPS system relies on precise time provided by atomic clocks onboard the satellites. If the position of a satellite, the time a signal originates from the satellite, and the time the signal is received are known, the distance between the satellite and a GPS receiver can be determined. Distances measured to each of a minimum of four satellites provide a point intersection that defines the receive location. This is quite similar to the manner in which earthquakes are located.

At its best, a single GPS receiver provides positioning accuracy of about 15 to 25 meters. In order to limit use of the system by hostile entities, the U.S. Department of Defense has recently implemented "selective availability" of the GPS signal. Through introduction of clock errors and incorrect satellite positions, selective availability reduces the positioning accuracy of GPS to about 100 meters. For more precise applications (surveying and geodetics, for example), the system is used in a somewhat different manner, commonly referred to as "differential GPS." Two GPS receivers operating simultaneously, and close to one another, are subject to the same sources of error (atmospheric effects, clock errors, and errors in satellite position). When the data collected simultaneously by two receivers are compared through computer post-processing, these errors cancel out and the relative difference of the two receivers can typically be determined to better than one centimeter.

To date, the GPS equipment at NBMG has been used primarily for three projects. The principal project for which this equipment was acquired is the implementation of a surveyed network in Las Vegas Valley for measurement and monitoring of subsidence due to groundwater withdrawal. Previously run level lines have shown that parts of Las Vegas Valley have subsided 5 feet or more since 1950. The GPS work is one part of a large study undertaken by NBMG to evaluate this subsidence. Other aspects of the subsidence project include computer modelling, mapping of surface fissuring, soils studies, and integration of various data through the use of GIS. Annual surveying of the GPS network will provide broader survey control and allow better delineation of areal extent and rates of subsidence than was afforded by prior leveling work.

In February and March 1990, NBMG participated in an extensive project aimed at measuring tectonic deformation along the San Andreas fault in southern California. This cooperative project between Riverside County, California and several academic institutions involved the use of 23 GPS receivers operating over a two-week period, and provided an extensive database over a broad region. The area surveyed during this project extends across all of Riverside County and south to the Gulf of California. By occupying each site for several hours and using sophisticated data postprocessing techniques, studies such as this are capable of obtaining accuracy of about 3 to 4 mm.

A third project recently undertaken, a cooperative study with the University of Texas at Austin, will provide a preliminary network of points for determining tectonic deformation across the Basin and Range province in Nevada and Utah. During this project, the Texas group occupied very long baseline interferometry (VLBI) sites in Quincy, California, the McDonald Observatory, Texas, and Platteville, Colorado in order to evaluate what level of accuracy can be obtained from GPS over very long baselines (more than 1,000 km). At the same time, NBMG occupied four stations spread across the Basin and Range in order to determine if actively extending areas coincide with the areas of high seismic activity.

Future plans for GPS work at NBMG include surveying of additional points for tectonic deformation studies and for monitoring of subsidence due to geothermal development or other causes.

---Alan Ramelli, Research Geologist