DESCRIPTION:
Global Positioning System (GPS) Measurements

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Global Positioning System (GPS) receiver at Cotopaxi Volcano, Ecuador. GPS uses data transmitted by orbiting satellites to locate points on the ground. The USGS has made baseline GPS measurements at several volcanoes in the United States and in Latin America. In the event of an awakening of one of these volcanoes, GPS receivers would be set up at these points again to determine whether or not measurable deformation had occurred and to monitor for precursory deformation that might herald an eruption.
-- USGS Photo by J. W. Ewert

In the early 1970's, work began on the NAVSTAR (NAVigation Satellite Time and Ranging) Global Positioning System (GPS) for the Department of Defense. ... The NAVSTAR satellite was launched in 1978. ... Satellites transmit signals which are observed by ground based receivers. Observed signal travel times from satellite to receivers are used to compute ranges from satellite to receiver. Ranges from four satellites are required to determine a position on the earth relative to a reference ellipsoid. In it's simplest civilian application, single point positioning, with a handheld receiver, accuracy in position is approximately 75 meters. For geodetic applications, with many hours of observation utilizing two or more sophisticated GPS receivers and rigorous data processing, accuracies of a few centimeters over baselines of 100 kilometers are routinely obtained. The use of satellite technology eliminates the need for survey points to be intervisible.

The USGS Volcano Hazards Program started using GPS receivers as a volcano monitoring tool on silicic volcanoes in 1989. A program of making baseline measurements at all volcanoes in the Western United States and at select active volcanoes in Alaska was started at that time. In 1991, the Cascades Volcano Observatory, in cooperation with the Alaska Volcano Observatory, installed the program's first permanent GPS stations on Augustine Volcano in Cook Inlet, Alaska. GPS observations from these stations are routinely retrieved via modem to provide a record of three-dimensional ground movement.

The Global Positioning System (GPS) uses information broadcast by orbiting satellites to accurately monitor changes in the horizontal and vertical position of survey points on volcanoes -- data on deformation needed to forecast future eruptions. The radio signals transmitted by GPS satellites include time, ranging data, and information on the predicted position of the satellites in space. Deformation studies on volcanoes often involve a procedure called relative positioning. To carry out this procedure, one receiver is situated at a stable control station and the other is set up at a point where the change in relative position is to be determined. Although a detailed description of how the system works is beyond the scope of this article, the central core of GPS operations is the simultaneous determination of distances between ground station and set of four or more satellites. This is accomplished by distance ranging. The ground-station receiver simultaneously records time-coded ranging signals from the satellites. The ground station also continuously generates a time-tagged replica of the ranging signal sent by the satellites. The time codes are embedded in the signals in such a way that the source-to-receiver transit time can be recovered from the data stream. Multiplication by the speed of light gives the corresponding distances.

The Global Positioning System -- Dvorak, 1992

In addition to recording seismic data, the portable volcano observatory also includes instrumentation that is used to measure horizontal and vertical movements as well as tilting of a volcano's surface in response to subsurface movement of magma. Measurements of horizontal deformation are made with electronic distance meters (EDMs) and global positioning system (GPS) receivers. Vertical deformation and ground tilt are measured with levels and electronic total stations (theodolite/EDM combinations). Telemetered tiltmeters provide tilt measurements in near-real time.

VDAP's equipment cache includes EDMs, theodolites, and reflectors needed to survey key points (benchmarks) on a volcano for detection of surface deformation. This traditional surveying method requires a clear line of sight between the benchmarks on the volcano and the instrument site at its base, which is often difficult to achieve. Increasingly, GPS receivers are being used to measure horizontal deformation because this technique does not require a clear line of sight between benchmarks. Compared to conventional surveying, the ease with which deformation data can be collected with GPS makes this a very attractive method.

A typical GPS survey deploys several receivers at benchmarks on the volcano to collect satellite data simultaneously at several points. The data are then downloaded and processed on a PC. This strategy works well in non-hazardous situations to gather baseline location data for comparing to future measurements. However, a GPS campaign requires someone to repeatedly deploy and retrieve the receivers. Such repeated entry into a zone of high hazard on a volcano that is threatening to erupt may represent an unacceptable risk. For this reason scientists at CVO designed a telemetered GPS monitoring system, using unattended stations, that is being tested on Augustine Volcano in Alaska. As we continue to develop the system, we expect it to become part of the portable volcano observatory.

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