| Most, if not all, volcanic eruptions are preceded by surface movements near the volcano. These ground movements are the response of the shallow crust tot he accumulation of magma or the buildup of magma pressure within a subterranean reservoir beneath the volcano. As the magma reservoir expands, the summit and the flanks of the volcano rise and spread apart. Measurements made at many volcanoes show that slow ground movement may precede an eruption by as many as several years. Sudden increases in the rate of ground movement often precede an eruption by a few hours or days. ... 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. -- Dvorak, 1992 |
