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GPS APPLICATIONS AND REQUIREMENTS 48 with useful accuracy.36 Other oceanographic positioning applications, such as the location of objects on the ocean floor, which is essential for drilling and sampling activities, require real-time accuracy of about 10 meters. Geodynamics In geodynamics, GPS is used to study relative motions on the surface of the Earth. The tectonic plates of the Earth's outer layers move relative to each other at rates within the range of 1 centimeter per year to 20 centimeters per year. Many earthquakes occur along the plate boundaries, a recent example being the earthquake in Northridge, California, which occurred on January 17, 1994. A few permanent GPS reference stations provided important data for the early determination of the Northridge earthquake mechanism, which had a displacement on the order of 1 meter. Arrays of permanent GPS stations, coupled with a few interferometric strain meters, can be used to study crustal deformation in the time intervals between earthquakes. This information could be used to estimate the varying amounts of seismic risk in, for example, different parts of the Los Angeles area. The risk assessment could be used to determine appropriate local variations in building codes, freeway and subway construction, and other public projects. In a very speculative vein, if GPS arrays and associated strain meters reveal premonitory or precursory signals for earthquakes, and if the signals are detected early enough to provide meaningful warnings to a region's population and public authorities, then it would become important to measure these signals in as near real time as possible, that is, with minimal post-processing. Only time will tell whether GPS arrays will become useful in this very speculative vein. If not, the improved study and understanding of the deformation of the Earth's crust and of the rupture process of earthquakes will still provide ample reason to establish and operate permanent geodetic GPS arrays. Airborne Geophysics Many of the measurement tools used historically by Earth scientists for regional studies are not sufficiently accurate to model physical processes and improve the understanding of natural hazards and the distribution of nonrenewable resources. Physical barriers, such as inaccessibility by land due to hazardous terrain, and limited resources which prevent the surveying of large areas by conventional means, pose other difficulties. Collecting data remotely from satellites or aircraft can overcome some of the sampling problems. Satellite missions, however, require long lead times between concept and realization, making airborne platforms an attractive alternative for regional Earth studies. 36 Tomography is the use of acoustic travel time to infer changes in acoustic wave speed due to changes in sea temperature and composition. A tomographic lagrangian drifter is a neutrally buoyant buoy equipped to record the arrival of acoustic pulses for use in tomography studies.
