AN INTRODUCTION TO THE GLOBAL POSITIONING SYSTEM AND SOME GEOLOGICAL APPLICATIONS

Receivers equipped to measure dual frequency carrier phase signals from satellites of the Global Positioning System (GPS) have been capable, under special conditions, of determining relative horizontal positions among stations separated by one to a few hundred kilometers with a precision of one to several millimeters since the early 1980s. The major obstacles to making this capability routine, extending it to all parts of the globe, and extending it to longer station separations, have been equipment cost, limitations in the GPS satellite constellation, arduous data analysis, uncertainties in satellite orbits, uncertainties in propagation delays associated with variable tropospheric water vapor, and difficulties in resolving carrier phase cycle ambiguities. Recent improvements have occurred in all these areas. The increasing ease and reduced cost of GPS data acquisition and analysis are having a significant impact on studies of near-fault crustal deformation and earthquake processes, until recently the province of conventional terrestrial geodetic techniques. The enhanced satellite constellation, improved models, and establishment of global tracking networks have extended several millimeters horizontal positioning capability to station separations of 1000 km or more in virtually all parts of the world. This enables study of new classes of tectonic problems that previously were difficult to attack with any geodetic technique. Examples include a complete kinematic description of ongoing crustal deformation in broad, complex continental plate boundary zones, and measurement of relative plate motion at convergent boundaries where global models may be poorly constrained.