Dunbar, N.W., New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, Socorro, NM, 87801 (

Global Positioning System, or GPS, is a satellite-based technology that allows location of a precise position on, or above, the earth's surface by triangulation of signals from 3 or more satellites. The satellites, which orbit at approximately 20,000 km above the earth's surface, broadcast signals that include information about the satellite's position, and the time at which the signal was broadcast. Interpretation of these signals allows the precise position of the receiver on, or above, the earth's surface to be determined. GPS receivers range from simple, inexpensive, hand-held models that allow determination of positions to within ± 30 m in the horizontal direction to high-end multi-receiver systems that provide positions accurate enough to track continental motion.
There are many applications of GPS technology to geosciences. Among the most basic is recording and storing sample locations as latitude and longitude, or as UTM coordinates, making translation of sample locations into an ARCINFO format more seamless. Many GPS receivers can download stored data directly to a personal computer. An application of relatively high-end GPS analysis to geology is precise mapping of locations of geological contacts or features in the field. This can be done to centimeter-level precision, if necessary, by using two receivers, one as a base station which remains at a fixed location, and one as a mobile receiver that is carried around and used to map the features of interest. By processing the base and mobile data sets together, a high-precision map can be obtained. The co-processing of the two data sets, called "differential GPS" analysis, allows error-producing noise in the satellite-transmitted data to be corrected, allowing high-precision positions (±1 cm) to be obtained. Two examples of this type of work include mapping contacts between subaerial and subglacial lava flows in Marie Byrd Land, Antarctica, and mapping the distribution of volcanic ash layers in a Antarctic ice fields. These types of maps can be used to precisely determine the paleo-positions of the West Antarctic ice sheet, and also to study and monitor local ice flow processes. Both of these data sets were collected and processed in remote field locations, using solar power to recharge the GPS batteries for data collection, and the PC batteries for data processing. As discussed in Nielsen et al. (1999), GPS has the potential to become an integral part of geologic mapping and other field work.