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GPS APPLICATIONS AND REQUIREMENTS 49 Aeromagnetic surveys have been used for half a century with great effect, but airborne gravity and topographic mapping depend on the ability to determine the aircraft's motion and position.37 Positioning to 100 meters horizontally and 3 meters vertically is required to provide useful measurements of gravity and terrain. GPS and DGPS are ideal for such positioning and a combination of GPS and an inertial navigation system to provide the acceleration of the aircraft, could enable studies of dynamic changes in topography and gravity, such as the expansion of a volcanic dome caused by the injection of magma. Using GPS and radar altimetry to obtain precise gravity anomaly maps, the regional prospecting for ore bodies, salt domes (petroleum reservoirs), or large anticlines (big domes that contain petroleum) can also be accomplished quickly and economically. Accuracy Requirements In general, the Earth science applications described above require much better positioning accuracy than was ever anticipated or intended from GPS, as Table 2-8 clearly illustrates. However, any static GPS reference station equipped with a dual-frequency geodetic receiver can currently be positioned "absolutely" at the centimeter level with respect to the international terrestrial reference frame with less than 24-hours of data. Relative positions between stations at regional scales can be determined at the few-millimeter level with very short observation times. This capability is due to major improvements in GPS software packages and the availability of very precise satellite ephemerides (10-centimeter accuracy) determined by the International GPS Service for Geodynamics (IGS). The ephemeris information available from the IGS can also be used for post-processed dynamic positioning applications. Moving platforms up to several hundred kilometers away from a fixed DGPS base station can achieve 10-centimeter to 20-centimeter positioning accuracy using corrections based on IGS ephemeris data. Satellite clock information distributed by the IGS also is helpful for mitigating SA effects in post-processing of position, particularly in airborne and oceanographic applications. 37 Airborne Geophysics is the subject of a recent NRC report titled: Airborne Geophysics and Precise Positioning: Scientific Issues and Future Directions, Board on Earth Sciences and Resources, National Research Council (Washington, D.C.: National Academy Press, 1995).
