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GPS APPLICATIONS AND REQUIREMENTS 46 to multipath. Improving the GPS orbit information (ephemeris) available to SPS users also would have a significant impact on the surveying and mapping community as longer and longer baselines could be measured in real-time with centimeter accuracy. Additional challenges to the use of GPS in the mapping, surveying, and GIS community deal with receiver cost, service and maintenance, user friendliness, and interfaces with other software and hardware. For example, at the Center for Mapping at the Ohio State University, researchers have developed a real-time positioning capability accurate to 1.5 centimeters with as few as five satellites in view. The system is interfaced with software developed by the construction service industry that displays "cuts and fills" on a screen so that an operator of earth-moving equipment can grade earth in a prescribed fashion. One problem with the overall system is the necessity to use two GPS receivers that currently cost $25,000 each. The competitive technology costs about $40,000. If the price of the GPS receivers falls to $10,000 each, however, the GPS technology will dominate the market. This is especially true because GPS offers coordinates in three dimensions without line of sight requirements. The rival technology (laser plane) is one dimensional and requires line of sight. Hence, the challenge to using GPS for earth moving is focused on software integration and the costs of receivers. This same scenario is applicable to many other potential GPS applications envisioned at the moment. Findings Greater geodetic accuracy for mapping and surveying will be pursued in part because of the challenge of obtaining it. A few applications, such as determining the position of the blades of earth- moving equipment in real time will demand increased accuracy. Most applications, however, will be enhanced by cost savings from quicker acquisition of the same data. The elimination of SA and A-S, and the use of dual-frequency user equipment can improve data acquisition time. For surveying, the weakest link in the utilization of GPS, aside from SA and A-S, is the precision of the GPS satellite orbits. While improving ephemerides will not significantly enhance positioning over short baselines, they will have a noticeable impact over baselines greater than 50 kilometers. GPS EARTH SCIENCE APPLICATIONS One's ability to measure the Earth, including its atmosphere and its ocean surfaces, has been greatly enhanced by GPS. New departures in scientific endeavor and commercial enterprise have begun, and initial results are very promising.
