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GPS APPLICATIONS AND REQUIREMENTS 41 b. Precision farming requirements were derived from information provided by the U.S. Department of Agriculture and the Deere & Company, Precision Farming Group. c. The values listed for these applications are not firmly established requirements. They are estimated useful values determined by the committee. Challenges to Full GPS Utilization The major challenges for most uses of GPS in the land transportation sector include the need to meet the desires and requirements of an ever-increasing number of creative applications and the need to do so with technologically integrated equipment that is affordable, reliable, and reasonably durable. Each of these challenges deserves further explanation. Accuracy Versus Other Requirements For many civil land transportation purposes, the issue of accuracy of location will be dominated by right-of- way and construction surveying needs, not by vehicle, cargo, or personnel position-location or navigation issues. Where positioning and navigation accuracy is important, it is often related to requirements such as availability and integrity. For example, it is important in the cities for both people and freight movements, and dispatch control, to have good accuracy resolution not compromised by loss of signal lock due to tall buildings or interference from other radio sources. Similarly, the problem of resolution to a few meters, essential in mountainous terrain for numerous applications such as avalanche search and rescue, and forest fire control, is made more difficult by terrain and foliage, which can mask GPS signals. For trucking and shipping, where vehicle and cargo location, and fleet dispatch and management are important, it seems clear that availability and coverage may be a greater challenge than greater accuracy. For example, a truck traveling from Boston, Massachussets to Seattle, Washington will not need to be located to within 25 meters of its actual position throughout the entire trip; however, it will need to be located. Satellite or terrestrial-based DGPS augmentation techniques must be adopted that provide better availability and coverage to the entire nation, not just the densely populated areas. Users who do perceive a need for higher navigation and position-location accuracy than is available from the GPS SPS can generally meet this requirement by utilizing one of several commercially provided DGPS services. Systems that provide the differential correction via FM subcarrier seem especially suited to land transportation users, although their networks do not yet cover all of the nation. Additional enhancements to GPS receivers, such as the integration of small solid-state inertial gyroscopes and accelerometers, can also improve accuracy and other performance characteristics. Those users with accuracy requirements in the 20-meter range, however, which includes the rapidly growing automobile navigation market, would not need to augment or enhance GPS with DGPS or inertial gyroscopes if SA were eliminated.
