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GPS APPLICATIONS AND REQUIREMENTS 43 Improved integrity, availability, and resistance to RF interference are as important to many land transportation GPS users as defeating the accuracy degradation caused by SA. GPS-based automobile navigation systems, which require accuracies in the 5 to 20-meter range, would no longer require DGPS if SA were eliminated and further improvements were made to the basic GPS as suggested in Chapter 3. The elimination of SA would also improve the performance of those DGPS systems required for higher-accuracy applications, such as collision avoidance, that are important to the future Intelligent Transportation System. MAPPING, GEODESY, AND SURVEYING APPLICATIONS Currently, the fields of mapping, surveying, and geodesy are being transformed by a number of new and innovative technologies, including Geographic Information Systems (GIS), high-resolution remote sensing, and GPS. Of these, GPS has had the most important and immediate impact because of both cost savings and accuracy improvements over previous positioning technologies and techniques. The single most powerful feature related to GPS, which is not true of traditional mapping and surveying techniques, is that its use does not require a line of sight between adjacent surveyed points. This factor is paramount in understanding the impact that GPS has had on the surveying and mapping communities. Current and Future Applications and Requirements GPS has been used by the surveying and mapping community since the late 1970s when only a few hours of satellite coverage were available. It was immediately clear that centimeter-level accuracy was obtainable over very long baselines (hundreds of kilometers). In the early 1980s, users of GPS faced several problems: the cost of GPS receivers; poor satellite coverage, which resulted in long lengths of time at each survey location; and poor user-equipment interfaces. Today, instantaneous measurements with centimeter accuracy over tens of kilometers and with one part in 108 accuracy over nearly any distance greater than 10 kilometers can be made. The cost of "surveying-level" receivers in 1994 ranged from $10,000 to $25,000, and these costs are falling rapidly. Practitioners are developing numerous new applications in surveying, such as the use of GPS in a kinematic mode to determine the elevation of terrain prior to grading it for a storm water basin.34 Traditional land surveying is increasingly being accomplished using GPS because of a continuous reduction in receiver costs, combined with an increase in user friendliness. This 34 "Kinematic" GPS surveying is accomplished using a reference receiver and one or more moving remote receivers. The carrier-phase measurements observed by the remote receivers and the static receiver are used in an interferometric mode to allow the positions of the remote receivers to be determined to the centimeter level in real time. More information on carrier- phase (interferometric) GPS techniques can be found in Appendix C.
GPS APPLICATIONS AND REQUIREMENTS 44 trend towards the use of GPS has enhanced the volume of survey receiver sales because land surveyors outnumber geodesists (control surveyors) by at least one order of magnitude.35 This usage has also increased the accuracy and accuracy requirements of surveying in general. GPS is also increasingly being used as the core technology for integrated mapping systems. These systems are usually mobile (e.g., a van, train, airplane or any other vehicle) and contain a combination of sensors. These sensors include vision or imaging systems, laser ranging and profiling systems, ground penetrating radars, and other navigation sensors such as inertial navigation units. GPS provides positioning data when satellites are visible, and other sensors provide the spatial location data required for map making. The inertial systems, and sometimes the vision systems, are used to interpolate between GPS positions through periods when GPS satellites are lost from the vehicle's field of view. These mapping systems provide the surveying and mapping community with powerful new ways of acquiring accurate and current digital data. In general, the availability of higher GPS accuracy has influenced various mapping and surveying requirements for three reasons: (1) people want the latest and the best; (2) past requirements were in some cases dictated by the cost of acquisition; and (3) if higher accuracy can be obtained, multiple purposes can be satisfied. As an example of requirements changing as a function of new capability, consider a problem of facilities management which deals with the inventory of transportation features such as the location and attributes (type, condition, and so forth) of a guardrail along a highway. Previously, the location was "required" by transportation departments to be accurate to Â± 6 meters (20 feet), which is generally the best that is possible from scaling or plotting on a 1/24,000 USGS quadrangle. Using differential techniques a GPS position can easily be obtained in a real-time, dynamic environment to within Â± 1.5 meters (5 feet). Users now realize that if accuracies of Â± 0.3 meters (1 foot) can be obtained (and they can), the length of the guardrail, in addition to its location, can be obtained so that if the guardrail needs to be upgraded or replaced, an accurate estimate of the cost is available. This kind of analysis is growing rapidly as GPS becomes understood and applied to various problems. Clearly, concepts of this kind are widespread in GIS applications in natural resource planning, environmental problems, civil infrastructure enhancements, an so on. Analogous examples can be given for surveying and geodesy. Accuracy requirements for surveying applications are generally satisfied at this time. The quest for better and better accuracy will continue, but any reasonable distance can currently be measured, with significant care, to one part in 108. In each of the categories in Table 2-7, the most stringent accuracy requirements are adopted because of the potential for multipurpose applications. 35 Land surveying usually ignores the curvature of the Earth (except in leveling) and assumes that the Earth's surface is a plane. Control surveying does not make this assumption and is generally performed with an accuracy an order of magnitude better than land surveying.