GPS Availability for Aviation Applications : How Good Does it Need to Be?
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2000-09-19
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NTL Classification:NTL-AVIATION-Air Traffic Control
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Abstract:The availability of GPS for airborne applications is driven by the accuracy, integrity, and continuity of the system. The International Civil Aviation Organization (ICAO) Standards and Recommended Practices (SARPS) for GNSS spells out all of these requirements for each phase of flight. Continuity and availability requirements are provided as a range of values instead of one specific number. A range of values is given so there is flexibility for the intended operation, traffic density, complexity of airspace, and availability of alternative navigation aids. Based on the requirements in the SARPS, availability for GNSS aviation applications is specified to range from 99% to 99.999%. For domestic U.S. airspace, the FAA has established a requirement of 99.999%. However, it is very difficult to satisfy this requirement without considerable augmentation. Also, what does this level of availability mean from an operational perspective since the associated outage duration is equivalent to a total of 5 min. of outage time per year? Are availability requirements for GPS being set more stringent than existing requirements for aviation navaids? Individual countries need to determine what is acceptable for their airspace and this paper examines how this analysis is performed. In Australia, for example, general aviation pilots currently are allowed to fly with a single Automatic Direction Finder (ADF). Obviously, GPS can perform much better than this. Other issues that are addressed in the paper include how the differences in performance requirements affect receiver design. If most of the GPS aviation receivers are manufactured in the U.S. under FAA specifications, what impact does this have on other countries who want to adopt FAA certification, but do not have requirements which are as stringent as the U.S.? Also, differences in requirements for high-end air transport vs. general aviation operations are examined. Finally, the transition from supplemental use of GPS under TSO C129 to primary means with TSO C145/146 equipment and the benefits of gaining operational experience vs. satisfying stringent requirements are discussed.
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