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(IALA) have deployed a marine beacon differential system internationally, known in the United States as National Differential GPS (NDGPS). The ArmyCorps of Engineers is currently deploying additional beacons that are compatible with the U.S. Coast Guard differential system and cover the entire continental United States (see Fig. 5). The FederalAviation Administration (FAA) is currently deploying the Wide Area Augmentation System (WAAS). WAAS is intended to provide enroute navigation and nonprecision approaches for aviation users (see Fig.4). The FAA is also developing a Local Area Augmentation System (LAAS) for Category I, II, and III precision landing capability at airports (see Fig. 11). This will require local ground monitoringstations to ensure the integrity of the system in addition to the nominal reference receivers. The U.S. Department of Defense is currently developing a new Military Landing System (MLS) to operate likethe LAAS system but will be used on aircraft carriers and at forward bases. The system, called the Joint Precision Approach and Landing System (JPALS), has already demonstrated fully autonomous carrierlanding using a specially equipped Navy F/A-18 (19).

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Figure 11. The LAAS system under current development by the FAA will provide precision approach capability using GPS.Due to tbe exacting requirements of Category II and III landings, tbe LAAS requires many cross-checks of tbe GPS system to ensure integrity. If one of tbese cross-cbecks fails, tbe time to alarm ofthe LAAS is specified at less tban 6 seconds. Tbis figure is available in full color at bttp://www. mrw. interscience.wiley.com/ esst.

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Tbere aremany additional private and international systems under development or deployed. Various private companies sell tbeir own proprietary carrier pbase differential GPS systems for use in sucb diverse...