LAAS
GPS/LAAS is currently under research and development by the Federal Aviation Administration (FAA), which will serve as the next generation navigation aid for aircraft precision approach and landing with the objective to replace current Instrument Landing System (ILS). There are two services provided by LAAS: an approach service and a differentially-corrected positioning service.
 The approach service provides vertical and lateral deviation guidance with respect to a defined final approach segment, while the positioning service provides horizontal position, velocity and time information to support area navigation (RNAV) operations in the airport terminal area. The LAAS system is composed of three primary subsystems: a) satellite subsystem, which produces ranging signals; b) ground subsystem, which provides a VHF Data Broadcast (VDB) containing differential corrections and other pertinent information; c) airborne subsystem, which consists of aircraft equipment used to receive and process the LAAS/GPS signals in order to compute and to output position solutions, deviations relative to a desired reference path, and appropriate annunciation of the system status. All associated LAAS system requirements are specified in the Minimum Aviation System Performance Standards for the Local Area Augmentation System (MASPS) [RTCA/DO245A].
LAAS system design is driven by the desire to replace the current ILS. The navigation performance requirements for LAAS are, therefore, derived to meet the three-tiered structure (Category I, II, and III) of ILS Approach Services and ICAO Ground-Based Augmentation System (GBAS) standards. In order to best specify LAAS ground and airborne subsystem performance, LAAS approach service performance is classified in terms of defined levels of service called GBAS service levels. A GBAS Service Level (GSL) defines a specific level of required Accuracy, Integrity, and Continuity. In short, LAAS GSL C is equivalent to Category I (CAT I) precision approach service, GSL F supports Category IIIb (CAT IIIb), and GSL D is the standard for aircraft augmented with navigation equipment in addition to GPS airborne receivers.
Because the design of the LAAS system is meant to replace the current ILS for precision approach, LAAS system integrity basically inherits system requirements from ILS. The total system level integrity is allocated into the airborne subsystem and SIS integrity requirements. While the LAAS airborne subsystem integrity remains the same as ILS airborne subsystem, the LAAS SIS integrity is adopted from the requirements and recommendations for ILS in ICAO Annex 10 with the consideration that error sources for LAAS SIS are different from ILS.
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