DFMC SBAS Approach Concept

Abstract

During the development of the Dual Frequency Multiple Constellation (DFMC) Satellite Based Augmentation System (SBAS) standards and recommended practices (SARPs), the International Civil Aviation Organization (ICAO) Navigation Systems Panel (NSP) discussed potential operational issues associated with approach procedures intended for use in dual-frequency mode. Those operational issues come from the considerations of an airspace with a mixed fleet where some aircraft are equipped with L1 SBAS receivers and other aircraft are equipped with DFMC SBAS receivers. The authors participated in the development of a concept of operation to enable aircraft to properly handle different types of SBAS service to support approach operations. The concept identifies a means to encode the SBAS signal-in-space performance into the Final Approach Segment (FAS) Database (DB). This enables receivers built to the DFMC SBAS standard to select an operating mode compatible with the SBAS Signal in Space (SiS). Additionally, it provides a means to identify FAS DB entries intended for L1 SBAS receivers and to identify procedures that need multiple constellations to provide the desired integrity and continuity. This paper describes the concept of operation. The paper opens with a description of the operational environment; namely differences in SBAS systems and differences in new and legacy avionics capabilities. SBAS systems may augment different signals and may augment different core constellations. SBAS service areas associated with specific performance levels may differ for DFMC SBAS and L1 SBAS services. From these constraints associated to the operational environment and the key drivers for the concept of operation agreed at ICAO level, the paper describes the Approach Performance Designator (APD) coding developed in the DFMC SBAS SARPs providing a means to differentiate the SBAS capability needed for a given precision approach. The APD parameter is coded in the Final Approach Segment data block (FAS DB) and is used to indicate that the ANSP has validated the performance of the indicated SBAS service type for the approach defined by the FAS DB. The APD enables ANSPs responsible for the publication of the SBAS approach procedures to communicate to the aircraft the suitability of each SBAS service types at the destination airport. The paper reviews several use cases of APD coding that identify the SBAS signal capability supporting an approach (APV or CATI performance levels) covered by L1 SBAS and/or DFMC SBAS to illustrate how ANSPs may use the APD coding when publishing SBAS-based procedures. The paper also introduces other ramifications of the proposed approach on operations such as chart publication, use of APD in dispatch or flight planning and in the generation of NOTAMs.