RTCA DO-246

Description / Abstract: INTRODUCTION

This Interface Control Document (ICD) defines the Signal-in-Space for the Global Navigation Satellite System (GNSS) based Local Area Augmentation System (LAAS). The LAAS has global application as a Ground-Based Augmentation System (GBAS) to the GNSS.

The content of this ICD that supports Category I Precision Approach and the differential positioning service is intended to be consistent with standards developed by the International Civil Aviation Organization (ICAO) and The European Organization for Civil Aviation Equipment (EUROCAE).

Within this document, the terms “LAAS” and “GBAS” are not synonymous. "LAAS” refers to the system being proposed by the RTCA to meet the operational objectives for the United States (US) National Air Space (NAS) as documented in RTCA/DO-245(). This system may also be suitable for implementation in other areas. “GBAS” refers to any system compliant with the ICAO standards (through and including Amendment 91 to ICAO Annex 10).

LAAS may implement the GBAS approach service and/or the positioning service based on GPS and, optionally, visible SBAS/WAAS geostationary satellites. The GBAS approach service has an optional broadcast of ephemeris error bound parameters. LAAS requires the broadcast of the ephemeris bound parameters. LAAS requires the broadcasting of additional information when supporting GBAS Approach Service Type (GAST) D (see Section 1.3.2).

Additional broadcast features have been included in this ICD for LAAS to support operations that have been identified as beneficial by some operators but they have not yet been harmonized or standardized internationally.

• Format definitions for the optional broadcast of a subset of RTCA/DO-236()-compatible terminal area path (TAP) data to provide additional support for terminal area operations have been included. This initial TAP broadcast development was intended to be compatible with the GBAS CAT I approach and positioning services and CAT II/III services, but further development may be needed.

• RTCA has developed requirements to support multiple Approach Service Types intended to support precision approach operations in all-weather minimums. Consequently, there are now two basic classes of ground stations and airborne user equipment: equipment that is single Approach Service Type capable and equipment that is multiple Approach Service Type capable.

The requirements and the corresponding ICD modifications necessary to support TAP have not been harmonized internationally. This version of the ICD includes no new changes to TAP related signal format definitions.

This document should be useful to equipment designers, installers, manufacturers, service providers, and LAAS users.

Figure 1-1 shows the three required components of GBAS including the GNSS satellite subsystem, the airborne subsystem, and the GBAS ground subsystem. Figure 1-2 shows similar subsystem components for LAAS. The GNSS satellite subsystem satellite constellation for LAAS differs from GBAS in that it only includes GPS and SBAS satellites with provisions to add other core satellite constellations in the future. In addition to these three subsystems, additional components are required to maintain the operations of GBAS or LAAS (e.g., the GPS control segment); however, these additional GNSS components are not considered to be part of the LAAS for the purpose of this ICD.

The GNSS/GBAS Signal-in-Space is composed of three signals:

1. the navigation signal transmitted from the GNSS satellites to the ground subsystem,

2. the navigation signal transmitted from the GNSS satellites to the airborne subsystem, and

3. the VHF Data Broadcast (VDB) transmitted from the GBAS ground subsystem to the airborne subsystem.