SBAS Interoperability

EGNOS
Title	SBAS Interoperability
Edited by	GMV
Level	Basic
Year of Publication	2011

In addition to EGNOS, there are other Satellite Based Augmentation Systems (SBAS) developed or under development, such as the Wide Area Augmentation System (WAAS) in USA, the multi-functional transport satellite (MTSAT) satellite-based augmentation system (MSAS), in Japan or the GAGAN system in India, KASS in Korea, SBAS ASECNA in Africa, the System for Differential Corrections (SDCM) in Russia or the Australian SBAS test system (SPAN). Although all SBAS are currently defined as regional systems, it is commonly recognized the need to establish adequate co-operation/co-ordination among the different systems, so that their implementation becomes more effective and part of a seamless world-wide navigation system.

To guarantee seamless and worldwide system provision, it is essential that the existing systems do meet common standards and interoperability requirements. SBAS service providers are regularly meeting through the so called interoperability working group (IWG) to conclude on a precise understanding of the term interoperability, and on the identification of the necessary interfaces among SBAS that each conceivable interoperability scenarios may imply. Nowadays, IWG discussions focus on SBAS dual-frequency multi-constellation (DFMC) standards, civil aviation/maritime region applications, minimum operating standards (MOPS), and the Concept of Operations (CONOPS).

The combination of SBAS Interoperability and SBAS expansion concepts should allow providing a true global world-wide navigation seamless service.

SBAS Systems

SBAS Indicative Service Areas. Source: GSA User Technology Report 2020

From all the SBAS systems in the world, some are already operational (WAAS in North America, MSAS/QZSS in Japan, EGNOS in Europe,GAGAN in India), other are under implementation (BDSBAS (formerly SNAS) in China, SDCM in Russia, KASS in South Korea) while others are under feasibility studies, for instance SACCSA, Australian SBAS (SPAN) or SBAS-ASECNA in Africa. As regards the operational SBAS:

• The Wide Area Augmentation System (WAAS) was jointly developed by the United States Department of Transportation (DOT) and the Federal Aviation Administration (FAA), beginning in 1994, to provide performance comparable to category I instrument landing system (ILS) for all aircraft possessing the appropriately certified equipment. On July 10, 2003, the WAAS signal was activated for safety-of-life aviation, covering 95% of the United States, and portions of Alaska. At present, WAAS supports en-route, terminal and approach operations down to a full LPV-200 (CAT-I like Approach Capability) for the CONUS, Mexico and Canada.
• The Multi-functional Satellite Augmentation System (MSAS) is the Japanese SBAS. NEC manufactured and delivered MSAS under contract with the Civil Aviation Bureau, Ministry of Land, Infrastructure, Transport and Tourism. MSAS is operational since 2007 supporting en-route, terminal and non-precision approach operations. MSAS evolutions will take advantage in the short term of the Quasi Zenith Satellite System deployed by Japan to broadcast radio signals from high elevation angle into urban canyons.
• EGNOS is the European satellite-based augmentation service (SBAS) that complements the existing satellite navigation services provided by the US Global Positioning System (GPS). EGNOS provides the first European GNSS services to users. The Safety of Life Service (SoL), which provides the most stringent level of signal-in-space performance to all communities of Safety of Life users over Europe, was officially started on 2 March 2011.EGNOS V3 will provide augmentation to both GPS and Galileo in L1/E1 and L5/E5a signals.
• The GPS Aided Geo Augmented Navigation System (GAGAN) is the SBAS implementation by the Indian government. The project has established 15 Indian reference stations, 3 Indian navigation load uplink stations and 3 Indian mission control centres, in order to navigate in the Indian airspace by an accuracy of 3m. On 30th December 2012, the Directorate General of Civil Aviation (DGCA), India provisionally certified the GAGAN system to RNP0.1 (Required Navigation Performance, 0.1 Nautical Mile) service level. The certification enabled aircraft fitted with SBAS equipment to use GAGAN signal in space for navigation purposes. Testing and validation of these procedures were finished in 2016.

The new civil signals suitable for aviation both in GPS and Galileo, offers great number of dual frequency ranging measurements. Because of that, the Satellite Augmentation Systems can also be updated to exploit these new signals and be available for multi-constellation performances. Such updates offer a variety of improvements over existing single frequency systems. These dual frequency systems are fully robust against ionospheric disturbances. Furthermore, they offer improve resistance against interference as operations can proceed when aircraft lose access to one frequency or the other. However, the largest benefit to a user taking advantage of both frequencies is that their availability can extend much farther away from the reference station network, in addition to the eliminated ionospheric behavior, allow this increase in coverage. An SBAS utilizes a network of precisely surveyed reference receivers, located throughout its coverage region. The information gathered from these reference stations monitors the GNSS satellites and their propagation in real-time. Availability of SBAS service is a function of two quantities: the arrangement of the pseudorange measurements used to determine the user`s position, referred to as geometry; and the quality of each individual measurement. Referred to as confidence bound. Although very small confidence bounds can make up for poor geometries, and strong geometries can overcome large confidence bounds, both values are generally required it be good to obtain high availability.

SBAS Standardisation

There are two sets of International Standards which SBAS’s shall be compliant in order to be used by Civil Aviation Authorities:

• The Standards and Recommended Practices (SARPS) Standard for SBAS systems established and controlled by the International Civil Aviation Organization (ICAO) and which provides Standards regarding the type and content of data which must be generated and transmitted by an SBAS system. In general, the SBAS provider shall broadcast a SBAS Signal in Space (SIS) compliant to this standard in terms of radio-frequency characteristics, and data content and format.
• The Minimum Operational Performance Standard (MOPS) DO229 established and controlled by the US Radio Technical Commission for Aeronautics (RTCA) and which provides standards for SBAS receiver equipment. It must be taken into account that MOPS RTCA 229 is only for GPS and another MOPS standard is being developed for the dual frequency and multi-constellation systems.

For more information, please refer to the article SBAS Standards.

SBAS System Cooperation

SBAS Interoperability refers to the ability of SBAS systems and the services they provide to be used together to provide better capabilities at the user level than those achieved by relying solely on one of the systems. The SBAS interoperability has always been a prerequisite for delivering a global seamless Safety-of-Life service.

Although all SBAS are regional systems, it is commonly recognised the need to establish adequate co-operation/coordination among SBAS providers so that their implementation becomes more effective and part of a seamless world-wide navigation system. SBAS co-operation is currently coordinated through the so-called Interoperability Working Groups (IWG). Although interoperability implies a large variety of complex issues (such as certification, standards, safety, operations,…), EGNOS, WAAS and MSAS SBAS providers among others agreed on the following list of objectives concerning technical interoperability and co-operation among SBAS:

• Objective 1 – Validate SBAS Performance consistency and SARPs compliance.
• Objective 2 – Improve the service level available in the regions outside the nominal SBAS service volumes.
• Objective 3 – Improve individual system performance through SBAS data interchange.
• Objective 4 – Improve SBAS prediction capability through SBAS data interchange.
• Objective 5 – Identify possible future improvements.

SBAS interoperability studies and discussions are currently been focused on two main set of topics:

• Interoperability issues concerning existing SBAS systems, in particular transition issues as observed by users crossing two SBAS service areas (GEO selection mechanism, activation of GEO ranging, etc).
• Interoperability issues for future SBAS evolutions:
  • Evolution of SBAS to become multi-constellation (e.g., GPS and GLONASS or GPS and Galileo).
  • Evolution of SBAS to become multi-frequency (L1, L5, L1/L5).
  • Potential evolution towards a combination of SBAS and RAIM techniques.