SBAS Standards

Fundamentals
Title	SBAS Standards
Edited by	GMV
Level	Intermediate
Year of Publication	2011

Twenty four hours a day, 365 days of the year, an airplane takes off or lands every few seconds somewhere on the face of the Earth. Every one of these flights is handled in the same, uniform manner, whether by air traffic control, airport authorities or pilots. Behind the scenes there are millions of employees involved in manufacturing, maintenance and monitoring of the products and services required in the never-ending cycle of flights.

This clockwork precision in procedures and systems is made possible by the existence of universally accepted standards.

Introduction

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.

For the aeronautical user international community, the documents listed in the following table constitute the current version of the core set of documents to be used for the development of a new system devoted to provide services to these users or for the development of aeronautical user terminals.

Document Item	Reference	Issue	Date	Standardisation Body
SBAS Elements
SARPS Volume 1 – Annex 10	Including Amendments 1-92	7	July 2018	ICAO
MOPS for GPS/WAAS airborne equipment	MOPS DO-229	F	2020	RTCA SC 159
MOPS for GNSS Airborne Antenna Equipment	MOPS-DO-228	1	January 2000	RTCA SC 159
MOPS for GNSS Airborne Active Antenna for the L1 Frequency Band	MOPS-DO-301	1	December 2006	RTCA SC 159
Primary Constellations
GPS
GPS L1, L2	IS-GPS-200	M	May 2021	GPS Wing (GPSW)
GPS L5	IS-GPS-705	H	May 2021	GPSW
GPS L1C	IS-GPS-800	H	May 2021	GPSW
GLONASS
GLONASS L1, L2 ICD	GLONASS ICD	5.1	2008	Russian Institute of Space Device Engineering
GLONASS L1, L2, L3 (CDMA) ICD	GLONASS ICD	1.0	2016	Russian Institute of Space Device Engineering
Galileo
Galileo Open Service SIS ICD	OS SIS ICD	2.0	January 2021	European Union
MOPS for Airborne Open Service Galileo Satellite Receiving Equipment	ED-259	1.0	February 2019	EUROCAE WG62

Standardization Bodies

ICAO

(Note: most of the information presented here has been extracted from ICAO web site.)

ICAO's Standardization

One of ICAO's chief activities is standardization, the establishment of International Standards, Recommended Practices and Procedures covering the technical fields of aviation: licensing of personnel, rules of the air, aeronautical meteorology, aeronautical charts, units of measurement, operation of aircraft, nationality and registration marks, airworthiness, aeronautical telecommunications, air traffic services, search and rescue, aircraft accident investigation, aerodromes, aeronautical information services, aircraft noise and engine missions, security and the safe transport of dangerous goods. After a Standard is adopted it is put into effect by each ICAO Contracting State in its own territories. As aviation technology continues to develop rapidly, the Standards are kept under constant review and amended as necessary. In keeping pace with the rapid development of international civil aviation, ICAO is conscious of the need to adopt in its specifications modern systems and techniques. In recent years, extensive work has been undertaken by ICAO in the areas of reporting aircraft accident and incident data, all-weather operations, automation of air traffic services, the application of computers in meteorological services, aircraft noise, engine emissions and the carriage of dangerous goods by air. ICAO has dealt with the subject of unlawful interference with civil aviation and with questions regarding aviation and the human environment.

The International Civil Aviation Organization (ICAO)

Creating and modernizing SARPs is the responsibility of the International Civil Aviation Organization, or ICAO, the specialized agency of the United Nations whose mandate is to ensure the safe, efficient and orderly evolution of international civil aviation. ICAO has its headquarters in Montreal, Canada, with seven regional offices throughout the world. From its beginning in 1944 it has grown to an organization with over 180 Contracting States. ICAO's aim is the safe and orderly development of all aspects of international civil aeronautics. It provides the forum whereby requirements and procedures in need of standardization may be introduced, studied and resolved. The charter of ICAO is the Convention on International Civil Aviation, drawn up in Chicago in December 1944, and to which each ICAO Contracting State is a party. The principal body concerned with the development of technical Standards and other provisions is the Air Navigation Commission. Its primary role is to advise the Council of ICAO on air navigation issues. It is composed of fifteen experts with appropriate qualifications and experience in various fields of aviation. Its members are nominated by Contracting States and are appointed by the Council. They are expected to function as independent experts and not as representatives of their States.

Forms of Standards and Recommended Practices

Sixteen out of eighteen Annexes to the Convention are of a technical nature and therefore fall within the responsibilities of the Air Navigation Bureau and its sections. The remaining two Annexes, Facilitation and Security, are under the purview of the Air Transport Bureau. Since the majority of the Annexes concern technical issues, it is focused on them when the development process is described. ICAO standards and other provisions are developed in the following forms:

• Standards and Recommended Practices - collectively referred to as SARPs;
• Procedures for Air Navigation Services - called PANS;
• Regional Supplementary Procedures -referred to as SUPPs; and
• Guidance Material in several formats.

A Standard is defined as any specification for physical characteristics, configuration, material, performance, personnel or procedure, the uniform application of which is recognized as necessary for the safety or regularity of international air navigation and to which Contracting States will conform in accordance with the Convention; in the event of impossibility of compliance, notification to the Council is compulsory under Article 38 of the Convention.

A Recommended Practice is any specification for physical characteristics, configuration, material, performance, personnel or procedure, the uniform application of which is recognized as desirable in the interest of safety, regularity or efficiency of international air navigation, and to which Contracting States will endeavour to conform in accordance with the Convention. States are invited to inform the Council of non-compliance. SARPs are formulated in broad terms and restricted to essential requirements. For complex systems such as communications equipment, SARPs material is constructed in two sections: core SARPs material of a fundamental regulatory nature contained within the main body of the Annexes, and detailed technical specifications placed either in Appendices to Annexes or in manuals.

The differences to SARPS notified by States are published in Supplements to Annexes. Procedures for Air Navigation Services (or PANS) comprise operating practices and material too detailed for Standards or Recommended Practices -they often amplify the basic principles in the corresponding Standards and Recommended Practices. To qualify for PANS status, the material should be suitable for application on a worldwide basis. The Council invites Contracting States to publish any differences in their Aeronautical Information Publications when knowledge of the differences is important to the safety of air navigation. The provisions for Annex 18, Dangerous Goods, are supplemented by Technical Instructions for the Safe Transport of Dangerous Goods by Air. While these detailed instructions do not have the status of SARPs or PANS, they do have a special status by which the Contracting States are requested to achieve compliance. Regional Supplementary Procedures (or SUPPs) have application in the respective ICAO regions. Although the material in Regional Supplementary Procedures is similar to that in the Procedures for Air Navigation Services, SUPPs do not have the worldwide applicability of PANS. Guidance Material is produced to supplement the SARPs and PANS and to facilitate their implementation. Guidance material is issued as Attachments to Annexes or in separate documents such manuals, circulars and lists of designators/addresses. Usually it is approved at the same time as the related SARPS are adopted.

Manuals provide information to supplement and/or amplify the Standards and Recommended Practices and Procedures for Air Navigation Services. They are specifically designed to facilitate implementation and are amended periodically to ensure their contents reflect current practices and procedures. Circulars make available specialized information of interest to Contracting States. Unlike manuals, circulars are not normally updated.

The sections of the ICAO SARPS Annex 10 more related to SBAS standardisation are described next:

• CHAPTER 1 INTRODUCTION
• CHAPTER 2 GENERAL PROVISIONS FOR RADIO NAVIGATION AIDS
  • Section 2.4 “Global navigation satellite systems”
• CHAPTER 3 SPECIFICATIONS FOR RADIO NAVIGATION AIDS
  • Section 3.7 “Requirements for the Global Navigation Satellite System (GNSS)”
• Appendix B TECHNICAL SPECIFICATIONS FOR THE GLOBAL NAVIGATION SATELLITE SYSTEM
• SECTION 1: DEFINITION
• SECTION 2: GENERAL
• SECTION 3: GNSS ELEMENTS
  • SECTION 3.1: GLOBAL POSITIONING SYSTEM (GPS) STANDARD POSITIONINGSERVICE (SPS) (L1)
  • SECTION 3.2: GLOBAL NAVIGATION SATELLITE SYSTEM (GLONASS) CHANNELOF STANDARD ACCURACY (CSA) (L1)
  • SECTION 3.3: COMBINED USE OF GPS AND GLONASS
  • SECTION 3.4: AIRCRAFT-BASED AUGMENTATION SYSTEM (ABAS)
  • SECTION 3.5: SATELLITE-BASED AUGMENTATION SYSTEM (SBAS)
    • Section 3.5.1: General
    • Section 3.5.2: RF characteristics
    • Section 3.5.3: Data Structure
      • Section 3.5.3.1 Format summary
      • Section 3.5.3.2 Preamble
      • Section 3.5.3.3 Message type identifier
      • Section 3.5.3.4 Data Field
      • Section 3.5.3.5 Ciclic redundancy check (CRC)
    • Section 3.5.4: Data Content
      • Section 3.5.4.1 PRN mask parameters
      • Section 3.5.4.2 Geostationary orbit (GEO) ranging function parameters
      • Section 3.5.4.3 GEO almanac parameters
      • Section 3.5.4.4 Satellite correction broadcast parameters
      • Section 3.5.4.4.1 Long-term correction parameters
      • Section 3.5.4.4.2 Fast correction parameters
      • Section 3.5.4.5 Fast and long-term correction integrity parameters
      • Section 3.5.4.6 Ionospheric correction parameters
      • Section 3.5.4.7 Degradation parameterS-GMV-4100-O-021-U-V1.0 05/02/2008 17/34
      • Section 3.5.4.8 Time parameters
      • Section 3.5.4.9 Service region parameters
      • Section 3.5.4.10 Clock-ephemeris covariance matrix parameters
    • Section 3.5.5 Definitions of protocols for data application
      • Section 3.5.5.1 GEO position and clock
        • Section 3.5.5.1.1 GEO position estimation
        • Section 3.5.5.1.2 GEO clock correction
      • Section 3.5.5.2 Long-term corrections
        • Section 3.5.5.2.1 GPS clock correction
        • Section 3.5.5.2.2 GLONASS Clock correction
        • Section 3.5.5.2.3 Satellite position correction
      • Section 3.5.5.3 Pseudo-range corrections
      • Section 3.5.5.4 Range rate corrections (RRC).
      • Section 3.5.5.5 Broadcast ionospheric corrections
        • Section 3.5.5.5.1 Location of ionospheric pierce point (IPP)
        • Section 3.5.5.5.2 Ionospheric corrections
        • Section 3.5.5.5.3 Interpolated vertical ionospheric delay estimate
      • Section 3.5.5.6 Protection levels
    • Section 3.5.6 Message tables
    • Section 3.5.7 Non-aircraft elements
      • Section 3.5.7.1 General
      • Section 3.5.7.2 Ranging function
      • Section 3.5.7.3 GNSS satellite status function
      • Section 3.5.7.4 Basic differential correction function
      • Section 3.5.7.5 Precise differential correction function
      • Section 3.5.7.6 Optional functions
      • Section 3.5.7.7 Monitoring
    • Section 3.5.8 Aircraft elements
      • Section 3.5.8.1 SBAS-capable GNSS receiver
      • Section 3.5.8.2 RANGING FUNCTION
      • Section 3.5.8.3 GNSS satellite status function
      • Section 3.5.8.4 Basic and precise differential functions
    • Section 3.5.9 Interface between SBAS
  • SECTION 3.6: GROUND-BASED AUGMENTATION SYSTEM (GBAS)
  • SECTION 3.7 RESISTANCE TO INTERFERENCE
  • SECTION 3.8 GNSS AIRCRAFT SATELLITE RECEIVER ANTENNA
  • SECTION 3.9: CYCLIC REDUNDANCY CHECK

RTCA, Inc.

RTCA Aims

RTCA, Inc. is a private, not-for-profit corporation that develops consensus-based recommendations regarding communications, navigation, surveillance, and air traffic management (CNS/ATM) system issues. RTCA functions as a Federal Advisory Committee.

Its recommendations are used by the Federal Aviation Administration (FAA) as the basis for policy, program, and regulatory decisions and by the private sector as the basis for development, investment and other business decisions. Organized in 1935 as the Radio Technical Commission for Aeronautics, RTCA today includes roughly 250 government, industry and academic organizations from the United States and around the world. Member organizations represent all facets of the aviation community, including government organizations, airlines, airspace user and airport associations, labor unions, plus aviation service and equipment suppliers. A sampling of our domestic membership includes the Federal Aviation Administration, Air Line Pilots Association, Air Transport Association of America, Aircraft Owners and Pilots Association, ARINC Incorporated, The Boeing Company, Department of Commerce, Department of Defence, GARMIN International, Honeywell International, Inc., The Johns Hopkins University, Lockheed Martin, MIT Lincoln Laboratory, MITRE/CAASD, NASA, National Business Aviation Association, and Raytheon.

Because RTCA interests are international in scope, many non-U.S. government and business organizations also belong to RTCA. RTCA is currently supported by approximately 60 International Associates such as Air services Australia, Airways Corporation of New Zealand, the Chinese Aeronautical Radio Electronics Research Institute (CARERI), EUROCONTROL, NAV Canada, Pilatus Aircraft Limited, Smiths Industries, Society of Japanese Aerospace Companies, Thales Avionics Limited, the United Kingdom Civil Aviation Authority, GMV and many more.

RTCA has proven to be an excellent means for developing government / industry consensus on contemporary CNS/ATM issues.

Special Committees

Essentially all RTCA products are developed by issue-oriented Special Committees staffed by volunteers. As with all Federal Advisory Committee activities, Special Committee meetings are publicly announced and open to participation by anyone with an interest in the topic under consideration. During Special Committee meetings, volunteers from government and industry explore the operational and technical ramifications of the selected topic and develop consensus recommendations. These recommendations are then presented to the RTCA Program Management Committee, which either approves the Special Committee report or directs additional Special Committee work. Approved recommendations are published and made available for sale to members and to the public. Easy access to updates on committee activities and related subjects is available on the RTCA web site and in the Digest, which is published every two months.

Global Positioning System Special Committee 159

SC-159 develops minimum standards that form the basis for FAA approval of equipment using GPS as a primary means of civil aircraft navigation. The committee's most recent publications are DO-245A, Minimum Aviation System Performance Standards for Local Area Augmentation System (LAAS) issued 9 December 2004 and DO-246C - GNSS Based Precision Approach Local Area Augmentation System (LAAS) - Signal-in-Space Interface Control Document (ICD), issued 7 April 2005. In addition to ongoing activities with GPS/WAAS, GPS/LAAS, GPS/Inertial and GPS/Galileo applications, the committee is completing a MOPS for a Ground-based Regional Augmentation System (GRAS) aeronautical receiver and developing a new MOPS for L1/L5 antennas.

DO-229D, Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment

This document contains Minimum Operational Performance Standards (MOPS) for airborne navigation equipment (2D and 3D) using the Global Positioning System (GPS) augmented by the Wide Area Augmentation System (WAAS).

The standards cover numerous topic and regulatory aspects for the certification of an SBAS airborne equipment, such as:

• System Overview.
• Operational goals.
• Equipment classes.
• Equipment performances requirements for each operational navigation mode.
• Equipment SBAS message processing.
• Equipment Test method and procedures.
• Installed equipment general requirements, test procedures and flight test procedures.
• WAAS SIS.
• Standard GPS/WAAS Assumptions.
• Standard Signal and Interference Environment.
• Algorithm description for Precision Approach Navigation solutions.
• Algorithm description for Protection Level computation for all phase of flight.
• Fault detection and Exclusion.

The regulatory application of these standards is the responsibility of appropriate government agencies. In the United States, the Federal Aviation Administration (FAA) plans to publish Technical Standard Order (TSO) C-145 and C-146 for GPS/ WAAS equipment. TSO C-146 will reference the requirements and bench tests procedures in Section 2. A TSO is a minimum performance standard for specified materials, parts, and appliances used on civil aircraft. When authorized to manufacture a material, part, or appliances to a TSO standard, this is referred to as TSO authorization. Receiving a TSO authorization is both design and production approval. Receiving a TSO Authorization is not an approval to install and use the article in the aircraft. It means that the article meets the specific TSO and the applicant is authorized to manufacture it.

EUROCAE

The European Organisation for Civil Aviation Equipment was formed at LUCERNE on the 24th April, 1963. At that time, there was no regular forum in Europe where administrations, airlines and industry could meet to discuss technical problems. EUROCAE was created to fill this gap.

EUROCAE started the preparation of minimum performance specifications for airborne electronic equipment. This work was noted and supported from 1967 by the European Civil Aviation Conference (ECAC). ECAC later proposed to European National Airworthiness Authorities to take EUROCAE specifications as the basis of their national regulations. Today, EUROCAE documents are considered by Joint Aviation Authorities as means of compliance to Joint Technical Standard Orders and other regulatory documents. EUROCAE has extended its activity from airborne equipment to complex CNS/ATM systems including their ground segment. The related documentation is also considered by EUROCONTROL and by the European Commission.

The main European administrations, aircraft manufacturers, equipment manufacturers and service providers are members of EUROCAE, and they actively participate in the Working Groups which prepare these documents.

The primary task of Working Groups is to prepare performance specifications and similar documents which may be referenced by the Aviation Authorities in Europe. They are most frequently Minimum Operational Performance Specifications (MOPS) or Minimum Aviation System Performance Specifications (MASPS), although Users Guides and other types are also published.

The Council of EUROCAE is responsible for approving the establishment of new Working Groups and their work programmes, taking account of submissions made by EUROCAE members. It also monitors their activities and approves all documents for publication.

The EUROCAE Working groups, and especially their chairmen, are following a methodology, which is defined in the EUROCAE Working Group Chairmanship Guide. Functions available for all the members of the working groups, and particularly for their chairmen and secretaries, in using the EUROCAE website workspaces are described in the EUROCAE Website Workspace User Guide.

Galileo Working Group 62

EUROCAE WG 62 was created in early 2002 to deal with the standardization of GALILEO. Its terms of reference were endorsed by the EUROCAE Council on July 17th ,2002 as follows :

1. be a forum to discuss and make recommendations to the Galileo project on issues of concern to civil aviation airborne and ground equipment. To achieve efficiently this task, close relationship with Galileo project management should be established.
2. produce a list of working assumptions for the operational use of GNSS in the European and the worldwide airspaces. If appropriate, WG62 will draft an operational concept document to be submitted to the relevant body.
3. produce a MOPS for airborne receiver equipment. consider, in conjunction with EUROCAE WG 28, the existing standards related to precision approach for both ground and airborne equipment and, if appropriate, update these standards to take into account GALILEO use (including joint GALILEO/GPS use).

In March 2003, the terms of reference of the EUROCAE WG 62 was extended to encompass all activities pertaining to the future SBAS L5 signals. Later on, during 2005, it was also agreed to extend the group mandate to discuss also open issues concerning SBAS L1 approved ICAO or MOPS standards.

One of the first priorities of WG 62 was to prepare an operational concept identifying the most likely airborne architecture for the future. The outcome of the work performed by the group is that the most probable architecture for future GNSS receivers is a combined receivers able to receive and use the following signals : GPS L1/L5-SBAS L1/L5 (providing integrity data for GPS) and Galileo E1/E5. This architecture provides extremely good robustness in case of interference on one frequency or in case of a complete system failure.

Based on these conclusions, the group agreed to extend the WG 62 mandate to include also SBAS System considerations. It was therefore agreed to extend the Terms Of Reference of WG 62 to address, as required, the standardization need related to the future introduction of dual frequency SBAS services. In particular, the working group will contribute to the elaboration of the signal in space ICD for the future SBAS L5 signal.

More recently, due to delays in the Galileo programme, a new roadmap is being defined. In particular, WG62 is preparing recommendations on:

• Operational use of GNSS.
• Airborne GPS/Galileo/SBAS receiver equipment (MOPS).
• Precision approach to CAT I, II, III for a combined GALILEO/GPS system (MOPS).
• The use of SBAS L5 signal in space (ICD).

The main developed or revised documentation in the last few years have been:

• ED-131 Interim MOPS for airborne GALILEO receiving equipment.
• ED-134 Signal Specification for SBAS L1/L5.
• ED-157 SBAS L1/L5 ICD.

The SBAS Interoperability Working Group (IWG)

Although all SBAS are regional systems, there is a commonly recognized need to establish adequate co-operation/co-ordination among SBAS providers so their implementation becomes more effective and part of a seamless worldwide navigation system.

Through the different Interoperability Working Group (IWG) meetings, a consensus has been reached to identify the following interoperability objectives:

• 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.

US-EU Cooperation group in SBAS matters (WG-C)

The United States and the European Union and its member states have been close partners in the area of satellite navigation since 2004, when the parties signed a historic agreement establishing cooperation between GPS and Europe's planned Galileo system. The cooperation aims to ensure that GPS and Galileo will be interoperable at the user level for the benefit of civil users around the world. The cooperation is also intended to maintain fair trade in the global satellite navigation market. The GPS-Galileo Agreement established four working groups for cooperation on:

1. Radio frequency compatibility and interoperability;
2. Trade and civil applications;
3. Design and development of the next generation of systems; and
4. Security issues related to GPS and Galileo.

Recently, WG-C has completed an assessment of the global, combined performance for GPS Space-Based Augmentation System (SBAS) receivers using the European Geostationary Navigation Overlay Service (EGNOS) and the GPS Wide Area Augmentation System (WAAS) supporting safety-of-life applications. The results confirmed improved availability for a wide range of aviation services in both hemispheres and significantly improved robustness to GPS satellite outages.

The working group also completed an assessment of receivers integrating planned interoperable GPS III and Galileo open civil services. The study compares GPS, Galileo, and GPS/Galileo combined performance for three receiver types using four study cases. The combination of GPS and Galileo services provided noteworthy performance improvements particularly in partially obscured environments, where buildings, trees or terrain block large portions of the sky. Dual-frequency receivers provide additional improvements in most environments. This study illustrates benefits expected from future broadband signals on GPS and Galileo and other future GNSS systems.

The result of these consultations is the public release of two papers:

• Combined Performances for SBAS Receivers Using WAAS and EGNOS
• Combined Performances for Open GPS/Galileo Receivers

Credits

Except for updates and small modifications, the information presented in this article has been mostly extracted from chapter 1.8 of ESA SP-1303 book. The information therein was mainly obtained from the following homepages:

• ICAO website.
• EUROCAE website.
• RTCA website.