If you wish to contribute or participate in the discussions about articles you are invited to contact the Editor
The SBAS Integrity Concept Standardised by ICAO: Application to EGNOS: Difference between revisions
No edit summary |
|||
| Line 26: | Line 26: | ||
==Integrity Requirements== | ==Integrity Requirements== | ||
The elaboration of a high level fault tree for all phases of flight leading to a given objective in term of Target Level of Safety (TLS) and further decomposition for a number of phases of flight into aircraft, airborne database and signal in space (SIS) contribution to this risk has been provided by the ICAO All Weather Operational Panel<ref name="ICAO AWOP15">ICAO AWOP/15 Report, 15th meeting, Montreal 26 September- 12 October 1994.</ref> (AWOP) [2]<ref name="ICAO AWOP16">ICAO AWOP/16 Report, 16th meeting, Montreal 23 June- 4 July 1997.</ref> [3]. | The elaboration of a high level fault tree for all phases of flight leading to a given objective in term of Target Level of Safety (TLS)<ref>The top TLS objective is that the probability of accident leading to hull loss should be inferior to 1.5 10<sup>-7</sup> per flight.</ref> and further decomposition for a number of phases of flight into aircraft, airborne database and signal in space (SIS) contribution to this risk has been provided by the ICAO All Weather Operational Panel<ref name="ICAO AWOP15">ICAO AWOP/15 Report, 15th meeting, Montreal 26 September- 12 October 1994.</ref> (AWOP) [2]<ref name="ICAO AWOP16">ICAO AWOP/16 Report, 16th meeting, Montreal 23 June- 4 July 1997.</ref> [3]. | ||
[[File:Fault tree allocation for SBAS.jpg|none|thumb|400px|alt=Fault tree allocation for SBAS|'''''Figure 1:''''' Fault tree allocation for SBAS APV I, II and Cat I operations]] | [[File:Fault tree allocation for SBAS.jpg|none|thumb|400px|alt=Fault tree allocation for SBAS|'''''Figure 1:''''' Fault tree allocation for SBAS APV I, II and Cat I operations]] | ||
The fault tree for approach with vertical guidance (APVI,II and Category 1 approach type) corresponding to the most demanding operations supported by SBAS derived from AWOP work is shown in Fig. 1. | The fault tree for approach with vertical guidance (APVI,II and Category 1 approach type) corresponding to the most demanding operations supported by SBAS derived from AWOP work is shown in Fig. 1<ref>The AWOP 2.10-7 figure for SIS integrity risk by approach (150 s) has been further decomposed by GNSSP into a 10-7/approach allocation for the ground system integrity risk and a 10-7/approach allocation for the fault free case.</ref>. | ||
This paper will focus on Non Aircraft, signal in space (SIS) integrity risk corresponding to the bottom right part allocations of Fig. 1. | This paper will focus on Non Aircraft, signal in space (SIS) integrity risk corresponding to the bottom right part allocations of Fig. 1. | ||
AWOP work has been used as input by GNSSP to define the high level integrity requirements summarised in Fig. 2. | AWOP work has been used as input by GNSSP to define the high level integrity requirements summarised in Fig. 2. | ||
Revision as of 13:04, 14 November 2010
| EGNOS | |
|---|---|
| Title | The SBAS Integrity Concept Standardised by ICAO: Application to EGNOS |
| Author(s) | Benoit Roturier, DGAC/STNA, France; Eric Chatre GSA, GNSS Supervisory Authority, Brussels, Belgium and Javier Ventura-Traveset, ESA, European Space Agency. |
| Level | Medium |
| Year of Publication | 2006 |
Abstract
There have been a lot of debates, within the International Civil Aviation Organisation (ICAO) GNSS Panel (GNSSP) group of experts[1], on the proper way to ensure SBAS user safety while at the same time respecting the high availability requirement. The group finally validated a method at the GNSSP Seattle meeting in June 2000 which is reproduced in the GNSS Standards And Recommended Practices (SARPs), published in November 2002[2] [1]. Although the technical relevant information for a SBAS system designer to implement the SBAS integrity concept is fully described in the SARPs, only the strict necessary information is reported there, and it is quite difficult to a non specialist to properly understand this important concept. Since the SBAS integrity concept is quite specific and new, some kind of complementary information to the SARPs was felt desirable. This is the main motivation of this paper, which will also illustrate how the integrity is being managed through the European EGNOS SBAS project.
Introduction
The integrity service of ICAO compliant GNSS systems may currently be provided by the three normalised augmentations known under the terms ABAS (Airborne Based Augmentation System), GBAS (Ground Based Augmentation System) and SBAS (Satellite Based Augmentation System)[2] [1]. ABAS integrity concept relies on the single observation through the airborne user receiver of redundant pseudo range information, while GBAS (resp. SBAS) integrity elaboration relies on the use of a single (resp. a network) of ground reference stations.
In addition to integrity service, GBAS and SBAS also provide to the user differential corrections to improve the precision in a restricted area around a single reference station for GBAS and over a wide area defined by a network of reference stations for SBAS. Finally, the SBAS geo satellites also transmit a ranging navigation signal similar to a GPS satellite.
Therefore, the SBAS integrity service which is addressed here should protect the user from both:
- failures of GPS/GLONASS/GEO satellites (drifting or biased pseudo ranges) by detecting and excluding faulty satellites through the measurement of GPS signals with the network of reference ground stations;
- transmission of erroneous or inaccurate differential corrections. These erroneous corrections may in turn be induced from either:
- undetected failures in the ground segment;
- processing of reference data corrupted by the noise induced by the measurement and algorithmic process.
This last type of failure, which may occur when the system is in a nominal state (no GPS/GLONASS/GEO satellite failure, no ground segment/user equipment failure) is usually known as “fault free case”. Protection of the user against noise effects has been quite demanding during the process of definition and validation of the ICAO SBAS integrity concept. In fact, the potential for such non integrity events generated in fault free conditions is inherent to data measurement and processing, to provide users with basic and precise correction messages and is thus a permanent risk which has to be carefully managed. This has involved the definition of statistical error bounds called horizontal or vertical protection levels (HPL or VPL) which will be discussed in depth in section V.
Before dwelling in depth into the details of the elaboration of adequate parameters to protect users from non integrity events which might occur from system failure (section IV) or noise (section V), we will recall integrity requirements (section II) and integrity definitions (section 3).
Integrity Requirements
The elaboration of a high level fault tree for all phases of flight leading to a given objective in term of Target Level of Safety (TLS)[3] and further decomposition for a number of phases of flight into aircraft, airborne database and signal in space (SIS) contribution to this risk has been provided by the ICAO All Weather Operational Panel[4] (AWOP) [2][5] [3].
The fault tree for approach with vertical guidance (APVI,II and Category 1 approach type) corresponding to the most demanding operations supported by SBAS derived from AWOP work is shown in Fig. 1[6]. This paper will focus on Non Aircraft, signal in space (SIS) integrity risk corresponding to the bottom right part allocations of Fig. 1. AWOP work has been used as input by GNSSP to define the high level integrity requirements summarised in Fig. 2.
| Typical operation | Time to Alarm | Integrity | Hor. alert limit | Vert. alert limit |
|---|---|---|---|---|
| En-route | 5 mn | 1-10-7/h | 4 NM | N/A |
| En-route | 15 s | 1-10-7/h | 2 NM | N/A |
| En-route, Terminal | 15 s | 1-10-7/h | 1 NM | N/A |
| NPA | 10 s | 1-10-7/h | 0.3 NM | N/A |
| APV I | 10 s | 1-2x10-7/app | 40.0 m | 50 m |
| APV II | 6 s | 1-2x10-7/app | 40.0 m | 20 m |
| CAT I | 6 s | 1-2x10-7/app | 40.0 m | 15 - 10 m |
References
- ^ Currently (2006) known as Navigation System Panel (NSP).
- ^ a b ICAO Amendment 77, Annex 10 to the Convention on International Civil Aviation, Aeronautical Telecommunications: International Standards and Recommended Practices, Volume 1, Radio Navigation Aids, November 2002.
- ^ The top TLS objective is that the probability of accident leading to hull loss should be inferior to 1.5 10-7 per flight.
- ^ ICAO AWOP/15 Report, 15th meeting, Montreal 26 September- 12 October 1994.
- ^ ICAO AWOP/16 Report, 16th meeting, Montreal 23 June- 4 July 1997.
- ^ The AWOP 2.10-7 figure for SIS integrity risk by approach (150 s) has been further decomposed by GNSSP into a 10-7/approach allocation for the ground system integrity risk and a 10-7/approach allocation for the fault free case.
[1] ICAO Amendment 77, Annex 10 to the Convention on International Civil Aviation, Aeronautical Telecommunications: International Standards and Recommended Practices, Volume 1, Radio Navigation Aids, November 2002.
[2] ICAO AWOP/15 Report, 15th meeting, Montreal 26 September- 12 October 1994.
[3] ICAO AWOP/16 Report, 16th meeting, Montreal 23 June- 4 July 1997.
[4] Liu Fan, “Analysis of Integrity Monitoring for The Local Area Augmentation System Using The GNSS”, PhD. Report, Ohio University, August 1998.
[5] RTCA, “Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment”, RTCA-DO 229 C, November 2001.
[6] Bruce DeCleene, “Defining Pseudo Range Integrity – Overbounding” ION Conference, September 2000.
[7] M. Tossaint, J. Samson, F. Toran, J. Ventura-Traveset, A Tadjine, I. Delgado, “The Stanford – ESA Integrity Diagram: Focusing on SBAS Integrity,” Part 1 of this book.