Ground-Based Augmentation System (GBAS) Safety Assessment

Background and Overview
The Holistic Approach and the GBAS Safety Assessment Process
Initial Deliverables
Need for Statistical Data and Link with GBAS Operational Validation Activities
Timescale

Background and Overview

The Ground-Based Augmentation System (GBAS) is a safety-critical system that augments the GPS Standard Positioning Service (SPS) and provides enhanced levels of service. It supports all phases of approach, landing, departure, and surface operations within its area of coverage. The current Instrument Landing System (ILS) suffers from a number of technical limitations such as, VHF interference, multipath effects (for example due to new building works at and around airports), as well as ILS channel limitations. GBAS is expected to play a key role in maintaining existing all-weather operations capability at CATI/II and III airports. GBAS CAT-I is seen as a necessary step towards the more stringent operations of CAT-II/III precision approach and landing.

The operational benefits of GBAS include:

Figure 1: GBAS Components

The Holistic Approach and the GBAS Safety Assessment Process

Until now, aviation has relied almost exclusively on systems developed to its own specifications for positioning and navigation services. This is not to be the case for GNSS, the control of which largely lies outside aviation. Aviation therefore has to use services of third party providers to meet performance requirements and shall have regard to the safety significance of those externally provided services. This leads to a change in the traditional approach to safety management and the organisation thereof. EUROCONTROL has been the mainspring of the development of common safety management policy, procedures, methods and tools and the EATMP GNSS Programme is instrumental in promoting the holistic end-to-end safety assessment approach for space-based navigation services. Following the recent experience related to the development of a Safety Case for the European Geostationary Navigation Overlay Service (EGNOS), the EUROCONTROL GBAS Project has adopted a pro-active approach to the safety activities that will accompany the overall lifecycle of GBAS. In line with EUROCONTROL Safety Regulatory Requirements (ESARRs) 3 and 4 and in close co-operation with the Safety Regulation Commission (SRC), a risk-based approach is applied to GBAS with regard to the total aviation system. Material developed within the framework of the GBAS Safety Assessment activities provides guidance to the EUROCONTROL/ECAC States / Air Navigation Service Providers and/or Airport Authorities on how to conduct a specific GBAS Safety Assessment at a national/local level to provide evidence that GBAS-based CAT-I operations are tolerably safe.

Initial Deliverables

Following the ‘Guidelines for the safety assessment of EATMP Programmes, the first two major deliverables of the GBAS Project were the GBAS Safety Policy and the GBAS Safety Plan. Both were developed in close co-operation with the Stakeholders who are actively participating in the Project. These range from airlines, airline associations to Air Navigation Service Providers and Airport Authorities.

The Safety Policy provides an approach to facilitate the safety regulation process of the GBAS CAT-I Project and defines and produces a harmonised safety regulatory position on the acceptability of changes that the implementation of GBAS in each of the EUROCONTROL/ECAC Member States will bring to the ATM system. The Safety Plan outlines a "route map" for the safety assessment of GBAS CAT-I approaches. It supports service providers by (i) Indicating what safety assessment activities will be addressed at a European level; (ii) Demonstrating techniques for conducting safety assessments that need to be done at national/local level; and (iii) Illustrating the roles and responsibilities of other GBAS Stakeholders. The GBAS Safety Policy and Safety Plan were concurrently endorsed by the SRC in January 2002 and these make up the pillars on which the subsequent activities are based. Those activities are the so-called Functional Hazard Assessment (FHA), Preliminary System Safety Assessment (PSSA) and System Safety Assessment (SSA). The final stage of the safety assessment process will consist in providing the outline of a GBAS Safety Case that will: (i) set out the safety objectives and requirements broken down to the level of total aviation system elements; and (ii) show by means of argument and supporting evidence that the concept and implementation of GBAS CAT-I precision approach is tolerably safe. This will serve as a template for States Air Navigation Service Providers (ANSPs) own Safety Cases. The latter will need to provide the evidence that the provision of GBAS CAT-I operations are tolerably safe in a particular State or particular airport. This Safety Case will need to be submitted to the National Regulators for approval, who, in turn, will closely liaise with the SRC.

Need for Statistical Data and Link with GBAS Operational Validation Activities

To date the GBAS Project is carrying out the FHA process and the outputs from this stage will include identification of hazards and analysis of their severity. To assess risk, the likelihood of these hazards being realised needs to be determined. Hence during PSSA and SSA the frequencies of hazards and their possible outcomes need to be estimated. Once this has been done frequencies and consequences can be combined together to provide measures of risk. These risk estimates can then be assessed against appropriate criteria. In parallel with the FHA process, fault trees and event trees are being developed to model the combination of events that could lead to a hazard and ultimately to accidents. Events and nodes within these trees will need to be quantified. Numerical values will be based on a mixture of actual data and expert judgement. For the former, the safety assessment activities will make great use of statistical data generated by a post-processing tool developed to support the GBAS CAT-I Operational Validation axis within the EUROCONTROL GBAS Project.

This tool, called GBAS Modular Analysis and Research System (G-MARS) is being developed to assist Air Traffic Service Providers to aid site approval and obtain operational approval of GBAS installation for supporting precision approach under CAT-I conditions at an airport from their respective safety regulation authorities.

Figure 2: Functional Diagram of GBAS-MARS

To make sure that statistically significant representative data can be provided within realistic timescales there will be an effective pooling of information across Europe. In order to maximise the data size and ensure that it comes from a representative range of airports, the GBAS Project will aim at capturing every GNSS/GBAS trial. Consequently, the expertise gained within the framework of the safety assessment process will contribute to the definition of the flight campaigns, the results of which will go back to the safety activities. This results in an iterative and incremental lifecycle since safety and operational validation activities will proceed in synergy within the GBAS Project.

Timescale

Implementation of GBAS for GBAS CAT-I could be achieved in ECAC as early as 2005/2006 and the main deliverables of the GBAS Project to ensure that relevant information and material are at hand at the European level are:

As mentioned, GBAS CAT-I is seen as a strategic step to gain knowledge and experience that will benefit CAT-II/III operations. It is expected that GNSS Landing System (GLS) operational experience will evolve from CAT-I to CAT-II/III as the GNSS infrastructure evolves. The GNSS Programme has always promoted the generic nature of activities being carried out and tools being developed (SAPPHIRE, Pegasus, Pegasus * Plus, etc.) and this will ensure that activities for GBAS CAT-I will benefit new investigations (e.g. CAT-II/III).

For further information about the EUROCONTROL GBAS Project, please contact:

Eric PERRIN (Project Manager)

Tel: + 33 1 69 88 74 01

Fax: + 33 1 69 88 73 07