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Validating the Interoperability of Ground-Based Augmentation Systems (GBAS) |
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The Ground-Based Augmentation System (see Links 1 and 7) to satellite navigation is an airport-based, local system. Manufacturers use prototype installations for development and testing, but these test systems rarely interact.before being deployed more widely. To assess interoperability early in the process, multiple avionics installations must be brought to each prototype ground installation or all ground systems must be set up at a single location – a difficult task to achieve for a precision landing aid because of the significant installation effort.
GBAS has the potential to reduce cost with respect to the other alternatives (Figure 1), the classical Instrument Landing System (ILS) and the Microwave Landing System (MLS) - of which a first public installation was recently approved at London Heathrow. Each has its own strengths and weaknesses, but GBAS is especially mentioned as a target in both SESAR and the ICAO navigation objectives.
The EUROCONTROL European Air Traffic Management (EATM) Navigation Domain has several activities in the area of precision approach and landing, notably for GBAS. These include the development of a Concept of Operations and guidelines for an Operational Safety Assessment, the review of requirements for Optimised Capacity Operations in low visibility operations and the review of Phraseology and Flight Plan issues.
When asked by our stakeholders (Link 2) to support the validation of GBAS as a precision approach system, the experience of the Communications, Navigation and Surveillance (CNS) team pointed to a solution with several components:
- A data evaluation process and software allowing reproduction of key algorithms and access to intermediate results for detailed analysis;
- A reconfigurable, flying testbed to visit key installations;
- Access to existing GBAS installations and, if required, temporarily installing and testing otherwise inaccessible equipment at a central site;
- Own test flights and co-operation with other ongoing research, notably EC-activities, to obtain a consistent presentation and results that can be directly compared between all evaluations.
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| Figure 1: GBAS as one of the available precision approach options |
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The data evaluation software plays a key role, since it ensures that data gathered in the different experiments can be analysed in a repeatable way and exchange of results becomes possible.
Past activities in the validation of other satellite navigation augmentations:
- first Airborne Augmentation Systems (ABAS) of which many are based on Receiver Autonomous Integrity Monitoring (RAIM) concepts,
- then the operational validation of the European Satellite Based Augmentation System SBAS system, EGNOS,
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have led to the PEGASUS (Prototype EGNOS and GBAS Analysis System Using SAPPHIRE) toolset, a proven, modular and partially automated software suite developed at the EEC for analysis of satellite-navigation data. An extension of the toolset to GBAS use was completed in 2005, providing a baseline for the experiments. It now reproduces significant elements of both ground and airborne GBAS algorithms. Performance and standards compliance can in this way be compared for static and flight tests with accessibility to all intermediate parameters. Figure 2 shows its key components.
The PEGASUS system has recently been declared the baseline tool for GBAS data evaluation worldwide by the International GBAS Working Group and is currently in use by analysts of more than 15 countries.
The data analysis is open and fully documented. Access to all processing steps allows extrapolation to future CAT III systems by adapting monitoring algorithms and performing error simulations, thereby contributing to the development of detailed performance and interface descriptions at ICAO and European level. This work has already helped to verify a first set of performance requirements recently published by EUROCAE as ED-144.
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| Figure 2: PEGASUS Tool Components |
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| Figure 3: The GBAS testbed aircraft |
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The DO128 aircraft of the Technical University of Braunschweig (Figure 3) has been involved in research on GBAS and its precursors since 1991, with a sister aircraft having performed the worldwide first automatic landing based on Differential GPS in Braunschweig in 1989.
It provides a very flexible, relatively low cost experiment platform and has been fitted with supplemental antennas, cockpit displays and specific reception and recording systems for the GBAS interoperability experiments. GBAS avionics for both Boeing and Airbus aircraft have been installed, as well as GBAS equipment of Russian manufacture.
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Assuring accessibility to European GBAS ground installations |
Prototypes of US-manufactured GBAS ground systems are currently installed in Frankfurt (early prototype), Bremen and Malaga, while Airbus in Toulouse uses a GBAS system provided by a European manufacturer for certification. A Russian manufacturer operates several installations near Moscow and, through facilitation by the Russian Ministry of Transport, has agreed to install and support one station in Braunschweig for a 2 year period.
Through cooperation with DFS and DSNA/DTI, permission to use the Frankfurt, Bremen and Toulouse stations was granted. As EC GBAS studies were already planned in Malaga (Figure 4) in the frame of the OPTIMAL project, based on AENA and Airbus aircraft, harmonised results could be obtained by participating in the data evaluation.
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| Figure 4: GBAS installation at Malaga airport |
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Two more stations are in preparation in Palermo and Braunschweig and will be considered once available.
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Test and evaluation activities |
After initial ground trials in 2005, several flight trial campaigns have been conducted in 2006 and 2007, with the aircraft performing more than 50 approaches to a regional airport near Frankfurt and a similar number in Toulouse (Figure 5). The experience gained has permitted the optimisation of trajectories for later tests in Bremen and Malaga and the preparation of the Malaga installation for the more involved Airbus testing in the frame of OPTIMAL. As had been hoped, no significant interoperability issues were detected and it was possible to confirm FAA results of a performance significantly exceeding the specification.
A short video (Link 15) of one of the approaches shows the possibility of GBAS to serve approaches to a regional airport (Egelsbach – 20km distance) based on signals from a ground system at a major hub airport (Frankfurt).
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| Figure 5: Cockpit view of the GBAS approach to Toulouse |
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Testing with the Russian GBAS system took a different turn however. Both the European and the US systems are the fruit of a long development history based on a US initiative and have thus many shared requirements, including the use of EUROCAE and RTCA MOPS (Minimum Operational Performance Specifications). The Russian system covers the use not only of GPS, but also of GLONASS – which for GBAS is included in ICAO Annex 10, but not the different MOPS. The design was started independently, using MOPS only where Annex 10 needed completion to define functionality.
Our first trials showed no ability to decode other system’s signals, and the investigation revealed that Annex 10 defines navigation data to be modulated “in a mathematically positive sense” onto the signal – a wording that was interpreted differently in Russia. In the MOPS, different, unambiguous wording had been used and as consequence an update to Annex 10 is now proposed to clarify the modulation.
Other results include the necessity to adapt processes of frequency coordination between European states, as the digital, timeslot based GBAS system allows several stations in close proximity (in this case Bremen and Braunschweig) to transmit on the same frequency. The automatic messages exchanged for the frequency coordination process do not contain appropriate fields for the additional information required. As a consequence, updates to several ICAO documents are now being proposed.
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Up to now, no significant interoperability issues have appeared that would have compromised system safety for operations. The experiments and analyses have, however, supported the detection of several smaller issues that can be corrected in the current development phase. Some of them are still being investigated and clarifications will be proposed to be able to ensure that landings with GBAS will become as commonplace in the future as with ILS now.
Partly based on the test-bed experiences and safety assessments supported by our EATM colleagues, the station in Bremen has in the meantime obtained a limited operational approval from the German authorities and one aircraft has been performing initial commercial operations since September 2007. Final approval is planned for 2009 after the conclusion of FAA-led validation exercise.
As these first implementations are completed, GBAS-equipped aircraft are becoming more commonplace. The International GBAS Working Group was initiated by EUROCONTROL in 2004 and is co-chaired with the FAA. It brings together airlines, service providers, authorities and researchers from over 20 states with the goal of harmonising GBAS implementation and validation.
This also means that interoperability validation activities continue and for 2008 we foresee further verifications with new prototype equipment, manufacturers having requested the use of our testing environment. Completion of the data analysis and application of the experience gained in validating the evolving ICAO standards are also key tasks.
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Links and Further Reading |
EUROCONTROL Navigation Domain Website: |
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