A novel avionics based GNSS integrity augmentation system for manned and unmanned aircraft

Sabatini, Roberto (2017) A novel avionics based GNSS integrity augmentation system for manned and unmanned aircraft. PhD thesis, University of Nottingham.

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Abstract

The aviation community has to implement very stringent navigation integrity requirements in a variety of manned and unmanned aircraft applications. This thesis presents the results of the research activities carried out by the Italian Air Force Research and Flight Test Centre (CSV-RSV) in collaboration with the Nottingham Geospatial Institute (NGI) and RMIT University in the area of Avionics Based Integrity Augmentation (ABIA) for mission-essential and safety-critical Global Navigation Satellite Systems (GNSS) applications in the civil/military aviation context. Space and Ground Based Augmentation Systems (SBAS/GBAS) have been developed in recent years to improve GNSS integrity, accuracy and availability for aircraft navigation and particularly for landing applications. SBAS satellites broadcast correction messages back to the earth, where suitably enabled receivers use the information to improve accuracy and integrity. The US, Europe and other nations have developed their own SBAS systems. In the US, the Wide Area Augmentation System (WAAS) exists and is operational. In Europe, SBAS coverage is provided by the European Geostationary Navigation Overlay Service (EGNOS), in Japan by the Multi-functional Satellite Augmentation System (MSAS) and India is developing the GNSS Aided Geo Augmented Navigation (GAGAN) system. An alternative approach to GNSS augmentation is to transmit integrity and correction messages from ground-based systems. An example is the American Local Area Augmentation System (LAAS), which allows a suitably equipped receiver to derive enhanced accuracy and integrity information in a local area. The combination of WAAS and LAAS is targeted to provide the Required Navigation Performance (RNP) in all phases of aircraft navigation, including en-route, terminal, approach/landing and surface operations. Along with SBAS and GBAS, GNSS augmentation may take the form of additional information being provided by other avionics systems. In most cases, the additional avionics systems operate via separate principles than GNSS and, therefore, are not subject to the same sources of error or interference. A system such as this is referred to as an Aircraft Based Augmentation System (ABAS). The additional sensors used in ABAS may include Inertial Navigation Systems (INS), TACAN/VOR-DME, Radar, Vision Based Sensors, etc. Unlike SBAS and GBAS technology, research on ABAS is limited and mainly concentrates on additional information being blended into the position calculation to increase accuracy and/or continuity of the integrated navigation solutions. Additionally, no significant attempts have been made of developing ABAS architectures capable of generating integrity signals suitable for safety-critical GNSS applications (e.g., aircraft precision approach and landing) and no flight certified ABAS products are available at present.

During flight test activities with GNSS and Differential GNSS (DGNSS) systems, it was observed that one or more of the following conditions was prone to cause navigation data outages or severe performance degradations:

• Antenna obscuration due to aircraft manoeuvring;

• Bad satellite geometries and low carrier-to-noise ratios (C/N0);

• Doppler shifts caused by aircraft-satellites relative motion;

• Interference, at the airborne GNSS antenna, caused by non-GNSS RF signals;

• Multipath caused by GNSS signals reflected by the earth surface or the aircraft body.

The last two problems can be mitigated by existing technology solutions (i.e., choosing a VHF/UHF Data Link, filtering the radio frequency signals reaching the GNSS antenna, identifying suitable locations for the GNSS antenna and providing adequate shielded of the antenna itself, either by physical devices or via dedicated software masks, etc.). However, there is little one can do in order to prevent critical events during realistic test/training manoeuvres and particular approach procedures (e.g., curved and segmented approaches) performed with high performance military aircraft. Furthermore, although in some cases a careful mission planning may significantly reduce the number of GNSS outages, the adoption of specific aircraft piloting strategies (using the information currently available in the cockpit) cannot effectively avoid the occurrence of these events.

ABIA is a new concept that progressively evolved based on research with GNSS-based Time and Space Position Information (TSPI) systems. TSPI research activities included design, integration and ground/flight testing carried out on MB-339CD, TORNADO and TYPHOON military aircraft. As soon as the validity of the TSPI-ABIA (T-ABIA) concept was established, a prototype system was developed for use in flight test applications. This system is capable of alerting the pilot when the critical conditions for GNSS signal loss are likely to occur (within a specified maximum time-to-alert). In this T-ABIA prototype, the aircraft on-board sensors provide information on the aircraft relevant flight parameters (navigation data, engine settings, etc.) to an Integrity Flag Generator (IFG), which is also connected to the on-board GNSS receiver. The IFG can be incorporated into one of the existing airborne computers or can be a dedicated processing unit. Using the available data on GNSS and the aircraft flight parameters, integrity signals are generated which are displayed on one of the cockpit displays and sent to an Aural Warning Generator. At the same time, an alternate flight path is computed taking into account the geometry and the tracking status of the available GNSS satellites, together with the current mission requirements and the information provided by the aircraft Flight Test Instrumentation (FTI) and standard on-board sensors.

Based on the results of T-ABIA research a more advanced ABIA system was developed suitable for manned and unmanned aircraft applications. Detailed mathematical algorithms were developed to cope with the main causes of GNSS signal outages and degradation in flight, namely: obscuration, multipath, interference, fading due to adverse geometry and Doppler shift. Adopting these algorithms, the ABIA system is able to provide steering information to the pilot and electronic commands to the aircraft flight control system, allowing real-time avoidance of safety-critical flight conditions and fast recovery of the required navigation performance in case of GNSS data losses. This is achieved by implementing both caution (predictive) and warning (reactive) integrity flags, as well as 4-Dimensional Trajectory (4DT) optimisation models suitable for all phases of flight.

The detailed design of the ABIA IFG module was completed and validation activities were performed on TORNADO-IDS, A-320 and AEROSONDE UAV simulated platforms to determine the Time-to-Alert (TTA) performances of the ABIA system in various flight phases from departure to final approach. The results of these activities were encouraging, showing that the system TTA performance is in line with current ICAO, FAA and CAA requirements for the different flight phases, with a potential synergy with SBAS and GBAS systems to support departure, en-route and TMA operations, including CAT-I/III precision approach. Further research concentrated on the 4DT computation module and extended the scope of ABIA applications to Unmanned Aircraft Systems (UAS). In particular, an initial investigation was accomplished to identify the potential synergies of ABIA with UAS Sense-and-Avoid (SAA) architectures for mid-air collision avoidance tasks.

In conclusion, although current and likely future SBAS/GBAS augmentation systems can provide significant improvement of GNSS navigation performance, it is shown that the novel ABIA system developed in this research can play a key role in GNSS integrity augmentation for mission-essential and safety-critical applications such as aircraft precision approach/auto-landing and UAS sense-and-avoid. Furthermore, using suitable data link and data processing technologies, a certified ABIA system could play a key role as part of a future GNSS Space-Ground-Aircraft Augmentation Network (SGAAN).

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Moore, Terry
Hill, Chris J.
Keywords: Satellite Navigation; Global Navigation Satellite System; Aircraft Based Augmentation System; Ground Based Augmentation System; Space Based Augmentation Systems; Integrity; Aviation; Air Navigation; Aircraft; Navigation; Guidance; Unmanned Aircraft System; Avionics; Air Traffic Management
Subjects: T Technology > TL Motor vehicles. Aeronautics. Astronautics
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 43912
Depositing User: Sabatini, Roberto
Date Deposited: 15 Oct 2017 04:40
Last Modified: 15 Oct 2017 18:39
URI: http://eprints.nottingham.ac.uk/id/eprint/43912

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