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Global Navigation Satellite Systems Performance Analysis And Global navigation satellite systems performance analysis and augmentation strategies in aviation Roberto Sabatini1*, Terry Moore2 and Subramanian Ramasamy1 1RMIT University, School of Engineering – Aerospace Engineering and Aviation Discipline, Bundoora, VIC 3083, Australia 2University of Nottingham, Nottingham Geospatial Institute, Nottingham, NG7 2TU, United Kingdom ABSTRACT In an era of significant air traffic expansion characterised by a rising congestion of the radiofrequency spectrum and a widespread introduction of Unmanned Aircraft Systems (UAS), Global Navigation Satellite Systems (GNSS) are being exposed to a variety of threats including signal interferences, adverse propagation effects and challenging platform-satellite relative dynamics. Thus, there is a need to characterize GNSS signal degradations and assess the effects of interfering sources on the performance of avionics GNSS receivers and augmentation systems used for an increasing number of mission-essential and safety-critical aviation tasks (e.g., experimental flight testing, flight inspection/certification of ground-based radio navigation aids, wide area navigation and precision approach). GNSS signal deteriorations typically occur due to antenna obscuration caused by natural and man-made obstructions present in the environment (e.g., elevated terrain and tall buildings when flying at low altitude) or by the aircraft itself during manoeuvring (e.g., aircraft wings and empennage masking the on-board GNSS antenna), ionospheric scintillation, Doppler shift, multipath, jamming and spurious satellite transmissions. Anyone of these phenomena can result in partial to total loss of tracking and possible tracking errors, depending on the severity of the effect and the receiver characteristics. After designing GNSS performance threats, the various augmentation strategies adopted in the Communication, Navigation, Surveillance/Air Traffic Management and Avionics (CNS+A) context are addressed in detail. GNSS augmentation can take many forms but all strategies share the same fundamental principle of providing supplementary information whose objective is improving the performance and/or trustworthiness of the system. Hence it is of paramount importance to consider the synergies offered by different augmentation strategies including Space Based Augmentation System (SBAS), Ground Based Augmentation System (GBAS), Aircraft Based Augmentation System (ABAS) and Receiver Autonomous Integrity Monitoring (RAIM). Furthermore, by employing multi-GNSS constellations and multi-sensor data fusion techniques, improvements in availability and continuity can be obtained. SBAS is designed to improve GNSS system integrity and accuracy for aircraft navigation and landing, while an alternative approach to GNSS augmentation is to transmit integrity and differential correction messages from ground-based augmentation systems (GBAS). In addition to existing space and ground based augmentation systems, GNSS augmentation may take the form of additional information being provided by other on-board avionics systems, such as in ABAS. As these on-board systems normally operate via separate principles than GNSS, they are not subject to the same sources of error or interference. Using suitable data link and data processing technologies on the ground, a certified ABAS capability could be a core element of a future GNSS Space-Ground-Aircraft Augmentation Network (SGAAN). Although current augmentation systems can provide significant improvement of GNSS navigation performance, a properly designed and flight-certified SGAAN could play a key role in trusted autonomous system and cyber-physical system applications such as UAS Sense-and-Avoid (SAA). 1. Introduction experimental flight testing, flight inspection/certification of ground-based radio navigation aids, wide area navigation and The origins of Global Navigation Satellite Systems (GNSS) date precision approach). back to the early 1960s, when the United States Department of Defense initiated the development of systems for three– GNSS signal deteriorations typically occur due to antenna dimensional position determination [1]. After the US Navy obscuration caused by natural and man-made obstructions present successfully tested the first satellite navigation system called in the environment (e.g., elevated terrain and tall buildings when TRANSIT, the Space Division of the US Air Force initiated a flying at low altitude) or by the aircraft itself during manoeuvring program, known as Project 621B that evolved into the Navigation (e.g., aircraft wings and empennage masking the on-board GNSS Signal Time and Range (NAVSTAR) program. In 1973, the US antenna), ionospheric scintillation, Doppler shift, multipath, Defense Navigation Satellite System (DNSS) was created, which jamming and spurious satellite transmissions. Anyone of these was later referred to as NAVSTAR Global Positioning System phenomena can result in partial to total loss of tracking and possible (GPS). Various GNSS systems are currently in service or under tracking errors, depending on the severity of the effect and the development. The US GPS and the Russian GLONASS receiver characteristics. Tracking errors, especially if undetected (Globalnaya Navigazionnaya Sputnikovaya Sistema) have by the receiver software, can result in large position errors. Partial achieved their Full Operational Capability (FOC) back in the loss of tracking results in geometry degradation, which in turn 1990’s [2]. Other GNSS systems that are currently at the advanced affects position accuracy. Consequently, GNSS alone does not development or deployment stages include the European always provide adequate performance in mission-essential and GALILEO and the People’s Republic of China BEIDOU safety-critical aviation applications where high levels of accuracy Navigation Satellite System (BDS). GNSS systems typically use and integrity are required. signals 20 dB below the ambient noise floor and, despite several research efforts devoted to interference detection and mitigation GNSS augmentation can take many forms but all share the same strategies at receiver and platform level (mostly for military fundamental principle of providing supplementary information applications), no effective solution has been implemented so far in whose objective is improving the performance and/or the civil aviation context. Thus, there is a need to characterize trustworthiness of the system. GNSS augmentation benefits in the GNSS signal degradations and assess the effects of interfering aviation domain can be summarized as follows: sources on the performance of avionics GNSS receivers and Increased runway access, more direct en-route flight Differential GNSS (DGNSS) systems used for an increasing paths and new precision approach services; number of mission-essential and safety-critical aviation tasks (e.g., Reduced and simplified avionics equipment; Potential elimination of some ground-based navigation at nominal altitudes of 19,100 km (GLONASS), 20,184 km (GPS) aids (VOR, ILS, etc.) with cost saving to Air Navigation and 23,222 km (GALILEO) from the Earth’s surface. Service Providers (ANSPs). BDS also employs satellites in Geostationary Orbit (GEO) and This paper is organised as follows: Section 2 provides a detailed Inclined Geosynchronous Orbit (IGSO). The user segment includes discussion on GNSS aviation applications, followed by a the large variety of GNSS receivers developed for air, ground and description of models for GNSS performance threats in Section 3; marine navigation positioning applications. The control segment Section 4 presents the different augmentation strategies and the includes one or more Control and Processing Stations (CPSs) identification of a pathway to a future GNSS Space-Ground- connected to a number of Ground Monitoring Stations (GMSs) and Avionics Augmentation Network (SGAAN) is discussed in Section antennae located around the globe for Telemetry, Tracking and 5; an investigation of the potential of GNSS augmentation Command (TT&C) signals down/uplink and navigation/integrity techniques to support trusted autonomous Unmanned Aircraft signals uplink to the satellites. The GMS antennae passively track System (UAS) applications is presented in Section 6; The all GNSS satellites in view collecting ranging signals from each conclusions of this article are summarised in Section 7 and satellite. This information is passed on to the CPS where the recommendations for future research are highlighted in Section 8. satellite ephemeris and clock parameters are estimated and predicted. Additionally, satellite integrity data are analysed and 2. GNSS Aviation Applications appropriate integrity flags are generated for faulty/unreliable satellites. The ephemeris/clock and integrity data are then up- Although different GNSS systems employ diversified hardware linked to the satellite for retransmission in the navigation message. and software features, all systems are composed by a space The satellite clock drift is corrected so that all transmitted data are segment, a control segment and a user segment (Fig. 1). The space synchronised with GNSS time. segment includes the satellites required for global coverage. These satellites are predominantly in Intermediate Circular Orbit (ICO), Space Satellite Navigation Signals Segment Satellite TT&C Data Navigation/Integrity Data Constellation Monitoring Data User Segment Legend: Control/Processing Station Ground Monitoring Station Control Satellite TT&C Link Segment Navigation/Integrity
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