GNSS Threats and Aviation, Mitigation Techniques, Alternatives and Regulation (2011)

Since its introduction in Aviation in the mid 90s, GPS has never stopped spreading around the world, increasing aircraft capacity to navigate on more direct routes, independently of the ground navaids network and enabling precision approach and landing. The International Civil Aviation Organization has recommended the wide use of GNSS for Navigation and its use for Surveillance is also promoted. Future ATM concepts as drawn up by European SESAR and US NextGEN, identify performance based operations, that cover all phases of flight, gate to gate, including the Cruise phase, Precision Approach in all weather conditions as well as surface navigation. As these functions are developed, Aviation community must face two challenges and one opportunity. First of all, Aviation Operations are more and more demanding as they require enhanced accuracy, integrity and continuity while maximizing availability, in order to enable more stringent operations in terms of aircraft separations and minima. This has been made possible thanks to data accumulation and confidence gained in GPS over the past 30 years, as well as the more accurate knowledge of their real performance and failure modes. Secondly, as we become increasingly reliant on GNSS technology, we must take account of several threats which the technology faces. One major threat comes directly from the fact that the GPS signal power is weak and must be acquired and tracked in the middle of other sources of radiation in-band or out-of-band such as VHF, SATCOM, DME, mobile phones, TV, UWB devices, Terrestrial Emitters and other GNSSs signals (i.e. intra and inter-system interferences). This leads to interference masks and installation rules for receivers, set by Industrial standards, as well as band protections for aeronautics. But, there are also other sources of interference much more out of control of the aviation community. One threat is linked to abnormal atmospheric propagation effects including tropospheric storms, ionospheric storms, ionospheric scintillation, worsen by the peak of Solar Cycle. The second threat is due to unintentional/intentional interference such as jamming and spoofing, due to personal privacy devices, high power jammer and abnormal tuning of transmitters. It appears that these threats, sometimes very harmful to satellite navigation, are more and more studied and known at least in terms of effects. The validation of the GNSS technology for Aviation has never been so extensive for the benefit of safety, but at the same time, this validation risks delaying the benefits for Aviation stakeholders. All the threats to GNSS are affecting the performance of the users as they can deny the service, affecting availability and/or continuity of the operations based on GNSS, all the more so since all users would be affected at the same time. This could be the case for scintillation or broadband interference over a large area. In some complex scenarios, the threat could become an integrity threat in the case of spoofing or ionospheric storm. At the same time, a fantastic and promising opportunity is given by the emergence of new, stronger and diverse signals from international GNSS Programs such as Galileo, the European Satellite Navigation System, Glonass from Russian Federation and Compass from China, but also GPS modernization, planned to be operational around 2020 timeframe. Besides, augmentations systems are providing enhanced performance and sometimes barriers against these threats like Satellite Based Augmentation Systems (SBAS) including WAAS covering the US Air Space, EGNOS covering European Civil Aviation Conference Airspace, MSAS covering the Japanese Airspace, GAGAN covering the Indian Airspace and SDCM covering the Russian Airspace. Ground Based Augmentation System (GBAS) has also some features against these threats while providing Category III capability in the near term. These systems, thanks to a new signal structure, frequency diversity, increased number of satellites and integrity monitoring, are partially protecting the Aviation users from GNSS threats. However, it is not always sufficient to mitigate the GNSS threats with GNSS only. Indeed, there are some cases where hybridization with additional independent sensors, alternate back-up and to a greater extent increased regulation and security features such as authentication would be needed. After having described the operational context of GNSS utilization and the potential of new GNSS signals, the GNSS threats will be described from a root cause perspective and above all from the effects perspective, focusing on the operations where these threats are more sensitive and harmful. In response to these threats, the existing mitigation techniques as well as alternatives means used on Air Transport aircraft will be detailed including their efficiency limitations and operational coverage. The paper will then suggest some recommendations and mitigation techniques for Air Transport aircraft as well as draw the line between the technical capacity to be provided by the Industry, and the regulations to mitigate the effects of GNSS threats so as to ensure that GNSS increased utilization does not become too constraining for the users, maximizing the expected benefits. Finally, this paper will give some insights into the activity performed inside Airbus to tackle these issues and support the above proposed recommendations.