Global Navigation Satellite Systems (GNSS), including GPS, GLONASS, Galileo, and BeiDou, are critical to modern aviation and society at large. GNSS has become an enabler for all facets of daily life, from finding the shortest route within a city on foot, by car, bicycle, etc. to being a cornerstone of the global financial market. Behind all this is accurate Position, Navigation, and Timing (PNT) information provided by satellites in space and sent to GNSS receivers on land, at sea and in the air.
Recent events have highlighted the vulnerabilities and consequences of and revealed our dependency on GNSS.
Stakeholders are increasingly facing the consequences of interference with and disruption of GNSS. This article looks at the aviation sector, provides an overview of recent incidents and consequences before addressing some ongoing and future developments in support of handling such scenarios.
Types of GNSS interference, symptoms and consequences
GNSS jamming and spoofing are the two types of interference best known to the public:
- Jamming is the intentional interference with GPS signals using radio frequency noise, preventing receivers from accurately determining their location. Therefore, a jammer transmits signals on the same frequency as GNSS, overpowering legitimate signals.
- Spoofing is the act of transmitting fake GPS signals to deceive receivers into calculating incorrect positions or times. Therefore, a spoofer send fake GNSS signals to deceive receivers.
In addition, GNSS interference can also happen through natural or unintentional technical causes. Natural causes include ionospheric and tropospheric interference happening in the Earth’s atmosphere. Unintentional technical causes can emerge from faulty electronic devices, radio transmissions or the inappropriate handling of SDR (Software Defined Radio) devices.
SDRs use programmable software to modulate, demodulate, and process radio signals, allowing them to adapt to different frequencies, protocols, and applications without requiring physical modifications.
Such equipment is widely used in telecommunications, research, military, and GNSS interference analysis, is widely available and inexpensive, depending on its capability and frequency range. The effective geographical range and potential adverse effect is linked to the power output, antenna and environmental conditions, e.g. urban, vs. rural areas and proximity to critical or sensitive infrastructures.
GNSS interference has far-reaching consequences for aviation. Such interference can happen during all phases of flight leading to diversion or re-routing to ensure safety. At times the effects or symptoms of GNSS interference can be difficult to identify. Symptoms include differences between ground speed and true air speed, TAWS (Terrain Awareness Warning System) alerts and time shifts.
In practice this can lead to following consequences:
- Airlines: Flight delays, rerouting, increased fuel consumption, and passenger safety concerns.
- Air Traffic Control: Reduced situational awareness and increased workload
GNSS interference can appear anywhere however, experience has shown that it happens disproportionally more often in and around conflict zones and where geopolitical tensions are high. Detailed information on affected IFRs can be retrieved from EASA who recently created a website dedicated to GNSS Outages and Alterations where you can also download the SIB (Safety Information Bulletin) 2022-02R3. The FAA (Federal Aviation Administration) issued a SAFO (Safety Alert for Operators) on Recognising and Mitigating GPS/GNSS disruptions.
Moreover, jamming is also used for the protection of high-profile figures like presidents when they are traveling to protect their safety through disguising their exact location. A great piece of research has been undertaken by C4ADS on this and can be retrieved here: Above us only the stars
The role of PNT and its wider consequences
PNT services are integral to aviation and have broad societal implications. A lack of PNT availability affects not only aviation but also sectors like agriculture, finance, and telecommunications. The interdependence of these industries on GNSS underscores the importance of maintaining system integrity and resilience. For example, precision agriculture relies on GNSS for accurate planting and harvesting, while the financial sector depends on precise timing for transaction records.
Recent incidents of GNSS interference
Over the past few years, several high-profile incidents have underscored the growing threat of GNSS interference. In 2022, multiple airlines reported navigational disruptions in the Eastern Mediterranean region, affecting flight paths and causing delays. Similarly, in 2021, flights over the Baltic Sea experienced GNSS signal loss, leading to reroutes and safety concerns. Military aviation has also faced challenges; for instance, during NATO exercises, participants experienced jamming in Northern Europe, leading to operational difficulties and safety concerns.
In 2016, an Embraer Phenom 300 which went into a so-called “Dutch Roll” (an aircraft rolls in one direction while yaws into the opposite direction) and an emergency descent due to the reaction of its AHRS (Attitude and Heading Reference System) reacted differently to a GPS outage situation. Here the link to the FAA Telegraphic Message.
Some airports like Tartu in Estonia and airlines bound to land there experienced the operational impact of GNSS interference in April 2024 where Finnair announced the suspension of flights “until alternative solutions have been established“. Tartu airport solely relied on GPS for approach and landing.
Reports from tracking service GPSJam showed that 46000 aircraft have shown potential signs of jamming between August 2023 and March 2024. (Source: wired.com)
On spoofing, 15000 aircraft had their location spoofed to Beirut Airport, more than 10000 to Cairo Airport and over 2000 to Yaroslavl in Russia. (Source: wired.com)
GNSS and Cybersecurity
Cyber-attacks pose a significant threat to GNSS infrastructure. Instances of cyber interference include the attack on ViaSat infrastructure happening during the invasion of Ukraine in February 2022 with the objective to deny Ukrainian Armed Forces the use of effective Command and Control (C2).
This objective was achieved by crippling ViaSat modems and routers with a wiper software. However, this attack had wider consequences on infrastructure beyond military use and beyond Ukraine, showing that spill-over effects can affect society and industries at large.
In addition, we also must keep in mind that assets in space rely on GCS (ground control stations). In some instances, the equipment in those GCS is not appropriately protected, often for operational reasons.
Recommendations and outlook
Reducing the impact of GNSS interference requires a systemic approach involving different elements, all contributing to a higher level of system’s resilience and reduce impact to operations and safety
Multi-Layered Approach:
- Incorporate diverse navigation systems beyond GNSS, such as INS and terrestrial radio navigation systems.
- Some solutions suggest following a so-called hybrid approach where GPS information is augmented with other technologies like a caesium clock (atomic clock) that provides a reliable frequency thus, allows to continue measuring accurate timing in case GPS is disrupted.
- Other navigation approaches investigate downward looking imaging technologies that scan the terrain and compare that information with available mapping information to determine the current position and trajectory of the aircraft.
Invest in Technology:
- Research and develop new technologies to counteract jamming and spoofing is ongoing, and some solutions are in the pipeline. For the time being such solutions merely cater military use cases however, derivatives may over time also find their way into commercial usage.
- Some companies and researchers also invest significant resources into jamming and spoofing resistant equipment.
Cooperation:
- GNSS interference has a global dimension, particularly in aviation as all airlines can be affected when overflying FIRs or landing at aerodromes where such interference is ongoing.
- International collaboration and information sharing can significantly contribute to situational awareness and enable a more proactive approach to GNSS interference. Some EU institutions and organisations, often in collaboration with representatives from the military have already established adequate forums facilitating the exchange of information, experiences and solutions. Examples include the European Defence Agency, Eurocontrol and EASA.
- Moreover, collaboration on best practices and standards can be another critical enabler not only reducing financial burden on operators but also ensuring a more coherent approach to GNSS interference beyond national borders. ICAO in 2023 for example, through its NSP (Navigation Systems Panel) adopted new standards covering DFMC (Dual Frequency Multi Constellation) permitting the combined leverage of dual frequency signals from up to four GNSS constellations simultaneously (GPS, GLONASS, BeiDou, Galileo). This shall promise a higher level of resilience against GNSS interference.
Training and Awareness:
- Fortunately, aircraft can still fly safely even in denied GPS environments however, such a situation leads to an increased workload on the flight deck. To enable flight crew to be better prepared, widespread awareness campaigns should be conducted about such incidents and how those can best be identified.
- Ideally, this is accompanied by training and updated procedures enabling appropriate handling of such situations to reduce the impact of GNSS interference on Flight Operations.
As we look to the future, the aviation industry must prioritize resilience and adaptability. By addressing the challenges of GNSS interference head-on, stakeholders can safeguard the skies and ensure continued progress and safety in aviation operations. The ongoing investment in resilient technologies and international cooperation will be key to overcoming these challenges and maintaining the integrity of global aviation systems.
