RESILIENCE Reading The Signals

James Miller and Mitch Narins discuss the need for resilient Positioning, Navigation, and Timing Services for the safe, secure, and efficient movement of aircraft in the National Airspace System and beyond

Since the dawn of flight in the early twentieth century, aviation has steadily progressed towards a better means of navigation, both in terms of precision and robustness. This has been enabled by new technologies and techniques being integrated into the cockpit to accommodate safer and more efficient flight operations.

This natural progression, emerging from the initial limitations of an airman’s eye following the line-of-sight rolling curves of rivers, rails, and roads to guide flightpaths, quickly advanced as these curvilinear landscape paths were augmented and ‘straightened-out’ with manmade navigation aids consisting of strategically placed signal fires and large concrete markers.

Besides making flight safer, always a high priority of pilots, these improved routes also prompted great efficiencies and economic value to the postal service and other users as both time and money was saved with more direct routeing to additional destinations.

By the middle of the twentieth century, through the advent of electronics derived from World War II, aviation quickly transitioned from pilotage visuals and kneeboard dead-reckoning to the seemingly mystical benefits of radio navigation – where dials and glowing gauges with jumping needles came to lead the pilot’s way. These newly emerging radio aids became the foundational era of beacons and four-course ranges that evolved into the very-high frequency (VHF) and ultra-high frequency (UHF) azimuth and ranging elements that remain the backbone of global air traffic management (ATM) today.

These advancements were not to stall however. Aviation, being a most dynamic sector, continued its march forward so that improving flexibility and resiliency with additional radio sources and sensors were always core characteristics. Thus, shortly before the twentieth century rolled into the twenty-first, the world aviation community was introduced to a powerful and incredibly accurate positioning, navigation, and timing tool – the space-based Global Positioning System (GPS), developed by the United States Air Force (USAF).

The advent of GPS brought immense, amazing new capabilities to many industries and segments of society worldwide, and to aviation in particular. The system’s global coverage, extreme accuracy and repeatability, and precise time dissemination prompted the development of many new aircraft systems and procedures that would truly revolutionise how aircraft would operate thereafter.

For the USAF, GPS was known as a force-multiplier. For civil aviation, GPS became a ‘leap-frog’ game-changer, dramatically increasing airspace safety, efficiency, and capacity almost overnight. These new applications, now standard equipment on many flight decks, range from Enhanced Ground Proximity Warning Systems (EGPWS) to Automatic Dependent Surveillance Broadcast (ADS-B), among many others.

GPS thus became the great enabler for both air traffic managers as well as pilots, providing much greater situational awareness and timely precision control, which in turns allowed for reduced separation distances, more direct, accessible and flexible flight paths, and lower operating costs for both the Federal Aviation Administration (FAA) and airlines as infrastructure requirements were minimised and aircraft flew more efficiently.

As these new applications were being incorporated in the 1990s, there was much technical and policy discussion on just how far to adapt and integrate the use of GPS signals. This is because although satellite-based navigation removed many constraints derived from using terrestrial radio navigation aids, airline executives are traditionally quite conservative and prefer to invest in proven technologies where there is a high probability of a quick return on investment. GPS was not such a known quantity, however, and its adoption would be shaped by fits and starts.

Initially, equipage with GPS was thus a relatively expensive and potentially risky endeavour, as even with the radio signals being available, standards and procedures had to be developed, pilots had to be trained, and the FAA always maintained the final call on the scope and pace in which these new investments could be implemented and utilised.

Even with all the benefits to be derived from GPS for civil aviation as demonstrated by numerous successful worldwide military applications in the field, there were still early lingering questions as to just how reliable weak signals from space would be when used within the many diverse and challenging aviation environments in busy metropolitan areas with high-powered ground transmitters scattered throughout. These transmitters, of course, had the potential to overpower GPS signals, making them unavailable during critical phases of flight such as approach and landing.

So, although GPS could greatly enhance aeronautical operations, it was also found that the adoption of satellite navigation (satnav) for safety-of-life flight required a careful, incremental approach as risk assessments and mitigation approaches were incorporated throughout the transition periods.

Sometimes this even required a few dance steps, and on more than one occasion the FAA had to ‘step back’ from the precipice. A good example of this is when the FAA, under pressure to reduce ground infrastructure costs by Congress, and being constantly lobbied by the airline industry for more cost efficient flight operations, moved full speed ahead to use GPS as a ‘sole-means’ navigation tool, only to reverse course when it became apparent that signals from space may not be robust or resilient enough, to be used alone.

A good example of this historic ‘push and pull’ to balance benefits with exposures as GPS was being adopted can be observed through a 2001 report on the vulnerabilities of GPS issued by US Department of Transportation Volpe Transportation System Center. In particular, this detailed technical report cited the potential risks of relying solely on an extremely low-power positioning, navigation, and timing (PNT) signal that could be interfered with – either by unintentional jamming or intentional spoofing.

Aviation and other sectors, which had assumed that their legacy, ground-based PNT systems could be replaced by satellite-based PNT services, needed to revisit their strategic planning to ensure that the necessary PNT services would remain available to users in the event of a loss of GPS signals. The end result was that GPS was approved by regulators as a tool for ‘primary’ means of navigation rather than ‘sole’ means, requiring both air navigation service providers (ANSP) and airlines to retain the capacity to process traditional terrestrial radio aids even as they became more reliant on space-based services.

This seems to have provided an acceptable outcome for both service providers and users during what has become a multi-decadal transition process, and has become the basis for ensuring that enough redundancy is built into the navigation architecture such that while GPS use is rewarded with the most precise and efficient PNT outputs, other, less accurate terrestrial options (VORs, DMEs, etc.,.) remain available if space services degrade or become unavailable.

Through several years of development and experience gained since the initial 2001 DOT Volpe GPS Vulnerability Report [1], and during a parallel period where other nations modernised and/or began developing their own Global Navigation Satellite Systems (GNSS), including Russian GLONASS, the European Galileo, China’s BeiDou, and even regional systems transmitting GNSS signals such as Japan’s Quasi-Zenith Satellite System (QZSS) and India’s NavIC, the US government issued a Presidential Policy Directive (PDD) in 2013 entitled ‘Critical Infrastructure Security and Resilience’.

This more recent policy advanced ‘a national unity of effort to strengthen and maintain secure, functioning, and resilient critical infrastructure’, and defined resilience as ‘the ability to prepare for and adapt to changing conditions and withstand and recover rapidly from disruptions’. In the formalised national recognition of the need for such a definition, the policy also noted that ‘resilience includes the ability to withstand and recover from deliberate attacks, accidents, or naturally occurring threats or incidents’. In harmony with this PDD language and subsequent calls for action, the USG’s latest Federal Radionavigation Plan (FRP), released in 2017, also recognises the vulnerability of GNSS PNT services and states that ‘no one PNT technology may meet all PNT user needs’.

Thus, decades after the initial ‘push-pull’ dances of adopting GPS for aviation, other sectors are now realising that they may be overly reliant upon GNSS, and therefore vulnerable to the impacts of service disruptions or outages. These other sectors, ranging from transportation modes to communications systems, must now also wrestle with ensuring they have adequate back-up options for continuity, just as aviation refined the scope of their GPS adoption to ensure redundancy.

While these other critical infrastructure sectors now search for resiliency in their use and dissemination of GNSS-based PNT services, it is reassuring to know that commercial aviation remains the most resilient of all the nation’s critical infrastructure, having the capacity to use both terrestrial and satellite-based signals for all manner of flight operations.

The challenges for the aviation sector has not diminished though, even as GNSS use continues growing amid overt threats of jamming, spoofing, and even radio frequency interference (RFI) from newly proposed terrestrial broadband networks. The goal then, remains to protect and promote the safety, efficiency, and capacity of GNSS space-based services, all while maintaining operational resilience by retaining access to an appropriate combination of enhanced ground-based services and avionics tools, and appropriate procedures that capitalise on both.

PNT Resilience through Ground-Based Systems

During the last decade, many ANSPs have developed routes and associated procedures based on the combined interaction and performance of ground and satellite based systems, and avionics. These Performance-Based Navigation (PBN) initiatives marked a dramatic departure from previous strategies that presumed that GNSS services would eventually become the ‘sole means’ of supporting all aircraft operations in all domains.

The examined vulnerabilities of GNSS highlighted the importance of maintaining some core elements of the ground-based infrastructure to ensure resilience, but also sparked the realisation that these same ground-based systems could serve far beyond a back-up role to actually further enhance many of the same PBN procedures originally derived through the use of GNSS for improved safety, efficiency, and capacity.

For instance, in 2016 the FAA published its PBN National Airspace System (NAS) Strategy, which included the commitment ‘to ensure that the NAS navigational infrastructure remains secure, sustainable and resilient’. It also recognised that ‘a key component of the navigational infrastructure is a resilient position, navigation and timing capability independent of GNSS that will ensure safety while minimising the impact of a GNSS disruption’.

To ensure that the elements needed for safety, recovery, and continued aviation operations would be maintained, the FAA PBN NAS Strategy supports the continuation of most Ground Based Navigation Aides (GBNA), including a Minimum Operational Network (MON) of Very High Frequency Omnidirectional Radios (VOR), an increased Distance Measuring Equipment (DME) network to support area navigation (RNAV) and the PBN Route Structure (PBN RS) concept, and Instrument Landing Systems (ILS) to ensure the safe recovery of aircraft in all weather conditions. ‘Rationalisation’ studies continue at the FAA and other ANSPs to determine the extent of the legacy navigational infrastructure needed to support resilient PBN and ensure safe and secure operations.

A number of ANSPs are also exploring other Alternative Position, Navigation, and Timing (APNT) possibilities that would continue to support safe RNAV operations, as well as potential procedures requiring Required Navigation Performance (RNP). Specifically, the US Federal Radionavigation Plan notes ongoing research ‘looking at technologies capable of delivering PNT data…including a study of eLoran PNT capabilities’.

Additionally, both the US and Europe have recognised the extreme value of the electromagnetic spectrum used by DMEs and Tactical Navigation (TACAN) and are exploring means of utilising this spectrum more efficiently, while maintaining current legacy distance and RNAV operations for the foreseeable future. There are also ongoing studies to determine how DME-based navigation could be more accurate, provide integrity to support RNP procedures, and even distribute precise and trustable time.

The need for trusted time has become even more critical as some more visible attacks on GNSS services have focused on ways to corrupt or spoof a GNSS receiver’s clock, which in turn affects the position being reported. While interest in the potential of providing independent and robust time from enhanced Loran (eLoran) system continues, research into other means of providing resilient precise time is also ramping up.

PNT Resilience through Avionics Systems

PNT Resilience in aviation is no longer the sole province of processing the signals from ground-based systems any longer, as described earlier. It has long been recognised that an aircraft’s avionics system plays an equally important role in ensuring safe and efficient operations. With the uptake of GNSS technology into the cockpit and its use to support most RNAV and all RNP procedures, especially in non-commercial fleets with fewer redundant systems, the need to ensure the ability of these aircraft to continue to dispatch, operate in terminal, enroute, and oceanic airspace, and safely land at their intended destinations during potential disruptions of GNSS services, is of great importance. Appreciatively, most of the commercial fleet has maintained and will continue to support their legacy onboard navigation capabilities and remain, for the most part, independent from GNSS systems.

This is in many ways a positive architecture evolution when one considers the need to process different signal sets. Physical separation of systems maintains separate electrical interfaces to critical systems, prevents problems that could arise from software/firmware updates, and supports ‘domain guards’ on networked aircraft systems.

Multiple, dissimilar radionavigation systems, as well as onboard including inertial systems, can support redundancy and consistency checks that could detect accuracy issues or verify the integrity issues of GNSS provided PNT services.

Antenna Technology

Advances in antenna technology and concepts, and the ability of aircraft to use what has been non-exportable technologies (e.g., Controlled Reception Pattern Antennas (CRPA)) can also help aircraft to attain PNT resilience. In its simplest of forms, larger aircraft could install multiple GNSS antennas at relatively “large” (e.g., 3 – 5 meters), calibrated spacing. Thanks to the precise accuracy of GNSS, a spoofing attack could be detected when the GNSS positions of each antenna merged. Research has also been done that would allow a single GNSS antenna to determine whether received GNSS signals were emanating from above or below an aircraft.

Enforcement of Spectrum Laws

  • Receiver Autonomous Integrity Monitoring  –  The development and continued advancements of Receiver Autonomous Integrity Monitoring (RAIM) is arguably the most significant contribution of avionics towards achieving aviation’s PNT resilience. RAIM was developed to assess the integrity of GNSS signals in a GNSS receiver system and is of special importance in safety-critical GNSS applications, such as in aviation or marine navigation. RAIM detects faults with redundant GNSS pseudorange measurements. A pseudorange that differs significantly from the expected value may indicate a fault of the associated satellite or another signal integrity problem. With the addition of the GPS L5 civil signal, the abilities of RAIM to both detect and exclude will be much improved.
  • Address civil navigation security from a holistic perspective
    • Employ processes and technologies developed for networked aircraft
    • Security measures and software/firmware design assurance requirements are derived from risk assessment
  • Enhancing Redundancy and Consistency Checking
    • Enhanced integrity algorithms
    • Additional Satnav signals
    • Retaining and enhancing terrestrial navigation sources
    • Exploiting aviation networks
    • Limited RFI mitigation could effectively address fratricide impact on civil aviation (e.g., PP devices near airport)
  • Open GNSS services (GBAS, SBAS, ADS-B) could buttress security of legacy signals through deterrence (more complex signal environment)
    • Would require modifications to use GNSS and legacy signals in parallel
    • Future security improvements to GNSS (integrity, availability, confidentiality) would support transition away from legacy signals

Resilience through Satellite-Based Services

Multi-GNSS use will result in new capabilities available for PNT. NASA’s mission to pioneer space exploration, scientific discovery, and aeronautics research includes a number of GNSS application areas.

At least four GPS satellites are needed to be within line-of-sight at any given time to enable onboard real-time autonomous navigation in space. However, as we move up beyond 3,000 km altitude, the number of available GPS signals decreases and, because of poor geometry and blockage of main beam reception from the Earth. However, if accessing the signals from all GNSSs, and regional navigation systems, there would be much improved capabilities to perform real-time onboard navigation at higher altitudes and, also, to obtain improved accuracy from using GNSS signals that are more optimally distributed in space – best geometry.

This has led NASA to lead efforts towards developing interoperability among GNSS. Real-time onboard autonomous navigation is vitally important to users in space because it supports increased satellite autonomy for missions – thus lowering mission operation costs, significantly improved vehicle navigation performance, and quick mission recovery after spacecraft trajectory manoeuvres. Similarly, access to signals from multiple interoperable GNSS constellations provides all users – space, aviation, and surface – resilience in the event service from one GNSS is disrupted (Fig. 1). In summary, GPS capabilities to support users everywhere will be further improved by pursuing compatibility and interoperability with other GNSS. [Ref.2]

Fig 1: NASA GNSS-based On-Board Autonomous Nav Components (LEO: Low Earth Orbit; MEO: Medium Earth Orbit; GEO: Geosynchronous Orbit; HEO: High Earth Orbit)

Way Ahead

We have described a number of technologies available for improving navigation resilience, including ground-based systems, avionics systems and satellite-based systems. The way ahead for improving overall navigation resilience is to find an optimal combination of these techniques, as we all multi-sensor fusion, into a systems of systems.

The US is undertaking a major transformation of air traffic control, known as Next Generation Air Transportation System (NextGen), by the year 2025. It is a multi-billion-dollar technology modernisation effort that will make air travel safer, more flexible and more efficient. As the system gets better, its capacity will grow and the demand for different types of air transportation – even unmanned aircraft – will increase. Figure 2 depicts the major organisational and operational elements of NextGen.

Fig 2: Next Generation Air Transportation System – Credit: NASA


James J Miller is the deputy director of the Policy & Strategic Communications Division with the Space Communications and Navigation Programme (SCaN) at the National Aeronautics and Space Administration (NASA) Headquarters in Washington, DC. Miller is responsible for advising NASA leadership on US and international Positioning, Navigation, and Timing (PNT) policy and technology issues. He is also the executive director of the National Space-Based PNT Advisory Board. Previously he was deputy director of the Office of Navigation and Spectrum Policy, Office of the Secretary, at the US Department of Transportation. Miller is a commercial pilot with degrees in aviation flight, aviation management, Master of Public Administration degree from Southern Illinois University, and Master of International Policy and Practice from George Washington University.

Mitch Narins is principal consultant/owner of Strategic Synergies, LLC. Formerly he was chief systems engineer for navigation at the Federal Aviation Administration. Mitch also served as the FAA’s Programme Manager for Terminal ATC Automation, Programme Lead for Next Generation Air-to-Ground Communications, Programme Lead for NAS Infrastructure Management System Master Planning, Team Lead for Electronic Navigation Systems, and Programme Manager for Systems Engineering and Integration Support. Prior to the FAA he headed the US Marine Corps Special Project Office’s Communications Terminal and Electronic Warfare branches at the Naval Electronic Systems Command and worked as a standards and facilities engineer at the Federal Communications Commission’s Field Operations Bureau.

Narins holds a Bachelor of Engineering (EE) degree from the City College of New York and a Master of Engineering Administration/Management degree from the George Washington University. He is a Certified Information Systems Security Professional (CISSP), an active member of the US Institute of Navigation, and a Fellow of the Royal Institute of Navigation.

[1] “Vulnerability Assessment of the Transportation Infrastructure Relying on the Global Positioning System.” Prepared by the John A. Volpe National Transportation Systems Center for the Office of the Assistant Secretary for Transportation Policy, U.S. Department of Transportation. August 29, 2001.