Hazards to navigation

Considerations of GPS interference to aircraft.

By David Bjellos and Dejan Damjanović
Contributing Writers

This is not an article describing GPS spoofing. Rather, it addresses the limitations, vulnerabilities, and opportunities of what is fast becoming a growing threat to our primary navigation source. Credit for the genesis of this analysis goes to Ops Group founder Mark Zee and his team.

They have done an exceptional job reporting the issues since spoofing became a regular occurrence in early September 2023. GPS is our most critical piece of navigational equipment – and it is being compromised. The hardware and software are susceptible to both spoofing and jamming (see sidebar below for definitions), the detrimental effects of which have been well known since the NavStar (GPS) constellation was established in 1973.

Most GPS applications require precise accuracy. As various disciplines evolved, 3 distinct components emerged – position, navigation, and timing (PNT). Eventually, each of these 3 will have individual, dedicated applications (but we’ll probably still call it GPS).

The NavStar constellation is planned to continue launching replacement satellites (busses) through 2034 and remain operational well into the 2040s. Spoofing exposes a vulnerability that affects nearly everyone, but the threat to aviation is far more immediate and alarming. There has been a 400% increase in spoofing, with some 1500 flights/day affected since first reported.

Degraded/denied performance and safety considerations

Avionics sub-systems are affected to varying degrees (in descending order of greatest impact – enhanced ground proximity warning system [EGPWS], automatic dependent surveillance – broadcast [ADS-B], controller-pilot data link communications [CPDLC], traffic alert and collision avoidance system [TCAS], head-up displays [HUDs], transponders, some weather radars, and navigational accuracy).

Required navigation performance (RNP) capability can become degraded for extended periods of time, eliminating operations in defined airspace (eg, NAT HLA RNP-4 vs RNP-10 considerations). Worse, we are seeing a shift in safety risk – EGPWS alerts are now being questioned and discussed instead of followed automatically.

The extraordinary advances our industry has made in controlled flight into terrain (CFIT) avoidance is now in danger of being compromised because of false position information. Crews are not following EGPWS cautions and warnings by rote memory (ie, escape maneuvers) and we stand to shatter the benefits of CFIT training with a single mishap. Can a simple preflight briefing mitigate this hazard? Almost certainly not.

The weaknesses of civilian unencrypted GPS signals are very well known in government and academia, but they had been rarely exploited until recently. In 2001, the US DOT’s Volpe Center publicized the vulnerabilities and the pace of launching more spoof-resistant satellites increased beginning in 2016, upgrading Block III busses and L5 transmission.

However, one significant susceptibility that remains for older Block I and II busses is power. GPS signal transmission starts at 45 watts at the transmitter and degrades to 10–16 watts at the receiver due to signal degradation. The bulk of NavStar busses on orbit are Block I and II. Only 6 Block III are operational from a nominal constellation of 24 operational satellites, plus on-orbit spares. Thus, terrestrial spoofing transmitters need very little power output to override older orbiting GPS satellites.

Spoofing/jamming evolved into tools of war, essentially a weapon designed to deny a combatant PNT solutions offensively and defensively. The removal of selective availability by President Clinton in 2000 was not without controversy. On balance, it has benefited civil and commercial users immeasurably.

The first Presidential efforts to protect GPS signals from intentional disruptions came in 2021 by Space Policy Directive 7 during the Trump administration, which also formally established the US Space Force (USSF) in 2019 as an additional branch of the military.

The threat to GPS/GNSS integrity for both military and civilian use was so powerful that the USSF became a national security imperative.

At present, the aviation community is uniquely at risk for GPS interference. Efficiency initiatives such as ACAS Xo (active during closely-spaced parallel approaches and high-density environments), performance-based communication and surveillance (PBCS) in the North Atlantic High-Level Airspace (NAT HLA), in-trail spacing (ITSP), and required time of arrival (RTA) will be compromised or delayed.

We are increasingly dependent on space-based navigation, and are poorly equipped to revert to ground-based navaids if necessary. Today’s generation of inertial navigation and inertial reference systems (INS/IRS, respectively) are increasingly miniaturized, but most are also far too reliant on GPS for corrections (hybrid IRS).

Unlike the military, commercial authenticated multi-constellation receivers are far less prevalent or available to choose from. When our GPS/GNSS signals are compromised, safety of flight issues become critical. There are credible academic studies pointing out that the StarLink (Ku-band) constellation may be able to provide PNT solutions. Whether StarLink, too, is vulnerable to spoofing remains to be seen. The chances that StarLink will be open to discussions on an alternative to GPS are promising.

Additional threats

While spoofing and jamming are getting their due attention, more traditional threats remain or are increasing, including:

• China and Russia continue to expand their space capabilities and want to dominate the LEO/MEO/GEO domain with their busses – including those capable to eliminate other military/civilian vehicles (hunter/killer satellites).

• It has been reported that Russia may have launched a satellite fitted with a nuclear weapon.

• The proliferation of satellites in LEO is becoming a debris hazard, and collision avoidance maneuvers are now commonplace among operators. Experts predict “when” not “if” accident scenarios will occur.

Summary

Bad actors can build and deploy GPS spoofing transmitters from readily available materials very inexpensively. We have seen what bad weather and computer problems  can do to an entire airspace system like our NAS, even when the weather is localized.

The Crowd Strike software crash that cost Delta an estimated $500 million and countless cancellations and delays serves as an example. Imagine the economic and commercial damage that a determined entity could inflict if simultaneous spoofing events occurred at just a handful of major US airports, such as LAX (Los Angeles CA), DFW (Dallas Fort Worth, TX), MIA (Miami FL), ATL (Hartsfield-Jackson, Atlanta GA), or JFK (John F Kennedy, New York NY).

The entire NAS would effectively shut down, and the attendant safety of flight risks increased exponentially. Just because the spoofing incidents are occurring in contested/conflict zones at present, the potential for disruption to domestic US interests (and other global economies) is tremendous.

Existing global and regional GNSS coverage (includes most but not all augmented systems – some are incorporated within the global constellation).

This scenario is well known to the US Department of Defense and the federal government. Hundreds of startups planning GPS-alternative PNT solutions and support equipment are rushing to fill the need, populated by bright young engineers and entrepreneurs.

With the exception of the US military, who have multiple constellations to choose from in GPS-denied conflict zones (including their own proprietary satellites), civilian choices at present are limited. StarLink services became operational in 2019 and are far ahead of any competition, but their diligence and buildup began over a decade earlier.

How long could it take for an alternative? A more appropriate question is, Can we afford to wait 15-plus years for another – as yet undetermined – solution?

As it has been since the dawn of aviation, the party who controls the high ground decides who prevails among the combatants. Satellites and the attendant supporting technology being developed are expensive and difficult to implement, but they’re extremely valuable and necessary tools.

Alabama, California, Colorado, Florida, and Texas have essentially become space-centric start-up hubs with hundreds of new entrants. Their collective talent and vision will produce safer and more reliable PNT solutions. Half a century old, GPS has had an outstanding run and served with distinction.

Its effects on the economy, transportation, commerce, and our lives in general is incalculable. New solutions are coming and they should be embraced and implemented just as we did in 1973 with NavStar. The next iteration will be extraordinary.

The authors would like to thank Mark Zee and his team at Ops Group for their significant input and very proactive approach to aviation safety. A working group was established in July 2024 to address the spoofing and jamming threats and it is tasked with providing recommendations and solutions – the results will be completed sometime in September 2024 and made available to PP readers as a Part 2 to this article.

Senior contributor David Bjellos has been writing for PP since 2004. He is an active airman flying a G650 based in south Florida.

Dejan Damjanović is the Managing Director for The FANS GROUP, a PANS-AIM  Consulting firm. Damjanovic is a senior enterprise architect who has worked with Jeppesen and other world-class geospatial service providers.