Enhanced Ground Proximity Warning systems have saved numerous lives since the 1990s. The commercial aviation industry must act to prevent GPS spoofing from undermining that value.
In a recent blog, I looked at the impact of GPS spoofing on ADS-B, the system most aircraft use to communicate their position and other key information to air traffic control.
But ADS-B isn’t the only aircraft system that’s affected by GPS spoofing. In this blog I want to look at the impact on the enhanced Ground Proximity Warning System (eGPWS); a critical safety application.
As we’ll see, the effect of a spoofing attack on eGPWS can seriously compromise situational awareness in the cabin, causing stress for flight crew and disruption to flight schedules.
A quick recap of GPS spoofing
But first, a recap of what’s meant by GPS spoofing. Like many other vehicles and devices, modern aircraft rely on radio signals from the Global Positioning System, a constellation of satellites orbiting the Earth at a distance of 12,550 miles. The signals contain data that allow the receiver to accurately calculate the position, heading, and groundspeed of the aircraft.
However, these signals are very faint by the time they reach the receiver, and can be drowned out by stronger signals transmitting on the same frequency. In a spoofing attack, a malicious actor uses radio equipment to broadcast false GPS signals in the GPS frequency band, usually at a higher power than GPS. If an unprotected receiver locks on to these signals instead of the real ones, it may calculate an incorrect position based on the co-ordinates given by the false data.
GPS spoofing has become easier to carry out in recent years, and it’s now a staple navigation warfare (NAVWAR) tactic used by nation states to disrupt adversaries’ operations in conflict zones. Spoofing is becoming more commonplace in other areas too, and its impact on commercial aviation is growing. OPSGROUP found that around 1,500 commercial flights a day experienced GPS spoofing in July–August 2024, five times more than in the first half of the year.
eGPWS has saved countless lives since the 1990s
One system that can be seriously affected by GPS spoofing is eGPWS. This is a safety system, installed in commercial aircraft, which alerts flight crew to approaching terrain. eGPWS was introduced in the 1990s to reduce the risk of Controlled Flight Into Terrain (CFIT) incidents, in which flight crew either miscalculate the aircraft’s altitude and collide with the ground or water, or fail to notice mountainous terrain ahead and collide with it.
Early GPWS systems used a radio altimeter to calculate the aircraft’s altitude and rate of change to indicate when the aircraft was over rising terrain. But because the radio altimeter only measured vertical distance from the ground, it wasn’t effective at warning of terrain ahead. It also lacked an understanding of different phases of flight to help avoid CFIT on approach and landing. eGPWS, by contrast, compares the aircraft’s GPS position with a digital map to provide warnings in all phases of flight, and by “looking ahead” it can provide more time for the crew to perform an escape maneuver.
When GPS signals are unaffected by interference, eGPWS does its job extremely well. In fact when its inventor, Don Bateman, died in 2023, his obituary in the Seattle Times credited him with having saved more lives than anyone else in aviation history. But if the aircraft’s GPS receiver is compromised by a spoofing attack, eGPWS can become unhelpful at best.
What happens to eGPWS when an aircraft experiences a GPS spoofing attack?
The problem is that eGPWS typically takes its GPS position and altitude directly from the aircraft’s GPS receiver, usually with no additional checks to ascertain whether or not that data is accurate.
If the receiver is spoofed, the eGPWS will assume the aircraft is at the spoofed position and altitude. If that is near the ground or close to a mountain, the eGPWS will sound an alert. Other eGPWS errors include incorrect terrain and runway proximity, flight path deviation alerts, and incorrect Mode 6 advisories (smart callouts).
Under normal conditions a crew is required to act on every eGPWS alert. But alerts are easy to ignore when they are clearly false, such as when the aircraft is in cruise with no terrain in sight. This has the potential to create situations where crews are slow or hesitant to respond to genuine eGPWS alerts—or may not even respond at all.
Crew desensitization to such a critical safety system is a clear risk for the industry. Exposing flight crew to alerts that repeat for as long as the aircraft is in a spoofing-affected area (or until the eGPWS is disabled) also adds unnecessary stress to an already high-pressure job.
New safety procedures impact landing schedules in areas of GPS spoofing
The impact of spoofing on eGPWS is particularly significant on landing approaches, where both flight crew and ATC must have a clear and accurate understanding of where the aircraft is in relation to the ground. False terrain warnings can disrupt landing procedures and cause heightened stress, especially at times of low visibility.
This has prompted the industry to develop specific guidance and procedures for pilots landing in an area of suspected GPS spoofing. Unless the crew can visually verify that there is no threat, the crew must respond to the eGPWS alert. This most commonly results in a go-around. The crew must then determine if a second approach can be done and if it is safe to ignore an eGPWS alert. If not, they may end up diverting to a different airport.
The risks arising from a spoofed GPS receiver feeding false data to eGPWS, then, can include delays, diversions, higher fuel burn, reduced safety and increased stress on flight crew and ATC.
The commercial aviation industry must address the risk of spoofing
The impact on eGPWS alone makes GPS spoofing a risk that the commercial aviation industry can’t afford to ignore. The combined impact on eGPWS, ADS-B and alternative aircraft navigation systems makes it vital to take action to mitigate the risk of spoofing.
Short-term risk mitigation measures might include subscribing to a GPS spoofing detection and alerting service like the one we offer at Spirent. This can enable the crew to make safer decisions in areas of GPS spoofing.
Longer-term measures might include upgrading GPS receivers to support more constellations and more frequencies, upgrading antenna hardware to CRPA technology, or utilizing other sources of PNT.
Spirent eBook: Protecting Commercial Aircraft Against GPS Spoofing Threats
To help inform risk mitigation decisions, we’ve produced an eBook that dives deeper into the impact of GPS spoofing on commercial aviation. It looks at the risks to airlines and aircraft, and sets out a mitigation roadmap with recommended actions for the immediate, medium and longer term.
You can download it here: Spirent eBook: Protecting Commercial Aircraft Against GPS Spoofing Threats.
Source: spirent blog
