On 31 August, European Commission president Ursula von der Leyen was en route from Warsaw to the Bulgarian city of Plovdiv aboard a luxaviation Dassault Falcon 900. During the initial approach to Plovdiv, the pilots reported an “issue with GPS” and requested an alternate approach. The pilots coordinated with the controller, who provided instructions for an Instrument Landing System (ILS) approach and the aircraft landed without incident. So where did a multi-day discussion of GPS jamming come from?
Jammed up — where did the headlines come from?
The aircraft carrying von der Leyen and other European Commission officials and journalists departed Warsaw at 12:37 UTC. At 13:45 the flight began its initial descent into Plovdiv.
As the flight neared Plovdiv and the pilots prepared to land, they reported issues with their ability to fly a GPS-based arrival and asked if an ILS approach was possible instead. Coordinating with ATC, the pilots requested to fly to the VOR near the airport and begin the ILS approach to land on Runway 30. The flight landed at 14:32 UTC without incident
Circling for hours and paper maps
On 1 September, the Financial Times published an initial article claiming that the aircraft lost the ability to navigate with GPS, circled the airport for an hour and that the pilots had to rely on paper maps to land the plane. Certainly, such extensive degradation of the aircraft’s systems is great cause for concern, especially in light of the dramatic rise of GPS interference around the world over the past few years.
As we began to look into the data from this flight, it was immediately clear that the aircraft did not circle for an hour, the flight time appears to have been extended by only a few minutes due to the need for an alternative approach. But what does the data tell us about GPS jamming?
How’s your NIC?
ADS-B data sent by the aircraft contained no clearly anomalous positions that would indicate GPS spoofing, so we looked at the raw data to investigate the possibility of GPS jamming. ADS-B data contains a field called Navigation integrity category (NIC) that encodes the quality and consistency of navigational data received by the aircraft. Throughout the flight from Poland, OO-GPE sent NIC values of 8, indicating a good signal. The map below shows the aggregate of all NIC values received as part of ADS-B messages by the Flightradar24 network during the six-hour period from 12:00-18:00 UTC on 31 August. The red areas indicate high levels of interference with many aircraft reporting degraded NIC values
Looking into the data further, this aircraft also transmitted the Navigation accuracy category for position (NACp). This value provides a 95% accuracy bound on the horizontal position of the aircraft, meaning that for any position, the probability of the aircraft being outside a given radius of a circle centered on that position is 5%. OO-GPE transmitted a NACp value of 10 for the entire flight from Warsaw to Plovdiv, which provides a 95% accuracy bound of 10 meters. An NACp value of 10 does not indicate the presence of GPS jamming.
A changing story
On 4 September, the Bulgarian government clarified that it had no evidence of GPS jamming affecting the aircraft as it landed in Plovdiv. Politico quotes Bulgarian Deputy Prime Minister and Transport Minister Grozdan Karadzhov saying
According to empirical data, according to the radio detection, the records of our agencies, civilian and military, there is not a single fact supporting the claim to silence the GPS signal that affected the plane
However, just hours later, Bulgarian Prime Minister Rosen Zhelyazkov held a press conference suggesting that while there is no current evidence of jamming, it could still have been jamming.
Politico quotes Zhelyazkov as saying, “In this regard, I have instructed the Civil Aviation Authority to contact the airline that is operating the flight in order to conduct an additional check regarding the instruments and computers in the aircraft.”
GPS interference is a real problem
In the map above, you can easily see the pervasiveness of GPS interference. The problems of satellite navigation interference are often seen in flight paths in the area around the Black Sea and the Baltics.
At Flightradar24, we unfortunately can’t do anything directly about the causes of GPS jamming, but we can use data to correct the flight paths as they appear on our service. In an effort to counteract the erroneous positions generated by GPS interference, we now perform Multilateration (MLAT) calculations on all flights, not just those without ADS-B transponders. In areas with good MLAT coverage, this allows us to plot the true path of a flight rather than the disrupted path.
Countering GPS jamming with MLAT
GPS interference on the previous flight
Prior to flying from Warsaw to Plovdiv, von der Leyen’s plane had traveled from Helsinki to Warsaw via an air base in Estonia the day before. En route to Warsaw the aircraft experienced GPS interference over southwestern Estonia near the Gulf of Riga. Because of the calculated MLAT positions, this was not evident when viewing the flight on Flightradar24, but is visible in the raw ADS-B data
During this flight, the NIC value transmitted by the aircraft dropped from 8 to 0 and the NACp value dropped from 11 to 0, indicating the presence of GPS interference. The disruption lasted from approximately 14:33 to 14:44 UTC.
How pilots react to GPS interference
Pilots are trained to deal with navigational failures and disruptions and the redundant systems both within the aircraft itself and in coordination with air traffic control enable the continued safe operation of the aircraft. For a detailed look at how crews manage the aircraft in areas affected by GPS interference, see the video below. We joined the SAS crew aboard the A350 from Copenhagen to Bangkok and encountered GPS jamming for a large portion of the flight
So what actually happened in Plovdiv?
At this point, we don’t know what specific fault the crew encountered that prevented them from using the GPS-based approach, but we do know the data indicates it wasn’t GPS jamming directed at the aircraft. Perhaps the investigation by Bulgarian authorities will be able find definitive answers [end]
The original article
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