Modern commercial aircraft, such as the Airbus A330, utilize a sophisticated architecture to determine their spatial position, this process is managed primarily by the FMS (Flight Management System), but they are affected by GNSS spoofing aircraft navigation systems.
The FMS integrates data from multiple independent sources to calculate a “High Integrity” position and the primary external data source is the MMR (Multi-Mode Receiver), a specialized radio unit that processes signals from the GNSS (Global Navigation Satellite System).
GNSS is the collective term for satellite constellations, including the United States’ GPS (Global Positioning System) and Europe’s Galileo, which provide three-dimensional coordinates by measuring the time-of-flight of radio signals from orbiting satellites.
To ensure accuracy even when satellite signals are unavailable, the aircraft utilizes the ADIRU (Air Data Inertial Reference Unit). The ADIRU consists of two distinct components: the Air Data system, which measures barometric altitude and airspeed, and the IR (Inertial Reference) system.
The IR system uses ring-laser gyroscopes and accelerometers to calculate the aircraft’s position through dead reckoning—measuring motion from a known starting point; this internal calculation is immune to external radio interference but is subject to a phenomenon called “drift,” where small mathematical errors accumulate over time.
To counteract this, the FMS performs a process called hybridization, blending the stable motion data from the IR with the absolute coordinate data from the GPS to create a GPIRS (GPS-Inertial Reference System) position.
Electronic interference with this architecture occurs through either jamming or spoofing.
Jamming is a “denial of service” where a high-power transmitter broadcasts radio noise on the GNSS frequency (1575.42 MHz), overwhelming the MMR and preventing it from locking onto satellites; in this state, the FMS simply flags the GPS data as invalid and reverts to purely inertial navigation.
Spoofing, however, is a “deception” attack, the spoofer transmits a counterfeit signal that mimics the structure of legitimate satellites and if the fake signal is mathematically consistent, it passes the RAIM (Receiver Autonomous Integrity Monitoring)—an internal software check within the MMR that ensures satellite signals are not providing conflicting distances.
Because the spoofed signal is consistent, the MMR accepts the false coordinates as “real” data. This data is then passed to the ADIRU, which uses the fake GPS coordinates to “correct” its inertial track. This contaminates the GPIRS solution, leading the aircraft’s computers to believe they are at a false location.
The FMS compares this false position against the programmed flight plan, if the spoofed data suggests the aircraft is five miles off-course, the Flight Guidance (FG) computer generates a roll command.
If the AP (Autopilot) is engaged in NAV (Managed) mode, it will execute a turn to “regain” the path, physically steering the aircraft away from its actual route while the cockpit displays indicate the plane is perfectly on track.
This corrupted position also flows into the EGPWS (Enhanced Ground Proximity Warning System) and the EGPWS contains a global Terrain Database—a digital map of every mountain and obstacle on Earth.
The system constantly compares the aircraft’s current MMR position and altitude against this map so if a spoofing event “walks” the aircraft’s perceived position over a mountain range in the database—even if the actual aircraft is over open water—the EGPWS will detect a collision risk.
This triggers a high-priority “PULL UP” or “TERRAIN” aural warning, because the EGPWS is designed to save the aircraft from “Controlled Flight Into Terrain” (CFIT), it is programmed to prioritize these warnings, which can be highly distracting to a crew.
Flight crews identify these anomalies through “Cross-Checking” against terrestrial aids and these include VOR (VHF Omnidirectional Range), which provides a bearing to a ground station, and DME (Distance Measuring Equipment), which provides a physical distance.
These systems operate on different frequencies and are unaffected by GNSS interference, if the GPS position disagrees with the VOR/DME position, the crew can manually “deselect” the GPS on the MCDU (Multipurpose Control and Display Unit).
This action isolates the MMR, forcing the FMS to ignore the spoofed signals and rely exclusively on the untainted Inertial Reference data to maintain safe flight.
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