FAA's GNSS Interference Resource Guide: A Technical Analysis for the PNT Industry

On December 4, 2025, the FAA’s Flight Technologies and Procedures Division quietly released a 69-page document that should be mandatory reading for every GNSS professional: the GPS/GNSS Interference Resource Guide, Version 1.0.

This isn’t a typical regulatory advisory. It’s the first comprehensive acknowledgment from the U.S. aviation authority that GNSS interference has evolved from a theoretical threat to an operational crisis. With over 1,000 daily interference incidents globally and the specter of the Azerbaijan Airlines Flight 8243 crash hanging over the industry, the FAA has moved from cautious monitoring to active intervention.

For PNT engineers, avionics manufacturers, and navigation system designers, this document reveals both the depth of the problem and the regulatory direction ahead. Let’s dissect what it means for the industry.

The Scale of the Crisis: By the Numbers

The FAA’s guide opens with sobering statistics that frame the current operational reality:

1,000+ daily GNSS interference incidents reported globally (up from ~700 in 2024)
220% increase in GPS signal loss events between 2021 and 2024
69 aircraft experienced GPS spoofing during a single representative event analyzed by EASA
9,000+ flights daily affected in the Eastern Mediterranean alone during peak interference periods
38 people killed in the Azerbaijan Airlines Flight 8243 crash on December 25, 2024—directly linked to GNSS spoofing

Global hotspots for GNSS interference GNSS interference hotspots span the Eastern Mediterranean, Black Sea, Middle East, Baltic region, and the India-Pakistan border.

The geographic concentration is notable. The guide identifies primary hotspots including:

Eastern Mediterranean / Cyprus FIR: The most intense zone, with interference extending hundreds of nautical miles from conflict areas
Black Sea region: Heavy activity near Ukraine and Russian-controlled territories
Middle East: Iraq, Syria, Iran, and the broader Arabian Peninsula
Baltic Sea / Russia border: Escalating activity affecting Northern European airspace
India-Pakistan border: Ongoing interference affecting South Asian routes

These aren’t isolated incidents. They represent a sustained, multi-vector assault on GNSS infrastructure by state actors engaged in electronic warfare, with civil aviation as collateral damage.

Defining the Threat: Jamming vs. Spoofing

The FAA document provides clear technical definitions that are worth internalizing:

Jamming

“Electromagnetic emission aimed at denying the use of GNSS signals by overpowering authentic signals, or by significantly increasing the noise floor, resulting in the pilot or system receiving denial of service for GNSS Position, Navigation, and Timing (PNT) services.”

Jamming is the brute-force approach. It doesn’t require sophisticated signal generation—just enough RF power to overwhelm the weak GNSS signals arriving from 20,200 km altitude. The result is typically a complete loss of GPS position, which modern avionics handle through reversion to inertial navigation or other backup systems.

Critically, jamming is self-announcing. Pilots know immediately when GPS is unavailable because the receiver reports signal loss. This allows procedural responses.

Spoofing

“Electromagnetic emission of counterfeit GNSS signals in order to mislead recipients, making them calculate an incorrect PNT solution, potentially including an increased hazard to aircraft.”

Spoofing is the more insidious threat. It doesn’t deny service—it corrupts it. The receiver continues reporting position and time, but the data is wrong. The pilot may have no indication that the navigation solution is compromised.

The FAA guide emphasizes a crucial point that many GNSS professionals already understand but that hasn’t penetrated aviation operations: RAIM does not protect against spoofing.

Receiver Autonomous Integrity Monitoring (RAIM) is designed to detect faulty satellites or abnormal error conditions within a GNSS constellation. It operates by checking consistency among multiple satellite measurements. But a well-crafted spoofing attack presents a consistent, self-reinforcing set of signals. Every satellite the receiver sees tells the same (false) story. RAIM passes all checks while the position is completely wrong.

This is why the FAA document repeatedly emphasizes cross-checking with independent, non-GNSS sources—particularly inertial reference systems (IRS).

The Cascade Effect: How Spoofing Propagates Through Aircraft Systems

Perhaps the most valuable section for avionics engineers is the detailed analysis of how GNSS corruption propagates through integrated aircraft systems. The FAA doesn’t just list affected equipment—it traces the infection vector.

Primary Affected Systems

ADS-B Out (Automatic Dependent Surveillance-Broadcast)

ADS-B Out transmits the aircraft’s position to air traffic control based on GNSS data. When GNSS is spoofed, ADS-B broadcasts the wrong position. This creates:

False traffic separation: ATC sees aircraft where they aren’t
Collision risk: Actual aircraft positions become unknown to the surveillance network
Regulatory implications: TCAS advisories based on false ADS-B data may be incorrect

The guide notes that spoofed ADS-B positions have been observed up to 200+ nautical miles from actual aircraft location.

ADS-B In and CDTI (Cockpit Display of Traffic Information)

Aircraft equipped with ADS-B In receive traffic data from other aircraft. If multiple aircraft in an area are spoofed to report the same false position, the cockpit display shows impossible clustering—multiple aircraft at identical coordinates.

During actual spoofing events, pilots have reported CDTI showing 50+ aircraft stacked at a single point.

TAWS (Terrain Awareness and Warning System)

TAWS uses GNSS position to compare against a terrain database. If the aircraft believes it’s 100 nm from where it actually is, TAWS terrain warnings become meaningless—or dangerous.

The FAA specifically addresses the Egpws (Enhanced Ground Proximity Warning System) issue: spoofed position data can cause:

False PULL UP warnings when no terrain threat exists
Failure to warn when actual terrain is present but the system thinks the aircraft is elsewhere
Pilot desensitization from repeated false warnings leading to ignored genuine warnings

The Azerbaijan Airlines Flight 8243 crash exemplifies this cascade. Initial reports indicate spoofing-induced navigation errors contributed to the crew’s spatial disorientation during approach to Aktau, Kazakhstan, after diverting from Grozny.

TCAS (Traffic Collision Avoidance System)

While TCAS primarily uses transponder interrogation rather than GNSS, the guide notes that TCAS-equipped aircraft receiving spoofed ADS-B data may generate false Resolution Advisories (RAs) directing pilots to climb or descend to avoid phantom traffic.

IRS/FMS Integration

This is where spoofing becomes particularly dangerous. Modern Inertial Reference Systems (IRS) are not purely self-contained. They accept GNSS updates to correct long-term drift.

If GNSS is spoofed during IRS alignment or during flight, the false position data can be absorbed into the inertial solution. Even after GNSS is restored, the IRS may continue reporting incorrect position until realignment.

The FAA guide terms this the “Lingering Effects” problem:

“GNSS spoofing may have lingering effects on aircraft position, including the aircraft IRU/IRS position solution and the aircraft system clock.”

For hybrid navigation systems, this contamination can persist for hours.

Aircraft cockpit with navigation displays Modern glass cockpits rely heavily on GNSS-fed displays. When the underlying data is corrupted, every dependent system inherits the error.

Secondary Effects

System Clocks

GNSS provides timing to numerous aircraft systems. Spoofed time can affect:

CPDLC (Controller-Pilot Data Link Communications): Message timestamps become unreliable
ADS-C (Automatic Dependent Surveillance-Contract): Position reports carry incorrect timing
FMS time synchronization: Calculated arrival times become invalid
PBCS (Performance-Based Communication and Surveillance): RCP/RSP compliance may be violated

ELT (Emergency Locator Transmitter)

If an aircraft crashes after prolonged spoofing, the ELT may transmit the last known (false) GNSS position—sending search and rescue to the wrong location.

Weather Radar and Predictive Windshear

Some advanced weather systems use GNSS position for georeferencing radar returns. Spoofed position data can cause:

Weather displayed at wrong relative bearings
Failure to correlate actual weather with displayed position
Incorrect predictive windshear calculations

Human Factors: The Pilot’s Dilemma

The FAA guide dedicates significant attention to human factors—an acknowledgment that technology solutions alone are insufficient.

The Startle Effect

GNSS interference events often begin without warning. Pilots may suddenly face:

Navigation flags appearing on primary flight displays
Uncommanded autopilot disconnection
Multiple simultaneous system alerts
TCAS RAs with no visible traffic
TAWS PULL UP warnings over open ocean

The cognitive load of processing these conflicting inputs while maintaining aircraft control creates significant risk, particularly during high-workload phases like approach and landing.

The Habituation Problem

In interference hotspots, pilots may experience GNSS degradation on every flight. The guide warns that this can lead to:

“Crews becoming complacent about GNSS outages, with resultant habituation affecting normal reactions to warnings and alerts.”

When TAWS false alarms become routine, pilots may begin dismissing genuine warnings.

Verification Challenges

The guide emphasizes that pilots should cross-check GNSS position against independent sources—primarily IRS and ground-based navaids. But this creates practical challenges:

Workload: Manual position verification is time-consuming
Skill decay: Many pilots have limited recent experience with non-GNSS navigation
VOR/DME reduction: Ground-based navaid infrastructure continues shrinking globally
IRS limitations: Standalone IRS drifts ~1 nm/hour; extended GPS denial accumulates significant error

The FAA’s recommended procedure is deceptively simple: “Compare FMS position to IRS position.” If they diverge significantly with no obvious cause, suspect spoofing. But operationally, this requires pilots to understand the normal behavior of their specific IRS-FMS integration—knowledge that varies by aircraft type and isn’t always well-documented in training curricula.

EPU, ANP, and FOM: Technical Indicators of Spoofing

For GNSS engineers, the guide’s discussion of Estimated Position Uncertainty (EPU) and Actual Navigation Performance (ANP) provides valuable operational context.

The EPU/ANP Paradox

Under spoofing, the receiver continues calculating EPU/ANP based on received signal characteristics. Since the spoofer typically transmits clean, high-SNR signals, the receiver may report excellent accuracy even while the position is completely wrong.

The FAA notes:

“Spoofing signals may even be cleaner than normal signal reception, so GNSS EPU/ANP may not ‘balloon’ like it does during jamming.”

This means the normal pilot cue—an expanding uncertainty circle on the navigation display—may be absent during spoofing.

Figure of Merit (FOM) Degradation

The FOM is a composite navigation accuracy indicator used for PBCS compliance. Under spoofing:

FOM may remain at high values (indicating good performance)
Actual position error may exceed RNP tolerances by orders of magnitude
Aircraft may lose PBCS eligibility without proper system indication

The guide recommends pilots disregard FOM values in areas of known interference and revert to procedural separation.

Altitude Anomalies

One subtle spoofing indicator is barometric-GNSS altitude divergence. The aircraft’s pressure altimeter derives altitude from atmospheric pressure—independent of GNSS. If GNSS-derived altitude suddenly diverges from barometric altitude by hundreds of feet, spoofing is likely.

However, this requires pilots to actively monitor both altitude sources, which may not be standard practice.

Industry Implications: What This Means for GNSS Professionals

For Avionics Manufacturers

The FAA guide signals coming regulatory requirements. Key implications:

1. Spoofing Detection Requirements

While current TSO’d GPS receivers aren’t required to detect spoofing, the FAA’s detailed treatment of spoofing indicators suggests forthcoming standards. Manufacturers should anticipate:

Requirements for multi-source position comparison
Automated spoofing alert systems
Integration of non-GNSS integrity sources

2. System Isolation Architectures

The cascade propagation described in the guide highlights the need for containment architectures that prevent corrupted GNSS data from infecting other systems. Future designs may require:

Hard partitioning between GNSS-dependent and GNSS-independent systems
Automatic reversion to inertial navigation upon anomaly detection
Prevention of GNSS position from updating IRS during suspected spoofing

3. Replay and Meaconing Defense

The guide mentions meaconing (receiving and rebroadcasting GNSS signals with delay) as an interference variant. This has implications for receiver designs that rely solely on signal authentication—meaconed signals retain authentic cryptographic signatures.

For Airlines and Operators

1. Training Investment

The FAA explicitly recommends:

“Operators should consider adding training materials to address the risk of GNSS interference.”

This includes:

Recognition of spoofing vs. jamming symptoms
Non-GNSS navigation proficiency
Procedure modification in interference areas
Post-event position recovery techniques

2. Procedural Development

The guide includes sample procedures that operators should adapt:

Pre-flight briefing items for interference regions
In-flight response checklists
Position recovery procedures after extended spoofing
Reporting templates for interference events

3. Route Planning

Operations departments should consider:

Routing to avoid known hotspots where practical
Fuel planning for potential diversions
Alternate airport selection accounting for GNSS availability
NOTAMs and interference advisories as standard briefing items

For Regulators and Policymakers

The FAA guide implicitly acknowledges that GNSS interference is beyond aviation’s ability to solve independently. The interference sources are state-level actors engaged in geopolitical conflict. Aviation is experiencing externalities from wars it has no part in.

This has policy implications:

1. International Coordination

ICAO and EASA have published parallel guidance. Harmonization is essential for operators flying international routes through interference regions.

2. Alternative PNT Investment

The guide’s repeated emphasis on backup systems supports arguments for:

Continued funding for ground-based navaid infrastructure
Development of GNSS-independent positioning systems
Research into LEO-based PNT alternatives

3. Liability Frameworks

As interference incidents increase, questions arise:

What constitutes reasonable preparation for a known threat?
When does navigation system failure become a design defect vs. unforeseeable attack?
How should insurance frameworks account for GNSS risk?

The Path Forward: What’s Missing from the Guide

While the FAA document is comprehensive, several gaps are notable:

Lack of Mandate

The guide is explicitly non-regulatory. It provides recommendations, not requirements. This leaves compliance voluntary—and potentially inconsistent across the industry.

The document states:

“This document is not a directive and does not constitute a regulation.”

For some operators, this means the guidance may receive limited implementation priority.

Limited Technical Detail on Detection

The guide describes spoofing symptoms but provides limited guidance on automated detection methods. This leaves avionics manufacturers without clear performance targets.

Emerging detection techniques—including:

Antenna diversity methods (comparing signals across multiple antennas)
Receiver motion consistency checks (comparing Doppler with INS-derived velocity)
Cryptographic signal authentication (GPS III’s upcoming authentication feature)

—receive minimal treatment.

No Alternative PNT Roadmap

The document focuses on recognizing and responding to interference, not on solving the underlying vulnerability. There’s no discussion of:

LEO-based positioning augmentation
eLoran or other terrestrial backup systems
Multi-constellation receiver requirements
Future GPS III capabilities

This is perhaps appropriate for an operational guidance document, but it leaves the fundamental fragility unaddressed.

Conclusion: A Pivotal Moment for GNSS Resilience

The FAA’s GNSS Interference Resource Guide represents a maturation point in aviation’s relationship with satellite navigation. For two decades, GPS moved from supplementary to primary to sole-means navigation for many operations. That trajectory has now reversed.

The industry is being forced to confront an uncomfortable reality: GNSS, as deployed, is not resilient against motivated adversaries. The signals are weak. The receivers are trusting. The system dependencies cascade catastrophically.

For GNSS professionals, this document should serve as both validation and warning:

Validation: The threats we’ve discussed in technical forums for years are now officially acknowledged at the regulatory level. The industry is listening.

Warning: The current architecture is inadequate. The research, the standards development, the alternative systems work—these are no longer academic exercises. They’re operational necessities.

The 1,000+ daily interference incidents aren’t decreasing. The geopolitical conflicts generating them aren’t resolving. And the aviation industry, along with every other GNSS-dependent sector, must adapt.

The FAA has provided the diagnosis. The prescription—robust, resilient, multi-source navigation—remains for the industry to write.

TL;DR

FAA released a landmark 69-page guide on GNSS interference (December 2025)—the first comprehensive U.S. regulatory acknowledgment of the operational crisis
1,000+ daily incidents globally: Up from 700 in 2024, with hotspots in Eastern Mediterranean, Black Sea, Middle East, and Baltic regions
Spoofing is the greater threat: Unlike jamming, it doesn’t announce itself. RAIM provides no protection. Corrupted data cascades through ADS-B, TAWS, TCAS, IRS, and clocks
Azerbaijan Airlines Flight 8243 (38 deaths, December 2024) exemplifies the real-world consequences of unchecked GNSS spoofing
Industry implications: Expect upcoming requirements for spoofing detection, system isolation architectures, training mandates, and alternative PNT investment
The guide is advisory, not regulatory—but the direction is clear: GNSS-sole-means navigation is no longer viable in contested environments