GNSS Interference in Aviation (Part II) Operational Checklists

 Recommended Operational Checklists for Pilots During GPS Interference

Pre-Flight Preparation

a) Review NOTAMs for GNSS interference zones along the planned route and destination.​

b) Prepare and load non-GNSS-based approaches (e.g., ILS, VOR/DME procedures) into the FMS.​

c) Verify navigation database validity to ensure alternate procedures are available.​

In-Flight Response to GNSS Loss

a) Monitor cockpit warnings such as “NAV GPS x FAULT,” “GPS PRIMARY LOST,” or “NAV GNSS x FAULT” for initial detection.​

b) Do not manually deselect GPS; allow the FMS to attempt automatic reacquisition when in a non-interference area.​

c) Switch navigation source to VOR/LOC/DME or use raw data monitoring on PFD/ND for positional information.​

d) Maintain aircraft control and use last known reliable heading; climb to a safe altitude if terrain clearance is required.​

e) Notify ATC immediately using standard ICAO phraseology, such as: “UNABLE GNSS POSITION – USING ALTERNATE NAVIGATION”.​

f) Report loss of ADS-B OUT capability if applicable.​

g) Request radar vectors or navigation assistance from ATC.​

h) Use visual references, charts, and terrain databases to supplement navigation.​

i) If RNAV is lost or position integrity errors (RAIM faults) arise, reset navigational sources and continue using published procedures or ATC direction.​

j) Disable GNSS position updates and terrain look-ahead functions if persistent nuisance alerts occur.​

Post-Flight Actions

a) Log GNSS interference events in the tech log, with time, location, and phase of flight.​

b) File a report with safety departments or regulatory authorities (DGCA, ICAO, FAA, or IATA as relevant).​

c) Coordinate with maintenance for follow-up and engineering checks.​

Example ICAO/EASA GNSS Loss Checklist (Summary Table)

Phase	Task	Reference
Pre-Flight	Check NOTAMs for GNSS RFI	​
Pre-Flight	Load non-GNSS approaches (ILS/VOR/DME)	​
In-Flight	Monitor for GNSS fault alerts	​
In-Flight	Switch to alternate navigation sources	​
In-Flight	Notify ATC, request radar vectors if needed	​
In-Flight	Use visual/terrain references	​
Post-Flight	Log the event, time, phase, details	​
Post-Flight	File report to authorities	​
Post-Flight	Notify maintenance for tech follow-up	​

In addition, always ensure enhanced crew briefing for GNSS contingency scenarios, include diversion airports with conventional navigation capability, and maintain readiness for real-time reporting as per regulatory mandates (e.g., DGCA's 10-minute requirement in India).

Cockpit Flow for GNSS Failure

1. Detect GNSS Failure

2. Observe GNSS/FMS warnings (“GPS PRIMARY LOST,” “NAV GNSS FAULT,” map shifts, or abnormal alerts such as unintended TAWS).​

3. Confirm and Cross-Check

4. Verify loss using standby/alternate navigation sources (e.g., IRS, radio nav aids, or visual references).​

5. Switch to Alternate Navigation

6. Select appropriate alternative (ILS, VOR/DME/LOC, INS) and update navigation mode on FMS and PFD.​

7. Advise ATC

8. Inform Air Traffic Control with standard phraseology (“UNABLE GNSS POSITION – USING ALTERNATE NAVIGATION”).​

9. Notify loss of ADS-B if applicable.

10. Maintain Situational Awareness

11. Use charts and visual references as needed.

12. Request radar vectors if required.​

13. Continue With Published Non-GNSS Procedures or ATC Guidance

14. Follow pre-briefed conventional approach or ATC instructions for routing/diversion.​

15. Log Event

16. Record occurrence details and report per regulatory requirements after landing.​

This flow ensures safe reversion to alternate procedures and effective coordination with air traffic control during declared GNSS outages.

Steps for Immediate Actions to Maintain Aircraft Control

Immediate actions to maintain aircraft control after GNSS (GPS) loss are focused on preserving situational awareness, ensuring safe flight operation, and reverting to reliable backup systems. The following steps are recommended:

Immediate Actions After GNSS Loss

a) Maintain Attitude and Heading
Monitor and trust primary flight instruments (attitude indicator, heading indicator, airspeed, and altimeter) for aircraft control. Do not attempt major navigational changes while diagnosing the failure; stabilize flight first.​

b) Cross-Check Navigation Inputs
Confirm loss using alternate sources such as inertial navigation systems (IRS/INS), radio navigation (VOR, DME, LOC), and visual references if available. Compare readings to identify false or drifting indications.​

c) Switch to Alternate Navigation
Select and activate ground-based navigation aids or inertial systems as primary reference. Update FMS or PFD to display conventional navigation data.​

d) Inform ATC Immediately
Declare "GNSS failure" to Air Traffic Control, stating your position based on the last known fix and current method of navigation. Request radar vectors or navigation assistance if needed.​

e) Monitor Terrain and Traffic
Ensure safe altitude and position especially in proximity to terrain and controlled airspace. Follow published minimum safe altitudes and use visual or radio references to avoid obstacles.​

f) Reduce Cockpit Workload
Prioritize essential flying tasks, minimize secondary activities, and delegate duties. Stay focused on aircraft control and navigation.​

This sequence preserves safe flight trajectory, quickly adapts navigation sources, and aligns communications for ongoing flight safety during a GNSS outage. Always follow company-specific and aircraft-manufacturer procedures where applicable.

Immediate ATC Calls to Make After GNSS Loss

After GNSS loss, the recommended immediate ATC calls focus on declaring the situation, ensuring operational safety, and enabling support from air traffic controllers. Use the internationally standard phraseology and concise reports:

Immediate ATC Calls After GNSS Loss

1. Declare GNSS Failure:
“UNABLE GNSS POSITION – USING ALTERNATE NAVIGATION”
This informs ATC that GPS-based navigation is lost and you are reverting to alternative means such as VOR, DME, INS, or radar vectors.​

2. Report Location and Situation:
State your aircraft’s last reliable position, present navigation method, and intentions.
Example: “ATC, [Callsign], unable GNSS position after [position], now using VOR/DME, request radar vectors”.​

3. Report Loss of Surveillance Capabilities (if applicable):
If ADS-B OUT is lost due to GNSS failure, immediately notify ATC:
“ATC, [Callsign], ADS-B OUT unavailable due to GNSS loss”.​

4. Request Assistance:
Request radar vectors, alternate clearances, or emergency support if required for terrain or traffic separation.​

5. Follow Regulatory Reporting Protocol:
As per DGCA and ICAO, file a mandatory real-time report (within 10 minutes in India) on the GNSS interference event through official channels.​

Note: Always use clear, internationally recognized phraseology and promptly communicate navigation impairments to ensure safety and regulatory compliance.

Declare UNABLE RNP phraseology to use with ATC

a) The standard ICAO-compliant phraseology for declaring inability to meet RNP requirements due to GNSS loss is:

"UNABLE RNP"

b) If needed, this can be clarified further with the cause:

"UNABLE RNP DUE TO GNSS FAILURE"

c) You may also expand using structured phraseology per the situation, for example:

“UNABLE RNP ON PRESENT STAR, REQUEST RADAR VECTORS”

“UNABLE RNP FOR APPROACH, REQUEST ALTERNATE CLEARANCE”

d) Declare this to ATC as soon as the UNABLE RNP alert appears in the cockpit, ensuring controllers understand the situation and can provide vectors or alternative navigation clearances.

Differences between UNABLE RNAV and UNABLE RNP

The key differences between “UNABLE RNAV” and “UNABLE RNP” phraseology relate to the underlying navigation requirements and what ATC should infer about the aircraft’s capability:

1. “UNABLE RNAV” Phraseology

a) Meaning: The aircraft cannot perform any area navigation (RNAV) per the current clearance, often due to equipment failure or database issues.

b) Pilot Action: State "UNABLE RNAV" to ATC and request radar vectors or conventional navigation alternatives (like VOR or DME routes).​

c) Implication: The aircraft must revert to traditional navigation methods, and ATC may assign conventional procedures or vectors.

2. “UNABLE RNP” Phraseology

a) Meaning: The aircraft cannot guarantee the specific Required Navigation Performance (RNP) level for the leg or procedure, often due to GNSS loss, RAIM issue, or onboard performance monitoring alerting.

b) Pilot Action: State "UNABLE RNP" with the reason (e.g., "UNABLE RNP DUE TO GNSS FAILURE"), clarify if area navigation can still be performed by other means, and request appropriate instructions.​

c) Implication: Loss of RNP does not always mean complete area navigation loss—alternate positioning sources (e.g., DME/DME) may still allow navigation, but not with the required RNP precision. ATC may need to assess whether vectors, alternate clearances, or contingency measures are necessary.

Phraseology	Description	Typical Cause	ATC Response
UNABLE RNAV	Unable to use area navigation as cleared	Equipment or database failure	Assign vectors or conventional SID
UNABLE RNP	Unable to meet the RNP for the procedure	GNSS/RAIM issue or alert	Clarify alternate nav capability, assign vectors or alternatives as needed

In essence,

“UNABLE RNAV” indicates total loss of area navigation capability, while

“UNABLE RNP” indicates a performance shortfall on a specified RNP operation, possibly with other navigation methods still available.​

Author: GR Mohan

GNSS Interference in Aviation—Threats, Responses, and Future Resilience

 The Evolving Threat Landscape

Global Navigation Satellite Systems (GNSS), primarily GPS, underpin modern aviation's precision navigation, surveillance, and timing. However, jamming—intentional signal overload—and spoofing—deceptive false signals—pose escalating risks, particularly as geopolitical tensions rise. Data from space-based surveillance provider Aireon indicates an 80% surge in GPS outage events from 2021 to 2024, with OPSGROUP estimating a staggering 500% increase in reported aviation incidents in 2024 alone. These disruptions, often indiscriminate, affect civilian flights over military hotspots like the Middle East, Eastern Europe, and the Black Sea, where state actors or non-state groups deploy portable jammers. While no direct fatalities have occurred, the evidence points to degraded safety margins, with the European Union Aviation Safety Agency (EASA) issuing urgent bulletins in 2024 highlighting inconsistent navigation and surveillance losses.

Impact of GPS Jamming and Spoofing

Jamming and spoofing erode GNSS reliability, cascading through aircraft subsystems. Jamming emits high-power noise on GPS frequencies (e.g., L1 band at 1575.42 MHz), overpowering faint satellite signals (as low as -160 dBW) and causing outright signal denial. Spoofing, more insidious, broadcasts counterfeit signals mimicking authentic ones, inducing false positions—e.g., an aircraft "teleporting" 100+ nautical miles or entering impossible circular loops at cruise altitude.

Core Aviation Impacts:

a) Navigation and Flight Management: Loss of GPS triggers autopilot disengagements, forcing manual reversion and increasing pilot workload by up to 300-500% in high-density airspace, per recent simulations. In performance-based navigation (PBN), this compromises RNP approaches, leading to go-arounds or diversions.

b) Surveillance and Collision Avoidance: Automatic Dependent Surveillance-Broadcast (ADS-B) fails, creating "ghost" tracks or duplicates, complicating air traffic control (ATC) separation. Airborne Collision Avoidance System (ACAS) may issue erroneous resolutions.

c) Safety Systems: TAWS/EGPWS generates false terrain warnings based on spoofed altitudes, risking unnecessary evasive manoeuvres. Honeywell reports potential distortions in weather radar overlays and flight planning.

d) Geopolitical Hotspots: Effects are most pronounced near conflict zones like the Middle East and Black Sea, where state or non-state actors deploy jammers, impacting civilian flights indiscriminately.

e) Operational and Economic Consequences: Delays average 30-60 minutes per incident, with 2024 hotspots causing thousands of diversions. Aireon's data shows over 10,000 flights flagged as anomalies monthly in affected regions, eroding on-time performance and passenger trust. Some of the operational consequences are:

i. Delays and diversions due to unavailable GPS approaches

ii. Higher fuel burn from rerouting and vectoring

iii. Increased ATC workload and flow restrictions

iv. Potential cancellations in airports relying heavily on PBN

v. Operational disruptions in mixed fleet operations

Quantitative Trends from Aireon (August 2024–January 2025) :

Anomaly Type	Description	Frequency Trend	Example Impact
Low Position Integrity (PIC < 7)	Degraded GPS quality, radius >0.6 NM	Steady (10% dip in Oct 2024)	Multiple aircraft lose RAIM integrity over hours
Field Type Code 0 (FTC0)	Unknown position due to GPS failure	Stable	Airborne FTC0 spikes near Boise, ID (13x increase, Jan 2025, tied to US military tests)
Duplicate Addresses	Position errors >6 NM in 30s	Rising (spoofing indicator)	Lahore, Pakistan (Dec 2024): High duplicates aligning with pilot jamming reports
Track Discontinuities/IPC Flags	Jumps >3 NM from reference track	Spiking (Nov 2024 update)	Black Sea spoofing: Bangkok-Vienna flight "jumped" to Bulgaria/Ukraine
Improbable Tracks (e.g., Circles)	Low velocity (<100 kts) at altitude	Declining (new methods?)	Circular patterns over Eastern Europe, detected via low-velocity flags

These metrics, derived from billions of daily ADS-B messages, correlate strongly with interference events, underscoring the shift from localized to widespread threats.

Case Studies:

a) Black Sea Incident (2024): A commercial flight reported spoofed positions across borders, triggering ATC alerts and a 45-minute delay; Aireon's Independent Position Check (IPC) validated the discrepancy.

b) Middle East Jamming (Ongoing): OPSGROUP logs show 80% of global reports here, with TAWS false alerts forcing climbs over safe terrain.

c) US Domestic Testing (Jan 2025): Military exercises near Idaho caused 13-fold anomaly surges, affecting Salt Lake City FIR traffic.

Critically, while media amplifies risks, aviation bodies like IATA stress that incidents remain manageable, with no evidence of targeted civilian attacks—yet the potential for escalation in contested airspace warrants vigilance.

Jamming vs. Spoofing: Technical Distinctions and Detection Challenges

1. Jamming

a) Mechanism: A jammer broadcasts strong RF signals on GNSS frequencies (L1, L2, etc.) to drown out or corrupt legitimate satellite signals.

b) Effect: Loss of signal, degraded signal-to-noise ratio, denial of position fix.

2. Spoofing

a) Mechanism: A spoofer transmits counterfeit GNSS signals that mimic real satellite signals. These Coherent fakes exploiting GNSS's unauthenticated design; subtler, as receivers may "lock" onto imposters, showing plausible but erroneous data (e.g., clock drifts or altitude mismatches). Both exploit GPS's civil signals' lack of encryption, though military P(Y)-code offers partial protection. Detection relies on cross-checks: e.g., inertial drift exceeding 1 NM/hour flags issues, or multi-antenna arrays spotting signal direction inconsistencies. Can mislead GNSS receivers into calculating false positions, altitudes, or time.

b) Spoofing-induced Hazards: Potentially more dangerous than jamming because the navigation system “thinks” the data is correct and may not declare an error. Spoofing may:

i. Mislead FMS position

ii. Trigger autopilot navigation on false waypoints

iii. Cause deviations without warning flags

This is significantly more dangerous than simple jamming.

3. Vulnerability

a) GNSS signals received by aircraft are extremely weak (very low power), making them susceptible to interference.

b) Many civil receivers do not have strong anti-jam or anti-spoofing protection.

c) In aviation, interference can propagate through FMS (Flight Management System), ADS-B, CPDLC (Controller-Pilot Data Link), and time-synchronization systems.

Oceanic and Remote Airspace Risks

Over oceans and deserts where ground-based navaids are absent, GNSS becomes a single point of failure. Jamming in these environments may cause:

a) Position uncertainty

b) Incorrect IRS drift corrections

c) Degraded CPDLC anchoring (time-stamp issues)

d) Contingency procedures being triggered (e.g., 15 NM offset or drift-down routes)

India’s Experience and Risks

India has recently reported significant GPS/GNSS interference, particularly spoofing, which has affected civil aviation operations in sensitive regions.

Incident Statistics & Geography

a) According to the Government of India, 465 GPS interference/spoofing incidents were reported between November 2023 and February 2025.

b) These incidents are concentrated in border regions — notably Amritsar and Jammu.

c) In parliamentary proceedings, the Minister of State for Civil Aviation confirmed these reports.

d) Some of these interference events are believed to be tied to cross-border electronic warfare, especially near conflict-prone zones. 4.2 Regulatory Response: DGCA’s Measures

e) The DGCA (Directorate General of Civil Aviation) has issued multiple advisories/circulars. In November 2023, it released an advisory circular on GNSS interference (jamming & spoofing), which outlines roles and responsibilities for airlines, ANSP (Air Navigation Service Providers), and ATC/aviation stakeholders.

f) DGCA formed a special committee (in October 2023) to monitor GNSS spoofing and make recommendations.

g) In November 2025, DGCA mandated a real-time reporting protocol: any pilot, ATC controller, or technical unit detecting “abnormal GPS behaviour” must report within 10 minutes.

Anti-Jamming Techniques: Engineering the Frontline Defence

Anti-jamming prioritizes signal preservation over full denial, leveraging physics and algorithms. These are mature in defence but emerging in civil aviation due to certification hurdles under RTCA DO-229 standards.

Primary Techniques:

1. Antenna-Based Solutions:

a) Controlled Reception Pattern Antennas (CRPA): Multi-element arrays (4-7 elements) adaptively nullify jammers (up to 40 dB attenuation) by steering beams—e.g., NovAtel's GAJT modules for aviation integration. Applicable to drones and airliners; CRFS notes efficacy in high-threat environments like Ukraine.

b) Directional Antennas: Fixed nulls toward ground-based threats, though less flexible than CRPAs.

2. Receiver-Level Processing:

a) Adaptive Nulling and Beamforming: Projects signals onto "jammer-free subspaces," suppressing interference while boosting satellites (DTIC research shows 30-50 dB gains).

b) Spread Spectrum and Frequency Hopping: Distributes power across bands (e.g., L1/L5 dual-use), resisting narrowband jammers; modern chips like those from u-blox enable this.

c) Power Minimisation: Dynamically lowers jammer influence via digital filtering, per NovAtel implementations.

3. Hybrid Approaches:

Infinidome-Style Dome Shields: Passive Faraday-like enclosures block ground jammers while passing skyward signals, ideal for low-altitude ops.

Technique	Pros	Cons	Aviation Maturity
CRPA	High gain (40+ dB), real-time adaptation	Cost ($10K+), power draw, certification delays	Military: High; Civil: Emerging (e.g., Boeing tests)
Spread Spectrum	Low cost, software-upgradable	Less effective vs. broadband jammers	Widespread in new GNSS receivers
Adaptive Filtering	Integrates with existing hardware	Computationally intensive	Proven in INS hybrids

These techniques, when layered, can extend GPS usability in 50-100 dB jamming fields, but spoofing demands orthogonal methods like authentication.

Comprehensive Mitigation Strategies: A Multi-Layered Framework

Mitigation spans technology, operations, and policy, as no single fix suffices—ICAO's 2025 paper advocates "defence in depth." Strategies address both jamming (denial) and spoofing (deception), with reversion to non-GNSS backups as the ultimate safeguard.

Technological Layers:

a) Receiver Enhancements: Dual-frequency multi-constellation (DFMC) receivers (GPS + Galileo + BeiDou) diversify signals, yielding 15 dB anti-jam margins and spoof detection via constellation cross-verification. Cryptographic authentication (e.g., Galileo's OS-NMA) verifies signals, though full rollout lags.

b) Sensor Fusion: Integrate GNSS with Inertial Navigation Systems (INS), baro-altimeters, and Doppler radars for hybrid PNT; Kalman filters detect outliers (e.g., >2σ position errors).

c) Detection Tools: Real-time monitors like GPSwise or NaviGuard apps flag anomalies via signal metrics; geofencing restricts ops in known hotspots.

d) PNT diversification: Long-term navigational resilience may depend on alternative PNT (Position, Navigation, Timing) systems. India’s indigenous GNSS (NavIC) could play a role, though its current civil aviation role is limited. Research in alternate nav technologies, including quantum-based navigation, may also contribute over the coming years. (See research trends in quantum navigation, though civilian aviation adoption remains nascent.)

Operational Protocols (Per CASA and EASA Guidelines):

a) Pre-Flight Planning: Assess route risks via NOTAMs; ensure backups for IMC approaches.

b) In-Flight Response:

a. Recognise: Monitor for TAWS anomalies, ADS-B drops, or inertial drifts.

b. Mitigate: Cross-check with VOR/DME/ILS; climb if below MSA.

c. Adapt: Notify ATC, vector to safe airspace, log for post-flight analysis. 

d. ATC Integration: Enhanced radar and multilateration for GNSS-denied surveillance.

Policy and Systemic Efforts:

a) ICAO standardisation for C-PNT (e.g., eLoran backups) and interference reporting.

b) Training: IATA workshops emphasise "GNSS hygiene"—e.g., avoiding sole reliance.

c) Emerging: AI-driven prediction models from Aireon forecast hotspots 24-48 hours ahead.

d) ICAO & AAPA / CANSO Engagement: At the 60th DGCA Asia-Pacific conference, a paper was presented on “safeguarding navigational safety and operational resilience amidst increasing GNSS interference.” This calls for reviewing over-reliance on GNSS, especially at airports lacking conventional navaids.

Strategy Layer	Jamming Focus	Spoofing Focus	Implementation Timeline
Technological	CRPA, DFMC	Authentication, multi-antenna	2-5 years (certification pending)
Operational	Backup aids, cross-checks	Anomaly alerts, crew drills	Immediate (via SOP updates)
Policy	Spectrum monitoring, NOTAMs	Global reporting networks	Ongoing (ICAO 2025-2030)

Challenges include cost (CRPAs add $50K/aircraft) and export controls, but incentives like FAA grants accelerate adoption. Future resilience hinges on hybrid ecosystems, reducing GNSS dependency to <50% of PNT.

Balancing Risks in a GNSS-Dependent Era

GPS interference underscores aviation's vulnerability to low-cost threats ($100 jammers vs. billion-dollar satellites), yet layered mitigations ensure safety continuity. As incidents trend upward—potentially doubling by 2026 per models—proactive investment in resilient tech and ops is essential. Stakeholders must collaborate, prioritising civil-military deconfliction to safeguard skies.

N.B. Part II of the article covers Flight crew procedures and checklists to counter GPS interference-induced errors in Navigation and Flight Operations.

Author: G R Mohan