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
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