ART-2025 • Module 7 • GNSS Contingency
ATCO Immediate Procedures • LGBI / AAI Use Only
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GNSS Threat & Why ATC Cares Context

  GNSS is now part of ATC separation logic Safety Critical

GNSS (Global Navigation Satellite System) provides Position, Navigation, and Time. Aircraft systems use it for:

  • FMS / RNAV routing and navigation display
  • ADS-B / ADS-C / MLAT surveillance feeds
  • EGPWS / TAWS terrain warning logic
  • ELT / SAR location reporting

If GNSS is jammed or spoofed, aircraft may drift off cleared track, lose RNP capability, or get nuisance “PULL UP” alerts. ATC must recognise this and stabilise traffic before it becomes an incident.

Interference Types
  • Jamming: Strong noise blocks real satellite signals → pilot may say “GPS FAIL”.
  • Spoofing: Fake but believable GNSS signal → aircraft believes wrong position/altitude.

Spoofing is more dangerous: ATC and crew may initially trust data that is wrong.

First Pilot Report Immediate ATCO Action

  When a pilot first says “GPS unreliable / map shift” COMPLETE
Ask pilot to confirm present position (radial/DME, fix, heading, or visual ref). If unsure, give heading/track to maintain separation.
Ask nearby traffic: “Confirm GNSS OK / any nav issues?” to detect if area-wide.
Broadcast advisory on frequency if required: “Reports of GNSS interference in sector. Use conventional navigation.”
Log time, callsign, sector, and nature of the fault (jam / spoof / RNP NAV LOST).
  High-Level Flow
1. Pilot: “GPS unreliable / map shift” 2. You: acknowledge and confirm current position
3. Apply procedural / heading separation if nav is affected 4. Ask nearby traffic if they see similar issues
5. Warn other aircraft in area if needed 6. Inform Supervisor / OCC / log
7. Prepare to use specific contingency (Radar / Non-Radar / Procedural)

Contingency in RADAR Sector Radar Available

  Controller Actions (Radar Available) COMPLETE
Confirm with the pilot what failed: “RNP NAV LOST?” “GPS FAIL?” “MAP DRIFT?”
Use the RADAR return as truth. Treat any drifting ADS-B target with caution if spoofing suspected.
If the aircraft was on an RNP/PBN approach: instruct to discontinue, then vector for ILS/VOR.
Temporarily suspend GNSS-reliant SIDs/STARs in the affected area.
  Radar Flow
1. Pilot report / ADS-B vs radar mismatch 2. Use primary/secondary radar as source of truth
3. Stop RNP approach; vector for conventional (ILS/VOR) 4. Suspend GNSS-dependent SIDs/STARs
5. Inform WSO / Supervisor 6. Advise other inbound/outbound traffic

Contingency in ADS-B / Non-Radar Sector Surveillance Degraded

  Controller Actions (ADS-B / No Radar) COMPLETE
Tell pilot: “ADS-B position may be unreliable in this area.” Do not promise radar-like separation if you don’t actually have radar.
Suspend surveillance service and revert to procedural separation. (Spacing by time / level / track.)
Cancel any active RNP/PBN approach. Offer conventional approach when traffic permits.
Suspend GNSS-dependent SIDs/STARs for that sector.
  ADS-B / Non-Radar Flow
1. GNSS interference → ADS-B becomes unreliable 2. Tell pilot ADS-B may be inaccurate
3. Stop using ADS-B for separation 4. Revert to procedural separation (time / altitude / geography)
5. Cancel RNP approach, clear conventional only 6. Inform Supervisor / brief next sector

Pure Procedural Control No Surveillance Support

  Controller Actions (Procedural Environment) COMPLETE
Ask pilot for last known fix / radial / DME / estimate. Confirm accuracy.
Provide guidance using ground-based navaids (e.g. “Track VOR radial 270, report DME 15”).
If on RNP approach: discontinue. Assign safe altitude. Then re-clear for an ILS/VOR or other conventional approach.
Maintain procedural separation using estimates and timing.
  Procedural Flow
1. Pilot: “GNSS unreliable” in procedural airspace 2. Get best position estimate (fix / radial / DME / visual ref)
3. Give conventional navaid guidance 4. Apply procedural separation (time / level / route)
5. If on approach: stop RNP, climb to safe altitude, clear conventional approach 6. Inform Supervisor and record

False GPWS / TAWS Alerts “PULL UP” Due to Spoofing

  False Terrain Alerts (Spoofed Data) COMPLETE
Acknowledge immediately: “Roger, continue climb…”
Assign a known safe altitude and give QNH. Use phrase: “CHECK YOUR ALTITUDE IMMEDIATELY.”
Ask intentions: continue approach / go around / divert?
If spoofing suspected: DO NOT clear RNP approach again. Offer only a conventional (non-GNSS) approach.
  GPWS / TAWS Flow
1. Pilot: “GPWS WARNING — GOING AROUND”
2. Acknowledge instantly 3. Give safe altitude + local QNH 4. Say: “CHECK ALTITUDE IMMEDIATELY”
5. Ask intentions (continue? divert?) 6. If spoofing suspected: vector for conventional approach only
7. Inform Supervisor, log event, brief following traffic

Reporting & Escalation Mandatory

  Post-Event Reporting Mandatory

Every GNSS interference event that affects navigation, separation, approach stability, or causes a GPWS/TAWS alarm is reportable. This is not optional.

  • Stabilise the situation first (safety comes before paperwork).
  • Inform Watch / Shift Supervisor immediately.
  • Make a clear log entry (time, callsign, sector, nature of interference).
  • Fill the GNSS Interference Occurrence format as per local SOP.
  • Forward to CNS/ATM Engineering, Safety Cell, and other addresses specified by AAI / DGCA.

Consistent reporting allows pattern detection across FIRs and supports national NOTAM / advisory action.