In case GNSS signal is lost, GBAS operations cannot be conducted. As, Radio Frequency Interferences are everyday more usual, ANSPs and regulators must plan in advance the contingency operations in case GNSS is not available.

This typical questions must be addressed:

A series of questions which would need to be considered for contingency operations in the event of a GNSS outage is detailed below. It should be noted that this is not an exhaustive list and individual states will have additional questions to be considered.

Probability of an outage?

• What are the threats?
  • Space weather – ionospheric blackout
  • Interference – Unintentional (mitigate by policing and education), intentional or malicious interference – impact area? (refer to attachments)
  • Constellation Outage – what and how long?

How long and/or how wide the impacted area?

• Area
  • Approach, terminal, ACC, all State airspace?
  • Awareness of outage? 
  • Knowledge of airspace impacted?
• Time
  • Short duration – up to one hour?
  • Longer duration – in hours or days – again societal impact?

What systems can be impacted by the loss of GPS in terms of CNS?

• Airborne
  • Nav – Position (if single sensor)
  • Comms – Time desync
  • Sur – ADS C and B
  • Ancillary safety – TWAS/EGPWS, geometric altimetry, synthetic vision loss, combined vision systems degraded
  • Additional considerations – Loss of Situational Awareness (SA), workload increase
  • In case of accident – loss of ELT leading to impact on SAR.
• Ground
  • Surveillance – ADS, possible desync of MLAT, multi-sensor tracking
  • Comms – De-syncing of time stamp (CPDLC)

Can full operations be maintained? (Yes/No)

If No, what is the operational impact on current operations?

• Percentage of aircraft impacted by loss of signal
• Reduction in capacity efficiency or access
• Safety implications 

What level of future service is needed?

• Get all aircraft safely on the ground
• Continue full operations
• Defined requirements on Capacity, Efficiency and/or Access whilst maintaining required safety levels. (what is the pressure to continue operations)

What is contingency operation airspace concept?

• Alternative operations (conventional, visual, diversion).
• Aerodrome capacity in the event of diversion or reduced operations

What current infrastructure exists to enable degraded operations?

• Is it sufficient to meet the contingency operation airspace concept?
• If not, what is required?
  • Ground infrastructure:
    • Additional NAVAIDs?
    • Alternative surveillance capabilities
  • Airborne infrastructure:
    • Are aircraft fitted with alternative equipment? (fleet analysis).
    • Additional equipment for reversion - weight
    • Retrofit?
    • Use of inertial – if fitted

Cost Benefit Analysis considerations:

• ANSPs – Amortisation of costs, controller re current training, licencing?
• AOs – Retrofit, certification and pilot re current training

Supplementary Considerations for pilots and controllers:

• Awareness of degraded environment
• Awareness of required reversion procedures
• Flight crew awareness of the impact of GPS outage on the specific aircraft type and the corresponding operational procedures.
• Do the controllers and pilots hold appropriate licences for the contingency environment?
• Maintenance of skill sets for the contingency environment, e.g.:
  • Controller - procedural control, radar vectoring
  • Pilot - flying NDB or VOR conventional procedures
• For ab-initio controllers and pilots, is the appropriate training in place for the contingency environment?