GNSS Reversion

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? (Examples are: GLONASS in 2014 (navigation data issue); GPS in  2016 (timing issue), Galileo 2019 (navigation data issue).  Is this more than aviation? What is the societal impact?

How long and/or how wide the impacted area?

• Area
  • Approach, terminal, ACC, all State airspace?
  • Awareness of outage? – How (see Section 2)
  • 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
  • Ancilliary 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)

(Refer to Appendix 1 for more detailed info on loss of services)

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?

No CNS enabler single-handedly resolves all an aircraft’s technical challenges in flight.  Whilst Communication, Navigation and Surveillance have historically been ‘separated’, primarily for safety and historical reasons, evolving systems are increasingly relying on the same key system i.e. GNSS.

In discussions about PBN, it often becomes evident that GNSS is used on several CNS systems, e.g. time-stamping of data transfers in message sets (COM), synchronisation of surveillance data processors (SUR), in some systems, Data Link (communication) timing (COM). These systems often have back-up timing sources or other reversion means. For back-up timing sources the GNSS being unusable is important where a longer problem will result in greater clock drifts.  This abridged list makes it clear that GNSS is a common point, a shared resource for Communication, Navigation and Surveillance and that a GNSS being unusable has the potential to disrupt operations depending on how much GNSS provides the backbone of various C-N-S elements.

In terms of navigation, the European fleet and Navaid Infrastructure is well equipped: Europe is fortunate to have a rich DME infrastructure and over 90% of the ECAC fleet is equipped with DME/DME RNAV capability. Indeed, in major European hubs, 98% of the fleet is equipped with DME/DME positioning capability. What this suggests is that continuing navigation as ‘normal’ for a while, after GNSS becomes unusable, is often feasible, though this statement is not absolute.