Operational benefits of Multi Constellation Dual Frequency GNSS for aviation
This paper summarises recent discussions of the 12th ICAO Air Navigation Conference and ICAO Navigation Systems Panel, and the work done within the SESAR project
GNSS is a key technology of the communications, navigation, and surveillance (CNS) infrastructure, essential for the introduction of Performance Based Navigation (PBN) and Automatic Dependent Surveillance-Broadcast (ADS-B). It is used in safety related systems such as GPWS (Ground Proximity Warning Systems), and provides the time reference that is used to synchronise many systems and operations in ATM.
Many navigation and surveillance applications (e.g. RNAV 5, RNP APCH or ADS-B) are already deployed on the basis of the excellent GPS service provided free of charge by the US. Around 90 % of the fleet operating in Europe is already equipped with GPS receivers and it is expected to reach 100% before 2020.
The ICAO ANC (Air Navigation Conference) 12th held in Montreal in November 2012 highlighted that as new constellations are deployed, and existing constellations are enhanced, signals from multiple constellations broadcasting in multiple frequency bands are becoming available to aviation. These developments lead to technical performance improvements, which create the potential for achieving significant operational benefits.
Recommendation 6/5 b) from the ICAO ANC 12th highlights the need that the ICAO work programme should address the “identification of operational benefits to enable air navigation service providers and aircraft operators to quantify these benefits for their specific operational environment “. At the same time, this Conference noted that the introduction of multi-constellation, dual frequency GNSS entailed a number of new technical and regulatory challenges beyond those already associated with current GNSS implementation.
The European roadmaps for Navigation and Surveillance show the plans for introducing more demanding applications based on GNSS. Taking into account the progressive dependency on GNSS for ATM/CNS applications and the operational benefits that MCDF GNSS will bring, EUROCONTROL policy on GNSS and the European ATM Master plan set a vision based on the combined use of signals coming from at least two constellations in diverse frequency bands that will provide technical capabilities to improve performance, robustness and coverage.
Technical capabilities of MCDF GNSS
The SESAR project 15.3.4 made a technical assessment of the MCDF GNSS capabilities based on performance simulations and the analysis of robustness against identified vulnerabilities (ionosphere and interference). The project also assessed the benefits of integrating GNSS sensors with inertial systems. This assessment identified the following technical capabilities resulting from having additional GNSS satellites from different constellations offering new and better signals in diverse frequency bands:
a) Increased availability and continuity in nominal conditions and in degraded degraded conditions (increased robustness).
Next generation of GNSS avionics will be robust against GPS constellation degradation or unavailability (RAIM holes). They will be able to calculate ionosphere delay in real-time, (due to dual frequency), effectively eliminating this major error source. Additionally it will be less likely that scintillation would result in loss of GNSS service due to the extra ranging sources available in multi constellation scenario. Additionally it will be less likely that un-intentional interference would result in loss of GNSS service.
New GNSS signals will be more resistant to interference due to higher power and improved signal designs. Better and more signals will be available in mountainous terrain/high latitude, making less likely that high terrain or lack of satellites in view would result in loss of GNSS service.
Dual-frequency and Multi Constellation capability will add robustness thanks to backup modes available in degraded modes (alternative frequency and additional satellites from different constellation).
b) Extended service area
For example, EGNOS v3 that is the evolution of EGNOS that will augment GPS L1/L5 and Galileo will enable to extend some services to all ECAC and even to Africa.
c) Supporting more demanding system performance levels
MCDF GNSS will support more demanding system performances that will support new applications or advanced operations that are expected to be developed over time. For example EGNOS V3 is being designed to support system performance levels with a Vertical Alert Limit of 10 meters that could be used to certify CAT I auto-land systems.
d) Independent time reference system
MCDF GNSS will improve availability and continuity of GNSS timing service making aviation less dependent on GPS for time distribution and synchronisation for an increasing number of CNS/ATM systems and applications (e.g. data link, ADS-B, terrestrial communication systems, WAM, 4D NAV,..).
As a conclusion, it can be said that MCDF GNSS will improve performance and enhance robustness to mitigate all the identified vulnerabilities with the exception of intentional interferences. The integration of GNSS with inertial systems as an on board augmentation (ABAS) would be mitigate the impact of interferences but during a limited time duration due to the drift of the inertial systems over time.
Operational benefits assessmemt
A generic assessment has been made to study how the technical capabilities identified in the previous section would result on operational benefits in terms of safety, cost efficiency, capacity and environment. This generic assessment has been customised by representatives of different aviation stakeholders: ANSPs (Aena and Avinor) and Airspace Users (Turkish Airlines, Ryanair, European Business Aviation Association, European Helicopters Association and International Council of Aircraft Owner and Pilot Association) to their individual operational needs. The main operational benefits that have been identified are summarised hereto:
• Improved availability, continuity and robustness for existing CNS applications based on GPS.
▪ En-route/TMA: Reduce likelihood of having to revert to DME/ DME and/or INS supporting only RNAV not RNP
▪ Final Approach: Reduce missed approach rates and likelihood to revert to conventional navaids
• Enable services at remote/oceanic/ high latitude areas lacking navaids and/or radar coverage.
• Improved performances to support advances concepts
• Extend service area coverage
• Enable further rationalisation of navaids
• Dual source for time distribution for CNS/ATM systems and applications.
• Possibility to relax airborne requirements (e.g. the possibility of not having to equip with INS to support RNP AR needs to be discussed with EASA position)
“During recent discussions within RTCA,EUROCAE, ICAO NSP on the subject the following additional benefits have been identified for aircraft equipped with MCDF GNSS:
1. No need for aircraft operators to run a RAIM prediction tool that is required today.
2. Possibility to remove the need to equip with sensors for some conventional navaids (e.g. NDB and VOR).
3. Possibility to comply with most stringent ADS-B requirements worldwide (in particular with FAA/US mandate on ADS-B).
4. Potential to benefit from LPV approaches in equatorial areas (e.g. EGNOS v3 could be expanded to Africa providing a LPV service that is not possible with EGNOS v2).
5. Aircraft that are not equipped with SBAS (most Airbus and Boeing) would benefit from a lower minima (LPV) and would avoid QNH setting issues. This would also enable new applications that today we just can imagine e.g. Geometric Vertical navigation in TMAs or use of geometric vertical positioning to support RVSM in the long term”
Conclusions: A cost efficient transition to MCDF GNSS
European aviation is transitioning to use multi-constellation dual-frequency GNSS with augmentations (ABAS, SBAS and GBAS) depending on the phase of flight. It is recommended that standardisation bodies (RTCA, EUROCAE and ICAO NSP) and industry work together to develop an architecture of future receivers integrating all GNSS capabilities (e.g. GPS L5, Galileo and possibly other constellations, new SBAS and RAIM standards) into one box (meaning one MOPS) in order to reduce costs for airspace users when upgrading to MCDF GNSS avionics.
It is also expected that most of the aircraft will be equipped with INS/IRS systems (e.g. assuming costs will become more affordable in particular for GA aircraft). It is recognised that one size does not fit all and this generic baseline has to be considered as a “common denominator” for all aviation stakeholders that will need to be tailored to specific stakeholders depending on individual business and operational needs.
Taking into account the fact that business case for retrofit is negative for airspace users, due to tangible benefits being small and costs associated with the equipment, installation and certification of future GNSS avionics being high, airspace users prefer to upgrade to next generation of GNSS receivers adopting a forward-fit approach. This, however, still assumes that incremental costs of future GNSS avionics when forward-fitting will be reasonable. When applying this forward-fit approach transition, it has to be considered that orders for new aircraft are typically made several years in advance. New generation of avionics/receivers can only be included in these orders when will be ready in terms of availability of approved MOPS, and EASA regulations and certification rules.
A forward fit approach is a cost efficient approach however it implies a long transition period until having the fleet operating in Europe equipped with MCDF GNSS avionics. There is a need that during the transition period GNSS systems offer a service backwards compatible with the current GNSS baseline (GPS L1 and ABAS, SBAS and GBAS augmentations) to provide a service to legacy users.
As a lesson learnt from the past and in order to reduce costs for airspace users these two recommendations are proposed for mandates to be issued in the next decade:
a) Considering that airlines operating outside Europe have to be compliant with mandates from other regions it is recommended that dates and requirements of European mandates are harmonised with other national/regional mandates (e.g. FAA rulemakings).
b) In Europe we have had several regulatory actions (e.g. Datalink, ADS-B and the PBN implementing rule that is under development ) that were a de factor GPS mandate but had different requirements and different retrofitting dates. It is proposed that future mandates will take into account all CNS requirements (e.g. 4D NAV with required time of arrival, advanced ADS B applications,..) and will indicate a single retrofitting date.
To take full advantage of the GNSS capabilities, a cost effective transition towards GNSS shall be pursued driven by operational needs with due consideration to safety, technical, security, economic and legal factors. GNSS implementation shall be based on cohesive benefit-driven technical choices backed by realistic system development plans and political commitments.
Considering that there is an effective navigation infrastructure in Europe (e.g. DMEs), the fleet operating in Europe is well equipped with navigation systems and the costs to airspace users of transitioning to a new system are high, it is anticipated that a cost effective transition towards a Multi Constellation Dual Frequency GNSS will be long. However, the positioning and timing performance requirements from different CNS systems/ applications may require mandating retrofitting to MCDF GNSS equipment in the 2030+ timeframe if justified from a performance based perspective.
The transition to MCDF GNSS should adopt a performance based approach and consider the following aspects:
• The operational impact of losing GPS in the 2020 timeframe (after PBN implementation) taking into account the capabilities of Alternative navigation systems (e.g. DME/DME and-or INS).
• Likelihood of having GPS L1 loss taking into account identified vulnerabilities.
The author would like to acknowledge the contribution of the SESAR Joint Undertaking and the SESAR partners contributing to the 15.3.4 project: AENA, INDRA, DFS, NORACON, SELEX Consortium, THALES ALENIA SPACE and Airspace Users representing Turkish Airlines, Ryanair (ELFAA), EBAA, EHA and IOPA), and also the EUROCONTROL colleagues and aviation stakeholders involved in the preparation and review of the 15.3.4 D8 document. Participants to ICAO NSP, EUROCAE and RTCA groups who contributed to the assessment of operational benefits.
For further reference visit http:// www.eurocontrol.int/ and http:// www.sesarju.eu/