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Global Navigation Surveillance System
Global Navigation Surveillance System (GNSS) is a revolutionary navigation technology and its accessibility and capability has the potential to offer a dominant part of civil aviation air traffic management, that is, communication, navigation and surveillance (CNS). Air Safety Support International (ASSI) is committed to accommodating the benefits of GNSS, but above all, it is committed to ensuring that the implementation of GNSS is safe. In pursuit of ensuring safety, ASSI has subscribed to the research that the UK Civil Aviation Authority has been conducting for the last ten years and has since commissioned its own work to ensure the safe implementation of certain GNSS based services for Instrument Approach Procedures (IAP).
Safety is clearly not new to the field of Air Traffic Management (ATM) where principles and practices are well established, and for some, there is difficulty in understanding why GNSS has triggered renewed and comprehensive research. To appreciate this, it is necessary to understand the fundamental differences between GNSS and traditional types of ATM facility.
1. GNSS is a System and not a Service
The basis of GNSS is a satellite constellation that provides signals in space whose timing component is interpreted by receivers in an aircraft to primarily provide a position (a timing and velocity component can also be extracted). The only fully operationally capable system in existence at the moment is the US Navstar Global Positioning System (GPS), but other globally available systems are progressing towards operational status (ie GLONAS, Galileo). These systems deliver only position and therefore GPS and emerging systems do not alone constitute an air traffic service or facility (eg track and guidance information has to be calculated outside GPS). Therefore components have to be added to a GNSS in the user domain to render a service compliant with navigation requirements expressed in terms of accuracy, integrity, availability and continuity. Consequently, GNSS is not a service and will never be singularly approved as a component fit for all purposes. A GNSS has to be designed to interface effectively and safety into each different ATM service separately. Furthermore, like all systems, it has failure modes whose significance varies depending upon ATM service in which it is applied. There is no denying that GPS has demonstrated remarkable performance in relation to compliance with its specification. However, the threat of insidious failures remain, and the consequences of this on worldwide users could be serious because of the one to many relationship. All these new factors need to be considered and this has created challenges for the industry in general.
2. Totally new types of Air Traffic Service
The nature of GNSS provision is creating opportunities in ATM that have hitherto been impossible. At the extreme end there are services such as Automatic Dependent Surveillance (ADS) which is a totally new technical concept revolutionising airborne surveillance. Such services clearly require complete design from scratch but also, where international traffic is concerned, there is a time lag before aircraft fleets will become fitted with the required avionics through the normal evolutionary cycle. Less radical, but no less effective are services such as non-precision approaches (NPA) based on GNSS, which permit more flexible IAP designs. Whilst GNSS based NPAs may resemble those based on traditional navaids, their failure modes are quite different and have to be thoroughly understood and accounted for in the IAP design phase before it can be approved.
3. Third party ownership of GNSS
A major component of safety is ownership and accountability. It is for this reason that approvals are granted to a person or organisation in relation to a navigation aid. This attests that there is accountability for the operation and maintenance of a system in relation to its specific use. This is in effect the process whereby the causes of potential failure modes are managed at source (and users need not account for them). This has developed into a specific practice now referred to as a Safety Management System (SMS) that is gradually being made mandatory by ICAO through various ICAO Annexes. If this is followed to its logical conclusion for GNSS, then any satellite provider should provide an SMS to every global safety regulator responsible for the regulatory oversight for each specific ATM worldwide purpose. This is clearly unworkable for GPS. The only solution is to shift the accountability burden to the user and the service provider of a GNSS based ATM service. Technically this has meant that users and providers are left to deal with the consequences of any system fault (because the provider cannot be regulated in relation to management and control of potential failures). This has led to technical systems being developed that either mitigate or at least detect a GNSS failure prior to it becoming a safety issue. Thus, services have to be designed to be fault tolerant.
ASSI has conducted research directly and also together with the UK Civil Aviation Authority.