Can Current DME Support PBN Operations with Integrity?

G. Berz, V. Vitan, I. Skyrda

Abstract:	Due to the space segment being operated by entities outside of aviation control, augmentation systems have been developed for GNSS to achieve the required level of navigation performance to support aviation operations. A key component and design driver of these systems is integrity. Over the past decade, significant intellectual investments have turned GNSS integrity service provision into a strength that provides benefits to all phases of flight. Due to its resulting worldwide operational suitability, the implementation of Performance Based Navigation (PBN) is primarily based on GNSS. Nonetheless, concerns about the impact of service outages remain. Consequently, Alternate means for the provision of Position, Navigation and Time (A-PNT) services during a GNSS outage are being investigated. For aviation, the first A-PNT choice is the significant base of currently installed equipment on aircraft and on the ground, e.g., conventional navigation aids. The most suitable existing terrestrial navigation aid to provide alternate PBN support is Distance Measuring Equipment (DME), because multi-DME ranging provides an Area Navigation service (RNAV) with performances up to at least 1NM accuracy. However, DME/DME positioning cannot currently support RNP navigation specifications, which require On-Board Performance Monitoring and Alerting (OPMA). For advanced applications envisaged in Europe, OPMA is a key ingredient to enable reduced route spacing and associated airspace capacity improvements. In recent PBN discussions, it has become clear that providing a DME-based RNP service with OPMA as currently understood and specified is not possible without equipment changes. What is being neglected in this view is that while formally, ICAO Annex 10 and aircraft DME and RNAV equipment standards do not require integrity analogous to GNSS, current DME ground and airborne system components nonetheless do provide some kind of heretofore unspecified level of integrity. This paper investigates the integrity provided by current DME and RNAV systems, and proposes an alternate, overall integrity concept which aims to support RNP operations despite the absence of the OPMA alarm. A key advantage of DME is that all system components are under aviation control. DME ground transponders have evolved over many years of service and contain a number of industry standard monitors that detect anomalies and terminate service if required. Some Air Navigation Service Providers in Europe have imposed specific integrity monitoring and manufacturers have consequently integrated them into their station designs. Next, the signal in space propagation issues facing DME are generally well understood – there is the potential of multipath, co-channel interference and other effects that can be controlled by a variety of ANSP efforts. This is also true for the historically most prevalent DME error effect, the map-shift due to station coordinate publication errors. Finally, aircraft interrogators and downstream FMS processing does include reasonableness checks and in many cases Inertial Navigation System (INS) integration that protect against several avionics-based failure modes. The paper will analyse each of these components, ground, signal-in-space and avionics to the extent possible, to see what kind of RNAV positioning integrity levels can be achieved by the current generation of DME equipment. The analysis will be based on a mix of empirical performance data, theoretical studies as well as generic design information from equipment manufacturers. This holistic overall DME integrity analysis will then be applied in the operational context of the envisaged RNP applications, where additional ATM system mitigations are available. For example, route spacing applications include route conformance monitors and other tools that may limit the impact of DME positioning failure modes, especially if the exposure time is limited to GNSS outages. This additional argumentation that will rely on available route spacing studies is considered necessary due to the absence of the OPMA integrity alarm. The sum of available effect mitigations may then make the achieved integrity level acceptable and exploitable. In some cases, the information available may be limited, and assumptions will need to be made. However, the goal of the paper is to investigate the feasibility of DME positioning integrity concepts by quantifying and obtaining credit for the integrity that is actually achieved by current systems. Ideally, this would provide an equipment based integrity similar to RAIM in terms of detection thresholds, even if the methodologies clearly have to be different given that a terrestrial service is by definition short on redundant measurements. If an initial feasibility can be demonstrated, recommendations for further analysis and development will be formulated. This work is especially significant for Europe, where a mature DME infrastructure exists. It is tempting to consider DME upgrades to overcome the identified limitations in supporting PBN. However, as soon as DME equipment changes are proposed, the limited spectrum efficiency of DME quickly leads to favouring more modern, new technologies. Consequently, the true potential of DME needs to be analysed and documented in order to decide on priorities in aeronautical navigation developments in the long term as well as terrestrial infrastructure evolution investment decisions in the near term. While it is regrettable from the perspective of current GNSS developments that conventional navigation aids have no formal integrity requirements (except of course the Instrument Landing System, ILS), this existing infrastructure and its established service record deserve a closer look at what kind of overarching and multi-disciplinary integrity service is actually being offered, and how this terrestrial service can serve the needs of airspace users until new alternate technologies are available.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	233 - 250
Cite this article:	Berz, G., Vitan, V., Skyrda, I., "Can Current DME Support PBN Operations with Integrity?," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 233-250.
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