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Aviation History
1997
1997 - 2212.PDF
AIR TRANSPORT Is the cockpit display of the future going to look more like this? ing aircraft separation remains on die ground side. New ATC clearances will be created, eg: "Aircraft A, maintain XXXnm behind Aircraft B." This type also exists under VMC. • Phase II: responsibility for maintaining air craft separation will be divided between the air borne and the ground sides, with the ground being responsible for air-traffic flow-manage ment planning, setting up priorities for conflict resolution and a general surveillance function. The aircraft's pilot would be responsible for handling complex and time-critical clearances for separation and collision avoidance. The basic concept has already been tried in IMC, during die in-trail climbs operated by the US Federal Aviation Administration beyond die West Coast of the USA and in the Oakland and Anchorage flight-information regions. In these trials, aircraft have been guided through die flight levels of preceding aircraft by the use of die secondary-transponder returns shown on cockpit TCAS traffic displays, with identification effected by die transponder of die aircraft in front being switched off and on. Separation has remained die responsibility of the controller, however. BROADCAST Work continuing at Malvern, at NASA Langlcy and at odier centres, supports die role of ICAO's secondary-surveillance radar (SSR) improve ments and collision-avoidance systems (SICAS) panel, which is a focal point for ASAS develop ments. This panel foresees die CDTI receiving the necessary data on aircraft position and status not by interrogation, but by broadcast. This relies on the Automatic Dependent Sur veillance - Broadcast (ADS-B) system, in which an aircraft's or vehicle's position, altitude, vector and odier information are broadcastvia datalink for use by other aircraft and vehicles and by ground sites. Separation assurance, whether provided by ground ATC or based on the ASAS, could draw on the high-accuracy navigation data thus made available. There remains a need for collision avoidance to provide protection which is independent of ADS-B and die ASAS function. Collision avoidance is a last-resort safety protection. Its purpose is to prevent collision when the prima ry means of separation assurance has failed. Logically, it must be independent of die ASAS' primary means of separation assurance, since a risk of collision implies a failure in that area. If the collision-avoidance function is to be based in any way on ADS-B data, it must be proved diat die overall design provides sufficient integrity, reliability and availability of the com mon elements. It may require time to obtain sufficient operational experience, taking account also of any risk of deliberate or acciden tal jamming. So the simplest and most cost- effective way to the integrity required will probably be by die provision of independendy measured data as die basis of collision avoidance. Aircraft not equipped widi SSR altitude reporting transponders, but fitted with satellite- navigation equipment, might be equipped to function with a base level of ADS-B. Traffic awareness would be enhanced if eidier or bodi aircraft were so fitted. Greater protection would be offered too by an ASAS which included a conflict-detection function. The generation of conflict-avoidance manoeuvres with such a basic ASAS is more problematic because dicy would need to be co ordinated and to be compatible widi die ACAS. There will be the potential need to co-ordinate its recommendations with diose of ATC. Full collision avoidance, as a last-resort safe ty system, cannot be achieved without an inde pendent means of validating the ADS-B data supporting die conflict detection - eg, active interrogation of transponders. The last-resort collision-avoidance protection would simply be missing, but die SICAS panel considers diat this may well not be required by general aviation. The ACAS cannot generate RAs against air craft which do not have altitude-reporting SSR transponders operating. Any unplanned and unco-ordinated reduction in the level of transponder equipment carried as a result of installing die ADS-B would increase die risk to aircraft which have the ACAS in operation. PARALLEL RUNWAYS An example of an ASAS application which could provide significant operational benefits in air space where there is radar coverage is die paired-approach concept. This proposes surveillance, based on received ADS-B aircraft-position data, of dual- runway approaches to airports with closely spaced parallel runways - especially runways which, during low-ceiling and poor-visibility weather conditions, would otherwise have to revert to a single-runway approach capability because of ATC separation requirements. In the final event, considering the range of potential applications, it is probable that die proposed ASAS will require surveillance per formance levels in excess of those now provided by die ACAS. The requirements will include high-integrity data on aircraft position, intent and further operational data to be exchanged via an appropriate air-to-air datalink. These data could be displayed eidier in new formats for surveillance-traffic presentation, suited to receive ADS-B aircraft-position data or, when showing extended Mode S data, by traffic displays shared with the existing ACAS. The ASAS, says the SICAS panel, should not be considered merely as an "improved ACAS". The ADS-B system could, however, eventually achieve sufficient availability and integrity that ADS-B-equipped users may no longer have to carry transponders. In regions where benefits can be shown for a co-operative ASAS, changes would be needed to realise its potential. These would have to be made progressively, within the controlled tran sition to a future air-navigation system. Where the ASAS is to be operated, all aircraft would have to be suitably equipped, procedures set in place to allow for equipment failure, and the effectiveness of failure and mode indications assessed. ATM providers will need to deal with users widi mixed-equipment levels in diese air spaces for die transition period. The SICAS panel stresses diat die ASAS, in its proposed pilot-operated surveillance role, will be a safety-critical system. Its performance and integrity, as a full-time system, will have to be even greater than those demanded of the ACAS. Identifying the type of airspace in which such a system can be applied, and setting the appropriate separation minima, are the next challenges for this promising concept. • 28 FLIGHT INTERNATIONAL 3 - 9 September 1997
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