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Aviation History
1991
1991 - 1698.PDF
SEPARATE Satnav and satcoms will soon combine to make David Learmount explains automatic dependent surveillance. • Oceanic skies have to be sparsely-used because radar cannot watch them Since present air traffic control (ATC) systems allow only a fraction of the airspace above the world's oceans to be used, airliners in Tokyo are held waiting to join processions of aircraft heading for Los Angeles and aircraft queue in London for New York. If minimum vertical and lateral separa tions between aeroplanes could be reduced without reducing safety, more aircraft could use the same sector of sky and queues could be eliminated. Accurate, reliable oceanic ATC surveil lance is needed to allow a higher trans oceanic traffic flow while maintaining safety levels. Radar, primary or secondary, cannot do it because of the ranges involved. Auto matic dependent surveillance (ADS) will be the ultimate answer to this need. ADS combines the capabilities of satellite naviga tion (satnav) and satellite communications (satcoms) to present Oceanic Control Cen tres (OCCs) with air traffic displays at which controllers can work. It is being tested by national aviation authorities on the Atlantic and the Pacific, but it is far from clear what the operational timescale is. To analyse how ADS — combined proba bly with traffic alert and collision avoidance system (TCAS) — will make reduced traffic separation distances safe eventually, it is essential to study the basics of current oceanic ATC systems and the safety stan dards on which the sights of the regulators are set. The reason for today's pre-oceanic queu ing is bifold: firstly, the primitive proce dural ATC methods by which oceanic air space is managed means that each aircraft has to file across the oceans in one of the several meteorologically determined, daily designated, one-way tracks in the Pacific or Atlantic Organised Track System (OTS); the airliners in each track fly at the same Mach number, each a minimum of ten flying minutes — about 165km (90nm) — behind the other. Also, the system's original need to make large allowances for primitive aircraft-navigational methods and simple mechanical altimeters means that, respec tively, OTS tracks within a given directional group (eastbound or westbound) must not be closer than one degree latitude (110km) and operational flight levels (FLs) must be separated by a minimum of 2,000ft (610m) — in airspace over FL290. Eastbound and westbound track groupings are well sepa rated geographically by the need to secure the best tailwinds or minimum headwinds. The ADS displays for the OCCs have been called "pseudo-radar" but, to be more accurate, they are computer-produced graphic displays of aircraft position as reported by the aircraft's navigation systems via satcom. Today's aircraft inertial navigation sys tems (INS) are far more accurate than the navigation methods in use when oceanic minimum separations were determined; Sat nav is even more accurate. Meanwhile, altimeter performance is being improved to enable height-keeping accuracy and reliabil ity above FL290, which would make 1,000ft vertical separation safe at both high and low altitude. SYSTEM RELIABILITY Before it can be used, however, redundancy has to be built in at all points of the complex and totally interdependent system constituting the ADS. Its prime components are the air mobile station (the aircraft),, satcoms, satnav and the ATC station. If each system had good redundancy though, and if TCAS was also available as a final "safety net" anti-collision system, separation reduction could be allowed logi cally and safely. If ADS could give control lers the same degree of real-time accuracy and reliability that primary and secondary radar permits now, then, theoretically, oce anic skies could have the same separations as aircraft under positive radar control — a 44 FLIGHT INTERNATIONAL 19 - 25 June, 1991
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