Airservices Australia and Qantas are working with Honeywell at Sydney Airport to supplement the signals broadcast by global navigation satellite systems. Those systems include the global positioning system that will provide the same level of precision approach guidance as a Category I instrument landing system–that is, to a height of 200 feet above the ground as long as visibility on the airport surface is adequate.

The ground-based augmentation system (GBAS), as it is known, monitors the GPS satellites’ signals, detects any errors resulting from ionospheric interference or other causes and broadcasts corrections. Suitably equipped aircraft can use the corrections to refine their own navigation solution, making it accurate and reliable enough to use for approach guidance.

In February 2007, Airservices announced an agreement with Honeywell to develop and commercialize GBAS and a ground-based regional augmentation system (GRAS). A single GBAS ground facility can provide approach and departure information to all runways within 23 nm. GRAS will extend coverage to the whole of mainland Australia using a network of transmitters and should support approaches with vertical guidance to an accuracy of 66 feet.

Existing regional systems such as the U.S. WAAS, European EGNOS and Japanese MSAS are space-based, using satellites to broadcast accuracy and integrity information derived from a network of ground stations. But air traffic in Australia is far smaller and more thinly spread, making a ground-based system more cost effective.

Because the two systems use similar communications and message formats, aircraft will require a single avionics unit to benefit from both. Modern air transport aircraft with multimode receivers should be able to use them without modification, and Honeywell is developing a more affordable receiver for regional aircraft.

Qantas has several Boeing 737s already equipped with the GNSS landing system (GLS) and has been using it operationally since November 2006. The airline’s Airbus A380s will be delivered with GLS. Airbus has been using GLS on both the A380 and the A320.

Initial certification of the GBAS will be for precision approaches down to Category I. The objective is to certify down to Category IIIb, which involves a decision height of less than 50 feet. That should enable airlines to maintain their blind landing capacity in Category III conditions, minimizing the number of diversions and delays caused by reduced visibility.

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