At a number of airports around the world, lower altitude operations in the terminal area are restricted by the presence of high terrain, which can block aircraft interrogations from nearby secondary radars. In turn, this prevents local controllers from monitoring terminal area traffic below a certain altitude. As a result, such airports experience high diversion rates in instrument weather conditions.
This was the problem facing authorities at Innsbruck, Austria and Ostrava in the Czech Republic, for example. At Innsbruck, surrounding mountains forced the minimum decision altitude (MDA) to be 3,100 feet above the airport. At Ostrava, aircraft were prevented from descending below 6,000 feet due to local high terrain.
One solution for those airports could have been the installation of one or more secondary radars at or near the airport. But economic analyses by both ANSPs showed there would be substantial cost and operational advantages in adopting multilateration surveillance systems. Not only would multilateration be cheaper in acquisition, installation and maintenance, but it would provide optimum terminal area coverage and — perhaps equally important — faster and more accurate tracking down to the airport surface.
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