Why ADS-B?

From the US Federal Aviation Administration's perspective, automatic dependent surveillance - broadcast is meant to reduce dependence on ground-based secondary surveillance radar systems by collecting GPS-derived position reports from ADS-B equipped aircraft at ground stations and sending the information (ADS-B "out"), to air traffic control facilities to be used for air traffic management.

atc radar 
© Rex Features
ITT's ADS-B will eventually replace most secondary surveillance radars

Given that ADS-B position reports are updated once a second, compared with once every 12s or more for ARTCC radar and once every 4s for terminal radar systems radar, it is likely that the technology will also be a prime candidate for decreasing aircraft spacing and thereby increasing capacity in the near future.

But first there has to be equipage. As envisaged by the FAA in its 2007 proposed rulemaking for ADS-B "out", most US aircraft will have to be equipped with one of two types of ADS-B equipment by 2020. Operators desiring to be above 24,000ft (7,315m) will have to be equipped with a 1090MHz extended squitter system.

Those flying below that altitude - typically lighter general aviation aircraft - will be required to use either a 1090ES unit or a universal access transceiver, a lower-cost device that operates at 978MHz and has more bandwidth capability.

The two-frequency solution requires that ADS-B prime contractor, ITT, collect signals from both types of users and rebroadcast an augmented report so that users of 1090ES and UAT systems can "see" each other. The notice of proposed rulemaking calls for either system to broadcast more than a dozen data elements, including aircraft position, velocity, barometric altitude, tail number and emitter category.

It is unclear whether the final ADS-B "out" rule, expected in 2010, will differ from the notice of proposed rulemaking due to industry concerns over whether the deadline is too far out and whether a business case can be made for equipping with ADS-B out only.

Although ADS-B out suits the FAA's surveillance needs, the more compelling argument for investing in new avionics from the aircraft operator's standpoint is the operational efficiencies and safety benefits provided by ADS-B "in", the capability that allows airborne aircraft to display other traffic, weather, and aeronautical information in the cockpit.


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