Plane & Pilot
Tuesday, August 3, 2010

Beyond Today’s Transponder


Demystifying ADS-B



Avidyne is upgrading their TAS-600/610/620 TCAS products to add ADS-B support, associating N-numbers and airline flight numbers with active targets.
Autonomous Dependant Surveillance-Broadcast (ADS-B) has to be one of the most misunderstood technologies of the 21st century. Among other things, it has been called a replacement for radar, an open invitation to attacks on the air traffic control system and a stalking horse for user fees, all of which are demonstrably false. ADS-B is, however, by the FAA’s own admission, an unfunded mandate on owner-pilots that will cost most aircraft owners several thousand dollars each over the next 10 years.

Today’s System
To operate in most controlled airspace in the U.S. today, you’re required to have a transponder, which responds to interrogation from primary and secondary surveillance radar signals, sending altitude and a four-digit “squawk code” to the radar site. Air traffic controllers use that to identify where aircraft are located, keep them separated if operating on IFR flight plans and offer traffic advisories if they’re operating VFR.

Radar accuracy varies: Traffic near a radar site can be located to within a few dozen feet, but at long distances, there can be errors of up to a mile or so. Worse, traffic position is updated only when an aircraft passes through the radar beam—typically every 12 seconds—and even light aircraft can move quite a distance in that time. The upshot is that today’s radar-based system requires a five-mile separation between aircraft.

In VFR conditions, ATC works around that by relying on pilots to see nearby traffic, but when the clouds roll in, they’re back to the five-mile rule, and traffic slows to a crawl. Even in visual conditions, the system can break down if there’s a radar outage, which I’ve personally encountered twice in Los Angeles airspace. And surveillance radars usually are located on (or near) busy Class B and C airports. That’s fine if you’re flying into those airports; but if you’re flying into a smaller airport, you may find that coverage is poor. It’s not unusual to have a controller say “radar service terminated, report closing your flight plan in the air or on the ground.” That generally means you’ve flown below the base of the radar coverage, which is limited by terrain or other factors. And until you close your IFR flight plan, nobody else can get into that airport.

NextGen Technology
The FAA has been working on a better system for more than a decade. It’s called the Next Generation air traffic system (NextGen), and instead of radar as the primary sensor, it’s based on satellite navigation systems like GPS. A majority of general aviation airplanes today have a GPS in the panel, and they’re being added to business jets and airliners. By providing a link between the GPS on the airplane and controllers on the ground, dependence on radar can be broken. ADS-B is the technology designed to provide that link. It’s autonomous, sending the aircraft position, a unique 24-bit identifier, your N-number (or airline flight number), altitude, transponder squawk, heading, velocity and other data every two seconds. It’s dependent on GPS (or an equally accurate system) for its position information. The data transmitted by ADS-B is used by ATC for surveillance of traffic in their airspace, broadcast using one of two radio links through a network of ground stations being installed by ITT Corporation, under contract to the FAA.



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