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From your friends at  2008 - Reproduction without permission is prohibited.

Editor's Note 12/31/2008:  The most recent FAA post-ADS-B/NextGen deployment road map calls for retaining approximately 50 percent of the current technology for backup and emergency purposes.

Of all the things that can be said about the Automatic Dependent Surveillance-Broadcast (ADS-B) system, only two can be uttered with any certainty: 1. It is coming (eventually) to an aircraft and airport near you; 2. It is highly controversial.  

On one side of the glide path, ADS-B supporters, including most policy makers at the FAA, believe that the global satellite-based ADS-B system should replace the current radar-driven ATC system in normal day-to-day operations., hopefully within the next two decades.

On approach in the middle of the runway are a large group of professional and private aviators who believe ADS-B should be deployed alongside the current radar system.  This group is about evenly divided between those who believe ADS-B should be used to supplement radar and those who believe ADS-B should become the dominant technology, with the existing system relegated to a backup role.

Finally, waiting for takeoff clearance on the taxiway, are a vociferous group of ADS-B opponents who claim that the current system, despite being "ancient" in high-tech terms, is operating brilliantly, that the skies are safer now than at any time in the history of aviation and that, to put it simply, if it ain't broke it shouldn't be fixed.

Before examining each of these three positions, let's take a brief look at how ADS-B works.
In flight, an ADS-B equipped aircraft obtains location information from GPS satellites, automatically labels it with the its call sign, attaches it to information about its altitude, velocity vector and velocity rate and broadcasts the continuously updated data stream at very short intervals.  This information is received by automated ground transponders, which combine it with weather and other data and rebroadcast it to aircraft for viewing on a graphical display; ATC controllers, who use it much the same way that they now use radar to position and direct traffic throughout airspace; and transponders in nearby aircraft where it can be viewed on a cockpit display and accessed by automated collision-avoidance systems. 

Proponents of full conversion to ADS-B claim that it will, in addition to adding air-to-air surveillance capability to the air-traffic control system, make it easier for ATCs to schedule arrivals and departures, enable reductions in FAA aircraft-separation requirements, reduce runway incursions, improve surface operations in poor visibility, deliver real-time traffic, weather, terrain and other information to pilots in an easily accessible form and -- perhaps most important to the nation's financially sagging commercial air carriers -- allow airlines to improve fleet utilization rates and return on investment. 

As noted, this position is strongly supported by the FAA, which defines ADS-B as "simply put, the future of air traffic control" and a cornerstone of its Next Generation air-traffic management initiative. 

In commenting on its October 2, 2007 proposed rule which would require all aircraft -- commercial and non-commercial -- passing through the nation's major air corridors to be ADS-B compliant by 2020, the Agency describes ADS-B satellite positioning as "ten times" more accurate than current navigation technologies and says it "may eventually allow air traffic controllers to reduce separation standards between aircraft, significantly increasing the number of aircraft that can be safely managed in the nation's skies." 

The agency also noted that "pilots viewing ADS-B cockpit displays are able to see, in real time, their location in relation to other aircraft, bad weather and terrain" and claims that "in Southwest Alaska, the fatal accident rate for ADS-B-equipped aircraft has dropped by 47 percent."

Opponents of totally replacing the existing ASR radar system with ADS-B generally acknowledge the technological virtuosity of the new technology, but question whether it has really been proven ready for the prime-time protection of millions of lives. 

They point out that the government's Alaska statistics, based upon an FAA research project -- the Capstone Program -- initiated in the early years of the 20th Century, did not offer conclusive evidence that ADS-B would be as safe and dependable as the current radar-based system in the highly congested air corridors of the lower 48 states.  The very richness of the data flowing back and forth between aircraft and ground transponders, they say, may compromise the system's integrity in high-traffic areas by overloading the available digital-communications bandwidth.

They further claim that the FAA's plans to "privatize" the ADS-B system by allowing outside contractors to set the hardware design specifications and operate the system after its deployment do not provide the kind of oversight necessary to ensure optimally secure and reliable operation. 

Noting that at least one potential contractor had proposed using commercial XMRadio broadcasts as a source of weather data for the system, an FAA opponent commented that "if the Pentagon had allowed the aircraft manufactures to build to their own specifications for piston-engined bombers we'd probably have lost World War II. I f NASA had let the aerospace industry spec the hardware we might never have made it to the moon."  Though ITT's modest $207 million contract runs only three years, detractors point out that it contains close to $2 billion in options that give the contractor operating and maintenance authority for the system through 2025. 

These opponents also note that the current air traffic management system has been federalized since virtually the dawn of the passenger air age and has never been safer than in the years since President Ronald Reagan dramatically increased the amount of governmental control over the system following the 1981 air traffic controller strike.  They ask whether a public corporation whose primary mission is to create maximum return for shareholders faces a conflict of interest in running a system where maintaining operational excellence may compromise profits, and they question the cost to taxpayers if ITT's contract has to be terminated prior to 2025 for performance-related issues.

Those opposing ADS-B altogether are split into two camps:  The larger camp is populated by those who say "why buy what we don't need," the smaller by what some folks would consider alarmists. (Defenders of the later group would point out that being paranoid doesn't mean someone isn't out to get you.)

According to the first camp, the basic assumptions about air traffic trends being put forth in support of ADS-B are questionable.  They claim that the major causes of poor airline fleet utilization and delayed flights are a lack of runways at commercial airports and "convenience" airline schedules that mimic their competitors takeoff and landing times. 

Reducing air-separation requirements to allow more planes per hour to approach an airport accomplishes nothing if there aren't enough runways for them to set down on, they say, and ADS-B will not allow significantly more closely timed or simultaneous landings than the current generation of Precision Runway Monitor (PRM) technology .

They also contend that a projected doubling in the number of annual air travelers of all types over the next two decades will not overtax the existing system because all those additional fliers will be transported in, according to the FAA's own numbers, only about 20 percent more aircraft than are already in service.

Which brings us to the final group of naysayers, those who consider ADS-B, in the words of one of them, "a terrorist's dream."

"Government policy using conventional radar makes it almost impossible for a terrorist or a criminal to locate the position and identity of an aircraft, because many millions of dollars are required to build a directional radar antenna," says Lindbergh Award winner Dick Smith, former chairman of Australia's Civil Aviation Safety Authority.  "With ADS-B, it's the opposite because all you need to track every aircraft is a small non-directional aerial, worth $5."
People who claim ADS-B can be used to empower terrorists set forth a number of futuristic scenarios to support their position. A common one, as described by pilot and frequent FAA critic Darryl H. Phillips, goes like this:

A lone Mideast terrorist comes to the United States and purchases (or rents or steals) a light aircraft such as a Cessna 172, Beech Bonanza, or Piper Arrow.  While selecting his plane, he is looking for just one item:  ADS-B collision avoidance equipment.  He knows that ADS-B, installed on all airliners and some business and pleasure aircraft, automatically reports the precise position of the aircraft and also the identity of the aircraft, twice per second. 

He has no particular target, his only desire is to cause fear and destruction ... he flies at low altitude and slow speed above the busy highway toward a major airport.  He knows that Air Traffic Control radar will not see him because he has disabled his transponder output, thereby assuring that there will be no secondary returns nor any ADS-B transmissions.  His aircraft is smaller than the tractor-trailers on the highway below, and the ATC primary radar has been programmed to eliminate highway clutter from the display.  He will not be seen. 

The terrorist also knows that the interval when a large aircraft is most vulnerable is on final approach.  It is moving slowly at low altitude, flaps, slats, and gear are extended, and engines spooled down.  This is the point in the flight when the plane is least maneuverable.  Using the ADS-B readout to spot his target, he flies up the glideslope and directly toward the doomed airliner.
Until the last moment he cannot see the oncoming plane but he knows it is there.  The display on his ADS-B is showing its altitude and position with an accuracy of a few meters.  The airline crew, monitoring their instruments and complying with pre-landing checklists, never sees him at all.  At the last instant he shouts "Allah Akbar" as he flies thru the windshield of the larger aircraft, taking hundreds of people to their death.

Not a very comforting vision, to be sure, but also not, to most aviation experts, a very potent argument against ADS-B.  In the vast majority of the nation's airspace, that out of proximity to Washington, D.C., New York and a few other "hot spots," they say, a terrorist can perform exactly the same suicide mission under VFR with only a minimal risk that an F16 will be scrambled fast enough to shoot him down before he dives on the jetliner.







FAA's depiction of current deployment of ADS-B in the U.S. 

There is, of course, much more, pro and con, that can be said about how well ADS-B works, whether it is needed right now, in 20 years or never, and if the cost-benefit projections for or against it are valid.

But none of that really matters.  ADS-B is coming and like the Iron Horse 150 years ago nothing is going to stop it.  The only open question -- unless some even better, cheaper technology is discovered the day after tomorrow -- is whether it will, like the Iron Horse and the stage coach, run in tandem with its predecessor for a few years or even a couple of decades.
Or will America's ATC radar dishes, once ADS-B is deployed, be consigned to the surplus peddlers as quickly as Pony Express saddlebags after the coming of the telegraph.

The owners and editors of encourage you to write your Senators and State Representatives and tell them you want the FAA to maintain the current Airport Surveillance Radar System for future generations.


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