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The Transition to the Next Generation Air Transport System
By J.D. Roy
August 8, 2006
Since the first successful flight of an aircraft in 1903 by the Wright Brothers, the U.S. has been the world's leader in the aviation. However, over the last few years, this country has been losing the battle to maintain that dominant position. Just recently, the Federal Aviation Administration, or FAA, made a great stride towards maintaining this country's position of leadership. In a giant leap forward, the FAA has selected a new system to manage the air traffic over this countries air space. In the fall of 2005, Avionics Magazine's editor David Jensen reported that automatic dependent surveillance - broadcast, or ADS-B, was selected to become the Next Generation Air Transport System, or NGATS (1). This new system has great advantages over the current air traffic control radar beacon system, or ATCRBS. The advantages of this new avionics system will be demonstrated through the terms and definitions of air traffic management and a description of the current system compared to ADS-B.
Before discussing any avionics system, it is important to explain several terms and abbreviations. These terms, along with several others, come from an instruction guide printed by Rockwell International, a major producer of aviation electronics equipment. Air traffic control, or ATC, is a part of the FAA and is in charge of safely directing all aircraft in the air over the United States. Air traffic control radar beacon system, or ATCRBS, is the current air traffic system used in this country. Radar, an acronym for radio detection and ranging and is how ATCRBS determines the location of an aircraft. SSR stands for secondary surveillance radar and requires that both the sender and the recipient have transmitters and receivers to operate. PSR or primary surveillance radar does not require a target to have any extra equipment to locate an aircraft in range. L-Band is the range of radio frequencies from 390 up to 1550 MHZ. These are the allocated frequencies for air traffic management systems worldwide (iv-vi).
With some terms and definitions cleared up, the next step is to describe the operations of the current system. James Wasson, author of several avionics textbooks, describes how the current system, ATCRBS, works. It consists of two major sub-systems interfaced together on ATC's radarscope. One segment is a PSR type system used to obtain the current location of all aircraft in range of the ground station. The other segment is a SSR type system, which sends out interrogations, at 1030 MHz, from the ground station to each aircraft and then waits for a reply, at 1090 MHz. This part of the system can ask for two types of information from the air traffic currently in range. Mode A interrogations ask the FAA assigned four digit identity assigned to each aircraft. Mode C interrogations ask all of the aircraft in the vicinity what their current altitude is. Once received, these interrogations are replied to by the transponder on each aircraft in range. When both the PSR and SSR inputs are combined ATC obtains a complete traffic picture of the area they control (145-148).While the operation of this system seems simple enough, there are some major drawbacks to the current air traffic system. Wasson continues explaining these issues in his textbook Avionic Systems: Operations & Maintenance. When the air traffic in an ATC area grow too large, the PSR segment of the ATCRBS system has a difficult time distinguishing between individual targets. In addition, PSR systems signal weakens significantly the farther away a target is along with detecting target data from tall object on the ground (146). These problems can lead to times when ATC loses the return signal from a small aircraft and cannot warn larger aircraft about the presence of all the targets in the area. The chair of the U.S. House Aviation Subcommittee, John Mica, has suggested other issues with the current system. The U.S recently spent forty-four billion dollars on upgrading and maintaining the venerable ATCRBS system. Additionally, the consistently overworked ATCRBS system threatens a major slowdown of air traffic in the U.S.; this slowdown would cause the loss of thirty billion dollars every year if it occurs. This slowdown seems inevitable since the FAA expects air traffic, to grow exponentially by the year 2025. It is only by the hard work and dedication of ATC workers that the U.S. continues to lead the way in aviation safety (1-2).
With a basic understanding of how ATCRBS works, one can more fully appreciate the features and benefits of NGATS. According to the editor of Avionics Magazine, David Jensen, the FAA had several options to choose from when selecting the new ATC system. In the fall of 2005, the FAA had made its decision by selecting ADS-B for this new system. The savings that ADS-B will provide the ATC over the long-term will be enormous. The FAA will be able to rid itself of the expense of 328 ground based radar stations along with other equipment by 2018 (1). In fact, Flying Magazine has stated that one ground radar station for ATCRBS cost as much to operate as twenty ground based transceivers, or GBTs, for ADS-B would cost. This upgrade would also double the coverage area for these stations at the same cost (Benenson 3). By selecting ADS-B as America's new traffic management system safety and capability will increase significantly with a more reasonable price.
Along with cost savings, ADS-B also brings a significant host of operational improvements over ATCRBS. Tom Benenson, of Flying Magazine, lists some of these improvements. ADS-B allows pilots to receive traffic information displayed on a screen in their cockpit with each aircraft showing its identification, direction, barometric and relative altitude. This system includes the ability to change the area the pilot is observing along with the option of taking a closer look at an individual aircraft's information. This allows pilots to have an increasing knowledge of traffic in the area without having to rely on a controller from ATC to be the eyes of the pilot. In addition to this increased situational awareness, the pilot of an ADS-B equipped aircraft can see the exact speed, location, and type of traffic in the area monitored (1-2). ADS-B is a significant improvement in features than that of ATCRBS, which provides no direct information to the pilot.
Even with these improvements of the current ATC situation, ADS-B does have a few minor drawbacks. Tom Benenson illustrates these drawbacks in his article "Mysteries of ADS-B", published in October's Flying Magazine.
As good as it is, there's a major disadvantage to ADS-B. Well, not to ADS-B per se, but to the lack of availability of ground-based transceivers (GBTs). In order to display non-ADS-B-equipped airplanes on a cockpit display of traffic (CDTI), the host airplane has to be within line-of-sight of a GBT that sends up traffic forwarded from ground-based air traffic surveillance sensors, typically radar. The uplink of non-ADS-B traffic is referred to as Traffic Information Service-Broadcast (TIS-B). Because the uplinked radar information is not as accurate as the ADS-B targets, the icon on the cockpit display for "degraded" non-ADS-B traffic looks a bit like a Pacman that's had a bite taken out of its backside; the ADS-B traffic is displayed as a chevron or arrow head pointing in its direction of movement. (1) This issue is only temporary and will be solved as the FAA begins installing more GBTs and the number of ADS-B equipped aircraft increase. Being the primary technology behind NGATS, ADS-B provides a strong foundation for developing new systems, which reduces the problems of coverage, security, emergency operation, and other issues.
With the benefits and features of ADS-B listed, how does it all work? Rockwell Collins answers that question in a booklet called Collins Automatic Dependent Surveillance - Broadcast (ADS-B). ADS-B operates by gathering precise information from the satellite based global positioning system, or GPS. This information is then relayed to other ADS-B equipped aircraft in range as well as to an uplink to a constellation of dedicated automatic dependent surveillance satellites. The satellites then send the data real-time to ADS-B ground based transceivers. Once the information enters the system it is available to any aircraft within range of any GBT. This process occurs every second, allowing an accurate traffic picture for both ATC and pilots. Pilots can choose from various models of ADS-B equipment, ranging from just transmit capability to a transceiver. The transmitter only allows an aircraft to be tracked, but does not give the pilot any information. If the pilot elects to install a transceiver, the full range of ADS-B information is available from the ATC network (2-3). ADS-B is based on reliable technology and sound theory, allowing advanced situational awareness along with several options from which the user can choose.
America has the opportunity to remain a world leader in the realm of aviation technology through the NGATS initiative. The current ATC system, ATCRBS, is overworked, outdated, and inaccurate. The FAA is correcting this problem by selecting ADS-B as the primary technology for the next air transport system. This solution offers a clear advantage over ATCRBS by reducing ATC workload and increasing the situational awareness of the pilot. It operates by using existing precise navigation systems and relaying that information to other aircraft. As the FAA solidifies NGATS requirements, they are revolutionizing the future of air travel in the U.S. and the rest of the world. This revolution over ATCRBS will come from the features and advantages that ADS-B and NGATS will posses.