Betting on the future
There are bright spots in the air traffic control picture. One technology, in the testing phase, seems to have everyone's hopes up as something that could increase safety and capacity. That technology is the Center-Tracon Automation System (CTAS) [ Fig. 8]. CTAS computers include tools to improve air traffic operations from departure point to touchdown.
CTAS was developed at NASA's Ames Research Laboratory in Moffet Field, Calif., in cooperation with Natca. It began in the mid-'80s with three or four researchers employed on an in-house project supported by the NASA basic research budget, recalled Ames senior scientist for air traffic management Heinz Erzberger. The initial plan was to look at problems with aircraft delays and cockpit procedures, as well as problems informally identified by (San Francisco) Bay Tracon and Denver (en route) Center controllers.
"The FAA didn't have anything to do with it," Phelps told Spectrum . "That's why it works."
The FAA came on the scene onlyin 1991, with a letter of understanding and some funding to continue CTAS research. At that point, the CTAS group, then numbering 20 researchers, had been able to demonstrate software driven by live radar data, transmitted from the Denver center. (Today, nearly 50 engineers and programmers are assigned to the CTAS project at NASA, and many more are working on it at the FAA and at its contractor companies.)
All CTAS automation has a common logical core, Erzberger explained. It computes aircraft trajectories approximately 40 minutes into the future, starting at the aircraft's current position, then factoring in the aircraft's intent as contained in itsflight plan, along with a model of the performance capabilities of individual aircraft, including lift, drag, and thrust, and atmospheric conditions.
Such trajectory calculations, Erzberger said, can be used to look ahead for conflicts (conflict probe), determine appropriate spacing for traffic arriving at an airport, and produce other controller tools. CTAS is written in C and C++, an object-oriented language, with some 400 000 lines of code, and runs on Sun UltraSparcworkstations and other Unix-based computers. The system interfaces with existing air traffic control computers to obtain relevant flight plan and track data.
CTAS Build 1, which began testing in 1992, is called traffic management advisor (TMA). This is a flow control product that helps traffic management coordinators (who determine aircraft flow into busy airports by issuing orders for speed restrictions and increased trailing distances between aircraft). TMA is now in operational prototype at Atlanta, Denver, Los Angeles, and Miami airports.
CTAS Build 2, now in the development phase, sends CTAS data from the Unix processors through the existing en route center computers (the Host computer system), which distributes it to the PVDs, telling controllers how much delay each aircraft has to absorb and counts down the delay as the aircraft is vectored--that is, as it is ordered into a series of zigzag turns to increase its travel time. This interface was designed by the FAA.
"This was excruciating to develop," said Erzberger, "as it involved the Jovial software [in the Host computers]. We do not have the Hosts do any processing, they merely pass the data on to the display processors. We couldn't send it to the display processor directly; there is no local-area network for these devices."
Build 2 also has a feature called final approach spacing tool (FAST), used in the Tracon environment. FAST looks at traffic approximately 40 miles from the airport and determines the best runway to use and most efficient landing sequence of aircraft. That information is displayed on the data tag that accompanies radar returns on the Tracon controller displays as, for example "3, 13R," meaning third in line for landing, assigned to runway 13 right. The CTAS Build 2 tools are operational at the DallasFort Worth Tracon and the Fort Worth Center, using NASA prototype code.
"With the new display systems, with local-area networks that use standard communications protocols," Erzberger said, "CTAS integration will be much simplified."
Spectrum has heard much praise for CTAS--a rare thing in air traffic control system development. Said Dick Swauger, national technology coordinator for Natca: "I never thought you could develop anything that could make a good controller better. A weak controller, sure, but not a good controller. Then I saw CTAS. It's like having a top controller at your side, whispering in your ear. And it does make good controllers better."
According to Ray Ellison, who is a controller at Phoenix Tower and Tracon representative to the CTAS development team, controllers love the CTAS tools being tested--TMA and FAST. During a recent test at DallasForth Worth Airport, CTAS was able to increase aircraft operations during normal conditions from a non-CTAS maximum of 102 operations per hour to more than 120 operations per hour.
An advanced version of FAST, which suggests headings and speeds, is running in a laboratory environment but has not as yet been tested in the field. Said Jack Ryan, vice president, air traffic management, of the Air Transport Association, an organization that represents commercial airlines: "The FAA refuses to test it citing a cost-benefit analysis that they won't show me." An FAA spokesman indicated that this advanced version, called Active FAST, needs the new generation of color displays to be operational.
Also looming on the horizon are automatic dependent surveillance and a technique that goes by the shorthand name ofdata link.
Automatic dependent surveillance (ADS) is based on the Global Positioning System (GPS). It makes use of the GPS receivers aboard aircraft that give pilots navigation information. ADS would take the GPS information, along with the aircraft's velocity, and digitally transmit it by satellite or some form of radio data link to ground control facilities. There the signal would be captured and displayed on controllers' workstations.
ADS-B is a broadcast version of ADS that could be read by other aircraft, giving pilots a situational display of traffic around them that is much more accurate than that provided by TCAS. (ADS-B traffic display systems have already been built and demonstrated in Sweden and by MIT's Lincoln Lab, and they are being developed by Seagull Technology Inc., Los Gatos, Calif., for the U.S. Forest Service for use in fighting fires.)
To be used for air traffic control, the accuracy of the existing civilian GPS signal would be enhanced by the wide-area augmentation system (WAAS), currently being developed by Hughes Electronics Co., El Segundo, Calif. This network of 24 ground reference stations, master processing stations, satellite communications links, and related software will correct clock, satellite location, ionospheric delay, and other errors in the GPS pseudorange measurements. The system's job is to compare known GPS receiver locations to the GPS-determined location of those positions, and then broadcast corrections for use by aircraft and other receivers.
Standard civilian GPS locates targets in a horizontal domain of 100 meters 70 percent of the time, compared with some 300 meters for ground-based radar surveillance systems; WAAS would bring that figure down to 7.6 meters, said a Hughes spokesman.
In testing, according to J. David Powell, professor of aeronautics and astronautics at Stanford University, GPS with a WAAS prototype system using 18 reference stations throughout the United States proved to be accurate to better than 3 meters (horizontally) 95 percent of the time.
GPS navigation is tailor-made for the ocean environment, where radar coverage is not available, and a $300 million program has been launched. (Today, aircraft flying over the oceans avoid conflicts by flying in predetermined corridors that are 100 miles apart over the Pacific and 60 miles apart over the Atlantic. Powell indicated that, given enough time and experience with ADS-B, separation standards could eventually be reduced.)
The first operational WAAS is scheduled to start up in early 1999, with nationwide rollout to be completed in 2001. A future version of differential GPS, the local-area augmentation system (LAAS), would enable even more accurate corrections for aircraft near airports. At present, it is still being tested and refined. Tests of a prototype system by Stanford University in 1995 demonstrated 100 automatic landings by a United Airlines B-737 with navigation accuracies better than 50 cm.
The idea of GPS-based surveillance makes some controllers nervous, however. "GPS isn't as wonderful as we think it is," Embry-Riddle's Smith said. "But it is the best thing that has come along in a long time. It works, and sooner or later we can anticipate that it will get through all the politics and will be certified. People will have to use it because the alternatives are too expensive."
One impetus for implementing GPS-WAAS is the problem that the FAA is facing with its aging very high-frequency omnidirectional range (VOR) navigation system. The 1000 VORs gridding the United States are beginning to fail and require replacement. Moving to GPS would lead to huge cost-savings in replacing the old VOR system.
Whether GPS-WAAS could permit the FAA also to abandon its radar surveillance is subject to debate. Said Jerry Welch, group leader of the air traffic automation group for MIT's Lincoln Laboratory: "The FAA's current position is that it will eventually get rid of that also as a cost-saving measure, but they have not conclusively shown that that is feasible."
Miami Center controller Lindholm, said that nondirectional beacons (NDBs), which are also ground-based navigation aids, may be phased out as well. Most experts agree that GPS-WAAS will allow elimination of en route radar surveillance. But, because of safety implications, GPS is not expected to replace terminal surveillance radars.
ADS when implemented on a large scale could also enable aircraft separation standards--now kept at 5 miles horizontally in the en route environment--to be reduced. According to L. Lane Speck, director of air traffic program integration for the FAA, the 5-mile standard comes from the technical limits of the radar, which completes a scan every 12 seconds. At the edge of radar ranges (250 miles), aircraft less than 5 miles apart could theoretically swap positions on radar monitors. Moving to satellite-based separation could end that limitation, although some in the industry are concerned that the delay times in satellite communication would compromise safety.
Less talk, more data
A continual complaint of controllers and pilots alike, and one that has been proven in accident analyses to have an impact on safety, is frequency congestion. Because controllers are talking rapidly to as many as 25 to 30pilots at a time, there is often no time available for standard readback of clearance information. For instance, a typical clearance might be: "TWA 242, descend and maintain flight level 350" (35 000 feet).Pilots then simply say "Roger," even if unsure that they heard a number correctly.Amistake might not be caught until the controller sees the blip on his screen zooming past the assigned altitude--or the plane crashes into a mountain.
The apparent solution to frequency congestion is adigital data link, which digitally transmits standard information--like speed, altitude, and heading assignments--from the controller keyboard to a display in the cockpit. Voice radio communications would be saved for anomalies.
"I'd love data link," an Indianapolis controller told Spectrum . "Half my conversations are bringing a pilot onto my frequency and then sending him off; if I could do that without talking, it would help me handle more planes. It also would eliminate misunderstandings. I send a lot of pilots to flight level 230 (23 000 feet); if a pilot reads back 330, I'm so used to hearing 230 that I just don't notice until I see him busting up into someone else's airspace."
While nearly everyone agrees data link is a good idea, implementation is not as obvious. One suggestion is to use the current aircraft communications and reporting system (Acars), a commercial service run by Aeronautical Radio Inc. (Arinc), Annapolis, Md., which airline management relies on to send messages such as connecting gate information to cockpit crews. Some control data, for example, predeparture clearances (time to start engines, push back from gate) have been moved to Acars at some airports.
But using Acars as currently implemented for real-time air traffic control communication would be a bad idea, said Ross Sagun, chairman of the Air Line Pilots Association's (ALPA's) Air Traffic Control Committee, Herndon, Va. "Acars is behind me [in the cockpit]; I often don't see messages for 10 minutes."
Another possibility is expansion of the Mode S transponders, used aboard aircraft to emit a 56-bit burst of data once per second at 1090 MHz. These are used to track aircraft from the ground as a replacement for the standard Mode C transponder and to identify aircraft to collision avoidance (TCAS) systems. (Mode S transponders can be selectively interrogated, relieving radar system capacity.)
It may be that neither Acars nor Mode S has enough bandwidth for extensive and rapid data transfer (though each have supporters), so other data link implementations are being considered. There are a number of technical options. For one, should the signal be transmitted by satellite or from the ground? At what frequency? Would a viable solution be time-division multiple-access (TDMA) coding? (In TDMA several data streams are transmitted simultaneously at one frequency, witheach stream sent in bursts in particular time slots and then reassembled at the end.)
"People are duking these issues out right now," Lincoln Lab's Spencer said.
There are also human factors concerns. In a recent report, the National Research Council, Washington, D.C., indicated that data link "may reduce the amount of information available to flight crews, often relayed via nonverbal cues in voice communications, and may have a deleterious effect on the development of teamwork between controllers and flight crews." The report also stated that the reduction of "party line" information (overheard transmissions between other pilots and controllers on the same frequency) may cause problems for pilots.
Data link must be intimately integrated with controller workstations, for a separate terminal would take controllers eyes from their radar displays too often for safety's sake. So full implementation must wait until the display system replacement and the standard terminal automation replacement system are in place. Then, theoretically, these systems could be upgraded with data link capability.