Publications
Volume 7, Number 2
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Air Traffic Control Development at Lincoln Laboratory Lincoln Laboratory-developed air traffic control technologies, which were described in the Fall 1989 issue of this journal, are now in operational use. These technologies include the Mode-Select beacon system, the Traffic Alert and Collision Avoidance System, the Precision Runway Monitor system, the Terminal Doppler Weather Radar, and the Moving Target Detector signal processor used in the current generation of Airport Surveillance Radars. Our newest efforts focus on utilization of the Global Positioning System for both navigation and surveillance, and on the development of automation aids for air traffic control and management. Preventing Runway Conflicts: The Role of Airport Surveillance, Tower-Cab Alerts, and Runway-Status Lights Runway incursions and conflicts present a persistent problem in airport ground operations. Numerous critical conflicts and several fatal accidents have occurred as a result of unauthorized or otherwise inappropriate entry of aircraft or surface vehicles onto an active runway. This article describes a detailed survey of runway-conflict accidents and high-hazard incidents resulting from inappropriate entry onto or movement on an active runway. The patterns that emerge allow us to determine the role that three different safety systems can be expected to play in reducing the incidence or consequences of runway incursions and conflicts. The three systems are a surface-surveillance system (such as a surface radar), a tower-cab alerting system, and runway-status lights. Judging from the history of runway conflicts, it appears that runway-status lights, operating automatically with inputs from a surface radar, can prevent over half of these conflicts. A surface radar alone or combined with tower-cab alerts promises to be effective in preventing another one-third. The three systems in combination can offer protection in an estimated 90% of high-hazard conflicts. Demonstration of Runway Status Lights at Logan Airport Lincoln Laboratory has developed a prototype runway-status light system (RSLS), designed to prevent runway incursions and accidents. These status lights will tell aircraft pilots and surface-vehicle operators when runways are unsafe to enter or unsafe for departure. This status information will improve the situational awareness of pilots and vehicle operators, thereby reducing the number of runway incursions and accidents. The goal of the RSLS Logan Demonstration is to use automatic processing of surface primary and approach secondary radar data to drive simulated runway-status lights in a real-time but off-line surface-traffic automation system. This article presents a description of the design motivation, methodology, and implementation for the RSLS Logan Demonstration; it also provides an overview of the entire system on a functional block scale and gives introductory descriptions of the various subsystems. Performance of the Runway-Status Light System at Logan Airport Runway incursions are a persistent problem in airport ground-movement operations. Numerous critical conflicts and several fatal accidents have occurred as a result of unauthorized or otherwise inappropriate entry of aircraft or surface vehicles onto an active runway. Many of these conflicts developed quickly, leaving little time for effective intervention by either the controller or the pilots involved. A reliable system of automatic runway-status lights would be an effective way to prevent such time-critical incursions. The runway-status light system (RSLS) at Boston's Logan International Airport is an off-line proof-of-concept technology-demonstration system designed to show that automatically operated runway-status lights can promptly and reliably transmit runway-status information to pilots and surface-vehicle operators, thereby preventing unsafe runway entry or unsafe takeoff. The demonstration system does not include actual lights on the airport surface but has relied instead on an illuminated airport model board, which has allowed system development to proceed in a realistic operating environment of live airport traffic without interfering with airport operations. The results of an initial proof-of-concept assessment indicate that the system performs well, even though it is an early prototype. Missed-detection and false-alarm rates are low, and interference with normal airport operations promises to be negligible. The demonstration has shown the technical feasibility of a system of automatic runway-status lights. Evaluation of Runway Assignment and Aircraft Sequencing Algorithms in Terminal Area Automation The Federal Aviation Administration has responded to the steady growth of air traffic and the ensuing increase in delays at airports by initiating new programs for increasing the efficiency of existing air traffic control facilities. The Terminal Air Traffic Control Automation (TATCA) program is intended to increase efficiency by providing controllers with planning aids and advisories to help them in vectoring, sequencing, and spacing traffic arriving at busy airports. Two important algorithms in this system allocate arrivals to multiple runways and set up the best sequences for landing aircraft. This article evaluates the potential for such algorithms to achieve higher throughput with less delay. The results show that, at airports with multiple active runways, the introduction of algorithms for systematic allocation of runways increases throughput considerably. These algorithms are in fact more effective than algorithms that aim at generating optimal landing sequences based on aircraft weight-class inputs. This result is fortuitous because algorithms for optimal sequencing are significantly more difficult to implement in practice than are algorithms for runway allocation. This study also provides a scientific basis for estimating future benefits of terminal automation by using traffic models patterned on actual recorded traffic-flow data, and by proposing a unified method for assessing performance. A Self-Organizational Approach for Resolving Air Traffic Conflicts The use of airways and navigational fixes to form fixed routes in the sky is central to modern-day air traffic control (ATC). Fixed routes, however, limit efficiency and airspace capacity in comparison to paths that are not constrained or predefined. This article describes a computational technique for determining collision-free time/space paths for multiple vehicles without the use of fixed routes. The technique applies a simple destination-seeking rule and a few conflict-avoidance algorithms to each vehicle individually such that the collective solution is determined by the calculated behavior of the individual vehicles; that is, the overall solutions are self-organizational in nature. This self-organizational approach has been tested in a variety of scenarios ranging from simple two-dimensional conflicts to the modeling of an ATC sector handling an unrealistically high traffic load. The simulations, implemented on a very modest computer workstation, have proven the self-organizational approach capable of finding solutions to complex traffic conflicts at rates faster than real time. Furthermore, the self-organizational solutions tend to require smaller speed/direction deviations than solutions obtained with human reasoning. Machine Intelligence for ATC Equipment Maintenance The normal duties of ATC equipment maintenance are to observe performance indicators and measurements, infer from these data whether the equipment is in good health, and, if a problem is found, diagnose its cause and perform the necessary repairs. Machine intelligence (MI) technology offers a means for automating much of this work load. Indeed, Lincoln Laboratory has developed and fielded several expert systems for similar applications involving the maintenance and control of military communications systems. This article describes three such expert systems, and then discusses opportunities for MI to automate the monitoring and maintenance of ATC equipment. A high-level design of an MI approach to the remote maintenance monitoring of the ASR-9 airport surveillance radar is presented, along with conceptual descriptions of other MI applications for the Federal Aviation Administration (FAA). GPS-Squitter: System Concept, Performance, and Development Program GPS-Squitter merges the capabilities of Automatic Dependent Surveillance (ADS) with the Mode S beacon system. In ADS, an aircraft determines its position on board by using satellite navigational data from a system such as the Global Positioning System (GPS). The information is then broadcast to ground and airborne users to provide surveillance of the aircraft. In GPS-Squitter, an aircraft would transmit the ADS information by using the Mode S squitter—a spontaneous periodic broadcast transmitted by all Mode S transponders. Currently, the Mode S transponders emit a 56-bit squitter once per second, and the squitters are used by the Traffic Alert and Collision Avoidance System (TCAS) to acquire the 24-bit address of the aircraft. For the GPS-Squitter system, the squitter broadcast is extended to 112 bits to provide for the transmission of a 56-bit ADS message field. This article defines the GPS-Squitter concept, describes its principal surveillance and data-link applications, and provides estimates of the expected performance in future moderate-to-high-density environments. The program under way to develop this concept is also described together with examples of measured performance data. TCAS: Maneuvering Aircraft in the Horizontal Plane The Traffic Alert and Collision Avoidance System (TCAS II) is now operating in all commercial airline aircraft to reduce the risk of midair collisions. TCAS II determines the relative positions of nearby aircraft, called intruders, by interrogating their transponders and receiving their replies. An intruder deemed a potential threat will trigger a resolution advisory (RA) that consists of an audible alert and directive that instructs the pilot to execute a vertical avoidance maneuver. Lincoln Laboratory has investigated the possibility of increasing the capability of TCAS II by incorporating the horizontal maneuvering of aircraft. Horizontal RAs can be computed if the intruder horizontal-miss distances at closest approach are known. Horizontal miss distances can be estimated with range and bearing measurements of intruders. With this method, however, large errors in estimating the bearing rates will result in large errors in calculating the horizontal miss distances. An improved method of determining the horizontal miss distances may be to use the Mode S data link to obtain state data (position, velocity, and acceleration) from intruder aircraft. Mode S Data-Link Applications for General Aviation The Mode S data link is a high-capacity air-ground digital communications system that can deliver information to the cockpit in a form that will significantly improve pilot situational awareness and aircraft utility. The Federal Aviation Administration is currently deploying Mode S surveillance sensors with data-link capability at 143 sites across the United States. Two Mode S data-link applications—Traffic Information Service and Graphical Weather Service—have been developed to meet the specific needs of general aviation. Traffic Information Service uses the surveillance capability inherent in the Mode S sensor to provide the pilot with a display of nearby traffic. Graphical Weather Service provides a means to deliver real-time weather graphics to the cockpit. Additional Mode S data-link applications, including the broadcast of local-area differential corrections for the Global Satellite Navigation System and the use of the Mode S squitter for Automatic Dependent Surveillance Broadcast, also offer significant benefits to general aviation. Low-cost avionics have been developed to support these and other Mode S data-link applications for general aviation. Precision Runway Monitor The prevalence of delay in scheduled airline flights in recent years has caused great interest in the use of new technologies that promise increased airport capacity, especially in poor weather. One consequence of this interest in new technologies is development of the Precision Runway Monitor (PRM) system. The PRM system uses enhanced radar and display capabilities combined with automatic safety alerts to allow safe operation of independently sequenced approaches, in instrument meteorological conditions, to parallel runways separated by less than 4300 feet (the current minimum separation without PRM). During the past several years, Lincoln Laboratory has carried out a development program for the PRM that has included field data collections, demonstrations, performance evaluation, and risk analysis. Partly on the basis of the results of this program, the FAA recently authorized independently sequenced approaches to parallel runways separated by 3400 feet or more, when these approaches are monitored with a PRM system. The FAA also initiated an implementation program to install PRM systems at several major U.S. airports. This article reports the results of field activities carried out by Lincoln Laboratory, the use of these results to verify the performance and safety of the PRM system, and continuing development that is part of the Lincoln Laboratory PRM program. The ASR-9 Processor Augmentation Card (9-PAC) Since 1990, the Airport Surveillance Radar-9 (ASR-9) has been commissioned and installed at more than sixty of the largest airports in the United States, and future installations are planned at more than sixty additional airports. After the first several systems were put into daily operation, air traffic controllers began to lodge complaints about the radar's performance. Problems included the detection of "phantom" aircraft caused by the reflection of beacon interrogation signals off buildings and other aircraft, the radar's losing track of targets during parallel approaches and departures, the inability to track highly maneuverable military aircraft through high-G turns, radar clutter caused by highways and weather, and system overloading as a result of signal returns from flocks of migrating birds. An initial investigation of the sources of these problems focused on the radar's post-processor. Nearly all of the problems could be addressed by additions to the post-processor software, but the post-processor was already running near capacity and there was no means for expansion. Thus a new processor—the ASR-9 Processor Augmentation Card (9-PAC)—was designed to augment the existing system to allow for a significant increase in processing power. New algorithms were developed to run in 9-PAC to address the problems cited by the controllers. Supporting the Deployment of the Terminal Doppler Weather Radar (TDWR) The Terminal Doppler Weather Radar (TDWR) program was initiated in the mid-1980s to develop a reliable automated Doppler-radar-based system for detecting weather hazards in the airport terminal area and for providing warnings that will help pilots avoid these hazards when landing and departing. This article describes refinements made to the TDWR system since 1988, based on subsequent Lincoln Laboratory testing in Kansas City, Missouri, and Orlando, Florida. During that time, Lincoln Laboratory developed new capabilities for the system such as the integration of warnings from TDWR and the Low Level Wind Shear Alert System (LLWAS). Extensive testing with the Lincoln Laboratory TDWR testbed system has reconfirmed the safety benefits of TDWR. Automated Microburst Wind-Shear Prediction We have developed an algorithm that automatically and reliably predicts microburst wind shear. The algorithm, developed as part of the FAA Integrated Terminal Weather System (ITWS), can provide warnings several minutes in advance of hazardous low-altitude wind-shear conditions. Our approach to the algorithm emphasizes fundamental principles of thunderstorm evolution and downdraft development and incorporates heuristic and statistical methods as needed for refinement. In the algorithm, machine-intelligent image processing and data-fusion techniques are applied to Doppler radar data to detect those regions of growing thunderstorms and intensifying downdrafts that lead to microbursts. The algorithm then uses measurements of the ambient temperature/humidity structure in the atmosphere to aid in predicting a microburst's peak outflow strength. The algorithm has been tested in real time as part of the ITWS operational test and evaluation at Memphis, Tennessee, and Orlando, Florida, in 1994. Automated Storm Tracking for Terminal Air Traffic Control Good estimates of storm motion are essential to improved air traffic control operations during times of inclement weather. Automating such a service is a challenge, however, because meteorological phenomena exist as complex distributed systems that exhibit motion across a wide spectrum of scales. Even when viewed from a fixed perspective, these evolving dynamic systems can test the extent of our definition of motion, as well as any attempt at automated tracking of this motion. Image-based motion detection and processing appear to provide the best route toward robust performance of an automated tracking system. The Integrated Terminal Weather System (ITWS) The Integrated Terminal Weather System (ITWS) is one of two major development projects sponsored by the FAA's Aviation Weather Development Program. Focused on the environment within the airport terminal area, ITWS integrates data from FAA and National Weather Service (NWS) sensors and systems to provide a suite of weather informational products for improving air terminal planning, capacity; and safety. This article provides an overview of the ITWS project, presenting the system concept, some of the design and engineering challenges, and plans for development that will lead to operational systems in the field. The Integrated Terminal Weather System Terminal Winds Product The wind in the airspace around an airport impacts both airport safety and operational efficiency. Knowledge of the wind helps controllers and automation systems merge streams of traffic; it is also important for the prediction of storm growth and decay, burn-off of fog and lifting of low ceilings, and wake vortex: hazards. This knowledge is provided by the Integrated Terminal Weather System (ITWS) gridded wind product, or Terminal Winds. The Terminal Wmds product combines data from a national numerical weather-prediction model, called the Rapid Update Cycle, with observations from ground stations, aircraft reports, and Doppler weather radars to provide estimates of the horizontal wind field in the terminal area. The Terminal Winds analysis differs from previous real-time winds-analysis systems in that it is dominated by Doppler weather-radar data. Terminal Winds uses an analysis called cascade of scales and a new winds-analysis technique based on least squares to take full advantage ofthe information contained in the diverse data set available in an ITWS. The weather radars provide sufficiently fine-scale winds information to support a 2-km horizontal-resolution analysis and a five-minute update rate. A prototype of the Terminal Winds analysis system was tested at Orlando International Airport in 1992, 1993, and 1995, and at Memphis International Airport in 1994. The field operations featured the first real-time winds analysis combining data from the Federal Aviation Administration TDWR radar and the National Weather Service NEXRAD radar. The evaluation plan is designed to capture both the overall system performance and the performance during convective weather, when the fine-scale analysis is expected to show its greatest benefit. |
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