Publications
Volume 16, Number 1
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Air Traffic Control at Lincoln Laboratory Foreword and overview of air traffic control at Lincoln Laboratory Advances in Operational Weather Radar Technology The U.S. aviation system makes extensive use of national operational Doppler weather radar networks. These are critical for the detection and forecasting of thunderstorms and other hazardous weather phenomena, and they provide dense, continuously updated measurements of precipitation and wind fields as inputs to high-resolution numerical weather-prediction models. This article describes recent Lincoln Laboratory activities that significantly enhance the operational effectiveness of the nation’s Doppler weather radar networks. An open radar controller and digital signal processor has been developed for the Terminal Doppler Weather Radar (TDWR), which provides safety-critical low-altitude wind-shear warnings at large airports. This processor utilizes a small computer cluster architecture and standards-based software to realize high throughput and expansion capability. Innovative signal processing algorithms—enabled by the new processor—significantly improve the quality of the precipitation and wind measurements provided by TDWR. In a parallel effort, the Laboratory is working with engineers in the National Weather Service to augment the national NEXRAD Doppler weather radar network’s algorithm suite. Laboratory staff develop and test enhancements directed at the aviation weather problem. They then provide plug-and-play software to the NEXRAD second-level engineering support organization. This effort has substantially improved the operational value of NEXRAD data for the aviation system. Finally, we discuss nascent efforts to define a future multifunction radar network using a single next-generation active array architecture, which could realize the capabilities of today’s multiple weather and air traffic control networks. Advanced Aviation Weather Forecasts The U.S. air transportation system faces a continuously growing gap between the demand for air transportation and the capacity to meet that demand. Two key obstacles to bridging this gap are traffic delays due to en route severe-weather conditions and airport weather conditions. Lincoln Laboratory has been addressing these traffic delays and related safety problems under the Federal Aviation Administration’s (FAA) Aviation Weather Research Program. Our research efforts involve real-time prototype forecast systems that provide immediate benefits to the FAA by allowing traffic managers to safely reduce delay. The prototypes also show the way toward bringing innovative applied meteorological research to future FAA operational capabilities. This article describes the recent major accomplishments of the Convective Weather and the Terminal Ceiling and Visibility Product Development Teams, both of which are led by scientists at Lincoln Laboratory. Corridor Integrated Weather System Flight delays are now a major problem in the U.S. National Airspace System. A significant fraction of these delays are caused by reductions in en route capacity due to severe convective weather. The Corridor Integrated Weather System (CIWS) is a fully automated weather analysis and forecasting system designed to support the development and execution of convective weather impact mitigation plans for congested en route airspace. The CIWS combines data from dozens of weather radars with satellite data, surface observations, and numerical weather models to dramatically improve the accuracy and timeliness of the storm severity information and to provide state-of-the-art, accurate, automated, high-resolution, animated three-dimensional forecasts of storms (including explicit detection of storm growth and decay). Real-time observations of the Federal Aviation Administration (FAA) decision-making process during convective weather at Air Route Traffic Control Centers in the Midwest and Northeast have shown that the CIWS enables the FAA users to achieve more efficient tactical use of the airspace, reduce traffic manager workload, and significantly reduce delays. A real-time data-fusion architecture to assist in national deployment of CIWS is under development, and the CIWS products are being used in integrated air traffic management decision support systems. Integrating Advanced Weather Forecast Technologies into Air Traffic Management Decision Support Explicit integration of aviation weather forecasts with the National Airspace System (NAS) structure is needed to improve the development and execution of operationally effective weather impact mitigation plans and has become increasingly important due to NAS congestion and associated increases in delay. This article considers several contemporary weather-air traffic management (ATM) integration applications: the use of probabilistic forecasts of visibility at San Francisco, the Route Availability Planning Tool to facilitate departures from the New York airports during thunderstorms, the estimation of en route capacity in convective weather, and the application of mixed-integer optimization techniques to air traffic management when the en route and terminal capacities are varying with time because of convective weather impacts. Our operational experience at San Francisco and New York coupled with very promising initial results of traffic flow optimizations suggests that weather-ATM integrated systems warrant significant research and development investment. However, they will need to be refined through rapid prototyping at facilities with supportive operational users. Surveillance Accuracy Requirements in Support of Separation Services The Federal Aviation Administration is modernizing the Air Traffic Control system to improve flight efficiency, to increase airspace capacity, to reduce flight delays, and to control operating costs as the demand for air travel continues to grow. Promising new surveillance technologies such as Automatic Dependent Surveillance Broadcast and multisensor track fusion offer the potential to augment the ground-based surveillance and controller-display systems by providing more timely and complete information about aircraft. The resulting improvement in surveillance accuracy may potentially allow the expanded use of the minimum safe-separation distance between aircraft. However, these new technologies cannot be introduced with today’s radar-separation standards, because they assume surveillance will be provided only through radar technology. In this article, we review the background of aircraft surveillance and the establishment of radar separation standards. The required surveillance accuracy to safely support aircraft separation with National Airspace System technologies is then derived from currently widely used surveillance systems. We end with flight test validation of the derived results, which can be used to evaluate new technologies. Operational Evaluation of Runway Status Lights To maintain safe separation of aircraft on the airport surface, air traffic controllers issue verbal clearances to pilots to sequence aircraft arrivals, departures, and runway crossings. Although controllers and pilots work together successfully most of the time, mistakes do occasionally happen, causing several hundred runway incursions a year—and, less frequently, near misses and collisions—in the United States. With this rate of incursions, it is imperative to have an independent warning system as a backup to the current system. Runway status lights, a system of automated, surveillance-driven stoplights, have been designed to provide this backup function. The lights are installed at runway-taxiway intersections and at departure points along the runways. They provide a clear signal to pilots crossing or departing from a runway, warning them of potential conflicts with traffic already on the runway. Existing FAA-installed radar surveillance is coupled with Lincoln Laboratory–developed algorithms to generate the light commands. To be compatible with operations at the busiest airports, the algorithms must drive the lights such that during normal operations pilots will almost never encounter a red light when it is safe to cross or depart from a runway. A minimal error rate must be maintained even in the face of inevitable imperfections in the surveillance system used to drive the safety logic. A prototype runway status light system has been designed at Lincoln Laboratory and installed at the Dallas/Fort Worth International Airport, where Laboratory personnel have worked with the FAA to complete an operational evaluation of the system, demonstrating the feasibility of runway status lights in the challenging, complex environment of one of the world’s busiest airports. Real-Time 3D Ladar Imaging A prototype image processing system has recently been developed which generates, displays, and analyzes three-dimensional (3D) ladar data in real time. It is based upon a suite of novel algorithms that transform raw ladar data into cleaned 3D images. These algorithms perform noise reduction, ground-plane identification, detector response deconvolution, and illumination pattern renormalization. The system also discriminates static from dynamic objects in a scene. In order to achieve real-time throughput, we have parallelized these algorithms on a Linux cluster. We demonstrate that multiprocessor software plus Blade hardware results in a compact, real-time imagery adjunct to an operating ladar. Finally, we discuss several directions for future work, including automatic recognition of moving people, real-time reconnaissance from mobile platforms, and fusion of ladar plus video imagery. Such enhancements of our prototype imaging system can lead to multiple military and civilian applications of national importance. Interactive Grid Computing at Lincoln Laboratory The Lincoln Laboratory Grid (LLGrid) project was initiated to provide Laboratory staff with an effective way to exploit cluster computing as a solution to the demand for computational power in large-scale algorithm development, data analysis, and simulation tasks. Because sensor capabilities and demands continue to increase, the dataset sizes and algorithm complexities of today’s challenging applications have outgrown the processing capabilities of single workstations. Cluster computing technology, where a networked set of workstations is used as a parallel processor, can provide the throughput and storage demands of these applications. Programming a cluster, however, requires algorithm developers to become parallel programmers, which is difficult, time-consuming, and distracting. To allow a large research community (who primarily use MATLAB) to exploit cluster computing, we have developed a parallel programming toolbox called pMatlab, which consists of a library of objects and routines for distributing numerical arrays onto multiple processors, and then carrying out parallel computations on these distributed arrays. A typical MATLAB programmer can use pMatlab to convert a program to a parallel implementation in a few hours or days, and can then run the application on a cluster. Since most Laboratory users do not have access to a cluster, the LLGrid enterprise cluster computing system was developed to provide users with desktop access to these resources. The LLGrid allows a pMatlab program to be run on a remote cluster as simply as it is to run a MATLAB program on the desktop. Several LLGrid satellite clusters dedicated to specific programs have also been established. To quantify the effectiveness of pMatlab on the LLGrid, we present pMatlab High Performance Computing Challenge benchmark results, which evaluate high-performance computing systems over a range of computations. Our results, including user experiences, illustrate the increased user productivity and high computational performance of pMatlab on LLGrid. We conclude with the future directions of both the LLGrid project and related advanced software developments. Superconducting Nanowire Photon-Counting Detectors for Optical Communications Photon-counting detectors can be used in optical communications links to provide a large enhancement in receiver sensitivity over conventional photodetectors at near-infrared wavelengths (1.3–1.55 μm). However, the highest data rate that has been demonstrated to date using these techniques is about 100 kbits/sec for a receiver consisting of a single InGaAs avalanche photodiode (APD), with projected maximum data rates for large arrays of these detectors in the tens of Mbits/sec. These data rates were limited by the intrinsic few hundred picosecond timing resolution and few nanosecond reset time of the APDs themselves. Applications in which higher data rates are required (and where size, weight, and power [SWAP] limitations for the laser transmitter and/or receiver necessitate a trade-off between SWAP and data rate) therefore constitute a new area of application parameter space for photon-counting receivers, which has heretofore remained inaccessible to current photon-counting detector technologies. Recently, however, a new technology based on superconducting NbN nanowires was demonstrated, which has shown promise for providing access to this regime. These devices have previously demonstrated a timing resolution of less than 50 psec, and detection efficiencies at 1550 nm as high as approximately 5%. In this article, we present an overview of the ongoing effort at MIT and Lincoln Laboratory to develop this detector technology into a practical solution for high-sensitivity, high-data-rate, photon-counting optical communications.
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