Volume 2, Number 3
Air Traffic Control Development at Lincoln Laboratory
For nearly two decades Lincoln Laboratory has carried out a series of major programs in air traffic control for the Federal Aviation Administration. The programs have developed systems that improve surveillance, communications, collision avoidance, and severe-weather sensing. These systems are now beginning to be installed at airports around the country in support of air traffic control. Recently. major new programs have been initiated to enhance the efficiency and capacity of aircraft operations in the terminal area and on the airport surface.
The Mode S Beacon Radar System
Air traffic controllers rely on primary and secondary radars to locate and identify aircraft. Secondary, or beacon, radars require aircraft to carry devices called transponders that enhance surveillance echoes and provide data links. Airports currently use a secondary-radar system known as the Air Traffic Control Radar Beacon System (ATCRBS). However, ATCRBS has limitations in dense-traffic conditions, and the system's air-to-ground data link is limited. In response to these shortcomings, Lincoln Laboratory has developed the Mode Select Beacon System (referred to as Mode S), a next-generation system that extensive laboratory and field testing has validated. In addition to significant surveillance improvements, Mode S provides the general-purpose ground-air-ground data link necessary to support the future automation of air traffic control (ATC). The Federal Aviation Administration (FAA) is currently installing the system with initial operation scheduled for 1991.
Advances in Primary-Radar Technology
Current primary radars have difficulty detecting aircraft when ground clutter, rain, or birds interfere. To overcome such interference, the Moving Target Detector (MTD) uses
adaptive digital signal and data processing techniques. MTD has provided
Cross-range measurements of aircraft travelling at distances of 50 to 200 miles include significant errors. Therefore, heading estimates for medium-to-long-range aircraft are not sufficiently accurate to be useful in conflict-detection predictions. Accurate cross-range measurements can be made—by using two or more sensors to measure aircraft position—but such measurements must compensate for the effects of system biases and aircraft turns. A set of algorithms has been developed that are resistant to system biases, that detect turns, and that track successfully through both biases and turns. These algorithms can be incorporated into a complete multisensor system, with good intersensor correlation of aircraft tracks and no added delays to the air traffic control processing chain.
Many airports across the United States will soon be equipped with Mode S, a next-generation beacon (or secondary) radar system. One feature of Mode S is that it provides a data link between airborne aircraft and air traffic controllers. If Mode S could be used to communicate with aircraft on the airport surface, the radar system would improve airport safety and efficiency on runways and taxiways. The airport surface, however, is a hostile propagation environment. This article outlines a candidate design for the propagation of Mode-S beacon signals on the airport surface. Data that support the feasibility of Mode S for surveilling runways and taxiways are presented.
Parallel Runway Monitor
The availability of simultaneous independent approaches to parallel runways significantly enhances airport capacity. Current FAA procedures permit independent approaches in instrument meteorological conditions (IMC) when the parallel runways are spaced at least 4,300 ft apart. Arriving aircraft must be dependently sequenced at airports that have parallel runways separated by less than 4,300 ft, a procedure that reduces the arrival rate by as much as 25%. The need for greater airport capacity has led to intense interest in new technologies that can support independent parallel IMC approaches to runways spaced as close as 3,000 ft. This interest resulted in several FAA initiatives, including a Lincoln Laboratory program to evaluate the applicability of Mode-S secondary surveillance radars for monitoring parallel runway approaches. This paper describes the development and field activities of this program.
TCAS: A System for Preventing Midair Collisions
To reduce the possibility of midair collisions, the Federal Aviation Administration has developed the Traffic Alert and Collision Avoidance System, or TCAS. This airborne system senses the presence of nearby aircraft by interrogating the transponders carried by these aircraft. When TCAS senses that a nearby aircraft is a possible collision threat, TCAS issues a traffic advisory to the pilot, indicating the presence and location of the other aircraft. If the encounter becomes hazardous, TCAS issues a maneuver advisory.
Multipath Modeling for Simulating the Performance of the Microwave Landing System
The Microwave Landing System (MLS) will be deployed throughout the world in the 1990s to provide precision guidance to aircraft for approach and landing at airports. At Lincoln Laboratory, we have developed a computer-based simulation that models the performance of MLS and takes into account the multipath effects of buildings, the surrounding terrain, and other aircraft in the vicinity. The simulation has provided useful information about the effects of multipath on MLS performance.
Modeling of Air-to-Air Visual Acquisition
A mathematical model of air-to-air visual acquisition has been developed and validated in a series of flight tests at Lincoln Laboratory. The model describes the visual acquisition process as a nonhomogeneous Poisson process in which the probability of visual acquisition per unit of time is proportional to the solid angle subtended by the target. The model has proven useful in the investigation of actual midair collisions, as well as in the evaluation of collision avoidance systems.
Wind Shear Detection with Pencil-Beam Radars
Abrupt changes in the winds near the ground pose serious hazards to aircraft during approach or departure operations. Doppler weather radars can measure regions of winds and precipitation around airports, and automatically provide air traffic controllers and pilots with important warnings of hazardous weather events. Lincoln Laboratory, as one of several organizations under contract to the Federal Aviation Administration, has been instrumental in the design and development of radar systems and automated weather-hazard recognition techniques for this application. The Terminal Doppler Weather Radar (TDWR) system uses automatic computer algorithms to identify hazardous weather signatures. TDWR detects and warns aviation users about low-altitude wind shear hazards caused by microbursts and gust fronts. It also provides advance warning of the arrival of wind shifts at the airport complex. Extensive weather radar observations, obtained from a Lincoln-built transportable testbed radar system operated at several sites, have validated the TDWR system. As a result, the Federal Aviation Administration has issued a procurement contract for the installation of 47 TDWR radar systems around the country.
Wind Shear Detection with Airport Surveillance Radars
Airport surveillance radars (ASR) utilize a broad, cosecant-squared elevation beam pattern, rapid azimuthal antenna scanning, and coherent pulsed-Doppler processing to detect and track approaching and departing aircraft. These radars, because of location, rapid scan rate, and direct air traffic control (ATC) data link, can also provide flight controllers with timely information on weather conditions that are hazardous to aircraft. With an added processing channel, an upgraded ASR can automatically detect regions of low-altitude wind shear. This upgrade can provide wind shear warnings at airports where low traffic volume or infrequent thunderstorm activity precludes the deployment of a dedicated Terminal Doppler Weather Radar (TDWR). Field measurements and analysis conducted by Lincoln Laboratory indicate that the principal technical challenges for low-altitude wind shear detection with an ASR—ground-clutter suppression, estimation of near-surface radial velocity, and automatic wind shear hazard recognition—can be successfully met for microbursts accompanied by rain at the surface.
Experimental Examination of the Benefits of Improved Terminal Air Traffic Control Planning
Airport capacity can be improved significantly—by precisely controlling the sequence and timing of traffic flow—even when airspace usage and procedures remain fixed. In a preliminary experiment, a plan for such sequencing and timing was applied in a simulation to a 70-min traffic sample observed at Boston's Logan Airport, and the result was a 13% increase in terminal throughput. A total of 2.2 aircraft flight hours were saved. Delays imposed upon arriving traffic in the simulation were much more equitably distributed than in the actual traffic sample.
An even greater improvement may be possible if controllers are able to space aircraft more precisely on final approach than was achieved in the simulation. If the plan had been followed precisely, the throughput increase would have been 23%.
We have developed a computer program that automates rudimentary air traffic control (ATC) planning and decision-making functions. The ability to plan, make decisions, and act on them makes this experimental program qualitatively different from the more clerical ATC software currently in use. Encouraging results were obtained from tests involving simple scenarios used to train air traffic controllers.
Using Aircraft Radar Tracks to Estimate Winds Aloft
In air traffic control, the wind is a critical factor because it affects, among other important variables, the amount of time an aircraft will take to reach its destination. The authors have developed a method for estimating winds aloft in which the radar tracks of aircraft are used; i.e., data beyond what are already available to terminal air traffic control are not required. The method, which has been implemented at Lincoln Laboratory, gives a useful estimate of wind fields.