Publications

A series of flight tests were flown to measure pilot air-to-air visual acquisition performance for pilots employing unalerted visual search. Twenty-four general aviation subject pilots flew a cross-country route while an intercepting aircraft was controlled to produce three intercepts with altitude separation of 500 feet. Pilots received no traffic advisory information to alert them to the possible presence of the intercepting aircraft. Results were analyzed to estimate the instantaneous rate of visual acquisition for a visual target of specified size and contrast. The results were used to calibrate a mathematical model of visual acquisition that can be used to predict pilot performance under a range of conditions.

This semiannual report for the Terminal Doppler Weather Radar program, sponsored by the Federal Aviation Administration (FAA), covers the period from 1 January 1990 through 30 June 1990. The principal activity of this period was the transport and reassembly of the FL-2 weather radar test site from Kansas City, MO to Orlando, FL and the change of radar frequency from S-band used in Kansas City to C-band for Orlando operations. Site operations to prepare the FL-2C radar site for summer testing began in January and continued through May, when testing began. This report describes the RF hardware, the data collection, the computer systems at site, and the networks between Orlando, FL and Lexington, MA. Also included are discussions of the microburst and gust front algorithm development, data collection, display terminals, and training for Air Traffic Control (ATC) supervisors and controllers.

This paper presents a study of military applications of advanced speech processing technology which includes three major elements: 1) review and assessment of current efforts in military applications of speech technology; 2) identification of opportunities for future military applications of advanced speech technology; and 3) identification of problem areas where research in speech processing is needed to meet application requirements, and of current research thrusts which appear promising. The relationship of this study to previous assessments of military applications of speech technology is discussed and substantial recent progress is noted. Current efforts in military applications of speech technology which are highlighted include: 1) narrow-band (2400 his) and very low-rate (50-1200 his) secure voice communication; 2) voice/data integration in computer networks; 3) speech recognition in fighter aircraft, military helicopters, battle management, and air traffic control training systems; and 4) noise and interference removal for human listeners. Opportunities for advanced applications are identified by means of descriptions of several generic systems which would be possible with advances in speech technology and in system integration. These generic systems include 1) an integrated multirate voice data communications terminal; 2) an interactive speech enhancement system; 3) a voice-controlled pilot's associate system; 4) advanced air traffic control training systems; 5) a battle management command and control support system with spoken natural language interface; and 6) a spoken language translation system. In identifying problem areas and research efforts to meet application requirements, it is observed that some of the most promising research involves the integration of speech algorithm techniques including speech coding, speech recognition, and speaker recognition.

The Federal Aviation Administration (FAA) is developing a number of terminal weather sensors and a terminal weather information system which can make important contributions toward an operational wake vortex advisory service. Although these systems have been developed to meet other important weather information needs, their existence/development offers the possibility of a more cost effective wake vortex advisory system than would be possible with a standalone system such as was tested in the 1970's. Specifically, we postulate an advisory system in which the aircraft separation during IFR conditions is adjusted to account for the advection of vortices by the wind on the approach path and/or the breakup of vortices due to air instability and in which the desired aircraft separation is achieved by the Terminal Air Traffic Automation (TATCA) system. When reduced separations are obtained with such a system, it is important to be able to anticipate that the winds/vortex stability in the terminal area will continue to meet the reduced spacing criteria for an appropriate time interval (e.g., at least 15 minutes) in the future.

Rotating winds aloft occurring with downdrafts often are associated with microbursts, which are serious aviation hazards. The Terminal Doppler Weather Radar system detects microbursts and warns pilots of windshear events, partly by its use of rotation as precursors. The role of the rotation region detection algorithm in this system is described, and the improvements to it are analyzed using measured data and simulated rotation regions. The final results show a substantial overall decrease in the number of false detections generated by the algorithm due to adjustment of thresholds and additional logic, while still retaining a good probability of microburst rotation region detection (84 percent). Ideas for future enhancement are explored through techniques such as discriminant analysis and environmental wind filtering.

One approach to the problem of representation of speech signals is to use the speech production model in which speech is viewed as the result of passing a glottal excitation waveform through a time-varying linear filter that models the resonant characteristics of the vocal tract. In many applications it suffices to assume that the glottal excitation can be in one of two possible states corresponding to voiced or unvoiced speech. In attempts to design high-quality speech coders at the midband rates, generalizations of the binary excitation model have been developed. One such approach is multipulse (Atal and Remde, 1982) which uses more than one pitch pulse to model voiced speech and a possibly random set of pulses to model unvoiced speech. Code excited linear prediction (CELP) (Schroeder and Atal, 1985) is another representation which models the excitation as one of a number of random sequences or "codewords" superimposed on periodic pitch pulses. In this chapter the goal is also to generalize the model for the glottal excitation; but instead of using impulses as in multipulse or random sequences as in CELP, the excitation is assumed to be composed of sinusoidal components of arbitrary amplitudes, frequencies, and phases (McAulay and Quatieri, 1986).

The Terminal Doppler Weather Radar (TDWR) microburst surface divergence detection algorithm has been under development and evaluation at Lincoln Laboratory since 1983. The TDWR program is sponsored by the Federal Aviation Administration (FAA), and the algorithm described in this report is a primary algorithm component of the TDWR system. The divergence algorithm processes radar velocity measurements taken near the earth's surface to identify the strong divergent outflow characteristic of microburst wind shear hazards. The algorithm uses a complex set of pattern matching and validation test criteria to locate microburst outflow signatures and to filter out false alarms from various data contamination sources. The divergence algorithm is primarily responsible for the detection of most microbursts, although the complete TDWR microburst algorithm consists of more than a dozen distinct algorithmic components. The divergence algorithm has demonstrated a very high probability of detection (POD) for strong microburst outflows, and its performance (as well as that of the complete microburst detection algorithm) was first formally assessed in the operational test and evaluation of the TDWR in Denver, CO (1988). Subsequent evaluations were performed in Kansas City, KS (1989) and Orlando, FL (1990). These evaluations have provid

Lincoln Laboratory, under sponsorship from the Federal Aviation Administration (FAA), is conducting a program to evaluate the capability of the newest Airport Surveillance Radars (ASR-9) to detect hazardous weather phenomena -- in particular, low-altitude wind shear created by thunderstorm-generated microbursts and gust fronts. The ASR-9 could provide coverage at airports not slated for a dedicated Terminal Doppler Weather Radar (TDWR) and could augment the TDWR at high-priority (high traffic volume, severe weather) facilities by providing a more rapid update of wind shear products, a better viewing angle for some runways, and redundancy in the event of a TDWR failure. An operational evaluation of a testbed ASR Wind Shear Processor (ASR-WSP) was conducted at the Orlando International Airport in Orlando, FL during August and September 1990. The ASR-WSP operational system issued five distinct products to Air Traffic Control: microburst detections, gust front detections, gust front movement predictions, precipitation reflectivity and storm motion. This document describes the operational system, the operational products, and the algorithms employed. An assessment of system performance is provided as one step in evaluating the operational utility of the ASR-WSP.

There is a need for a real-time volcanic ash advisory system for aviation which could provide improved accuracy and timeliness in warnings to planes in flight as well as to air traffic controllers for flight planning. To provide an operationally useful capability at reasonable cost, it is essential that the system elements take maximum advantage of the weather sensing and information dissemination system under development by the FAA and NWS. Volcanic ash should be treated as a type of weather hazard similar to regions of icing and thunderstorms. Real-time information from ground sensors, unmanned air vehicles, air carriers, and satellites would be used to estimate current and predicted ash systems to pilots in flight, ATC controllers, traffic management units and airlines. Future research should focus on defining the ash densities and time exposures of concern to aircraft, resolving the sensor mix for measuring the ash density and extent, and validating models for ash transport.

This paper describes an experimental system for cockpit display of Terminal Doppler Weather Radar (TDWR) wind shear warnings. The TDWR is a ground-based system for detecting wind shear hazards that pose a threat to aviation, During the Summer of 1990, wind shear warnings generated by the Lincoln-operated TDWR testbed radar at Orlando, Florida were transmitted in real-time to a research aircraft performing microburst penetrations. This test marks a milestone as being the first time that TDWR wind shear warnings were successfully transmitted and displayed in an aircraft in real-time. This effort was supported by NASA Langley Research Center as part of a program to investigate techniques for integrating airborne and ground-based wind shear information for aircrew alerting. The three main goals for 1990 were 1) to conduct microburst penetrations with an instrumented aircraft, 2) to compare a hazard estimate called the F factor (Bowles, 1990) for airborne and TDWR data, and 3) demonstrate real-time data link and cockpit display of TDWR warnings. All three of these goals were successfully carried out. The research aircraft, a Cessna Citation II operated by the University of North Dakota (UND) Center for Aerospace Sciences conducted over 80 microburst penetrations in Orlando over a six week period with TDWR testbed radar surveillance. Initial post-processing analysis in comparing the aircraft and TDWR F factors has begun. The cockpit display system was operated during the latter part of the flight test period, and proved useful in aiding the Citation crew in locating microburst and gust front events. There were three main objectives in the development of the cockpit display system. First, the real-time display was intended to aid the Citation crew in locating microburst and gust front events. This capability was desired both to aid the crew in locating events to penetrate, and to improve safety by providing a better information about the location of the wind shear events. A second objective was to demonstrate the feasibility of transmitting TDWR wind shear warnings to aircraft in real-time. This demonstration is an important element in the eventual development of an integrated aircrew alerting procedure incorporating both airborne and ground-based wind shear information. This study marks the first successful demonstration of real-time transmission of TDWR wind-shear warnings to an aircraft in flight. A third objective was to demonstrate the desirability of transmitting TDWR wind shear warnings to aircraft in real-time. Currently, the TDWR provides these warnings to controllers as textual messages, which are then relayed to pilots via voice communications. The TDWR also includes graphical displays of wind shear and precipitation products but these are only provided currently to the Tower and TRACON supervisors. A potential use of Mod S Data Link (or other ground-to-air data link systems) is to provide TDWR wind shear warnings directly to pilots, Automatic delivery of TDWR wind shear warnings potentially result in decreased controller workload and improved pilot information. Mode S Data Link is currently planned to provide textual wind shear warnings only. However, studies by Wanke and Hansman (1990) show that pilots substantially prefer graphical presentation of wind shear warnings over textual presentation. The paper will first describe the organization of the system, including the process of generating the display messages in the TDWR testbed and data linking them to the aircraft. Second, the display format and operation of the cockpit display will be described. Next, an example of the operational use of the cockpit display will be presented, along with initial F factor results. Finally, the paper will conclude with a summary and plans for future work.