Publications

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.

A gust front is the leading edge of a thunderstorm outflow. A gust frontal passage is typically characterized by a drop in temperature, a rise in relative humidity and pressure, and an increase in wind speed and gustiness. Gust front detection is of concern for both Terminal Doppler Weather Radar (TDWR) and Next Generation Weather Radar (NEXRAD) systems. In addition, airborne systems using radar, lidar, and infrared sensors to detect hazardous wind shears are being developed. The automatic detection of gust fronts is desirable in the airport terminal environment so that warnings of potentially hazardous gust front-related wind shears can be delivered to arriving and departing pilots. Information about estimated time of arrival and accompanying wind shifts can be used by an Air Traffic Control (ATC) supervisor to plan runway changes. Information on expected wind shifts and runway changes is also important for terminal capacity programs such as Terminal Air Traffic Control Automation (TATCA) and wake vortex advisory systems. In addition, the convergence associated with gust fronts is often a factor in thunderstorm initiation and intensification. Knowledge of gust front locations, strengths, and movement can aid forecasters with thunderstorm-specific predictions. Current gust front detection systems generally are reliable in that the probability of false alarms is low. However the probability of detecting gust fronts with these systems is less than desired. Improved characterization of gust fronts is a key element in improving detection capability. Typically, the basic products from the algorithms are the location of the gust front (for hazard assessment) and its propagation characteristics (for forecasting). This paper discusses the thermodynamic and radar characteristics of gust fronts from three climatic regimes, highlighting regional differences and similarities of gust fronts. It also compares propagation speeds, estimated by two techniques, to measured propagation speeds.

Gust front detection is of concern for both Terminal Doppler Weather Radar (TDWR) and Next Generation Weather Radar (NEXRAD) systems. The automatic detection of gust fronts is desirable in the airport terminal environment because warnings of potentially hazardous gust front-related wind shears can be delivered to arriving and departing pilots. Information about estimated time of arrival and accompanying wind shifts can be used by an Air Traffic Control (ATC) supervisor to plan runway changes. Information on expected wind shifts and runway changes are also important for terminal capacity programs such as Terminal Air Traffic Control Automation (TATCA) and wake vortex advisory systems. In addition, the convergence. associated with gust fronts is often a factor in thunderstorm initiation and intensification. Knowledge of their locations and strengths can aid forecasters with thunderstorm forecasts. Experienced radar meteorologists can identify gust fronts in single Doppler radar data by the presence or radial convergence, azimuthal shear, and thin lines of reflectivity. The radial convergence signature is the most reliable of all of the signatures. Therefore, the formally-documented TDWR gust front algorithm is designed to automatically detect gust fronts through radial convergence.

The Terminal Doppler Weather Radar (TDWR) is a ground-based system for providing automated warnings of aviation wind shear hazards. This paper describes a proposed new TDWR product for microburst prediction. The proposed Microburst Prediction (MBP) product provides the ability to predict microbursts prior to the onset of surface outflow. The MBP product uses the ability of the TDWR to scan aloft for precursor signatures which indicate that a microburst is about to occur. The proposed MBP product provides a complementary capability to the other TDWR wind shear detection and prediction algorithms. As shown in Figure 1, the Microburst and Gust Front algorithms provide safety benefits by detecting wind shear hazards. The Wind Shift Prediction product provides an economic benefit by predicting runway wind shifts up to 20 minutes in advance. The MBP product provides both safety and economic benefits by predicting microburst hazards about 5 minutes in advance. The development of the MBP product is intended to be evolutionary. The initial implementation of the product relies on TDWR radar data only. Later versions are expected to also employ thermodynamic information, as part of the Integrated Terminal Weather System (ITWS). However, the radar-only version discussed in this paper will provide a useful interim capability. The organization of the paper is as follows. Section 2 provides a discussion of the potential operational benefits of the MBP product in improving safety and reducing delay. Section 3 describes the current MBP product algorithm, and section 4 provides performance results for two environments: Kansas City, KS and Orlando, FL. Section 5 provides an example of the product operation in predicting a 50 knot microburst which had substantial impact on airport operations. Section 6 will provide a summary and discuss future work.

The estimation of air and particle motions in storms from multiple Doppler radar measurement is a long standing problem in radar meteorology. Our research interest in understanding the relationship of electrical change generation processes above the freezing level to thunderstorm life cycle, and in the detailed quantification of the eventual low altitude divergent outflow produced by the storm, demands an accurate retrieval of air and particle motions at essentially all altitudes within the storm. We found that existing approaches had deficiencies for our needs, and have developed an improved "hybrid" approach which attempts to provide high quality estimates throughout the storm volume.

Gust fronts are associated with potentially hazardous wind shears and cause sustained wind shifts after passage. Terminal Air Traffic Control (ATC) is concerned about the safety hazard associated with shear regions and prediction of the wind shift for runway reconfiguration. The Terminal Doppler Weather Radar (TDWR) system has a gust front detection algorithm which has provided an operationally useful capability for both safety and planning. However, this algorithm's performance is sensitive to the orientation of the gust front with respect to the radar radial. Due to this sensitivity, the algorithm is unable to detect about 50% of gust fronts that cross the airport. This paper describes a new algorithm which provides improved performance by using additional radar signatures of gust fronts. The performance of the current TDWR gust front algorithm for the various operational demonstrations has been documented in Klingle-Wilson et al. (1989) and Evans (1990). These analyses highlighted deficiencies in the current algorithm, which is designed to detect radial convergent shears only. When gust fronts or portions of gust fronts become aligned nearly parallel to a radial, the radial component of the shear is not as readily evident. In addition, gust fronts that are near or over the radar exhibit little radial convergence along their lengths and ground clutter can obscure the gust front near the radar. Thus, special handling is needed for fronts that approach the radar. Figure 1 illustrates the various components of a gust front as viewed by Doppler radar. The portion of the gust front in the figure labelled radial convergence is detectable with the current algorithm. Fronts, or portions of fronts, that are aligned along the radar radial and those that pass over the radar are examples of events which can exhibit little or no radial shear signature. These events are often detectable by variations in the radial velocities from azimuth to azimuth (i.e., azimuthal shear)., and/or by radar reflectivity thins lines. The new algorithm improves the detection and prediction of gust fronts by merging radial convergence features with azimuthal shear features, thin line features, and the predicted locations of gust fronts which are passing over the radar. The next four sections of this paper describe the individual components of the improved algorithm. Section 6 describes the rule base used to combine detections from the four components into single gust front detections and Section 7 discusses the output of the algorithm.

The Federal Aviation Administration (FAA) initiated the Terminal Doppler Weather Radar (TDWR) program in the mid-1980s in response to the need for improved real time hazardous weather (especially low altitude wind shear) detection in the terminal area. The program is designed to develop a reliable automated Doppler radar based system to detect terminal weather hazards and provide warnings that will help pilots successfully avoid these hazards. Following the successful operational evaluation of the TDWR concept which was described at the last conference, a production contract was awarded to the Raytheon Company, with the first deliveries scheduled for fall of 1992. This paper will describe the current status and deployment strategy for the operational systems and present recent results from the extensive testing of the radar system concept and weather information dissemination approach.

Aviation operations in the airport terminal area, where flights converge from a number of directions onto one or two active runways, create a fundamental limitation on the capacity of the national airspace system. The U.S. Federal Aviation Administration (FAA) has recognized that the throughput of existing terminals can be increased significantly by providing the terminal air traffic control team with Terminal Air Traffic Control Automation (TATCA) tools that increase the efficiency of individual controller tasks and provide a dynamic, overall plan for traffic management throughout the terminal control region (Andrews and Welch, 1989). This latter function relies on accurate projection of traffic flow into the future (0-30 minutes) in order to automatically examine the many possible permutations of control actions. The result is a coordinated plan for the multiple (four to ten) control positions involved in the decision making processes that determine end-capacity at the runways. The FAA has launched an intensive effort to develop and implement TATCA capabilities by taking advantage of preparatory work done at NASA Ames Research Center, MITRE Corporation, and M.I.T. Lincoln Laboratory. An initial TATCA configuration, the Final Approach Spacing Tool (FAST), will be evaluated in the field beginning in 1993 and will be scheduled for possible national implementation two years later. Estimates of the economic value of TATCA-generated operational improvements, when implemented at major airports nationwide, are expected to be over $1 billion yearly by the year 2000 in reduced fuel consumption, other air carrier operating costs, and passenger time (Boswell et al., 1990). Since TATCA is first and foremost a planning system, the primary impacts of weather upon T ATCA performance involve disruption of planning. This can occur because of sudden or unexpected changes in routing, runway availability, or separation standards. In addition, errors in estimated wind produce errors in time-to-fly predictions made by the TATCA planning logic. The TATCA system must be robust with respect to weather events that commonly occur in its region of operation. This paper describes an initial study of the weather information requirements for TATCA, and their relationship to current and future systems for measurement, integration, forecasting and dissemination of meteorological data in the terminal area. A major goal is to stress the need for close coupling between ongoing initiatives in weather sensing/forecasting in the airport terminal area, and air-space capacity enhancement programs.

The limitations to ultra-low sidelobe performance are explored using a 32-element linear array, operating at L-band, contianing transmit/receive (T/R) modules with 12-bit phase shifters. With conventional far-field calibrations, the average sidelobe level of the array was about-40dB. In theory, considerably lower sidelobe performance is expected from such an array. Initially, sidelobe performance was thought to be limited by inadequate calibrations. An examination of individual array element patterns showed a mirror-symmetric ripple which could be attributed to edge effects in a small array. Simulations indicated that more precise calibrations would not compensate for these element-pattern differences. An adaptive calibration technique was developed which iteratively adjusted the attenuator and phaser commands to create nulls in the antenna pattern in the direction of the nulls of a theoretical antenna pattern. With adaptive calibrations, the average sidelobe level can be lower to 60dB. The technique can be used for interference suppression by implementing antenna patterns with deep nulls in specified directions.

In this paper, we investigate an AM-FM model for representing modulations in speech resonances. Specifically, we propose a frequency modulation (FM) model for the time-varying formants whose amplitude varies as the envelope of an amplitude-modulated (AM) signal. To detect the modulations we apply the energy operator (psi)(x) = (x)^2 - xx and its discrete counterpart. We found that psi can approximately track the envelope of AM signals, the instantaneous frequency of FM signals, and the product of these two functions in the general case of AM-FM signals. Several experiments are reported on the applications of this AM-FM modeling to speech signals, bandpass filtered via Gabor filtering.