Publications
Clutter rejection in Doppler weather radars used for airport wind shear detection
June 1, 1990
Conference Paper
Published in:
Proc. Second Int. Symp. on Noise & Clutter Rejection in Radars & Imaging Sensors (ISNCR-89), 14-16 November 1989, PP. 275-280.
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Summary
Techniques for the suppression of ground and storm clutter to permit the detection of low altitude windshear by pulse Doppler radars are described. Novel features of the system include the use of clutter residue and range aliased weather echo editing maps which edit out the range-azimuth cells on a "data adaptive" basis.
Summary
Techniques for the suppression of ground and storm clutter to permit the detection of low altitude windshear by pulse Doppler radars are described. Novel features of the system include the use of clutter residue and range aliased weather echo editing maps which edit out the range-azimuth cells on a "data...
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Analysis of the potential benefits of Terminal Air Traffic Control Automation (TATCA)
May 23, 1990
Conference Paper
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Proc. 1990 American Control Conf., Vol. 1, 23-25 May 1990, pp.535-542
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Terminal Air Traffic Control Automation (TATCA) is an FAA research and development program to provide computer-aided sequencing, spacing, and management of air traffic flows in terminal areas. This paper discusses technical and national economic benefits that are attainable with such a terminal automation program.
Summary
Terminal Air Traffic Control Automation (TATCA) is an FAA research and development program to provide computer-aided sequencing, spacing, and management of air traffic flows in terminal areas. This paper discusses technical and national economic benefits that are attainable with such a terminal automation program.
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Microburst observability and frequency during 1988 in Denver, CO
May 14, 1990
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-170
Summary
The observability of microbursts with single-Doppler radar is investigated through comparison of radar data and surface weather sensor data. The data were collected during 1988 in Denver, CO as part of the FAA Terminal Doppler Weather Radar measurement program. Radar data were collected by both and S-band and C-band radar, while surface data were taken from a mesoscale network of 42 weather sensors in the vicinity of Denver's Stapleton International Airport. Results are compared with previous similar studies of observability using data from 1987 in Denver, and 1986 in Huntsville, AL. A total of 184 microbursts impacting the surface mesonet were identified. For those microbursts for which both radar and surface data were available, 97% were observable by single-Doppler radar. This compares to 94% observability during 1987 in Denver, and 98% during 1986 in Huntsville. Two strong microbursts (at lease 20 m/s differential velocity) were unobservable by radar throughout their lifetime: one due to low signal-to-noise ratio, and the other due initially to an asymmetric outflow with low signal-to-noise ratio also a contributing factor. Two other microbursts, with differential velocities from 10-19 m/s, were unobservable by radar: one due to shallow outflow with a depth limited to a height below that of the radar beam, and one due to asymmetric outflow oriented unfavorably with respect to the radar viewing angle. Consistent with previous observations, microburst occurrence was most frequent during June and July, when 94 microbursts were identified on 20 days. An anomalously high frequency was also seen in April, although the strength of these events was relatively modest. As expected, the diurnal distribution shows the late afternoon to be the most favorable time for microburst development; more than half of all events reached their maximum strength between the hours of 2-5 p.m. local time.
Summary
The observability of microbursts with single-Doppler radar is investigated through comparison of radar data and surface weather sensor data. The data were collected during 1988 in Denver, CO as part of the FAA Terminal Doppler Weather Radar measurement program. Radar data were collected by both and S-band and C-band radar...
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Terminal Doppler Weather Radar clutter control
May 10, 1990
Conference Paper
Published in:
Proc. IEEE 1990 Int. Radar Conf., 7-10 May 1990, pp. 12-16.
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The FAA is developing the Terminal Doppler Weather Radar system to automatically detect low altitude wind shear due to microbursts and gust fronts. Detection of this phenomenon presents a significant radar engineering challenge due to the need to observe low reflectivity events in the presence of strong clutter from ground objects and range aliased weather returns. This paper describes a number of unique approaches to clutter recognition which have been validated with the TDWR test bed radar.
Summary
The FAA is developing the Terminal Doppler Weather Radar system to automatically detect low altitude wind shear due to microbursts and gust fronts. Detection of this phenomenon presents a significant radar engineering challenge due to the need to observe low reflectivity events in the presence of strong clutter from ground...
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Short-time signal representation by nonlinear difference equations
April 6, 1990
Conference Paper
Published in:
Proc. IEEE Int. Conf. on Acoustics, Speech and Signal Processing, ICASSP, Vol. 3, Digital Signal Processing, 3-6 April 1990, pp. 1551-1554.
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The solution of a nonlinear difference equation can take on complicated deterministic behavior which appears to be random for certain values of the equation's coefficients. Due to the sensitivities to initial conditions of the output of such "chaotic" systems, it is difficult to duplicate the waveform structure by parameter analysis and waveform synthesis techniques. In this paper, methods are investigated for short-time analysis and synthesis of signals from a class of second-order difference equations with a cubic nonlinearity. In analysis, two methods are explored for estimating equation coefficients: (1) prediction error minimization (a linear estimation problem) and (2) waveform error minimization (a nonlinear estimation problem). In the latter case, which improves on the prediction error solution, an iterative analysis-by-synthesis method is derived which allows as free variables initial conditions, as well as equation coefficients. Parameter estimates from these techniques are used in sequential short-time synthesis procedures. Possible application to modeling "quasi-periodic" behavior in speech waveforms is discussed.
Summary
The solution of a nonlinear difference equation can take on complicated deterministic behavior which appears to be random for certain values of the equation's coefficients. Due to the sensitivities to initial conditions of the output of such "chaotic" systems, it is difficult to duplicate the waveform structure by parameter analysis...
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Noise reduction using a soft-decision sine-wave vector quantizer
April 6, 1990
Conference Paper
Published in:
Proc. IEEE Int. Conf. on Acoustics, Speech and Signal Processing, ICASSP, Vol. 2, Speech Processing 2; VLSI, Audio and Electroacoustics, 3-6 April 1990, pp. 821-824.
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The need for noise reduction arises in speech communication channels, such as ground-to-air transmission and ground-based cellular radio, to improve vocoder quality and speech recognition accuracy. In this paper, noise reduction is performed in the context of a high-quality harmonic serc-phase sine-wave analysis/synthesis system which is characterized by sine-wave amplitudes, a voicing probability, and a fundamental frequency. Least-squared error estimation of a harmonic sine-wave representation leads to a "soft decision" template estimate consisting of sine-wave amplitudes and a voicing probability. The least-squares solution is modified to use template-matching with "nearest neighbors." The reconstruction is improved by using the modified least-squares solution only in spectral regions with low signal-to-noise ratio. The results, although preliminary, provide evidence that harmonic zero-phase sine-wave analysis/synthesis, combined with effective estimation of sine-wave amplitudes and probability of voicing, offers a promising approach to noise reduction.
Summary
The need for noise reduction arises in speech communication channels, such as ground-to-air transmission and ground-based cellular radio, to improve vocoder quality and speech recognition accuracy. In this paper, noise reduction is performed in the context of a high-quality harmonic serc-phase sine-wave analysis/synthesis system which is characterized by sine-wave amplitudes...
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Automatic talker activity labeling for co-channel talker interference suppression
March 21, 1990
Conference Paper
Published in:
Proc. IEEE Int. Conf. on Acoustics, Speech and Signal Processing, Vol. 2, Speech Processing 2; VLSI; Audio and Electroacoustics, ICASSP, 3-6 April 1990, pp. 813-816.
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This paper describes a speaker activity detector taking co-channel speech as input and labeling intervals of the input as target-only, jammer-only, or two-speaker (target+jammer). The algorithms applied were borrowed primarily from speaker recognition, thereby allowing us to use speaker-dependent test-utterance-independent information in a front-end for co-channel talker interference suppression. Parameters studied included classifier choice (vector quantization vs. Gaussian), training method (unsupervised vs. supervised), test utterance segmentation (uniform vs. adaptive), and training and testing target-to-jammer ratios. Using analysis interval lengths of 100 ms, performance reached 80% correct detection.
Summary
This paper describes a speaker activity detector taking co-channel speech as input and labeling intervals of the input as target-only, jammer-only, or two-speaker (target+jammer). The algorithms applied were borrowed primarily from speaker recognition, thereby allowing us to use speaker-dependent test-utterance-independent information in a front-end for co-channel talker interference suppression. Parameters...
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Robust speech recognition using hidden Markov models: overview of a research program
February 26, 1990
Technical Report
Published in:
MIT Lincoln Laboratory Report TR-875
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This report presents an overview of a program of speech recognition research which was initiated in 1985 with the major goal of developing techniques for robust high performance speech recognition under the stress and noise conditions typical of a military aircraft cockpit. The work on recognition in stress and noise during 1985 and 1986 produced a robust Hidden Markov Model (HMM) isolated-word recognition (IWR) system with 99 percent speaker-dependent accuracy for several difficult stress/noise data bases, and very high performance for normal speech. Robustness techniques which were developed and applied include multi-style training, robust estimation of parameter variances, perceptually-motivated stress-tolerant distance measures, use of time-differential speech parameters, and discriminant analysis. These techniques and others produced more than an order-of-magnitude reduction in isolated-word recognition error rate relative to a baseline HMM system. An important feature of the Lincoln HMM system has been the use of continuous-observation HMM techniques, which provide a good basis for the development of the robustness techniques, and avoid the need for a vector quantizer at the input to the HMM system. Beginning in 1987, the robust HMM system has been extended to continuous speech recognition for both speaker-dependent and speaker-independent tasks. The robust HMM continuous speech recognizer was integrated in real-time with a stressing simulated flight task, which was judged to be very realistic by a number of military pilots. Phrase recognition accuracy on the limited-task-domain (28-word vocabulary) flight task is better than 99.9 percent. Recently, the robust HMM system has been extended to large-vocabulary continuous speech recognition, and has yielded excellent performance in both speaker-dependent and speaker-independent recognition on the DARPA 1000-word vocabulary resource management data base. Current efforts include further improvements to the HMM system, techniques for the integration of speech recognition with natural language processing, and research on integration of neural network techniques with HMM.
Summary
This report presents an overview of a program of speech recognition research which was initiated in 1985 with the major goal of developing techniques for robust high performance speech recognition under the stress and noise conditions typical of a military aircraft cockpit. The work on recognition in stress and noise...
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Low-sidelobe phased array antenna characteristics using the planar near-field scanning technique: theory and experiment
February 14, 1990
Technical Report
Published in:
MIT Lincoln Laboratory Report TR-870
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Characteristics of a low-sidelobe phased array antenna are investigated using the technique of planar near-field scanning. The theory associated with the planar near-field scanning technique, with and without probe compensation, is reviewed and an application of the theory is made. The design of an experimental low-sidelobe phased array antenna consisting of monopole elements which are corporate-fed using high precision transmit/receive modules is described. Accurate array radiation patterns are obtained both theoretically and experimentally using centerline scanning at less than one wavelength distance from the antenna. The effects of the antenna probe on the array near-field pattern, plane-wave spectrum, and far-field pattern are demonstrated theoretically using a method of moments numerical simulation. Comparisons of the array theoretical near-zone electric field and array received voltage due to a V-dipole near-field transmitting probe are made. It is shown that a V-dipole theoretical probe antenna can accurately model a practical near-field measurement probe consisting of an open-ended rectangular waveguide surrounded with anechoic material.
Summary
Characteristics of a low-sidelobe phased array antenna are investigated using the technique of planar near-field scanning. The theory associated with the planar near-field scanning technique, with and without probe compensation, is reviewed and an application of the theory is made. The design of an experimental low-sidelobe phased array antenna consisting...
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Use of clutter residue editing maps during the Denver 1988 Terminal Doppler Weather Radar (TDWR) tests
January 25, 1990
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-169
Summary
The Lincoln Laboratory Terminal Doppler Weather Radar (TDWR) testbed operated in Denver, CO in 1987-88. This radar is a prototype of the wind shear detection radars scheduled to be installed by the FAA to provide warnings of possibly hazardous wind shear conditions in airport terminal areas. To obtain the required coverage at low altitudes (down to 100-200 meters above ground level), the antenna beam is required to scan at or very near the earth's surface. Strong ground clutter returns at these low elevation angles present a major problem in the detection of low reflectivity wind shear signals and pose a significant challenge to the mission of these radars. To address this problem, steps along several fronts are taken to mitigate the effects of clutter contamination. These include the use of narrow pencil-beam antennas to minimize ground illumination, suppression by high-pass clutter filters, and the use of clutter residue map editing. This report deals with the latter step, and focuses on the clutter environment experienced at the testbed site during April-October 1988 and its effect on clutter residue map usage. Since the clutter environment is subject to change over time -- due either to man-made or natural causes -- the residue maps require periodic updates to reflect the changing nature of the clutter. This is particularly important for radar systems such as these which rely on automated algorithms to detect subtle patterns and features in the radar returns. To study the frequency with which residue maps required replacement in Denver, clutter measurements recorded during this period were analyzed and are presented in this report as a series of clutter residue maps. The maps are compared and the short and long term changes analyzed. It is concluded that the overall changes during this time were relatively small and gradual, and that map updates at one to two month intervals were sufficient. The generation of the residue maps is described and the importance of collecting clutter data on clear, weather-free days, without the presence of anomalous propagation conditions is addressed. This report also describes the use of median estimation in the construction of the maps as an effective method of eliminating the occasional strong returns from moving reflectors, such as aircraft and vehicles, which would otherwise distort the maps.
Summary
The Lincoln Laboratory Terminal Doppler Weather Radar (TDWR) testbed operated in Denver, CO in 1987-88. This radar is a prototype of the wind shear detection radars scheduled to be installed by the FAA to provide warnings of possibly hazardous wind shear conditions in airport terminal areas. To obtain the required...
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