Publications
A case study of the 24 August 1986, FLOWS microburst
November 28, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-162
Summary
From 1984 to 1986, Lincoln Laboratory under the sponsorship of the Federal Aviation Administration (FAA) collected wind shear measurements in the southeastern United States using a pulsed Doppler radar. The major emphasis of the measurement program and subsequent analyses is the development and testing of algorithms that will enable the Terminal Doppler Weather Radar (TDWR) to provide wind shear warnings to the aviation community by detection and tracking gust fronts and microbursts. An important phase of the program involves determining appropriate scan strategies and algorithms to detect other radar measurable features which precede or accompany the surface outflows of microbursts. The detection of features aloft such as convergence, rotation, divergence, storm cells, and descending reflectivity cores may permit advanced recognition of the wind shear while it is less than 10 m/s. In this report a microburst on 24 August 1986 in Huntsville is analyzed with single and dual-Doppler techniques to assess microburst precursors, asymmetry, and forcing mechanisms which could be used for futute algorithm development. The microburst producing storm formed within a moist adiabatic, unstable air-mass with weak wind shear at low to mid-levels of the atmosphere. Rotation, convergence, divergent tops, and a descending core were detected prior to the outflow attaining a divergence of 10 m/s. This storm is similar to other Huntsville microburst producing cells in exhibiting upper-level divergence prior to the initial microburst outflow. Previous analyses of wind shear in Denver and Oklahoma did not discuss divergent tops as a possible microburst precursor. However, its relation to storm severity and hailstorm intensity has been reported by Witt and Nelson (1984) and NEXRAD Program Office (1985). In this case-study, the 3-dimensional microburst detection algorithm provided an early declaration of the event while the radial velocity differential was less than 10 m/s.
Summary
From 1984 to 1986, Lincoln Laboratory under the sponsorship of the Federal Aviation Administration (FAA) collected wind shear measurements in the southeastern United States using a pulsed Doppler radar. The major emphasis of the measurement program and subsequent analyses is the development and testing of algorithms that will enable the...
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Development of an automated windshear detection system using doppler weather radar
November 1, 1989
Journal Article
Published in:
Proc. IEEE, Vol. 77, No. 11, November 1989, pp. 1661-1673.
Summary
The US Federal Aviation Administration (FAA) is developing the Terminal Doppler Weather Radar (TDWR) system to determine the location and severity of LAWS (low-altitude windshear) phenomena and other weather hazards (e.g. tornadoes and turbulence) and to provide the pertinent information to real-time air traffic control users. The FAA program for developing and evaluating the TDWR is described, with emphasis on the resolution of key technical issues such as separation of the radar return due to the low-altitude weather phenomena from that caused by various clutter sources and the automatic detection of the phenomena by means of pattern recognition applied to images depicting the weather reflectivity and Doppler shift. These technical issues have been addressed using experimental data obtained using a testbed radar in representative meteorological regimes. The system performance has been assessed using numerous experimental windshear data sets with corresponding 'truth' developed by experienced radar meteorologists from a number of organizations. It is shown that the system provides very reliable detection of strong microbursts in a variety of environments with a gust-front detection capability that supports effective planning of airport runway use.
Summary
The US Federal Aviation Administration (FAA) is developing the Terminal Doppler Weather Radar (TDWR) system to determine the location and severity of LAWS (low-altitude windshear) phenomena and other weather hazards (e.g. tornadoes and turbulence) and to provide the pertinent information to real-time air traffic control users. The FAA program for...
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The relationship between lightning type and convective state of thunderclouds
September 30, 1989
Journal Article
Published in:
J. Geophys. Res., Vol. 94, No. D11, 30 September 1989, pp. 13,213-13,220.
Summary
Thunderstorm case studies and earlier observations are described which illuminate the relationship between cloud vertical development and the prevalence of intracloud (IC) and cloud-to-ground (CG) lightning. A consistent temporal evolution starting with peak IC activity changing to predominant CG activity and concluding with strong outflow (microburst) suggests that ice is responsible for both the electrical (i.e., lightning) and dynamical (i.e., microburst) phenomena. The IC activity is attributed to the updraft-driven accumulation of graupel particles in the central dipole region, and the subsequent CG activity to the descent of ice particles beneath the height of the main negative charge. The subsequent descent and melting of ice particles beneath the height of the 0 degree C isotherm are associated with the acceleration of the downdraft and outflow. The IC lightning precursor can provide a valuable short-term (5-10 min) warning for microburst hazard at ground level.
Summary
Thunderstorm case studies and earlier observations are described which illuminate the relationship between cloud vertical development and the prevalence of intracloud (IC) and cloud-to-ground (CG) lightning. A consistent temporal evolution starting with peak IC activity changing to predominant CG activity and concluding with strong outflow (microburst) suggests that ice is...
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Mode S Beacon System: a functional overview
August 29, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-150
Summary
This document provides a functional overview of the Mode S Beacon System, a combined secondary surveillance radar (beacon) and ground-air-ground data link system capable of providing the aircraft surveillance and communications necessary to support ATC automation in future traffic environments. Mode S is capable of common-channel interoperation with the current ATC beacon system, and may be implemented over an extended transition period. Mode S will provide the surveillance and communication performance required by ATC automation, the reliable communications needed to support data link services, and the capability of operating with a terminal or enroute, radar digitizer-equipped, ATC surveillance radar. The material contained in this document serves as an introduction to the more detailed information contained in "Mode S Beacon System: Functional Description." DOT/FAA/PM-86/19, 29 August 1986.
Summary
This document provides a functional overview of the Mode S Beacon System, a combined secondary surveillance radar (beacon) and ground-air-ground data link system capable of providing the aircraft surveillance and communications necessary to support ATC automation in future traffic environments. Mode S is capable of common-channel interoperation with the current...
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Microburst detection with airport surveillance radars
August 1, 1989
Conference Paper
Published in:
34th Ann. Air Traffic Control Associsation., 30 October 1989 - 2 November 1989, pp. 514-522.
Summary
With the advent of fully digital signal processing for new airport surveillance radars (ASR-9), terminal air traffic control displays will be largely free of clutter from precipitation and ground scatterers [1,2]. Early acceptance testing of the ASR-9, however, indicated that working air traffic controllers actually made considerable use of the weather echo information on their displays. To reinsert weather data in a non-interfering manner, the ASR-9's signal processor was augmented with a dedicated channel for processing and displaying six quantitative levels of precipitation reflectivity (i.e. rain rate) [2,3]. This processor does not utilize tile radar's coherency, other than for Doppler filtering of ground clutter echoes. In this paper, we describe processing techniques that would allow airport surveillance radars to extend their weather measurement capability to the detection of microburst-generated low altitude wind shear. The two principal technical challenges are the development of (i) signal processing to suppress ground clutter and estimate the near surface radial wind component in each radar resolution cell; (ii) image processing to automatically detect hazardous shear in the resulting velocity field. The techniques have been evaluated extensively using simulated weather signals and measurements from an experimental airport surveillance radar in the southeastern United States. Overall our analysis indicates that microbursts accompanied by rain at the surface -- the predominant safety hazard in many parts of the U.S. --could be detected with high confidence using a suitably modified ASR. In the following section we describe briefly the background and potential operational role of an ASR-based wind shear detection system. We then discuss the primary technical issues for achieving this capability and our evaluations of processing methods that address these issues.
Summary
With the advent of fully digital signal processing for new airport surveillance radars (ASR-9), terminal air traffic control displays will be largely free of clutter from precipitation and ground scatterers [1,2]. Early acceptance testing of the ASR-9, however, indicated that working air traffic controllers actually made considerable use of the...
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The effectiveness of adaptive PRF selection in minimizing range obscuration in the TDWR system
July 27, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-164
Summary
An adaptive procedure for selecting radar pulse repetition frequency (PRF) has been developed as the primary means of minimizing the occurence of range aliased echoes within operationaly significant coverage aread (e.g., airport runways) of the Terminal Doppler Weather Radar (TDWR) system. This procedure underwent extensive testing at the S-Band TDWR testbed while located in Denver, CO, where it was judged to be highly successful at preserving the integrity of data collected within the vicinity of the Stapleton International Airport runways. The actual TDWR system will operate at a C-Band frequency, and an increase in potential range obscuration is expected over that experience by the S-band testbed. This report discusses the anticipated performance of the PRF selection procedure in the C-Band environment by extrapolating results obtained using S-Band testbed data. The results conclusively demonstrate the efficacy of adaptive PRF selection as a method by which to reduce potential range obscuration. A worst-case scenario, for example, indicates that over 20% of the TDWR radar collected about the airport runways has the potential for being contaminated with range aliased echoes at any given time during TDWR surveillance operations. With adaptive PRF selection, however, the expected obscuration is reduced to only 3%. (The corresponding figures for the S-Band testbed are shown to be 14& rather than 20% and 1% rather than 3%). While adaptive PRF selection can substantially reduce range obscuration, it cannot totally eliminate the problem. An enhancement to the PRF selection strategy, which further reduces the potential range obscuration, is introduced and recommended. Additionally, the complementary use of pulse-to-pulse phase modulation to exact valid velocity measurements in the presence of range contamination is discussed and preliminary experimental results presented.
Summary
An adaptive procedure for selecting radar pulse repetition frequency (PRF) has been developed as the primary means of minimizing the occurence of range aliased echoes within operationaly significant coverage aread (e.g., airport runways) of the Terminal Doppler Weather Radar (TDWR) system. This procedure underwent extensive testing at the S-Band TDWR...
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Dual-beam autocorrelation based wind estimates from airport surveillance radar signals
June 21, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-167
Summary
This report describes an efficient, autocorrelation based algorithm for estimating low altitude radial winds using signals from the two receiving beams of an airport surveillance radar (ASR). The approach seeks to achieve the accuracy demonstrated previously for spectral domain dual beam velocity estimators with significantly reduced computational requirements. Fundamental to the technique is the assumption that the power spectrum measured with an airport surveillance radar's broad elevation beam can be fitted by a two component Gaussian model. The parameters of this model are estimated using measured low-order autocorrelation lags from the low and high beam received signals. The desired near surface radial velocity estimate is obtained directly as one of these parameters -- the center frequency of the "low altitude" Gaussian spectrum component. Simualted data and field measurements from Lincoln Laboratory's experimental ASR-8 in Huntsville, Alabama were used to evaluate the accuracy of the autocorrelation based velocity estimates. Monte Carlo simulations indicate that biases relative to the near surface outflow velocity in a microburst would be less than 2.5 m/s unless the microburst were distant (range > 12 km) or very shallow (depth of maximum wind speed layer < 50 m). Estimate standard deviations averaged 0.5 m/s after the spatial filtering employed in our processing sequence. The algorithm's velocity estimate accuracy was sufficient to allow for automatic detection of measured microbursts during 1988 with a detection probability exceeding 0.9 and a false alarm probability less than 0.05. Our analyses indicates that the dual-beam autocorrelation based velocity estimator should support ASR with shear detection at approximately the same level of confidence as the low-high beam spectral differencing algorithm evaluated by Weber and Noyes (1988).
Summary
This report describes an efficient, autocorrelation based algorithm for estimating low altitude radial winds using signals from the two receiving beams of an airport surveillance radar (ASR). The approach seeks to achieve the accuracy demonstrated previously for spectral domain dual beam velocity estimators with significantly reduced computational requirements. Fundamental to...
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Far-echo cancellation in the presence of frequency offset (full duplex modem)
June 1, 1989
Journal Article
Published in:
IEEE Trans. Commun., Vol. 37, No. 6, June 1989, pp. 635-644.
Topic:
R&D area:
Summary
In this paper, we present a design for a full-duplex echo-cancelling data modem based on a combined adaptive reference algorithm and adaptive channel equalizer. The adaptive reference algorithm has the advantage that interference to the echo canceller caused by the far-end signal can be eliminated by subtracting an estimate of the far-end signal based on receiver decisions. This technique provides a new approach for full-duplex far-echo cancellation in which the far echo can be cancelled in spite of carrier frequency offset. To estimate the frequency offset, the system uses a separate receiver structure for the far echo which provides equalization of the far-echo channel and tracks the frequency offset in the far echo. The feasibility of the echo-cancelling algorithms is demonstrated by computer simulation with realistic channel distortions and with 4800 bits/s data transmission at which rate frequency offset in the far echo becomes important.
Summary
In this paper, we present a design for a full-duplex echo-cancelling data modem based on a combined adaptive reference algorithm and adaptive channel equalizer. The adaptive reference algorithm has the advantage that interference to the echo canceller caused by the far-end signal can be eliminated by subtracting an estimate of...
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Phase coherence in speech reconstruction for enhancement and coding applications
May 26, 1989
Conference Paper
Published in:
Proc. IEEE Int. Conf. on Acoustics, Speech and Signal Processing, ICASSP, Vol. 1, Speech Processing 1, 23-26 May 1989, pp. 207-209.
Topic:
R&D area:
Summary
It has been shown that an analysis-synthesis system based on a sinusoidal representation leads to synthetic speech that is essentially perceptually indistinguishable from the original. A change in speech quality has been observed, however, when the phase relation of the sine waves is altered. This occurs in practice when sine waves are processed for speech enhancement (e.g., time-scale modification and reducing peak-to-RMS ratio) and for speech coding. This paper describes a zero-phase sinusoidal analysis-synthesis system which generates natural-sounding speech without the requirement of vocal tract phase. The method provides a basis for improving sound quality by providing different levels of phase coherence in speech reconstruction for time-scale modification, for a baseline system for coding, and for reducing the peak-to-RMS ration by dispersion.
Summary
It has been shown that an analysis-synthesis system based on a sinusoidal representation leads to synthetic speech that is essentially perceptually indistinguishable from the original. A change in speech quality has been observed, however, when the phase relation of the sine waves is altered. This occurs in practice when sine...
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Speech-state-adaptive simulation of co-channel talker interference suppression
May 23, 1989
Conference Paper
Published in:
Proc. IEEE Int. Conf. on Acoustics, Speech and Signal Processing, ICASSP, 23-26 May 1989, pp. 361-364.
Topic:
R&D area:
Summary
A co-channel talker interference suppression system processes an input waveform containing the sum of two simultaneous speech signals, referred to as the target and the jammer, to produce a waveform estimate of the target speech signal alone. This paper describes the evaluation of a simulated suppression system performing ideal suppression of a jammer signal given the voicing states (voiced, unvoiced, silent) of the target and jammer speech as a function of time and given the isolated target and jammer speech waveforms. By applying suppression to select regions of jammer speech as a function of the voicing states of the target and jammer, and by measuring the intelligibility of the resulting jammer suppressed co-channel speech, it is possible to identify those regions of co-channel speech on which interference suppression most improves intelligibility. Such results can help focus algorithm development efforts.
Summary
A co-channel talker interference suppression system processes an input waveform containing the sum of two simultaneous speech signals, referred to as the target and the jammer, to produce a waveform estimate of the target speech signal alone. This paper describes the evaluation of a simulated suppression system performing ideal suppression...
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