Publications
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
Topic:
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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.
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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.
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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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ASR-9 weather channel test report
May 3, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-165
Summary
The ASR-9, the next generation airport surveillance radar, will be deployed by the FAA at over 100 locations throughout the United States. The system includes a weather channel designed to provide ATC personnel with timely and accurate weather reflectivity information as a supplement to normal aircraft information. This report presents results of an assessment of the ASR-9 weather channel performance. Two issues addressed are: (1) whether the ASR-9 weather channel performs according to FAA specifications, and (2) whether the ASR-9 weather channel adequately represents weather reflectivity for ATC purposes. These measurement results are intended to support the FAA in developing the operational use of ASR-9 weather information. Comparisons between data from an ASR-9 in Huntsville, Alabama, recorded during design qualification and testing, and data from two other "reference" radars were used as the basis for the assessment. Several storm cases were analyzed, comprised of stratiform rain, isolated convective storms, squall lines, and cold fronts containing multiple simultaneous convective storms. Results suggest that, with the exception of an apparent 3 dB discrepancy between the weather products of the ASR-9 and the "reference" radars, the ASR-9 weather channel seems to perform according to FAA specifications. Although the ASR-9 products give a reasonable representation of the extent and severity of potentially hazardous weather in Huntsville, the results suggest that the static storm model used to determine beamfill corrections for the ASR-9 should be optimized for the particular climatic region in which an ASR-9 will be operated.
Summary
The ASR-9, the next generation airport surveillance radar, will be deployed by the FAA at over 100 locations throughout the United States. The system includes a weather channel designed to provide ATC personnel with timely and accurate weather reflectivity information as a supplement to normal aircraft information. This report presents...
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ASR-9 weather channel test report, executive summary
May 3, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-168
Summary
The ASR-9, the next generation Airport surveillance radar, will be deployed by the FAA at over 100 locations throughout the United States. The system includes a weather channel designed to provide ATC personnel with timely and accurate weather reflectivity information as a supplement to normal aircraft information. Comparisons between data from an ASR-9 in Huntsville, Alabama, recorded during design qualification and testing, and data from two other "reference" radars, were used as the basis for assessment of ASR-9 weather channel performance. Results suggest that, with the exception of an apparent 3 dB discrepancy between the weather products of the ASR-9 and the "reference" radars, the ASR-9 weather channel seems to perform according to FAA specifications.
Summary
The ASR-9, the next generation Airport surveillance radar, will be deployed by the FAA at over 100 locations throughout the United States. The system includes a weather channel designed to provide ATC personnel with timely and accurate weather reflectivity information as a supplement to normal aircraft information. Comparisons between data...
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Storm models for end-to-end TDWR signal processing simulation tests
May 1, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-155
Summary
End-to-end qualification testing of teh Terminal Doppler Weather Radar (TDWR) contractor signal processing system will be accomplished by a signal processing simulation test. Government furnished storm models will be used to provide inputs to the signal processor. The corresponding hazardous weather product results will be compared to hte results determined by the detection algorithm developers. This report examines the role of the end-to-end tests in the context of overall TDWR qualification testing and concludes that the signal waveform/velocity ambiguity resolution should be the principal focus of the signal processing simulation testing. Salient characteristics of the initial pair of storm models (a high reflectivity microburst observed in Huntsville, AL, and a series of low-to-moderate reflectivity microburst storms observed in Denver, CO) are described as well as desirable characteristics of additional storm models to be provided later.
Summary
End-to-end qualification testing of teh Terminal Doppler Weather Radar (TDWR) contractor signal processing system will be accomplished by a signal processing simulation test. Government furnished storm models will be used to provide inputs to the signal processor. The corresponding hazardous weather product results will be compared to hte results determined...
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Notes and correspondence - Correcting wind speed measurements for site obstructions
April 1, 1989
Journal Article
Published in:
J. Atmos. Oceanic Technol., Vol. 6. No. 2, April 1989, pp. 343-352.
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Summary
The effects of obstructions on winds measured by the 30 station FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) mesonet and the 6 station FAA LLWAS (Low Level Wind Shear Alèrt System) near Memphis, TN in 1985 are analyzed. The slowing of surface winds by anemometer site obstructions is a continuing problem for scientific and operational wind shear measurement system This paper considers an improved version of the technique used by Fujita and Wakimoto for compensating the obstruction effects by the use of mathematical models relating the unobstructed wind speed to the measured wind speed and the observed obstructions at each site. Over eight million wind speed measurements gathered over 197 days (15 February–31 August) were used. The effects of obstructions at a particular site were evidenced by a strong negative correlation between the observed wind speed transmission factors and the obstruction angles as measured from panoramic photographs taken of the horizon around each station. The functional relationship between them was modeled as a decaying exponential plus a constant, and an iterative least squares regression technique was used on data from all of the stations at once in deriving the three parameters of the equation. It was found that the first 8° of obstruction have the greatest blockage effects, and that even a 2° or 3° high isolated clump of trees can have a pronounced effect on the measured wind speeds from that direction. The possibility that the transmission factors are scale dependent and time dependent is explored.
Summary
The effects of obstructions on winds measured by the 30 station FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) mesonet and the 6 station FAA LLWAS (Low Level Wind Shear Alèrt System) near Memphis, TN in 1985 are analyzed. The slowing of surface winds by anemometer site obstructions is a continuing problem...
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