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Variable-PRI processing for meteorologic Doppler radars
March 29, 1994
Conference Paper
Published in:
1994 IEEE Natl. Radar Conf., 29-31 March 1994, pp. 85-90.
Summary
In this communication we described how, with nonuniform sampling, the concept of bandlimited extrapolation can be used to obtain unambiguous Doppler velocity estimates in the supra-Nyquist region. The proposed method coherently processes a multi-PRI sample using a generalized form of periodogram analysis. The work is described in the context of meteorologic Doppler processing and includes a discussion of effective suppression for stationary ground clutter when multi-PRI schemes are used.
Summary
In this communication we described how, with nonuniform sampling, the concept of bandlimited extrapolation can be used to obtain unambiguous Doppler velocity estimates in the supra-Nyquist region. The proposed method coherently processes a multi-PRI sample using a generalized form of periodogram analysis. The work is described in the context of...
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Status of the Terminal Doppler Weather Radar with deployment underway
August 2, 1993
Conference Paper
Published in:
Proc. Fifth Int. Conf. on Aviation Weather Systems, 2-6 August 1993, pp. 32-34.
Summary
The Federal Aviation Administration (FAA) initiated the Terminal Doppler Weather Radar (TDWR) program in the mid-1980's in response to the need for improved real-time hazardous weather (especially low-altitude wind shear) surveillance in the terminal area (Turnbull, et al., 1989). The initial focus for the TDWR was to provide reliable, fully automated Doppler radar detection of microbursts and gust fronts and 20-minute warning of wind shifts which could effect runway usage. Subsequent operational demonstrations have shown that the overall terminal situational awareness provided by the TDWR color Geographical Situation Display (GSD) depiction of wind shear locations, weather reflectivity and storm motion also yields substantial improvements in terminal operations efficiency for air traffic managers and for airlines. In this paper, we will describe the current status and deployment strategy for the operational systems and recent results from the extensive testing of the radar system concept and of the weather information dissemination approach.
Summary
The Federal Aviation Administration (FAA) initiated the Terminal Doppler Weather Radar (TDWR) program in the mid-1980's in response to the need for improved real-time hazardous weather (especially low-altitude wind shear) surveillance in the terminal area (Turnbull, et al., 1989). The initial focus for the TDWR was to provide reliable, fully...
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Coherent processing across multi-PRI waveforms
May 24, 1993
Conference Paper
Published in:
Proc. 26th Int. Conf. on Radar Meteorology, 24-28 May 1993, pp. 232-234.
Summary
Meteorological Doppler radars have typically utilized constant pulse-repetition intervals (PRI) to facilitate clutter filtering and estimation of weather echo spectral moments via pulse-pair or periodogram-based algorithms. Utilization of variable PRIs to support resolution of velocity ambiguities has been discussed, for example by Banjanin and Zrnic, but not implemented owing to difficulties associated with clutter filter design. Recent work by Chornoboy presents design algorithms for time-varying finite impulse response (FIR) filters that achieve Chebyshev or mean-squared error (MSE) optimality when processing multi-PRI waveforms. This paper is a follow-on to that work, treating techniques for post-clutter filter processing (e.g. periodogram estimation) that are appropriate for such waveforms. Our approach involves a least-squares fitting of the signal - sampled at a nonuniform rate - to a weighted sum of uniformly spaces sinusoids. The sinusoids or "basis functions" are chosen to span a Nyquist interval consistent with the longest PRI in the transmitted waveform, and need not be centered at zero Doppler. Determination of the sinusoid weightings - effectively a discrete Fourier transformation (DFT) - and the associated residual between the harmonic fit and the data area accomplished via multiplications of the signal vector with precomputed matrices. The resulting spectrum estimate can be used directly for weather echo moment calculations, or can be inverse-Fourier transformed using conventional techniques to generate a time-domain signal representation. This work has been motivated by a specific application - estimation of weather spectrum moments for a Wind Shear Processor (WSP) modification to the Federal Aviation Administration's Airport Surveillance Radar (ASR-9). Our approach supports candidate low-altitude radial wind estimation algorithms that operate on frequency-domain signal representations and require that the radar's block-stagger PRI and the possibility of velocity ambiguities be accounted for in generating the spectrum estimates. In principle, however, these processing techniques are also applicable to weather radar systems such as WSR-88D and Terminal Doppler Weather Radar (TDWR) where range and Doppler ambiguities are an operational concern.
Summary
Meteorological Doppler radars have typically utilized constant pulse-repetition intervals (PRI) to facilitate clutter filtering and estimation of weather echo spectral moments via pulse-pair or periodogram-based algorithms. Utilization of variable PRIs to support resolution of velocity ambiguities has been discussed, for example by Banjanin and Zrnic, but not implemented owing to...
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Summary of triple Doppler data, Orlando 1991
April 7, 1992
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-186
Summary
Under Federal Aviation Administration (FAA) sponsorship, Lincoln Laboratory conducted an aviation weather hazard measurement and operational demonstration program during the summer of 1991 near the Orlando International Airport. Three Doppler radars were sited in a triangle around the airport, allowing triple Doppler coverage of thunderstorms and microbursts occurring there. This report contains a summary of all of the microburst producing thunderstorms that occurred within the triple Doppler region that were scanned in a coordinated fashion, during the months of June, July, August, and September, 1991. Statistics on the microburst events are presented to give an overall picture of the available data for use in analysis. The bulk of the report consists of detailed information about each triple Doppler day, including the time, location, and strength of microbursts within the triple Doppler period as well as the availability of data from supporting sensors including the ASR-9-WSR Doppler radar, radiosondes, LLWAS, Mesonet, AWOS, instrumented aircraft, ACARS, interferometer, and corona points.
Summary
Under Federal Aviation Administration (FAA) sponsorship, Lincoln Laboratory conducted an aviation weather hazard measurement and operational demonstration program during the summer of 1991 near the Orlando International Airport. Three Doppler radars were sited in a triangle around the airport, allowing triple Doppler coverage of thunderstorms and microbursts occurring there. This...
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Doppler mean velocity estimation - small sample analysis and a new estimator
March 25, 1992
Technical Report
Published in:
MIT Lincoln Laboratory Report TR-942
Summary
Optimal Doppler velocity estimation, under the constraint of small sample size, is explored for a standard Gaussian signal measurement model and thematic maximum likelihood (ML) and Bayes estimation. Because the model considered depends on a vector parameter [velocity, spectrum width, and signal-to-noise ratio (SNR)], the exact formulation of an ML or Bayes solution involves a system of equations that is neither uncoupled nor explicit in form. Historically, iterative methods have been the most suggested approach to solving the required equations. In addition to being computationally intensive, it is unclear whether iterative methods can be constructed to perform well given a small-sample size and low signal strength. This report takes a different approach and seeks to construct approximate (ML and Bayes) estimators based on the notion of using constrained adaptive models to deal with nuisance parameter removal. A Monte Carlo simulation is used to determine small-sample estimator statistics and to demonstrate true performance bounds in the case of known nuisance values. Performance comparisons between these optional forms and other standard estimators [pulse pairs (PP) and a frequency domain (WP) method] are presented. Performance sensitivity of the optimal algorithms, with respect to uncertainity in the values of model nuisance parameters, is explored.
Summary
Optimal Doppler velocity estimation, under the constraint of small sample size, is explored for a standard Gaussian signal measurement model and thematic maximum likelihood (ML) and Bayes estimation. Because the model considered depends on a vector parameter [velocity, spectrum width, and signal-to-noise ratio (SNR)], the exact formulation of an ML...
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High resolution microburst outflow vertical profile data from Huntsville, Alabama, and Denver, Colorado
April 10, 1991
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-163
Summary
The purpose of this report is to present detailed data on microburst outflows recorded by the TDWR testbed radar (FL-2) in Huntsville, Alabama (1986) and Denver, Colorado (1987-88). Whenever possible, a microburst detected within 10 km of the radar was scanned in a vertical direction (RHI) at 1 to 2 degree azimuthal intervals about the center of divergence. The vertical profile of the outflow is pertinent to the detection capability and siting strategy of a single Doppler radar observing the microburst from a horizontal viewing angle. Additionally, outflow features are important in assessing the hazard associated with microbursts as well as the capability of other wind shear detection (LLWAS or ASR). Of particular interest is the variability of outflows depths from case to case and site to site. If the depth across the maximum velocity differential is shallow, an outflow might go undetected or underestimated by a radar, the beam ot which was not viewing the axis of peak divergence. Previous research projects in Denver reported the highest winds in a microburst typically occur near the surface with an average outflow depth (1/2 peak velocity) ranging between 500 and 600 meters: however, the vertical resolution of these data was fairly crude due to the scan strategies utilized. This report provides detailed high resolution microburst outflow vertical profile data pertinent to TDWR system studies based on RHI and closely spaced PPI scans. The median observed outflow depth in Huntsville was 200 meters shallower than in Denver while the median height of the maximum velocity varied from 100 meters AGL in Huntsville to 200 meters AGL in Denver. For those Denver events presented here, we recommend that the TDWR microburst detection scan extend to at least 200 meters AGL and 100 meters if there is adequate clutter suppression.
Summary
The purpose of this report is to present detailed data on microburst outflows recorded by the TDWR testbed radar (FL-2) in Huntsville, Alabama (1986) and Denver, Colorado (1987-88). Whenever possible, a microburst detected within 10 km of the radar was scanned in a vertical direction (RHI) at 1 to 2...
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Clutter suppression for Doppler weather radars with multirate sampling schemes
December 11, 1990
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-149
Summary
Reliable weather parameter estimates are required of radars such as the Terminal Doppler Weather Radar (TDWR) - a Federal Aviation Administration project - which will automatically detect hazaradous weather phenomena in the vicinity of an airport. Velocity and range aliasing will degrade the quality of these estimates, as will contamination by ground clutter. For radars which operate at short ranges and at low elevation angles, as the TDWR will to detect windshears at the airport surface, clutter contamination is an especiallly severe problem. Multirate pulse trains - pulse trains containing multiple intersample spacings - can extend both the unambiguous velocity and range of a Pulsed Doppler Radar beyond those afforded by pulse trains with a constant intersample spacing; but the usual properties of conventional clutter filter architectures change radically when applied to data collected with a multirate sampling scheme. A brief introduction to the systems and weather considerations fo Doppler Weather Radars is provided and the Pulse-Pair spectral moment estimators are presented. This introduction is followed by a discussion of frequency domain clutter rejection tecniques for Batch PRT (Pulse Repetition Time) sequences - blocks of equispaced samples with the PRT alternating from block to block. The main topic of the report is clutter suppression for Staggered PRT sequences in which the PRT alternates from pulse to pulse. The Staggered PRT scheme has the advantage over the Batch PRT scheme of spatial coherency for estimates of the radar return signal's autocorrelation function at the lags corresponding to the two PRT's. A time-varying filter architecture with multiple transfer functions is presented and analyzed, and its interaction with the Pulse-Pair estimators is explored. Three design techniques for Staggered PRT filters are described and assessed in the context of clutter suppression. The final section of the report summarizes the results for the Batch and Staggered PRT schemes and provides suggestions for further research.
Summary
Reliable weather parameter estimates are required of radars such as the Terminal Doppler Weather Radar (TDWR) - a Federal Aviation Administration project - which will automatically detect hazaradous weather phenomena in the vicinity of an airport. Velocity and range aliasing will degrade the quality of these estimates, as will contamination...
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Preliminary results of the 1983 Coordinated Aircraft - Doppler weather radar turbulence experiment, volume 1
July 26, 1988
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-137-I
Summary
This report presents results of analyses of coordinated radar-aircraft data acquired form the 1983 experiment conducted at Hanscom AFB, Massachusetts. The objective of the experiment is to assess and validate the current NEXRAD algorithms for estimating aircraft turbulence from volume-scanned Doppler weather observations. Estimates of the turbulence severity index epsilon to the 1/3 power (a quantity used by NEXRAD) computed from radar and aircraft data are presented as a time series along each aircraft track. The radar point estimates of turbulence were averaged horizontally and vertically to yield layered Cartesian maps such as are intended for use by real time ATC controllers and pilots. The derived gust velocity (Ude), also used to indicate the intensity of aircraft encountered turbulence, was computed so that comparisons could be made of the turbulence intensity scales inferred from values of epsilon and U sub de. These quantitative comparisons indicate that for the turbulence generally encountered during the flights, both radar and aircraft estimates of epsilon to the 1/3 power significantly overstate the severity of turbulence as reported by the aircraft pilot. The data analysis also shows that radar-based estimates of epsilon to the 1/3 power, often significantly exceeded aircraft based estimates of epsilon to the 1/3 power. In contrast, the quantity Ude underestimates the aircraft reported turbulence intensity on all the flights. The uncertainty as to operationally useful thresholds for radar epsilon to the 1/3 power, aircraft epsilon to the 1/3 power and Ude is discussed as is the use of spectrum width as a turbulence indicator. It should be noted that the turbulence detection flights used in the study were conducted at ranges such that the radar resolution cell cross range extent was typically 1.5 to 3 km. With such resolution cell size extents, the hypothesis of spatially homogeneous turbulence may not hold and/or the assumed relationship of radar measured spectrum width to kinetic dissipation rate may not be fully accurate.
Summary
This report presents results of analyses of coordinated radar-aircraft data acquired form the 1983 experiment conducted at Hanscom AFB, Massachusetts. The objective of the experiment is to assess and validate the current NEXRAD algorithms for estimating aircraft turbulence from volume-scanned Doppler weather observations. Estimates of the turbulence severity index epsilon...
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TDWR clutter residue map generation and usage
January 29, 1988
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-148
Summary
The Terminal Doppler Weather Radar (TDWR) system is designed to provide high quality low altitude Doppler radar data near airports. Ground clutter suppression will be a major challenge to supplying such high quality Doppler data. To confront this challenge the FAA has specified stringent clutter suppression requirements in the TDWR technical specifications. These specifications are designed to provide an effective clutter suppression system. In particular, the specifications require an antenna with narrow beam width and low side-lobes to minimize ground target illumination. Also, a high pass frequency filter (with a stop attenuation in excess of 50 dB) is required to reduce stationary clutter. FInally, a clutter reisdue map editing system is used to remove remaining clutter. This report describes the algorithms used to generate and use the clutter residue editing system. The major issues are discussed followed by a description of the algorithms designed to address these issues. Finally, preliminary experimental results using a clutter residue map are presented.
Summary
The Terminal Doppler Weather Radar (TDWR) system is designed to provide high quality low altitude Doppler radar data near airports. Ground clutter suppression will be a major challenge to supplying such high quality Doppler data. To confront this challenge the FAA has specified stringent clutter suppression requirements in the TDWR...
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Automated detection of microburst windshear for terminal doppler weather radar
October 28, 1987
Conference Paper
Published in:
SPIE, Vol. 846, Digital Image Processing and Visual Communications Technolody in Meteorology, 27-28 October 1987, pp. 61-68.
Summary
An image analysis method is presented for use in detecting strong windshear events, called microbursts, in Doppler weather radar images. This technique has been developed for use in a completely automated surveil-lance system being procured by the Federal Aviation Administration (FAA) for the protection of airport terminal areas. The detection system must distill the rapidly evolving radar imagery into brief textual warning messages in real time, with high reliability.
Summary
An image analysis method is presented for use in detecting strong windshear events, called microbursts, in Doppler weather radar images. This technique has been developed for use in a completely automated surveil-lance system being procured by the Federal Aviation Administration (FAA) for the protection of airport terminal areas. The detection...
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