Publications
Anomalous propagation associated with thunderstorm outflows
May 24, 1993
Conference Paper
Published in:
Proc. 26th Int. Conf. on Radar Meteorology, 24-28 May 1993, pp. 238-240.
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Summary
Battan noted that ducting of radar energy by anomalous atmospheric refractive index profiles and resulting abnormally strong ground clutter can occur during three types of meteorological circumstance: (i) large scale boundary layer temperature inversions and associated sharp decrease in moisture with height -- these are often created by nocturnal radiative cooling; (ii) warm, dry air moving over cooler bodies of water, resulting in cooling and moistening of air in the lowest levels; (iii) cool, moist outflows from thunderclouds. In contrast to the first two types of anaomalous propagation (AP), radar ducting associated with thunderstorm outflows is quite dynamic and may mimic echoes from precipitating clouds in terms of spatial scale and temporal evolution. While non-coherent weather radars (e.g. WSR-57) are obviously susceptible to false storm indications from this phenomemenon, Doppler radars that select the level of ground clutter suppression based on "clear day maps" may also fail to suppress the AP-induced ground clutter echoes. Operational Doppler radar systems known to be susceptible to this phenomena are the National Weather Service's WSR-88D and the Federal Aviation Administration's Airport Surveillance Radar (ASR-9) six-level weather channel. In this paper, characteristics of thunderstorm outflow-generated AP are documented using data from a testbed ASR-9 operated at Orlando, Florida. The testbed radar's rapid temporal update (4.8 seconds per PPI scan) and accurate scan-to-scan registration of radar resolution cells enabled characterization of the spatial and temporal evolution of the AP-induced clutter echoes. We discuss implications of these phenomenological characteristics on operational systems, specifically the ASR-9. Algorithms for discrimination between true precipitation echoes and AP-induced ground clutter are discussed.
Summary
Battan noted that ducting of radar energy by anomalous atmospheric refractive index profiles and resulting abnormally strong ground clutter can occur during three types of meteorological circumstance: (i) large scale boundary layer temperature inversions and associated sharp decrease in moisture with height -- these are often created by nocturnal radiative...
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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.
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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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Airport Surveillance Radar (ASR-9) Wind Shear Processor - 1991 Test at Orlando, Florida
June 1, 1992
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-189
Summary
An operational test of a Wind Shear Processor (WSP) add-on to the Federal Aviation Administration's airport surveillance radar (ASR-9) took place at Orlando International Airport during July and August 1991. The test allowed for both quantitative assessment of the WSP's signal processing and wind shear detection algorithms and for feedback from air traffic controllers and their supervisors on the strengths and weaknesses of the system. Thunderstorm activity during the test period was intense; low-altitude wind shear impacted the runways or approach/departure corridors on 40 of the 53 test days. As in previous evaluations of the WSP in the southeastern United States, microburst detection performance was very reliable. Over 95% of the strong microbursts that affected the Orlando airport during the test period were detected by the system. Gust front detection during the test, while operationally useful, was not as reliable as it should have been, given the quality of gust front signatures in the base reflectivity and radial velocity data from the WSP. Subsequent development of a Machine Intelligent gust front algorithm has resulted in significantly improved detection capability. Results from the operational test are being utilized in ongoing refinement of the WSP.
Summary
An operational test of a Wind Shear Processor (WSP) add-on to the Federal Aviation Administration's airport surveillance radar (ASR-9) took place at Orlando International Airport during July and August 1991. The test allowed for both quantitative assessment of the WSP's signal processing and wind shear detection algorithms and for feedback...
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The 1990 Airport Surveillance Radar Wind Shear Processor (ASR-WSP) operational test at Orlando International Airport
July 17, 1991
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-178
Summary
Lincoln Laboratory, under sponsorship from the Federal Aviation Administration (FAA), is conducting a program to evaluate the capability of the newest Airport Surveillance Radars (ASR-9) to detect hazardous weather phenomena -- in particular, low-altitude wind shear created by thunderstorm-generated microbursts and gust fronts. The ASR-9 could provide coverage at airports not slated for a dedicated Terminal Doppler Weather Radar (TDWR) and could augment the TDWR at high-priority (high traffic volume, severe weather) facilities by providing a more rapid update of wind shear products, a better viewing angle for some runways, and redundancy in the event of a TDWR failure. An operational evaluation of a testbed ASR Wind Shear Processor (ASR-WSP) was conducted at the Orlando International Airport in Orlando, FL during August and September 1990. The ASR-WSP operational system issued five distinct products to Air Traffic Control: microburst detections, gust front detections, gust front movement predictions, precipitation reflectivity and storm motion. This document describes the operational system, the operational products, and the algorithms employed. An assessment of system performance is provided as one step in evaluating the operational utility of the ASR-WSP.
Summary
Lincoln Laboratory, under sponsorship from the Federal Aviation Administration (FAA), is conducting a program to evaluate the capability of the newest Airport Surveillance Radars (ASR-9) to detect hazardous weather phenomena -- in particular, low-altitude wind shear created by thunderstorm-generated microbursts and gust fronts. The ASR-9 could provide coverage at airports...
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Weather information requirements for terminal air traffic control automation
June 24, 1991
Conference Paper
Published in:
Fourth Int. Conf. on Aviation Weather Systems, 24-28 June 1991, pp. 208-214.
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Summary
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.
Summary
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...
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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.
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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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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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Dual-beam autocorrelation based wind estimates from airport surveillance radar signals
June 21, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-167
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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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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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Weather sensing with airport surveillance radars
February 3, 1989
Conference Paper
Published in:
Proc. Third Int. Conf. on the Aviation Weather System, 30 January - 3 February 1989, pp. 68-74.
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Summary
Modern airport surveillance radars (ASR) are coherent, pulsed-Doppler radars used for detection and tracking of aircraft in terminal area air space. The Federal Aviation Agency (FAA is procuring over 100 next-generation ASR-9 radars for major US. airports while relocating existing ASR-8s to secondary terminals. Thus within the next five years, almost every U.S. airport that supports commercial operations will be equipped with one of these sensitive, highly stable S-band radars. In view of their on- or near-airport location, rapid scan rate and direct data link to air traffic control personnel, it has been recognized that ASRs can also provide flight controllers with timely information on weather conditions that are hazardous to aircraft. An ASR's transmitted frequency, power, pulse-to-pulse stability and receiver sensitivity are well suited for weather sensing. Conversely, its broad elevation beamwidth, rapid antenna scan rate and non-uniform pulse transmission sequence introduce significant complications for the quantitative interpretation of echoes returned from weather. This paper reviews principal results of a four-year, FAA-sponsored program to evaluate the capabilities and limitations of ASRs for measuring storm severity.
Summary
Modern airport surveillance radars (ASR) are coherent, pulsed-Doppler radars used for detection and tracking of aircraft in terminal area air space. The Federal Aviation Agency (FAA is procuring over 100 next-generation ASR-9 radars for major US. airports while relocating existing ASR-8s to secondary terminals. Thus within the next five years...
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