Publications
Mode-S data link
June 1, 1993
Journal Article
Published in:
J. of ATC, June 1993, pp. 34-37.
Topic:
R&D area:
R&D group:
Summary
Mode-S is an enhancement of the ATCRBS secondary surveillance radar (SSR) system which adds selective interrogation of individual aircraft, monopulse processing of the replies and a digital data link between the ground station and the aircraft. These features result in greatly improved surveillance accuracy, virtual elimination of synchronous garble of the replies from closely spaced aircraft, and provide a high capacity digital communication link for a wide variety of ground/air/ground messages.
Summary
Mode-S is an enhancement of the ATCRBS secondary surveillance radar (SSR) system which adds selective interrogation of individual aircraft, monopulse processing of the replies and a digital data link between the ground station and the aircraft. These features result in greatly improved surveillance accuracy, virtual elimination of synchronous garble of...
READ MORE
LNKnet: Neural network, machine-learning, and statistical software for pattern classification
June 1, 1993
Journal Article
Published in:
Lincoln Laboratory Journal, Vol. 6, No. 2, Summer/Fall 1993, pp. 249-268.
Topic:
R&D area:
R&D group:
Summary
Pattern-classification and clustering algorithms are key components of modern information processing systems used to perform tasks such as speech and image recognition, printed-character recognition, medical diagnosis, fault detection, process control, and financial decision making. To simplify the task of applying these types of algorithms in new application areas, we have developed LNKnet-a software package that provides access to more than 20 pattern-classification, clustering, and feature-selection algorithms. Included are the most important algorithms from the fields of neural networks, statistics, machine learning, and artificial intelligence. The algorithms can be trained and tested on separate data or tested with automatic cross-validation. LNKnet runs under the UNM operating system and access to the different algorithms is provided through a graphical point-and-click user interface. Graphical outputs include two-dimensional (2-D) scatter and decision-region plots and 1-D plots of data histograms, classifier outputs, and error rates during training. Parameters of trained classifiers are stored in files from which the parameters can be translated into source-code subroutines (written in the C programming language) that can then be embedded in a user application program. Lincoln Laboratory and other research laboratories have used LNKnet successfully for many diverse applications.
Summary
Pattern-classification and clustering algorithms are key components of modern information processing systems used to perform tasks such as speech and image recognition, printed-character recognition, medical diagnosis, fault detection, process control, and financial decision making. To simplify the task of applying these types of algorithms in new application areas, we have...
READ MORE
ATCRBS Reply Environment at Memphis International Airport
May 28, 1993
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-198
Summary
This report demonstrates, through data and analysis, how the airport environment can affect ATCRBS surveillance. The Lincoln Laboratory ATCRBS Monopulse Processing Subsystem was used to collect reply data at Memphis International Airport during March 1991. These data show a correlation between aircraft density, potential reflectors, and ATCRBS reply integrity. The number of replies has been shown to be directly related to multipath from reflecting surface, including taxiing aircraft. Additionally, it is shown that conditions can exist during which not all of the replies from ATCRBS equipped aircraft can be processed when forming target report measurements. Finally, it is shown that the bunching of replies in both time and space can introduce reply decoder overloading.
Summary
This report demonstrates, through data and analysis, how the airport environment can affect ATCRBS surveillance. The Lincoln Laboratory ATCRBS Monopulse Processing Subsystem was used to collect reply data at Memphis International Airport during March 1991. These data show a correlation between aircraft density, potential reflectors, and ATCRBS reply integrity. The...
READ MORE
Adjoint-method retrievals of microburst winds from TDWR data
May 24, 1993
Conference Paper
Published in:
26th Int. Conf. on Radar Meteorology, 24-28 May 1993, pp. 433-434.
Topic:
R&D area:
R&D group:
Summary
The simple adjoint (SA) method of Qiu and Xu (1992, henceforth referred to as QX92) was recently upgraded and tested with the Phoenix-II data for retrieving the low-altitude winds from single-Doppler scans (Xu et al. 1993a,b henceforth referred to as XQY93a,b). The major results can be briefly reviewed as follows: (i) Using multiple time-level data with the adjoint formulation makes the retrieval more accurate and less sensitive to the observational error. (ii) Imposing a weak nondivergence constraint can suppress the spurious divergence caused by the data noise and improve the retrieval. (iii) Retrieving the eddy coefficients improves the wind retrieval. (iv) Retrieving the time-man residual term improves the wind retrieval. Although the results in XQY93a,b were encouraging, the Phoenix-II data used in XQY93a,b were collected on non-storm days with chaff dispensed from an aircraft. The real challenge is to test the SA method with storm data. A microburst case is selected for the test in this paper.
Summary
The simple adjoint (SA) method of Qiu and Xu (1992, henceforth referred to as QX92) was recently upgraded and tested with the Phoenix-II data for retrieving the low-altitude winds from single-Doppler scans (Xu et al. 1993a,b henceforth referred to as XQY93a,b). The major results can be briefly reviewed as follows...
READ MORE
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.
Topic:
R&D area:
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...
READ MORE
Clutter filter design for multiple-PRT signals
May 24, 1993
Conference Paper
Published in:
Proc. 26th Int. Conf. on Radar Meteorology, 24-28 May 1993, pp. 235-237.
Summary
The trade-off of range vs. velocity ambiguity is fundamental and operationally significant for many S- and C-band pulsed Doppler weather radars. Transmission schemes using multiple pulse repetition times (PRTs) (i.e., nonuniform pulse spacing) offer the potential for extending the unambiguous measurement range by resolving intervals of velocity ambiguity. Unfortunately, multiple PRT methods can be problematic with low-elevation scanning when ground clutter removal is required. We have constructed both Chebyshev and mean-squared error (MSE) desing algorithms (Choroboy, 1993) that deal with design in the complex domain; the MSE algorithms are described below.
Summary
The trade-off of range vs. velocity ambiguity is fundamental and operationally significant for many S- and C-band pulsed Doppler weather radars. Transmission schemes using multiple pulse repetition times (PRTs) (i.e., nonuniform pulse spacing) offer the potential for extending the unambiguous measurement range by resolving intervals of velocity ambiguity. Unfortunately, multiple...
READ MORE
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.
Topic:
R&D area:
R&D group:
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...
READ MORE
Dual-Doppler measurements of microburst outflow strength asymmetry
May 24, 1993
Conference Paper
Published in:
26th Int. Conf. on Radar Meteorology, 24-28 May 1993, pp. 664-666.
Topic:
R&D area:
R&D group:
Summary
The Federal Aviation Administration (FAA) has been sponsoring Lincoln Laboratory in its effort to develop and test weather detection algorithms for the Terminal Doppler Weather Radar (TDWR). An automated microburst detection algorithm operates on the TDWR radial velocity data and, based on the shear and velocity difference along the radial, outputs regions which are hazards to aviation. This algorithm has been operating since 1987 in Denver, Kansas City, and Orlando and is part of the operational TDWR being deployed across the country. One issue which continues to cause concern for automated windshear detection is microburst asymmetry. Asymmetry, or aspect angle dependence, in microbursts refers to outflows which have a divergent surface outflow strength or extent that varies depending on the viewing angle of the radar. The TDWR is a single-Doppler radar, therefore, an asymmetric microburst may be underestimated or go undetected if the radar is viewing the event from an aspect angle where the strength of the outflow is weak. Past work by Wilson et al., Eilts, and Hallowell has indicated that some microbursts are highly asymmetric. Strength asymmetries (maximum/minimum strength over all viewing angles) from these past studies ranged from 1.3 to as high as 6.0. Hallowell using Denver data examined 27 Denver microbursts (96 observations) and found strength asymmetries from 1.3 to 3.8 with a median of 1.9. However, this previous work has been limited in scope to Denver and Oklahoma (plains) microbursts, and may have used assumptions about the data which introduce false or apparent asymmetry.
Summary
The Federal Aviation Administration (FAA) has been sponsoring Lincoln Laboratory in its effort to develop and test weather detection algorithms for the Terminal Doppler Weather Radar (TDWR). An automated microburst detection algorithm operates on the TDWR radial velocity data and, based on the shear and velocity difference along the radial...
READ MORE
Estimating a windshear hazard index from ground-based terminal Doppler radar
May 24, 1993
Conference Paper
Published in:
26th Int. Conf. on Radar Meteorology, 24-28 May 1993, pp. 670-672.
Summary
In the past decade, a great deal of effort has been invested in developing ground based wind shear detection systems for major U.S. airports. However, there has been a lack of research in developing a quantitative relationship between the wind shear hazards detected by ground based systems and the actual hazard experienced by an aircraft flying through the affected air space. To date, the main thrust of the verification efforts for ground-based systems has been to ensure that the system accurately detect and report the presence of the meteorological phenomena that cause potentially important hazardous windshear. There is a subtle, but potentially important difference between detecting the presence or a microburst and detecting the presence of an aviation hazard. With this in mind, it would seem prudent to rigorously determine what correlation exists between the wind shear warnings that are generated from ground systems and the performance impact on aircraft flying through the impacted airspace. The operational demonstration of the testbed Terminal Doppler Weather Radar (TDWR) in Orlando, Florida along with the testing of airborne Doppler radar systems created a unique opportunity to compare extensively the ground based windshear reports with in-situ aircraft measurements. This paper presents the results from 69 microburst penetrations flown in 1990 and 1991 by the University of North Dakota (UND), the National Aeronautics and Space Administration (NASA) Langley Research Center, and Rockwell Collins under surveillance of the Lincoln-operated TDWR testbed radar. The primary goal of the research was to determine the relative accuracy of several methods designed to generate a numerical microburst hazard index, called the F factor, from ground-based Doppler radar data. It is hope that this work will provide both a qualitative and quantitative basis for the discussion and assessment of microburst hazard reporting for ground-based microburst detection systems. The Integrated Airborne Wind Shear Program is a joint NASA/FAA program with the objective to provide the technology base that will permit low altitude windshear risk reduction through airborne detection, warning, and avoidance. Additionally, the program aims to demonstrate the practicality and utility of real-time assimilation and synthesis of ground-derived windshear data to support executive level cockpit warning and crew-centered information display. Lincoln Laboratory joined this effort and provided the weather radar ground support and some of the post-flight data analysis for NASA's microburst penetration flights in Orlando, Florida.
Summary
In the past decade, a great deal of effort has been invested in developing ground based wind shear detection systems for major U.S. airports. However, there has been a lack of research in developing a quantitative relationship between the wind shear hazards detected by ground based systems and the actual...
READ MORE
Real-time multiple single Doppler analysis with NEXRAD data
May 24, 1993
Conference Paper
Published in:
26th Int. Conf. on Radar Meteorology, 24-28 May 1993, pp. 460-462.
Topic:
R&D area:
R&D group:
Summary
As part of the Aviation Weather Development Program of the Federal Aviation Administration, a high resolution winds analysis system was demonstrated at Orlando International Airport (MCO) in the summer of 1992. The purpose of this demonstration was to illustrate the winds analysis capability possible from operational sensors in the mid '90s. An important part of the design of this system was the development of a procedure for the assimilation of Doppler data from multiple radars. This procedure had to be able to automatically handle regions with missing data from one or more radars, as well as avoid baseline instability. The two operational radars scanning the analysis region were the National Weather Service WSR-88D (NEXRAD) radar located approximately 65 km east and slightly south of MCO, and the MIT prototype Terminal Doppler Weather Radar (TDWR) located 7 km due south of the airport. The base data from these two Doppler radars were the major information component for the analysis system. Our system includes the most recent improvements in the winds analysis portion of the Local Analysis and Prediction System (LAPS) developed by the Forecast Systems Laboratory (McGinely et al., 1991). LAPS is designed to run locally on systems affordable for operational weather offices and takes advantages of all sources of local data at the highest possible resolution. Our implementation for the airport terminal region id called the Terminal-area LAPS (T-LAPS). LAPS formerly had a technique for the assimilation of data from a single Doppler radar. We have modified that technique for the assimilation of data from the two available radars. Our approach, using a Multiple Single Doppler Analysis (MSDA) technique, is more suited for unsupervised operational analysis than traditional Dual Doppler Analysis (DDA), because it is able to handle such problems as incomplete data and baseline instability. We will describe the T-LAPS analysis, with particular attention to our implementation of ASDA, and give some examples from our demonstration.
Summary
As part of the Aviation Weather Development Program of the Federal Aviation Administration, a high resolution winds analysis system was demonstrated at Orlando International Airport (MCO) in the summer of 1992. The purpose of this demonstration was to illustrate the winds analysis capability possible from operational sensors in the mid...
READ MORE