Publications
The capabilities and limitations of using the ASR-9 as a terminal area precipitation sensor
September 7, 1997
Conference Paper
Published in:
28th Conf. on Radar Meteorology, 7-12 September 1997.
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Summary
The Airport Surveillance Radar (ASR-9) weather channel is an invaluable tool to air-traffic and flight management specialists. The precipitation data from this sensor is currently displayed on air-traffic specialists' radar scopes and is incorporated into the Integrated Terminal Weather System (ITWS). The data are used to determine optimum routes for aircraft operating in and near the tenninal airspace. Data from other terminal area precipitation sensors such as the Terminal Doppler Weather Radar (TDWR) and the Next Generation Weather Radar (NEXRAD) are also used for this same purpose. The primary advantage of using the ASR-9 as a precipitation sensor is its high update rate, e.g. thirty seconds versus about five minutes for TDWR and N EX RAD. The ASR-9 is also quite reliable, with limited down time. Finally, range folding is not a significant problem with this radar. However, during ITWS prototype testing over the past three years, we have identified several limitations of using this radar as a precipitation sensor. For one, the maximum reflectivity of cells can be significantly underestimated by the ASR-9 due to partial filling of its fan-shaped elevation beam and cell-to-cell spatial averaging. Also, the occurrence of underestimation seems to increase when the radar operates in circular polarization mode. In addition, we have analyzed cases where significant precipitation-induced attenuation has occurred. Finally, because most ASR-9s are located on the airport, rain cores developing aloft, above the airport, maybe underestimated or missed entirely. This paper focuses on the problems identified through the ITWS prototype testing.
Summary
The Airport Surveillance Radar (ASR-9) weather channel is an invaluable tool to air-traffic and flight management specialists. The precipitation data from this sensor is currently displayed on air-traffic specialists' radar scopes and is incorporated into the Integrated Terminal Weather System (ITWS). The data are used to determine optimum routes for...
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A modified transmission line model for cavity backed microstrip antennas
July 18, 1997
Conference Paper
Published in:
IEEE Antennas and Propagation Society Int. Symp. 1997 Digest, Vol. 4, 13-18 July 1997, pp. 2139-42.
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Summary
Spatial power combining of many MMIC amplifiers at millimeter wave frequencies using a fixed array of microstrip antenna elements places unique demands on dielectric media. The substrate must be relatively thick to allow space for MMIC placement, must provide rather high thermal conductivity to disipate MMIC heat, and be of high dielectric constant to shrink circuit element dimensions. Presently, microstrip antenna models require a low dielectric constant substrate to be valid. This paper presents a modified transmission line model on the model of Pues and Van de Capelle which addresses the problems of thick, high microstrip antenna elements. The goal of the model was to guide design of a microstrip array antenna suitable for a spatial power combined module.
Summary
Spatial power combining of many MMIC amplifiers at millimeter wave frequencies using a fixed array of microstrip antenna elements places unique demands on dielectric media. The substrate must be relatively thick to allow space for MMIC placement, must provide rather high thermal conductivity to disipate MMIC heat, and be of...
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Ambiguity resolution for machine translation of telegraphic messages
July 7, 1997
Conference Paper
Published in:
Proc. 35th Annual Meeting of the Assoc. for Computational Linguistics, 7-12 July 1997, pp. 120-7.
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Telegraphic messages with numerous instances of omission pose a new challenge to parsing in that a sentence with omission causes a higher degree of ambiguity than a sentence without omission. Misparsing reduced by omissions has a far-reaching consequence in machine translation. Namely, a misparse of the input often leads to a translation into the target language which has incoherent meaning in the given context. This is more frequently the case if the structures of the source and target languages are quite different, as in English and Korean. Thus, the question of how we parse telegraphic messages accurately and efficiently becomes a critical issue in machine translation. In this paper we describe a technical solution for the issue, and present the performance evaluation of a machine translation system on telegraphic messages before and after adopting the proposed solution. The solution lies in a grammar design in which lexicalized grammar rules defined in terms of semantic categories and syntactic rules defined in terms of part-of-speech are utilized together. The proposed grammar achieves a higher parsing coverage without increasing the amount of ambiguity/misparsing when compared with a purely lexicalized semantic grammar, and achieves a lower degree of ambiguity/misparses without, decreasing the parsing coverage when compared with a purely syntactic grammar.
Summary
Telegraphic messages with numerous instances of omission pose a new challenge to parsing in that a sentence with omission causes a higher degree of ambiguity than a sentence without omission. Misparsing reduced by omissions has a far-reaching consequence in machine translation. Namely, a misparse of the input often leads to...
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Speech recognition by machines and humans
July 1, 1997
Journal Article
Published in:
Speech Commun., Vol. 22, No. 1, July 1997, pp. 1-15.
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Summary
This paper reviews past work comparing modern speech recognition systems and humans to determine how far recent dramatic advances in technology have progressed towards the goal of human-like performance. Comparisons use six modern speech corpora with vocabularies ranging from 10 to more than 65,000 words and content ranging from read isolated words to spontaneous conversations. Error rates of machines are often more than an order of magnitude greater than those of humans for quiet, wideband, read speech. Machine performance degrades further below that of humans in noise, with channel variability, and for spontaneous speech. Humans can also recognize quiet, clearly spoken nonsense syllables and nonsense sentences with little high-level grammatical information. These comparisons suggest that the human-machine performance gap can be reduced by basic research on improving low-level acoustic-phonetic modeling, on improving robustness with noise and channel variability, and on more accurately modeling spontaneous speech.
Summary
This paper reviews past work comparing modern speech recognition systems and humans to determine how far recent dramatic advances in technology have progressed towards the goal of human-like performance. Comparisons use six modern speech corpora with vocabularies ranging from 10 to more than 65,000 words and content ranging from read...
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A description of the interfaces between the Weather Systems Processor (WSP) and the Airport Surveillance Radar (ASR-9)
June 16, 1997
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-259
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Summary
The Weather Systems Processor (WSP) is an enhancement for the Federal Aviation Administration's (FAA) current generation Airport Surveillance Radars (ASR-9) that provides fully automated detection of microburst and gust front wind shear phenomena, estimates of storm cell movement and extrapolated future postion, and 10- and 20-minute predictions of the future postion of gust fronts. The WSP also generates six-level weather reflectivity free of anomalous propagation induced ground clutter breakthrough. Alphanumeric and graphical displays provide WSP-generated weather information to air traffic controllers and their supervisors. This report describes the hardware, interfaces, timing and digital signal extraction from the ASR-9 necessary to support the WSP. The digital interface circuitry between the WSP and the ASR-9, the control functions associated with the WSP, and the strategies for performing system test functions are described
Summary
The Weather Systems Processor (WSP) is an enhancement for the Federal Aviation Administration's (FAA) current generation Airport Surveillance Radars (ASR-9) that provides fully automated detection of microburst and gust front wind shear phenomena, estimates of storm cell movement and extrapolated future postion, and 10- and 20-minute predictions of the future...
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Initial comparison of lightning mapping with operational time-of-arrival and interferometric systems
May 27, 1997
Journal Article
Published in:
J. Geophys. Res., Vol. 102, No. D10, 27 May 1997, pp. 11,071-11,085.
Summary
The mapping of lightning radiation sources produced by the operational Time-of-Arrival National Aeronautics and Space Administration/Lightning Detection and Ranging (NASA/LDAR) system is compared with that of the Interferometric French Office National D'Etudes et de Recherches Aerospatiales (ONERA-3D) system. The comparison comprises lightning activity in three Florida storms and also individual flashes in one of these storms. Although limited in scope, the comparison analysis show a significant difference in the representation of lightning radiation by each mapping system. During the duration of a flash, the LDAR data show a continuity in time and a three-dimensional structure of radiation sources. The ONERA-3D radiation source data are more intermittent in time and have a more two-dimensional structure. The distinction between the radiation sources mapped by the two systems is also reflected in the difference between their propagation speeds, 10^4-10^5 m s^-1, estimated by the LDAR system, and 10^7-10^8 m s^-1, estimated by the ONERA-3D system. We infer that this difference occurs because most of the radiation sources mapped with LDAR are associated with virgin breakdown processes typical of slowly propagating negative leaders. On the other hand, most of the radiation sources mapped with ONERA3D are produced by fast intermittent negative breakdown processes typical of dart leaders and K changes as they traverse the previously ionized channels. Thus each operational system may emphasize different stages of the lightning flash, but neither appears to map the entire flash.
Summary
The mapping of lightning radiation sources produced by the operational Time-of-Arrival National Aeronautics and Space Administration/Lightning Detection and Ranging (NASA/LDAR) system is compared with that of the Interferometric French Office National D'Etudes et de Recherches Aerospatiales (ONERA-3D) system. The comparison comprises lightning activity in three Florida storms and also individual...
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HTIMIT and LLHDB: speech corpora for the study of handset transducer effects
April 24, 1997
Conference Paper
Published in:
Proc. IEEE Int. Conf. on Acoustics, Speech and Signal Processing, ICASSP, Vol. 2, 21-24 April 1997, pp. 1535-1538.
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Summary
This paper describes two corpora collected at Lincoln Laboratory for the study of handset transducer effects on the speech signal: the handset TIMIT (HTIMIT) corpus and the Lincoln Laboratory Handset Database (LLHDB). The goal of these corpora are to minimize all confounding factors and to produce speech predominately differing only in handset transducer effects. The speech is recorded directly from a telephone unit in a sound-booth using prompted text and extemporaneous photograph descriptions. The two corpora allow comparison of speech collected from a person speaking into a handset (LLHDB) versus speech played through a loudspeaker into a headset (HTIMIT). A comparison of analysis and results between the two corpora will address the realism of artificially creating handset degraded speech by playing recorded speech through handsets. The corpora are designed primarily for speaker recognition experimentation (in terms of amount of speech and level of transcription), but since both speaker and speech recognition systems operate on the same acoustic features affected by the handset, knowledge gleaned is directly transferable to speech recognizers.
Summary
This paper describes two corpora collected at Lincoln Laboratory for the study of handset transducer effects on the speech signal: the handset TIMIT (HTIMIT) corpus and the Lincoln Laboratory Handset Database (LLHDB). The goal of these corpora are to minimize all confounding factors and to produce speech predominately differing only...
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Speech recognition by humans and machines under conditions with severe channel variability and noise
April 21, 1997
Conference Paper
Published in:
SPIE, Vol. 3077, Applications and Science of Artificial Neural Networks III, 21-24 April 1997, pp. 46-57.
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Summary
Despite dramatic recent advances in speech recognition technology, speech recognizers still perform much worse than humans. The difference in performance between humans and machines is most dramatic when variable amounts and types of filtering and noise are present during testing. For example, humans readily understand speech that is low-pass filtered below 3 kHz or high-pass filtered above 1kHz. Machines trained with wide-band speech, however, degrade dramatically under these conditions. An approach to compensate for variable unknown sharp filtering and noise is presented which uses mel-filter-bank magnitudes as input features, estimates the signal-to-noise ratio (SNR) for each filter, and uses missing feature theory to dynamically modify the probability computations performed using Gaussian Mixture or Radial Basis Function neural network classifiers embedded within Hidden Markov Model (HMM) recognizers. The approach was successfully demonstrated using a talker-independent digit recognition task. It was found that recognition accuracy across many conditions rises from below 50 % to above 95 % with this approach. These promising results suggest future work to dynamically estimate SNR's and to explore the dynamics of human adaptation to channel and noise variability.
Summary
Despite dramatic recent advances in speech recognition technology, speech recognizers still perform much worse than humans. The difference in performance between humans and machines is most dramatic when variable amounts and types of filtering and noise are present during testing. For example, humans readily understand speech that is low-pass filtered...
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Analysis of downstream impacts of air traffic delay
March 3, 1997
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-257
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Summary
Reduction of air carrier flight delay in the U.S. National Airspace System (NAS) has been a major objective of the Federal Aviation Administration (FAA) for many years. Much of the current delay arises from weather-induced delays at airports. When a plane is delayed on one of the day's flights, there can be a carryover delay that affects later flights by that aircraft. In this report, we develop statistical models to predict: 1. The "downstream" delays that occur when a flight experiences an initial delay, and 2. The likelihood of flight cancellation as a function of the initial delay. Using historical airline-reported delays for December 1993, we conclude that the mean "downstream" delay is approximately 80 percent of the initial delay, i.e., the net delay for an aircraft due to an initial flight delay is approximately 1.8 x the initial delay.
Summary
Reduction of air carrier flight delay in the U.S. National Airspace System (NAS) has been a major objective of the Federal Aviation Administration (FAA) for many years. Much of the current delay arises from weather-induced delays at airports. When a plane is delayed on one of the day's flights, there...
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Convective weather forecasting for FAA applications
February 2, 1997
Conference Paper
Published in:
7th Conf. on Aviation, Range, and Aerospace Meteorology, ARAM, 2-7 February 1997.
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Summary
The Convective Weather Product Development Team (PDT) was formed in 1996 as part of the reorganization of the FAA Aviation Weather Research Program, to provide an effective way to conduct critical applied research in a collaborative and rational fashion. Detecting and predicting convective weather is extremely important to aviation, since approximately half of the national airspace delay in the warm season is caused by thunderstorms. Reliable 0--6 hr storm predictions are essential for aviation users to achieve safe and efficient use of the airspace, as well as for future air traffic control automation systems. Our goal on this PDT is to direct our research and development activities toward operationally useful convective weather detection and forecast products, and delivery of those products, so that users can receive benefits on an immediate and continual basis. Given that we have many more initiatives than funding, we have chosen to prioritize our activities according to near-term achievable benefits to users. Our hope is that the success of initial planned demonstrations will help the FAA identify a consistent level of long-term R&D funding, so that we can make real progress towards achieving our full set of goals. In this paper, we present our statement of the FAA Convective Weather Forecasting problem, evidence of the need for forecasts in the National Airspace System (NAS), and an illustration of the air traffic delay caused by convective weather. We then discuss our research plan and rationale, and outline our main initiatives for the upcoming year.
Summary
The Convective Weather Product Development Team (PDT) was formed in 1996 as part of the reorganization of the FAA Aviation Weather Research Program, to provide an effective way to conduct critical applied research in a collaborative and rational fashion. Detecting and predicting convective weather is extremely important to aviation, since...
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