Publications
Optimal searches for asteroids
January 1, 1984
Journal Article
Published in:
ICARUS, Vol. 57, No. 2, February 1984, pp. 259-266.
Topic:
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Summary
Optimal searches for a fixed object are discussed and the rigorous analytical results of discrete search theory are presented. They show that the totally optimal, the uniformly optimal, the locally optimal, and the fastest searches are identical under not too restrictive assumptions. The mathematical formalism is illustrated by an Earth-approaching asteroid search and optimal searches for such objects are explicitly constructed. The approximation that Earth-approaching asteroids are fixed is equivalent to having a very high (>or=100 square degrees/hr) search rate. Generalizations to other types of astronomical search are briefly mentioned.
Summary
Optimal searches for a fixed object are discussed and the rigorous analytical results of discrete search theory are presented. They show that the totally optimal, the uniformly optimal, the locally optimal, and the fastest searches are identical under not too restrictive assumptions. The mathematical formalism is illustrated by an Earth-approaching...
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Experience with speech communication in packet networks
December 1, 1983
Journal Article
Published in:
IEEE J. Sel. Areas Commun., Vol. SAC-1, No. 6, December 1983, pp. 963-980.
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R&D area:
Summary
The integration of digital voice with data in a common packet-switched network system offers a number of potential benefits, including reduced systems cost through sharing of switching and transmission resources, flexible internetworking among systems utilizing different transmission media, and enhanced services for users requiring access to both voice and data communications. Issues which it has been necessary to address in order to realize these benefits include reconstitution of speech from packets arriving at nonuniform intervals, maximization of packet speech multiplexing efficiency, and determination of the implementation requirements for terminals and switching in a large-scale packet voice/data system. A series of packet speech systems experiments to address these issues has been conducted under the sponsorship of the Defense Advanced Research Projects Agency (DARPA). In the initial experiments on the ARPANET, the basic feasibility of speech communication on a store-and-forward packet network was demonstrated. Techniques were developed for reconstitution of speech from packets, and protocols were developed for call setup and for speech transport. Later speech experiments utilizing the Atlantic packet satellite network (SATNET) led to the development of techniques for efficient voice conferencing in a broadcast environment, and for internetting speech between a store-and-forward net (ARPANET) and a broadcast net (SATNET). Large-scale packet speech multiplexing experiments could not be carried out on ARPANET or SATNET where the network link capacities severely restrict the number of speech users that can be accommodated. However, experiments are currently being carried out using a wide-band satellite-based packet system designed to accommodate a sufficient number of simultaneous users to support realistic experiments in efficient statistical multiplexing. Key developments to date associated with the wide-band experiments have been 1) techniques for internetting via voice/data gateways from a variety of local access networks (packet cable, packet radio, and circuit-switched) to a long-haul broadcast satellite network and 2) compact implementations of packet voice terminals with full protocol and voice capabilities. Basic concepts and issues associated with packet speech systems are described. Requirements and techniques for speech processing, voice protocols, packetization and reconstitution, conferencing, and multiplexing are discussed in the context of a generic packet speech system configuration. Specific experimental configurations and key packet speech results on the ARPANET, SATNET, and wide-band system are reviewed.
Summary
The integration of digital voice with data in a common packet-switched network system offers a number of potential benefits, including reduced systems cost through sharing of switching and transmission resources, flexible internetworking among systems utilizing different transmission media, and enhanced services for users requiring access to both voice and data...
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Mode S surveillance netting
November 4, 1983
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-120
Summary
The surveillance performance of a single Mode S Sensor is degraded by several factors, including: poor crossrange accuracy at long range, diffraction-induced azimuth errors, missing of incomplete reports, and extraneous reports. The surveillance netting project reported here sought to overcome these difficulties by employing information from a secondary (and perhaps also a tertiary) sensor. The project was performed to determine what auxiliary information is most useful, how this information could be used for maximum effect, when help should be sought from other sensors, what form this inter-sensor communication should take, and where the netting algorithms should be implemented. It was also planned to include the construction of a real-time netting demonstration system to exercise and test the concepts developed. The central issue in this project was the approach to be used for multi-sensor azimuth determination. In particular, a new form of incremental bilateration, employing a flat earth model, is shown to be both accurate and bias-resistant. Altitude estimation methods and multi-sensor tracker design are also addressed, with new algorithms developed in each case. Finally, the deisgn of the netting subsystem for a Mode S sensor is presented.
Summary
The surveillance performance of a single Mode S Sensor is degraded by several factors, including: poor crossrange accuracy at long range, diffraction-induced azimuth errors, missing of incomplete reports, and extraneous reports. The surveillance netting project reported here sought to overcome these difficulties by employing information from a secondary (and perhaps...
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Frequency sampling of the short-time Fourier-transform magnitude for signal reconstruction
November 1, 1983
Journal Article
Published in:
J. Opt. Soc. Amer., Vol. 73, November 1983, pp. 1523- 1526.
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Summary
Unique recovery of a signal from the magnitude (modulus) of the Fourier transform has been of long-standing interest in image and optical processing in which Fourier-transform phase is lost or difficult to measure. We investigate an alternative problem of recovering a signal from the Fourier-transform magnitude of overlapping regions of the signal, i.e., from the short-time (or -space) Fourier-transform magnitude. Recently it was established that a discrete-time signal x (n) can be uniquely obtained under mild restrictions from its short-time Fourier-transform magnitude. In this paper we extend this result to the case when the short-time Fourier-transform magnitude is known at only one or two frequencies for each n. We also present a recursive algorithm for recovering a sequence from such samples and demonstrate the algorithm with an example.
Summary
Unique recovery of a signal from the magnitude (modulus) of the Fourier transform has been of long-standing interest in image and optical processing in which Fourier-transform phase is lost or difficult to measure. We investigate an alternative problem of recovering a signal from the Fourier-transform magnitude of overlapping regions of...
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The Experimental Integrated Switched Network - a system-level network test facility
October 31, 1983
Conference Paper
Published in:
Proc. 1983 IEEE Military Communications Conf., MILCOM, 31 October-2 November 1983.
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Summary
An Experimental Integrated Switched Network (EISN) has been developed to provide a system-level testbed for the evaluation of advanced communications networking techniques, including survivable network routing algorithms using a mix of transmission media, for application in the Defense Switched Network (DSN). EISN includes five CONUS sites linked by a wideband demand-assigned satellite channel and by dialed-up terrestrial trunks for alternate satellite/terrestrial routing experiments. Experiments to date have validated techniques for integration of circuit-switched terrestrial systems with the demand-assigned satellite system, and for the establishment of alternate routes over satellite and terrestrial paths. Currently, candidate routing algorithms for application in the DSN are being implemented and tested using external routing/controller processors attached to digital circuit switches at EISN sites. In addition, EISN is also being used to support data communication experiments using DoD standard data protocols in a combined satellite/terrestrial network environment. Work is ongoing both in system experiments and in testbed developments to include additional capabilities. This paper represents a description and status report on both the testbed and the experimental efforts.
Summary
An Experimental Integrated Switched Network (EISN) has been developed to provide a system-level testbed for the evaluation of advanced communications networking techniques, including survivable network routing algorithms using a mix of transmission media, for application in the Defense Switched Network (DSN). EISN includes five CONUS sites linked by a wideband...
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Airborne Intelligence Display (AID) phase I software description
October 24, 1983
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-123
Summary
The Airborne Intelligent Display is a microprocessor-based display capable of serving as a cockpit data terminal in a variety of FAA developmental applications. A prototype of this display was developed by Lincoln Laboratory during 1979-1980 in order to evaluate and demonstrate the use of the data link between Mode S ground sensor and Mode S transponder-equipped aircraft. The AID served as a data link interface allowing the pilot to see, respond to, and initiate communications from a ground sensor. Later, when Lincoln began testing the Traffic Alert and Collision Avoidance System (TCAS), the AID became the TCAS display device, showing position estimates for TCAS-tracked aircraft. More recently, a redesign effort, focused principally on software, was begun to extend the AID design so that it could be more quickly adapted to a variety of FAA developmental programs. This document describes the redesigned Airborne Intelligent Display, with special emphasis on software design.
Summary
The Airborne Intelligent Display is a microprocessor-based display capable of serving as a cockpit data terminal in a variety of FAA developmental applications. A prototype of this display was developed by Lincoln Laboratory during 1979-1980 in order to evaluate and demonstrate the use of the data link between Mode S...
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Ground clutter cancellation for the NEXRAD system
October 19, 1983
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-122
Summary
Returns from the ground and associated obstacles surroudning a NEXRAD weather radar (i.e., ground clutter) will contaminate the estimates of weather echo spectral features (e.g., reflectivity, mean velocity, and spectral width). The ground clutter returns are particularly large at low elevation angles and close range (e.g., within 40 km). Additionally, the pulse repetition frequency (PRF) values necessary to obtain the desired weather Doppler features result in ground clutter contamination at ranges that are multiples of the unambiguous range interval (e.g., 115-175 km for a typical NEXRAD). Fortunately, the groung clutter power spectrum is localized around zero velocity so that one can reduce its effect by appropriate Doppler signal processing. Automatic reduction of clutter contamination is essential if NEXRAD is to achieve the desired automatic weather product generation capability. The results of an analytical/experimental study oreinted toward development of a clutter cancellation specification and assiciated quality assurance tests for the NEXRAD system are described.
Summary
Returns from the ground and associated obstacles surroudning a NEXRAD weather radar (i.e., ground clutter) will contaminate the estimates of weather echo spectral features (e.g., reflectivity, mean velocity, and spectral width). The ground clutter returns are particularly large at low elevation angles and close range (e.g., within 40 km). Additionally...
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A traffic alert and collision avoidance system for general aviation
October 1, 1983
Conference Paper
Published in:
IEEE/AIAA 5th Digital Avionics Systems Conf., Seattle, WA, 31 October - 3 November 1983, pp. 20.
Summary
One component of the Federal Aviation Administration approach to independent aircraft separation assurance is known as the Traffic Alert and Collision Avoidance System I (TCAS I), which employs passive or active techniques for the detection of nearby transponder-equipped aircraft. This paper gives the results of a study conducted by Lincoln Laboratory of simple techniques for the passive and active detection of transponders. Filter criteria that may be used to restrict passive detections to potentially threatening aircraft are described and evaluated. These techniques and criteria were used in a candidate passive detector whose performance was evaluated in flight against targets of opportunity. A candidate low-power active interrogator was also evaluated through link calculations and airborne measurements. The results indicate that a low-power active interrogator can provide more reliable detection of nearby aircraft and a lower false alert rate than any of the simple passive techniques considered. The active technique generates insignificant levels of interference and, unlike a passive system, also provides protection in regions where there are no ground interrogators.
Summary
One component of the Federal Aviation Administration approach to independent aircraft separation assurance is known as the Traffic Alert and Collision Avoidance System I (TCAS I), which employs passive or active techniques for the detection of nearby transponder-equipped aircraft. This paper gives the results of a study conducted by Lincoln...
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An experimental GPS navigation receiver for general aviation: design and measured performance
September 27, 1983
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-121
Summary
This report describes work performed by MIT Lincoln Laboratory between 1 October 1979 and 1 March 1983, to evaluate the use of the Global Positioning System (GPS) for low-cost civil air navigation. The report describes a GPS Test and Evaluation System developed jointly by MIT Lincoln Laboratory, Stanford Telecommunications, Inc., and Intermetrics, Inc., using techniques that could lead to low-cost commercial avionics. System performance results obtained in the laboratory and during flight tests are provided which demonstrate compliance with current and future navigation accuracy requirements for enroute, terminal, and non-precision flight paths. The report also includes functional specifications for a low-cost GPS navigation system for civil aircraft. The GPS Test and Evaluation system design was based on two important features: 1) automatic tracking of all visible satellites (rather than a minimum set of four) and 2) a dual-channel GPS C/A code receiver. Tracking all visible satellites allows the system to maintain continuous navigation when a satellite sets or is momentarily masked during aircraft maneuvers. The dual-channel receiver dedicates one channel to pseudo-range measurements, and the other channel to acquiring new satellites as they become visible. These two features, validated by flight test, allow the system to provide continuous navigation updates during critical aircraft maneuvers, such as non-precision approaches, and during satellite constellation changes.
Summary
This report describes work performed by MIT Lincoln Laboratory between 1 October 1979 and 1 March 1983, to evaluate the use of the Global Positioning System (GPS) for low-cost civil air navigation. The report describes a GPS Test and Evaluation System developed jointly by MIT Lincoln Laboratory, Stanford Telecommunications, Inc...
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Doppler radar observations of an Oklahoma downburst
September 23, 1983
Conference Paper
Published in:
21st Conf., on Radar Meteorology, 19-23 September 1983, pp. 590-595.
Summary
"Downbursts", first discovered by Dr. T. T. Fujita, are small intense downdrafts at very low altitudes which impact the surface and cause a divergent outflow of wind. They can occur under a variety of meteorological conditions as was demonstrated during the JAWS 1982 field experiment which took place in the vicinity of Stapleton airport in Denver, CO. Many downbursts were detected but most of them were of the type now being called "dry" or "cumulus" or "virga" downbursts. A distinction must be made between these and the "wet" or "thunderstorm" downbursts which are the subject of this study. The two phenomena are very different. They are easy to distinguish: the former come from benign looking cumulus clouds and fall through a very deep and dry subcloud layer and the latter are associated with thunderstorms. Thunderstorm downbursts have been detected throughout the Great Plains and the Midwest, on the east coast, and in Florida, while the virga downbursts have been detected mainly over the high plains east of the Rockies. The word "downburst" was first introduced by Fujita (1976) after the investigation of a plane crash at JFK airport, to describe the situation in which a thunderstorm downdraft becomes hazardous to the operation of jet aircraft on take-off or landing. At first, Fujita (1979) thought that the downburst and the well known thunderstorm downdraft were essentially the same but that, in the same way a funnel cloud aloft is not called a tornado, a mid-level downdraft in a thunderstorm would not be called a downburst. The concept was later refined when it was decided that the downburst must induce "an outburst of damaging winds on or near the ground" (Fujita and Wakimoto, 1981) where "damaging winds" refers to winds that can be estimated on the F-scale (for which there minimum threshold is 18 m/s). These damaging winds can be either straight or curved but they must be highly divergent (Fujita, 1981). Thus, even in its most recent and more meteorological definition, the term downburst is meant to signify a potential human hazard. Whether of not it also signifies a dynamically distinct phenomenon in thunderstorms is a matter of some debate and one which will be investigated in the current work.
Summary
"Downbursts", first discovered by Dr. T. T. Fujita, are small intense downdrafts at very low altitudes which impact the surface and cause a divergent outflow of wind. They can occur under a variety of meteorological conditions as was demonstrated during the JAWS 1982 field experiment which took place in the...
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