Publications
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.
Topic:
R&D area:
R&D group:
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...
READ MORE
Mode S Beacon System: Functional Description (Revision C)
July 15, 1983
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-42,C
Topic:
R&D area:
R&D group:
Summary
This document provides a functional description of the Mode S Beacon Systme, a combined secondary surveillance radar (beacon) and ground-air-ground data link system capable of providing the aircraft surveillance and communications necessary to support ATC automation in future traffic environments. Mode S is capable of common-channel interoperation with the current ATC beacon system, and may be implemented at low user cost over an extended transition period. Mode S will provide the surveillance and communication performance required by the ATC automation, the reliable communications needed to support data link services, and the capability of operating with a terminal or enroute, radar digitizer-equipped, ATC surveillance radar. The material contained in this document updates and expands the information presented in "Mode S Beacon System: Functional Description", DOT/FAA/RD-82/52, 27 October 1982.
Summary
This document provides a functional description of the Mode S Beacon Systme, a combined secondary surveillance radar (beacon) and ground-air-ground data link system capable of providing the aircraft surveillance and communications necessary to support ATC automation in future traffic environments. Mode S is capable of common-channel interoperation with the current...
READ MORE
Short-term prediction of high reflectivity contours for aviation safety
June 9, 1983
Conference Paper
Published in:
Proc. Ninth Conf. Aerospace and Aeronautical Meteorology, 6-9 June 1983, pp. 118-122.
Topic:
R&D area:
R&D group:
Summary
Airspace utilization and safety could benefit significantly from accurate, real-time, short-term predictions of hazardous weather regions (e.g., 5-30 minutes). For some hazards, such as heavy turbulence, the detection process itself is in an immature stage. No universally accepted algorithm exists for indicating the regions of current turbulence - let alone predicting it. For other hazards, such as hail and more particularly for heavy rain, the detection process is in a more mature state. In fact heavy rain may be unambiguously associated with high dBZ (reflectivity), if no ice phases are present. Hail is also associated with high reflectivities. We have therefore chosen to place our initial emphasis on the prediction of reflectivity contours in the context of ATC (air traffic control) operations. For all or our prediction techniques, we begin by collecting fixed dBZ-level contours on a fixed-elevation scan by fixed-elevation scan basis, and then combining these elevation cell slices into volume cells as is done in the algorithm of Bjerkaas and Forsyth (1980). To these volume cells we attach translations vectors to make the desired prediction: at this time no provision is made for the growth or decay of reflectivity cells. We generate our translation vectors using each of several algorithms which have already been described elsewhere. Firstly, we use the centroid-tracking approach of Bjerkaas and Forsyth (1980). This is the current tracker of choice in the NEXRAD (Next Generation Weather Radar) program. Secondly, we use tracking vectors of clusters of volume cells, as described ny Crane (1979): much of this work was performed under the sponsorship of the Federal Aviation Administration (FAA). Thirdly, we generate translation vectors by cross-correlating low-altitude (0-4 cm) CAPPIs (constant-altitude plan position indicators): this correlation is done either for the entire storm, or for 30 km by 30 km segments of the storm. This approach has been motivated by the work of Rinehart and Garvey (1978), although we generally use a CAPPI of liquid water content. Fourthly, we use as a prediction the current, composite reflectivity map - our so-called status-quo prediction.
Summary
Airspace utilization and safety could benefit significantly from accurate, real-time, short-term predictions of hazardous weather regions (e.g., 5-30 minutes). For some hazards, such as heavy turbulence, the detection process itself is in an immature stage. No universally accepted algorithm exists for indicating the regions of current turbulence - let alone...
READ MORE
Traffic Alert and Collision Avoidance System (TCAS): a functional overview of minimum TCAS II
April 8, 1983
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-119
Topic:
R&D area:
R&D group:
Summary
The Traffic Alert and Collision Avoidance System (TCAS) is a beanon-based airborne collision avoidance system that is able to operate in all airspace without reliance on ground equipment. The TCAS concept encompasses a range of capabilities that include TCAS I, a low-cost, limited-perofrmance version, and TCAS II, which is intended to provide a comprehensive level of separation assurance in all current and predicted airspace environments through the end of this century. This document provides a functional overview of the TCAS II including operating features, a description of the avionics package, and examples of surveillance data obtained with experimental TCAS equipment.
Summary
The Traffic Alert and Collision Avoidance System (TCAS) is a beanon-based airborne collision avoidance system that is able to operate in all airspace without reliance on ground equipment. The TCAS concept encompasses a range of capabilities that include TCAS I, a low-cost, limited-perofrmance version, and TCAS II, which is intended...
READ MORE
Traffic Alert and Collision Avoidance System (TCAS): a functional overview of active TCAS I
April 8, 1983
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-118
Topic:
R&D area:
R&D group:
Summary
The Traffic Alert and Collision Avoidance System (TCAS) is a beacon-based airborne collision avoidance system that is able to operate in all airspace without reliance on ground equipment. The TCAS concept encompasses a range of capabilities that include TCAS I, a low-cost, limited-performance version, and TCAS II, which is intended to provide a comprehensive level of separation assurance in all current and predicted airspace enviornments through the end of this century. This document provides a functional overview of a TCAS I equipped with a low power active transponder detector. It includes a definition of TCAS I functions, calculated and measured surveillance performance for a low power interrogator and an implementation approach that incorporates the TCAS I functions in a Mode S transponder.
Summary
The Traffic Alert and Collision Avoidance System (TCAS) is a beacon-based airborne collision avoidance system that is able to operate in all airspace without reliance on ground equipment. The TCAS concept encompasses a range of capabilities that include TCAS I, a low-cost, limited-performance version, and TCAS II, which is intended...
READ MORE
Generation of the mode select sensor network coverage map
November 24, 1982
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-98A
Summary
This paper describes the technique of desiging the network management coverage map files necessary to coordinate a network of Mode S sensors. First, the concept of the Mode S network is defined, and the functions of Network Management are briefly described, as they relate to the coverage map. Then, the rationale for the coverage map is given together with definitions of the map structure and the information required in the file. Implementation of these definitions is illustrated in terms of a specific example: a network of four Mode S sensors in the Washington, D.C. area. As configured, each of the sensors provides service to only one of four ATC facilities (three TRACONs and one ARTCC). The resulting map generation process illustrates not only the general principles but also the significant effects of the ATC control are geometry. Finally, the procedure requored for automated map generation is defined. This procedure assumes the use of an interactive computer display terminal and is applicable to any sensor network and ATC facility configuration.
Summary
This paper describes the technique of desiging the network management coverage map files necessary to coordinate a network of Mode S sensors. First, the concept of the Mode S network is defined, and the functions of Network Management are briefly described, as they relate to the coverage map. Then, the...
READ MORE
FAA weather surveillance requirements in the context on NEXRAD
November 19, 1982
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-112
Summary
The Federal Aviation Administration (FAA), National Weather Service and Air Force Weather Service are currently engaged in a program to develop a next generation of weather radars (NEXRAD) capable of satisfying (to the greatest extent possible) the common weather information needs of these agencies. This report identifies the unique FAA weather radar surveillance requirements and examines the technical issues that arise in attempting to meet these requirements with the NEXRAD strawman radar sensors and siting. Current air traffic control (ATC) weather data usage and statistics of aviation weather hazards and system efficiency are used to prioritize products needed for ATC. The strawman NEXRAD capability is then reviewed in the context of the identified weather products and factors such as: (1) effects of front end noise and weather return statistics (2) resolution and low altitude coverage constraints (3) the clutter environment associated with various siting options, and (4) data quality required for real time automated display of hazardous weather regions to ATC controllers. It is concluded that significant problems will arise in attempting to simultaneously provide terminal and en route weather surveillance by a single radar as envisioned in the NEXRAD strawman. An analytical/experimental research and development program is described to resolve the identified technical uncertainties in the NEXRAD strawman design for FAA applications. The suggested research and development program includes an operationally oriented interactive data gathering program to evaluate weather products at an ARTCC and TRACON using existing pencil beam S-band radars (e.g., similar to that at MIT) to be followed by similar evaluations in other key geographical areas (e.g., the southeast) using a transportable testbed. Both radar systems would incorporate special features to minimize the likelihood of false targets (e.g., due to obscuration and/or clutter) as well as automated display and short term prediction of hazardous weather regions for use by ATC controllers.
Summary
The Federal Aviation Administration (FAA), National Weather Service and Air Force Weather Service are currently engaged in a program to develop a next generation of weather radars (NEXRAD) capable of satisfying (to the greatest extent possible) the common weather information needs of these agencies. This report identifies the unique FAA...
READ MORE
Experimental measurements of low angle ground reflection characteristics at L- and C-bands for irregular terrain
November 1, 1982
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-107
Summary
Field measurements of terrain reflection data at L band and C band have been taken during 1980 at several test sites at Hanscom airport, Fort Devens, and Camp Edwards, Massachusetts. These field data are used to (1) characterization of the L band and C band multipath environments, (2) validation of terrain mutlipath propagation models, and (3) assessment of TRSB elevation angle estimation performance with various aperture sizes. Conventional beamsum, maximum likelihood, and maximum entropy angular power spectral estimates have been employed as a means to characterize the multipath environment. The maximum entropy angular power spectral estimate offered highest resolution of the various multipath signals. The L-band field measurement results indicated that the terrain multipath were specular reflections with a high multipath levels. L-band M/D ratios ranging from -5 dB to 1 dB were observed in a variety of terrain conditions. At C band, diffuse ground reflections were evident at some measurement sites, especially at Camp Edwards J2 range site, where small scale terrain roughness was fairly visible. However, these C-band disffuse reflections appeared to be at fairly low levels, e.g., -15 dB to -20dB relative to the direct signal. The C-band peak specular multipath levels of -10dB to -2 dB were slightly lower than those of the L-band for the same terrain geometry. The phenomenon of hte "focusing" ground reflections, i.e., more than one specular ground reflection present at the same time, was observed at both L-band and C-band. For the L-band, this generally occurred in the rolling type of terrain. However, for the C-band, this also happened in the fairly flat terrain at Hanscom airport site.
Summary
Field measurements of terrain reflection data at L band and C band have been taken during 1980 at several test sites at Hanscom airport, Fort Devens, and Camp Edwards, Massachusetts. These field data are used to (1) characterization of the L band and C band multipath environments, (2) validation of...
READ MORE
Mode S Beacon System: Functional Description (Revision B)
October 27, 1982
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-42,B
Topic:
R&D area:
R&D group:
Summary
This document provides a functional description of the Mode S Beacon System, a combined secondary surveillance radar (beacon) and ground-air-ground data link system capable of providing the aircraft surveillance and communications necessary to support ATC automation in future traffic environments. Mode S is capable of common-channel interoperation with the current ATC beacon system, and may be implemented at low user cost over an extended transition period. Mode S will provide the surveillance and commucation performance required by the ATC automation, the reliable communications needed to support data link services, and the capability of operating with a terminal or enroute, radar digitizer-equipped, ATC surveillance radar. The material contained in this document updates and expands the information presented in "DABS: A System Description", FAA-RD-74-189, November 1974 and "DABS: Functional Description," FAA-RD-80-41, April 1980.
Summary
This document provides a functional description of the Mode S Beacon System, a combined secondary surveillance radar (beacon) and ground-air-ground data link system capable of providing the aircraft surveillance and communications necessary to support ATC automation in future traffic environments. Mode S is capable of common-channel interoperation with the current...
READ MORE
TCAS I design guidelines
September 24, 1982
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-114
Topic:
R&D area:
R&D group:
Summary
A description of the FAA airborne Traffic Alert and Collision Avoidance System known as TCAS I introduces the main topic of the report: results of an investigation of simple techniques suitable for the passive and active detection of nearby aircraft by TCAS I. This is followed by a review of the measurement facilities and data used to evaluate the detection techniques. Techniques for identifying passively detected returns from potentially threatening aircraft, i.e., the rejection or "filtering out" of non-threat aircraft, are described and evaluated. Alternatives for time-sharing the 1090 MHz channel between the TCAS I transponder and the passive detector are described. A candidate passive detector is defined and its performance is evaluated using flight test data. Predictions of the performance of a low-power TCAS I based on active detection are made via link calculations and flight test measurements. A summary of results concludes the report.
Summary
A description of the FAA airborne Traffic Alert and Collision Avoidance System known as TCAS I introduces the main topic of the report: results of an investigation of simple techniques suitable for the passive and active detection of nearby aircraft by TCAS I. This is followed by a review of...
READ MORE