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The requirement for a Discrete Address Beacon System (DABS) was highlighted by the Department of Transportation Air Traffic Control Advisory Committee to provide improved surveillance and ground-air communications in support of air traffic control automation. This document presents a technical development plan for such a system; this plan was developed in close colaboration with FAA personnel in the Office of System Engineering Management and the Systems Research and Development Service. The DABS Technical Development Plan identifies the critical issues and technical options, presents a program for their resolution, followed by the development and test of a feasibility model of the system, and suggests a management structure to coordinate and carry out the many tasks involved in the implementation of the plan.

Runway 4-R at Logan Airport cannot now be used with full effectiveness because of tall ships passing through the runway's approach clearance zone. The key to utilizing the full length of Runway 4-R at Logan Airport for longer periods of time is to establish a channel surveillance system that would allow both ships and aircraft to safely share the air space above the channel. A laser-gate alarm system across the channel, at the approaches to the clearance zone, has been proposed for alerting tower operators to the passage of tall ships. A surface detection (ASDE) radar would maintain surveillance during passage of the ship. This ship detection concept was explored and developed within Lincoln Laboratory with cooperation and encouragement from MASSPORT and members of the FAA Burlington office. This report completes the study and field measurements program, sponsored by MASSPORT, for the evaluation of an experimental laser-gate ship-detection system. The study and field Measurements program supports the conclusion that a reliable, fail-safe, laser-gate system can be designed, installed and operated to detect ships entering the clearance zone of Runway 4-R. System feasibility has been established for visual meteorological conditions both day and night. Ship masts of 1-inch diameter at speeds up to 15 knots can be reliably detected under visual meteorological conditions for laser-path-lengths up to 3000 feet. An operational system can be de signed for visibility ranges down to 1/4 mile with laser path lengths of 1800 feet and with the expectation that performance would be substantially the same. The system design is amenable to various balance s between mast-height margin, VFR runway length and displaced threshold duty factor. In the recommended system, the laser-gates may be placed at a height of about 45 feet, which will detect ships tall enough to intrude the clearance zone, while allowing small craft to pass without alarm. This configuration would permit full use of the 10,000-foot runway, while providing an 11- to 21-foot margin for narrow objects which might be less than one-inch wide at the laser-gate height. Analysis of ship traffic indicates that the use of a laser-gate system at the 40- to 50-foot height under nighttime visual meteorological conditions would permit the full-length use of Runway 4-R throughout the night except for the times of passage of 4 to 7 ships on the average. During the early nighttime hours having the heaviest nighttime air traffic, less than one ship alarm would be expected on the average. Operational system design relations are presented which permit a spectrum of design options. False alarms would be at a negligible level in a multiple-parallel-beam operational system and an infrequent false alarm could be checked with the ASDE radar. Recommendations are made for a program which would lead to an operational laser-gate ship-detection system fully integrated with airport facilities and activities. This program would initially involve the design and installation of one prototype laser-gate, which would be integrated with the airport surface detection radar to establish optimum system configuration, develop operational procedures, and to test more extensively the reliability and fail-safe features of the surveillance system. Upon satisfactory completion of the tests and studies, the prototype could be used as one element of an operational system. This report contains in Appendix B engineering guidelines for the recommended prototype laser-gate system.

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To develop plans for a viable ATC system over the next 25 years a whole spectrum of studies can be conducted, each concerned with a different time frame. The spectrum, when laid out over time, is bracketed by two extreme cases. 1. One extreme is analysis of the present ATC system to identify its shortcomings, followed by synthesis studies to identify evolutionary ways of overcoming these shortcomings. 2. At the other extreme one can study the ATC system sufficiently far into the future that decisions need not be constrained by existing equipment, airspace utilization and procedures. Between these two extremes are other studies concerned with developing plans for intermediate time frames. To be effective, study (1) must be done immediately. Study (2) should precede many of the studies for intermediate time frames since the results of study (2) should be available to influence what is done in intervening periods. In this report we view the Fourth Generation Concept Formulation Study as study (2). Thus the results are not strongly influenced by present day equipment and are influenced by present airspace utilization and procedures only where they appear to be as good or better than other ways of operating the system. The ATC system is designed to fulfill certain needs of the nation. To satisfy those needs the ATC system must achieve specific objectives. The major objective of the system is to provide safe, expeditious flow of air traffic at reasonable cost. It is generally accepted that to achieve this objective certain functions in the area of surveillance, navigation, and communication must be performed and that considerable data processing in the ATC system is required. The examination of ways of achieving various performance levels of these functions is the subject of concept formulation in -- the areas of surveillance, navigation, communication and data processing. Given that the surveillance, communication, and navigation functions are performed, there are other functions which are required in order to achieve the objectives of the ATC system. These functions, which include flow control, metering, sequencing, spacing, conformance and hazard monitoring, and conflict and hazard resolution make up the control aspects of the ATC system. In terms of the operation of the ATC system the surveillance, communication and navigation functions must be performed if the control functions are to be performed. In terms of the design of the system, however, the surveillance, communication, and navigation functions cannot be specified in detail until the required control functions are determined in detail. Thus, studies in the control area must be performed in a timely manner in order to insure that studies in the other areas will be conducted at a high level of efficiency. Control studies seek to determine the detailed characteristics of the functions which will be performed to achieve the objectives of the ATC system.

An airborne traffic situation display system which could be used as an adjunct to the evolving National Airspace System/Automatic Radar Control Terminal System (NAS/ARTS) is described. In the proposed system, a contemporary realization of an old concept, the NAS/ARTS data are broadcast. A small digital computer in an aircraft then selects from the message stream the data on its own aircraft, nearby aircraft, and a local map. These data, plus aircraft heading data from a directional gyro, are used to generate a situation display that can be aircraft-centered and heading-oriented.

The theories describing the effects of the troposphere on satellite communication systems operating in the microwave region are reviewed. The results of computations based upon the theories and atmospheric models are presented and compared with available experimental data. From the model computations it is seen that rain causes the major propagation problems for the frequency bands allocated to or proposed for allocation to the satellite communications service. Two effects are dominant: attenuation due to rainfall along the line-of-sight and interference between two systems operating at the same frequency and beyond each other's radio horizon due to rain scatter. The methods for calculating the magnitude of the effects of rain given the spatial distribution of rainfall intensity are available. The statistical data required for the prediction of the spatial distribution of rainfall intensity are not available.

The data recording of storm information as detected by a weather radar has been customarily made on photographic film. Research radars and an occasional U. S. Weather Bureau radar are fitted with scope cameras to record the radar plan position indicator (PPI) display. Over the past 15 years a large sample of weather radar data has been accumulated in this fashion. The photographic technique provides an easy, quick, and inexpensive way to record weather radar data. The major drawback of this technique is data reduction. Information on storm shape, size, and intensity is normally extracted from the photographic images by hand. This means that only the most interesting aspects of individual storms are analyzed and the vast majority of the collected radar data is not analyzed. A vast amount of climatological information could be obtained from the existing store of weather radar data if an automatic technique of data retrieval were available. The first part of this report describes the use of a computer-controlled programmable film reader to process weather radar PPI photographs to obtain digital maps of rainfall intensity for use in climatological studies.

By the mid-1970's, the evolving NAS/ARTS ground environment will provide the air traffic controllers with high quality computer-processed traffic situation displays. We believe it would be useful, particularly in busy terminal areas, to display some of this data in the cockpit. Systems with this objective have been constructed and flight tested at least 3 times during the past 25 years, but these earlier systems could not benefit from: 1) a source of computer-processed data of the quality to be available from NAS/ARTS; 2) aircraft altitude information; 3) contemporary digital data link techniques; and 4) airborne equipment capable of automatically selecting and displaying only information relevant to a particular airplane. It is believed that an effective cockpit display would permit pilots, under IFR conditions, to retain some of the station-keeping and navigation functions they perform during VFR conditions and thereby improve the efficiency of terminal area operation. The goals of the proposed program are: a) to evaluate the effectiveness of this class of system in reducing pilot and controller work loads, and b) to determine its potential for expediting traffic flow in busy terminal areas. A simulated cockpit display has been developed and experienced pilots and controllers who have "flown" it have endorsed enthusiastically the desirability of evaluating this class of system in an operational environment.

Radar measurements of thin turbulent layers in the clear atmosphere have been extensively reported in the literature and have recently been summarized by Hardy and Katz (1969). The majority of the thin turbulent layer detections reported have been for layers in the lower troposphere. Using the high power radar facilities at Wallops Island, Atlas, et al (1966) have detected layers at heights up to the tropopause. In this paper, layer detections at heights above the tropopause are discussed. The detection of layers in the lower 10 km of the stratosphere is made possible by using a radar system which has approximately 10 dB more sensitivity than the Wallops Island radars for the detection of turbulent layers. The program of radar measurements of thin turbulent layers was undertaken to provide basic information about the structure of scattering layers in the upper troposphere and lower stratosphere for use in the prediction of troposcatter field strengths. The radar measurements were accompanied by radiosonde soundings. For a limited series of measurements, a U-2 aircraft was also used to probe for turbulent layers.

Simultaneous measurements were made of the backscatter cross section and the bistatic scattering cross section of rain and thin turbulent layers. The radar measurements were made at a frequency of 1.3 GHz using the Millstone Hill Radar. The bistatic scattering measurements were made using CW transmission at 7.7 GHz with a 145-km separation between transmitter and receiver. The receive station was the Westford Communication Terminal with a 60-foot antenna. The transmitter was van-mounted and used either a 6-foot antenna or a standard gain horn. Stable frequency sources were used to allow Doppler shift measurements on the bistatic scattering link. The measurements were made by fixing the pointing angles of the transmit antenna and scanning both the receive antenna and the radar to investigate the dependence of the scattered signals both on scattering angle and on the location of the scatterers. The measurements of the scattering cross section of the thin turbulent layers were made in the near forward direction, the measurements of rain at a large number of scattering angles. System sensitivities allowed the measurement of scattering from turbulent layers at a 10-km height with a thickness, Cn^2 product of 10^-13 N^2 m^1/3 and from rain with a 0.1 mm/hr. rate. Comparisons between the radar and bistatic measurements were in good agreement with the appropriate scattering theories.