Publications

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.

This report documents the study of the lateral positions of aircraft on Instrument Landing System (ILS) approaches during the Memphis, Tennessee, Precision Runway Monitor (PRM) demonstration. The PRM is an advanced radar monitoring system that improved the arrival capacity of closely spaced parallel runways in poor weather conditions. The results of this study are used to assist in determining the minimum runway spacing that will he authorized for PRM. The objective of this study was to quantify the lateral character of ILS arrivals and the consequent impact on independent simultaneous ILS arrival to closely spaced parallel runways. The sensitivity of the arriving aircrafts' lateral positions to different variables such as visibility, wind runway, aircraft type, autopilot performance, and localizer beam width was determined. Also, the Memphis arrival data were compared to FAA Technical Center Chicago O'Hare approach data. The analysis was primarily based on surveillance reports of 4,000 ILS arrivals into Memphis International Airport, collected with the PRM AMPS sensor (ATCRBS Monopulse Processing System). A major result of the study was that lateral aircraft positions will not hamper independent arrivals to parallel runways spaced 3,400 feet apart, but will impede operations at 3,000 feet or smaller unless approach modifications are introduced. Lateral deviations were found to be most sensitive to reduced visibility and certain autopilots. Lateral deviations were also found to be somewhat more at Memphis relative to Chicago O'Hare. Recommendations for further data analysis and collection are discussed.

Researchers at MIT's Lincoln Laboratory reviewed GLONASS developments during 1992, focusing on the requirements of civil aviation and the issues related to position estimation. The results show that the overall performance remains substantially the same as observed in 1991.

Dependent Surveillance and the Mode S beacon radar. The result is an integrated concept for seamless surveillance and data link that permita equipped aircraft to participate in ADS or beacon ground environmenta. This offers many possibilities for transition from a beacon to an ADS based environment. The ADS-Mode S concept in baaed on use of the Mode S squitter. The Mode S squitter is a spontaneous, periodic (once per second) 56-bit Mode S broadcast containing the Mode S 24-bit address. This broadcast is provided by all Mode S transponders and in used by the Traffic Alert and Collision Avoidance System (TCAS) to acquire Mode S equipped aircraft. For ADS-Mode S use, this squitter broadcast is extended to 112 bits to provide for the transmission of a 56-bit ABS message field. The ADS squitter is transmitted in addition to the current TCAS squitter in order to maintain compatibility with current TCAS equipment. This paper defines the ADS-Mode S concept, describes its principal surveillance and data link applications and provides estimates of expected performance.

A receiver autonomous integrity monitoring (RAIM) algorithm is proposed, and used to analyze the integrity monitoring capabilities of potential sole-means (or stand-alone) systems based on integrated use of GPS and GLONASS, GPS supplemented with a geostationary overlay, and enhanced GPS constellations. As in the other RAIM algorithms, the idea is to take advantage of the redundant measurements. Our focus, however, is on the quality of the position estimate, rather than on diagnosing whether the system is working as intended. The proposed approach uses the redundant measurements to generate a position estimate and a measure of its quality. The latter, called integrity level, is defined as an upper bound on the position error. The estimation of the integrity level is the main innovation in the proposed scheme. The RAIM algorithm is tailored to an abundant redundancy of the measurements, and addresses the following issue: Given a snapshot of the pseudo range measurements, one of which may be in error, can we compute a position estimate that can be shown with high confidence to meet the user's accuracy requirement?

Increased air travel in recent years has resulted in a steady increase in the number and duration of flight delays. In an attempt to increase airport capacity, MIT Lincoln Laboratory, under the sponsorship of the Federal Aviation Administration (FAA), has supported the development of a Precision Runway Monitor (PRM). The PRM is an advanced radar monitoring system designed to increase utilization of closely-spaced, multiple, parallel runways during adverse weather conditions. The PRM consists of radar which has higher accuracy and a faster update interval than the current system, and a high resolution, color display that informs the Monitor Controller of the occurrence of hazardous flight path deviations by means of automated visual and vocal warning alerts. Studies of air traffic controller reaction to the PRM were conducted at Memphis Airport and Raleigh-Durham Airport in order to evaluate system effectiveness and to assess the effects of key variables on controller reaction time. This paper documents the results of the controller studies conducted at Memphis by MIT Lincoln Laboratory. The testing consisted of the presentation of real-time simulations, and measurement of air traffic controllers were surveyed regarding the acceptability of the PRM.

This paper describes an experimental system for cockpit display of Terminal Doppler Weather Radar (TDWR) wind shear warnings. The TDWR is a ground-based system for detecting wind shear hazards that pose a threat to aviation, During the Summer of 1990, wind shear warnings generated by the Lincoln-operated TDWR testbed radar at Orlando, Florida were transmitted in real-time to a research aircraft performing microburst penetrations. This test marks a milestone as being the first time that TDWR wind shear warnings were successfully transmitted and displayed in an aircraft in real-time. This effort was supported by NASA Langley Research Center as part of a program to investigate techniques for integrating airborne and ground-based wind shear information for aircrew alerting. The three main goals for 1990 were 1) to conduct microburst penetrations with an instrumented aircraft, 2) to compare a hazard estimate called the F factor (Bowles, 1990) for airborne and TDWR data, and 3) demonstrate real-time data link and cockpit display of TDWR warnings. All three of these goals were successfully carried out. The research aircraft, a Cessna Citation II operated by the University of North Dakota (UND) Center for Aerospace Sciences conducted over 80 microburst penetrations in Orlando over a six week period with TDWR testbed radar surveillance. Initial post-processing analysis in comparing the aircraft and TDWR F factors has begun. The cockpit display system was operated during the latter part of the flight test period, and proved useful in aiding the Citation crew in locating microburst and gust front events. There were three main objectives in the development of the cockpit display system. First, the real-time display was intended to aid the Citation crew in locating microburst and gust front events. This capability was desired both to aid the crew in locating events to penetrate, and to improve safety by providing a better information about the location of the wind shear events. A second objective was to demonstrate the feasibility of transmitting TDWR wind shear warnings to aircraft in real-time. This demonstration is an important element in the eventual development of an integrated aircrew alerting procedure incorporating both airborne and ground-based wind shear information. This study marks the first successful demonstration of real-time transmission of TDWR wind-shear warnings to an aircraft in flight. A third objective was to demonstrate the desirability of transmitting TDWR wind shear warnings to aircraft in real-time. Currently, the TDWR provides these warnings to controllers as textual messages, which are then relayed to pilots via voice communications. The TDWR also includes graphical displays of wind shear and precipitation products but these are only provided currently to the Tower and TRACON supervisors. A potential use of Mod S Data Link (or other ground-to-air data link systems) is to provide TDWR wind shear warnings directly to pilots, Automatic delivery of TDWR wind shear warnings potentially result in decreased controller workload and improved pilot information. Mode S Data Link is currently planned to provide textual wind shear warnings only. However, studies by Wanke and Hansman (1990) show that pilots substantially prefer graphical presentation of wind shear warnings over textual presentation. The paper will first describe the organization of the system, including the process of generating the display messages in the TDWR testbed and data linking them to the aircraft. Second, the display format and operation of the cockpit display will be described. Next, an example of the operational use of the cockpit display will be presented, along with initial F factor results. Finally, the paper will conclude with a summary and plans for future work.

Pursuant to a bilateral agreement signed in 1988, both US and USSR are currently in the process of examining integrated use of GPS and GLONASS for sole-means civil aviation navigation. This paper presents results from the initial phase of a program underway at MIT Lincoln Laboratory to support this effort. Specifically, we present results on satellite coverage and quality of the range measurements from GPS and GLONASS. The coverage results highlight the extent to which each system alone falls short of providing a self-contained system integrity check. In integrated use, however, there are enough redundant measurements to make receiver autonomous integrity monitoring (RAIM) practical. The data quality results are based on statistical analysis of the range measurements from GPS, at various levels of selective availability (SA), collected over extended periods. We present empirical cumulative distribution function of the range error, and RMS value of its component, defined as the 'effective' range error, relevant to position estimation. These results are used to project the position estimation. These results are used to project the position estimation accuracy achievable globally with GPS, when operational. Comparable results for GLONASS are being developed. The coverage and data quality results together provide a basis for development of the navigation and RAIM algorithms for the integrated use. This will be addressed in the next phase of the program. The important considerations in the design of these algorithms, including the differences in the reference systems for space and time employed by the two systems, are briefly reviewed.

This document provides a functional overview 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 over an extended transition period. Mode S will provide the surveillance and communication performance required by 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 serves as an introduction to the more detailed information contained in "Mode S Beacon System: Functional Description." DOT/FAA/PM-86/19, 29 August 1986.

We have developed a computer program that automates rudimentary air traffic control (ATC) planning and decision-making functions. The ability to plan, make decisions, and act on them makes this experimental program qualitatively different from the more clerical ATC software currently in use. Encouraging results were obtained from tests involving simple scenarios used to train air traffic controllers.