Publications

The lift-generated wake vortices trailing behind an aircraft present a danger to aircraft following the same or nearby path. The degree of hazard to the following aircraft depends on the nature of the wake encountered in its flight path and on the ability of the aircraft to counter its effects. This report describes the current state of understanding of the factors that influence the motion and dissipation of wake vortices. The relationships of these factors to parameters that are measurable through meteorological sensors and from a priori knowledge of the vortex generating aircraft characteristics are discussed as an aid to structuring development plans for the creation of wake vortex advisory products by the Integrated Terminal Weather System (ITWS) and by special wake vortex sensors.

A new Automatic Dependent Surveillance (ADS) system concept combining GPS satellite navigation with Mode S data communications is described. Several potential applications of this concept are presented with emphasis on surface surveillance at airports. The navigation and data link performance are analyzed. Compact ADS position formats are included. The results of the first tests at Hanscom Field demonstrating the feasibility of the spontaneous broadcast of ADS positions using Mode S messages are presented. Test aircraft, vehicles, avionics equipment and the ground system configuration are described. Avionics standards and GPS interface requirements are discussed. Multipath and airport surface coverage issues are addressed. Plans for further testing in an operational environment at Logan Airport are outlined.

Hazardous weather in the terminal area is the major cause of aviation system delays as well as a principal cause of air carrier accidents. Several systems presently under development will provide significant increases in terminal safety. However, these systems will not make a major impact on weather-induced delays in the terminal area, meet a number of the safety needs (such as information to support ground deicing decisions), or reduce the workload of the terminal controller. The Integrated Terminal Weather System (ITWS) will provide improved aviation weather information in the allocated TRACON area (up to 50 nmi from the airport) by integrating data and products from various Federal Aviation Administration (FAA) and National Weather Service (NWS) sensors and weather information systems. The data from these sources will be combined to provide a unified set of safety and planning weather products for pilots, controllers, and terminal area traffic managers. by using data from multiple sensors, ITWS can generate important new products where no individual sensor alone could generate a single, reliable product. In other instances, use of data from several sources can compensate for erroneous data from one sensor and thus improve the overall integrity of existing products. Major objectives of the ITWS program are to increase the effective airport acceptance rate in adverse weather by rpoviding information to support terminal automation systems, better terminal route planning, and wake vortex advisory services, and to reduce the need for controllers to communicate weather information to pilots via VHF voice. This report summarizes the work acocmplished during fiscal year 1992 on the development of the ITWS initial operational capability products; functional prototype design; operation of testbeds to acquire data for product development and testing; operation evaluation of products by ATC users; investigation of approaches for effective transfer of the technology to the production contractor; transfer of products to pilots via digital data links; and technical support for the ITWS documents required by the General Accounting Office (GAO).

GPS and GLONASS employ different geocentric Cartesian coordinate frames to express the positions of their satellites and, therefore, of their users. GPS uses WGS84; GLONASS, SGS85. Interest in the civil aviation community in using signals from both systems requires that a transformation between the two coordinate frames be determined. We present an estimate of the SGS85--WGS84 transformation.

An important part of the FAA Aviation Weather Development Program is a system, the Integrated Terminal Weather System (ITWS), that will acquire data from the various FAA and National Weather Service (NWS) sensors and combine these with products from other systems (e.g., NWS Weather Forecast Offices and the FAA Aviation Weather Products Generator). This wide variety of input data and products will enable the ITWS to provide a unified set of weather products for safety and planning/capacity improvement for use in the terminal area by pilots, controllers, terminal area traffic managers, airlines, airports, and terminal automation systems (e.g., Terminal Air Traffic Control Automation (TATCA) Center Tracon Advisory System (CTAS) [Andrews and Welch, 1989] and wake vortex advisory systems.

Flight crews need tiimely information about terminal weather conditions when approaching or departing airports. This paper describes a new concept in providing this information from new ground-based terminal weather sensors currently being deployed via new and existing data link systems. Currently, pilots rely on ATIS (Automatic Terminal Information System) for airport weather conditions. However, the Surface Observation (SAO) contained in the ATIS message is nominally only updated once per hour. Special observations are issued more frequently, but are difficult to keep current manually in rapidly changing conditions. The Automated Surface Observing System (ASOS) and Automated Weather Observing Systems (AWOS) are beginning to supplant manual surface observations in many locations. These automated systems offer the advantage of providing continuous , automated surface observations. However, the surface observations issued by these units lack the remarks section provided by manual observers, including such information as the location and motion of storm activity in the airport area. The shortcomings of the current ATIS system were illustrated by an incident at Kansas City International Airport (MCI) in the evening of September 8, 1989. An aircraft approaching from the west received an ATIS message indicating 10 miles visibility at the airport. However, unknown to the crew, an intense storm was approaching the airport from the East. By the time the aircraft reached the airport (about 30 minutes after the initial ATIS message was received), the visibility had dropped to 1/2 nmi, but the flight crew was not notified. The aircraft subsequently struck power lines while on final approach and was forced to make an emergency landing at an alternate airport. This example provides a vivid example of current shortcomings in the generation and dissemination of terminal weather information to the flight deck. Besides improving safety, improved access to terminal weather information would provide economic benefits by allowing more efficient flight planning and utilization of air space.

The Meteorological Data Collection and Reporting System (MDCRS) was designed for the Federal Aviation Administration (FAA) and the National Weather Service (NWS) to collect, decode, store and disseminate aircraft meteorological observations. The system, targeted primarily at improving upper air wind forecasts, was fielded in 1991.

Low level wind shear has been identified as a cause or contributing factor in a significant number of aviation accidents. Research has shown that the most dangerous type of wind shear is the microburst (Fujita, et al., 1977 and 1979). Briefly, a microburst is an intense local downdraft that results in a strong divergent outflow near the surface. The diameter of the outflow region may vary from 3 to 10 Km. Although many of these accidents were nonfatal, six of them resulted in a total of 550 lives lost. During the past 17 years, the mainstay of the effort by the Federal Aviation Administration (FAA) to provide wind shear warnings to pilots has been the Low Level Wind Shear Alert System (LLWAS). The system has been redesigned, based on extensive operational experience and new knowledge about the nature of the aviation wind shear hazard (Goff and Gramzow, 1989). In parallel development, the Terminal Doppler Weather Radar (TDWR) has provided a capable alternative for ground-based microburst detection (Turnbull, et al., 1989). Recent studies on the integration of LLWAS with TDWR have established the value of a combined TDWR/LLWAS wind shear detection system (Cole and Todd, 1993) The LLWAS system is being developed in four phases, I, II, III, and IV, which reflect the chronology of operational deployments. The original LLWAS, now called LLWAS I, was designed for the detection of frontal shears under the assumption that hazardous wind shear is associated with large-scale meteorological features (Goff and Gramzow, 1989). This system was deployed at 110 airports between 1977 and 1987. LLWAS I had no microburst detection capability and had excessive false alerts. LLWAS II was developed to reduce the false alert rate of LLWAS I and to provide a modest microburst detection capability. It is a direct response to recommendations by the National Research Council (NRS-NAS, 1983), following the 1982 microburst crash in New Orleans. This upgrade, deployed by modifying the software in LLWAS I, provided an improvement that would not suffer the delays and costs of the major construction that is required for off-airport LLWAS III sensors. These upgrades to LLWAS I were installed between 1988 and 1991. LLWAS II will be the operational wind shear detection system at many airports until the late '90s. LLWAS III was developed in response to the requirements that LLWAS have a microburst detection capability (NRS-NAS, 1983). This system was designed by a combination of computer simulation studies (Wilson and Flueck, 1986) and a successful field test of a prototype at Stapleton International Airport, Denver in Augist 1987 (Smythe, et al., 1989 and Wilson et al., 1991). LLWAS III combines a dense sensor network and a sophisticated Wind Shear/Microburst (WSMB) detection algoritohm to provide a substantial microburst detection capability. The prototype LLWAS III has continued to operate at Stapleton International Airport, Denver since 1987 and has been credited with the "save" of a commercial airliner on July 8, 1989. Nine LLWAS IIIs are being installed this year. LLWAS IV will be deployed at 83 airports in the late '90s. The LLWAS IV wind shear and microburst detection algorithms will be identical to LLWAS III. This system features a full hardware upgrade. Major imporvements include an ice-free sensor and hardware that is more reliable and maintainable. This report provides an evaluation of the effectiveness of LLWAS II and LLWAS III. The TDWR operational test bed at Orlando International Airport, Orlando (MCO) provides a unique data set for this evaluation. This test-bed features data from a 14-sensor LLWAS, the prototype TDWR, FL-2C, operated by MIT/LL, and the University of North Dakota meteorolgical radar (UND). Data from this test bed in the summers of 1991 and 1992 are used to provide an evaluation of LLWAS II and LLWAS III. Since LLWAS IV uses the same wind shear detection algorithm, it is expected that LLWAS III and LLWAS IV will have comparable wind shear detection capabilities.

Inadvertent engine ingestion of volcanic ash has caused expensive damage to a number of aircraft recently and could have caused accidents in at least two cases [Casadevall, 1993]. Consequently, there is great interest in a real-time air traffic control (ATC) volcanic ash advisory system which could provide timely warnings of operationally significant ash concentrations to planes in flight as well as information for flight planning. The current system (see figure 1) is characterized by non-automatic determination of ash eruption characteristics (especially altitudes) with trajectory analysis based on the National Meteorological Center (NMC) forecast winds being used to provide warnings of future locations. SIGNETS and Airport Weather Advisories are the principal means of providing information on the ash locations to pilots and controllers. After one to three days, volcanic ask from Alaska can be transported over major portions of the US aviation system (figure 2) [Heffter, et al. 1990]. The operational use of the ash trajectory predictions which do not provide information on hazard associated with the ask density has resulted in more frequent disruption of air traffic. The most recent example was an incident on 19 September 1992 where a 17 September eruption from Mt. Spurr in Alaska resulted in a significant disruption of air traffic in the Upper Midwest. A workshop in Washington, DC [Machol, 1993] discussed many of these issues associated with the Spurr disruption and the operational response to ash clouds which had been drifting for several days.

The Integrated Terminal Weather System (ITWS) is an FAA-sponsored program (Sankey, 1993; Ducot, 1993) whose objective is to acquire data and products from a variety of weather sensors, integrate the data and create aviation weather products for users, such as Air Traffic (AT) controllers and traffic managers, pilots, and airline and airport operations managers. The goal of ITWS is to increase capacity at airports, reduce controller workload, and enhance safety. The objective of the ITWS Storm Cell Information (StoCel) and Weather Impacted Airspace (WIA) Detection products is to identify storm cell characteristics (echo top, echo bottom, presence of heavy rain, hail, etc.) and airspace that pilots are likely to avoid because it contains hazardous weather. The StoCel/WIA products rely on the integration of pencil-beam data and products and Air Surveillance Radar (ASR-9) Weather Channel data. ASR-9 radars are useful because they cover the entire airspace of interest, perform a volume update at roughly 30-second intervals, and will be the weather representation most widely available to the Air Traffic Control (ATC) community. On the other hand, the ASR-9 has a 4.8° fan beam which results in a vertical integration over the depth of a storm, so information on the vertical structure of storms is lost. In addition, the current ASR-9 Weather Channel may produce false weather regions during ducting or anomalous propagation (AP) conditions. Nearby WSR-88D radars also cover the entire airspace of interest and provide indications of storm vertical structure. However, the volume update rate is typically on the order of 5 to 10 minutes, depending on the scanning strategy. TDWR radars perform volume updates about every 2.5 to 3 minutes, but perform sector scans that do not cover the entire airspace. Integration of the data from these various sensors produces a product that is superior to a product based on any single sensor. Field tests of components of this algorithm were conducted at Dallas-Ft. Worth (DFW) and Orlando (MCO) International Airports during the summer of 1993. The objectives of these tests are to evaluate the technical performance of the algorithm and the validate the operational concept. This paper will describe the algorithm, and discuss the operational concept and functional requirements for the product. A summary of the results and experiences of the Summer 1993 field tests, and a preliminary evaluation of the performance of the algorithm based on off-line and real-time tests will be provided at the conference.