The User Request Evaluation Tool, or URET, was developed at MITRE's Center for Advanced Aviation System Development (CAASD) to assist controllers with timely detection and resolution of predicted problems. By helping to manage workload and to allow more strategic planning, URET helps the system support a greater number of user-preferred flight profiles, increased user flexibility, and increased system capacity while maintaining the level of safety.  The prototype version of URET has been in daily use operation at Indianapolis and Memphis Air Route Traffic Control Centers (ARTCCs) since November 1997. Over 800 operational personnel are trained in URET operation at the two facilities. Both facilities are operating URET 22 hours a day, seven days a week. In early May 2001, URET will reach a combined sector operational use at the two facilities of 1,000,000 hours. The Free Flight Phase 1 version of URET is on schedule for deployment to Indianapolis, Memphis and five additional ARTCCs in late 2001 and early 2002.
URET processes real-time flight plan and track data with site adaptation, aircraft performance characteristics, and temperature and wind data to build four-dimensional flight profiles, or trajectories, for all flights within a facility or inbound to it. When a conflict (i.e., possible loss of separation) is detected, URET determines which sector to notify and displays an alert to that sector up to 20 minutes prior to the conflict. This longer look-ahead gives controllers more time for strategic planning. In addition to conflict detection capabilities, URET has introduced a new controller interface that supports flight data management and task prioritization at the sector using both textual and graphic displays. Background Each year, demand for air traffic services in the United States increases. The diverse community of airspace users (air carriers, general aviation, and military) has clearly expressed concerns that the current system is unable to keep pace with growing demand. They have articulated the need for continuous improvement in system services to keep pace with the desire for more flexible and cost-efficient operations. Current air traffic control (ATC) operations in many areas of the country remain highly structured and restrictive. This structured and restrictive system helps air traffic controllers manage their workload within the constraints imposed by system safety requirements. It is, however, in conflict with the users' need for less structure and more flexibility¿¿the freedom to fly more cost-effective routes and altitudes from origin to destination. In January 1995, the Report of the RTCA Board of Director's Select Committee on Free Flight was published as a result of the collaboration among government, industry, and the user community. This included defining the procedures, system architecture, and transition for Free Flight. Free Flight is a concept of air traffic management that permits pilots and controllers to share information and work together to manage air traffic from pre-flight through arrival without compromising safety. The use of on-board flight management systems, enhanced cockpit situation awareness, satellite-based navigation, and decision support systems coupled with procedures will permit pilots to fly more cost effective flight paths between takeoff and landing. With Free Flight, pilots will not have to fly routes structured around ground-based navigation systems. In order to achieve the benefits of Free Flight as early as possible, an evolutionary approach to developing and deploying several of the automation and decision making tools will lead to the final architecture. This incremental approach will bring user benefits to the system sooner, and it will allow the FAA to modernize the NAS gradually as a set of building blocks. As part of the evolutionary approach to achieving Free Flight, the RTCA has made recommendations for the priority implementation of several decision support system capabilities. (Link to RTCA documents on Free Flight) The first set of these capabilities is organized into Free Flight Phase 1 (FFP1). One of the capabilities to be deployed in FFP1 is a Conflict Probe, specifically based on the User Request Evaluation Tool (URET) prototype. The URET prototype represents the culmination of many years of analysis by the FAA and the MITRE Corporation's Center for Advanced Aviation System Development (CAASD). It is based largely on an earlier prototype, the Automated En Route Air Traffic Control (AERA) system, which was developed through close collaboration with field controllers. In January 1996, field evaluations of URET began at the Indianapolis Air Route Traffic Control Center (ARTCC). At first, the capabilities were somewhat limited, with the primary intention of enabling better responsiveness to user requests. Since then, its capabilities have significantly expanded and evolved. Today, it provides a broad set of conflict detection, resolution, and flight data management capabilities. URET Capabilities URET combines real-time flight plan and radar track data with site adaptation, aircraft performance characteristics, and winds and temperatures aloft to construct four-dimensional flight profiles, or trajectories, for pre-departure and active flights. For active flights, it also adapts itself to the observed behavior of the aircraft, dynamically adjusting predicted speeds, climb rates, and descent rates based on the performance of each individual flight as it is tracked through en route airspace. URET uses its predicted trajectories to continuously detect potential aircraft conflicts up to 20 minutes into the future and to provide strategic notification to the appropriate sector. Trajectories are also the basis for the system's trial planning capability. Trial planning allows the controller to check a desired flight plan amendment for potential conflicts before a clearance is issued. The controller can then construct the amendment from that trial plan with the click of a button. The system enables expeditious coordination of these plans and amendments among sectors and facilities with its auto-coordination function. The controller interface to these detection and resolution capabilities supports flight data management and task prioritization using both text and graphic displays. The text-based Aircraft List and Plans Display help manage current flight plan information, trial plan information, and conflict data. The Graphic Plan Display provides for a graphic view of aircraft routes and altitudes, predicted conflicts, and results of trial plan resolutions. The point-and-click interface enables quick access to system functions and entry of flight plan amendments. A key part of the URET infrastructure is its Interfacility, or IFA, capability. In an IFA mode of operation, the systems in neighboring en route facilities exchange critical flight and track data. This interfacility data exchange significantly improves the quality of the information used by each individual system, thus reducing uncertainties in its predictions, enhancing its look-ahead, and improving overall controller situational awareness at the sector. Prototype Status Since the first URET system was installed at the Indianapolis ARTCC in January 1996, it has been upgraded and evaluated several times. In June 1997, the system was installed in a second facility, the Memphis ARTCC. And by November 1997, both facilities began to use the system on a daily basis.  System capabilities are developed using an evolutionary model. At each step of its evolution, enhanced capabilities are introduced to a team of controllers at each facility and evaluated under simulated and live traffic conditions. If the capabilities are found to be operationally useful and acceptable, the daily-use system is upgraded. A process is also in place for the facilities to provide feedback from daily-use operations. The system evolves as feedback from its use is received and understood in the context of the operational concept upon which it is based¿updates are planned and implemented accordingly.
URET continues in daily use at the Indianapolis and Memphis ARTCCs to evaluate the Conflict Probe capabilities for operational acceptability and suitability. Over 800 operational personnel (controllers, supervisors, traffic management specialists, etc.) are trained in the use of URET at the Indianapolis and Memphis ARTCCs. Both facilities are operating 22 hours a day, 7 days a week. In early May 2001, URET will reach a combined sector operational use at the two facilities of 1,000,000 hours. The use of URET at the two facilities has achieved some near-term benefits. More than 20 altitude restrictions have been lifted allowing aircraft to fly higher, more efficient altitudes longer. It is estimated that lifting these restrictions will save almost 1 million gallons of fuel annually. URET has allowed the facilities to grant more direct clearances to flights in their airspace. The overall analysis to date shows an average savings of more than 0.5 mile per flight for every flight going through ZID and ZME airspace. This translates into a monthly economic benefit of approximately $1.5 million for both ARTCCs combined. Core Capability Limited Deployment The NAS Modernization Task Force, at their January 1998 meeting, suggested a way to evolve the early Free Flight capabilities and called it Core Capability Limited Deployment (CCLD). The first step in the evolution of capabilities is called Free Flight Phase 1 (FFP1). Conflict Probe capabilities are currently on schedule for deployment to Indianapolis, Memphis and five additional ARTCCs in late 2001 and early 2002. The seven sites are shown in Figure 1. Table 1 defines the functionality for Build 1 scheduled for 2001 and Build 2 scheduled for 2002. Build 1 includes the basic functions in the URET prototype being used at Indianapolis and Memphis at the end of 1999. Build 2 contains the functions deemed necessary for operational acceptability at all seven FFP1 facilities. 
Figure 1. URET FFP1 Implementation Sites
In addition to the increased functionality defined in the builds, incremental software upgrades are to be scheduled every six months to address bug fixes and enhancements based on field input and software problem reports. Currently, field input from Indianapolis and Memphis are collected into discrepancy reports (DRs), which are the basis for developing requirements for enhancements. The DR process will be expanded to include all seven sites to define the incremental upgrades.
BUILD 1 - Basic Conflict Probe
- Functions (Probing of Plans and Trial Plans
- 2-Way Host Interface
- Interfacility
- "Red Route" Processing
- Arrival Stream Filters
- Airspace Activation
- Automatic Resectorization
- Supervisory Capability
- Utilizes Existing DSR M&C. SAR, DR&A
| BUILD 2 - Aircraft List Direct Manipulation and Sorting
- Stop Probe
- Heading and Speed from Host
- Special Coding for Point Outs
- ATC Preferred Route Processing
| | Table 1. Conflict Probe Build Functional Capabilities | At the time of deployment of FFP1, en route ATC automation will consist of a centralized Host Computer System (HCS) that processes flight plan and radar data, and provides the backbone for automated en route operations. Networked to the HCS will be a set of display systems, (stationed at the sectors), known as the Display System Replacement (DSR) or simply the Display System (DS). The Radar Controller (R-Controller) position contains a 20x20 inch display and associated display hardware, and communications hardware used to display radar and aircraft position information. The Radar Associate Controller (D-Controller) position will consist of a 20" flat panel on an articulating arm, flight strip bays, and communications support. This position will provide the D-Controller with the ability to display conflict probe data and interact with the URET system, to receive and send messages to the HCS, and to manually manipulate and mark paper flight strips. As part of Conflict Probe deployment, a computer complex of processors will be networked to the HCS and to the DS D-Controller consoles. The network will include interfaces to the Weather and Radar Processor (WARP) for weather data, to Monitor and Control (M&C) positions for system support, and to DS Dynamic Simulation (DYSIM) positions to be used for training and evaluation. The Conflict Probe computer complex will also be connected to an external network router for communications with Conflict Probe systems at neighboring ARTCCs (See Figure 2). 
Figure 2. URET CCLD System Interface
Operational Benefits Controllers agree that URET capabilities provide many operational advantages. For example, consistently longer problem-identification lead times enhance safety; the problem prediction capabilities do not degrade as traffic complexity and volume increase; and reliance on flight strip marking and manipulation are reduced. In addition, there is less uncertainty about potential conflict situations, and confidence in the long-term effects of flight plan amendments increases. These advantages are expected to translate into benefits for airspace users as well. Reduction in separation uncertainty will result in less frequent maneuvering of aircraft. The use of the tool is expected to allow users to operate safely without some of the route, altitude, and speed restrictions currently in place. Today, preferred IFR high altitude routes provide structure and predictability in the ATC system, but do not flexibly account for varying wind conditions. Flying more wind-optimal routes would result in a significant reduction in flight time and fuel savings. Safety Although there has not been a mid-air collision between two aircraft under active radar control in over thirty years in the US, separation standards violations (simultaneous loss of both horizontal and vertical separation) do occur. There is a growing concern that with increasing traffic levels these operational errors may occur more frequently and that the risk of a mid-air collision is increasing. Several studies conducted for the FAA in recent years have identified the causes of most operational errors. Many operational errors occur because the controller fails to predict conflicts with sufficient time to avoid a separation violation. A controller's ability to detect a conflict is limited by his ability to accurately mentally project the future positions of the aircraft. This ability may also be affected by other tasks the controller is performing. Unlike the controller, URET is able to continuously check for conflicts independent of traffic volume or complexity. In addition, URET continues to provide information about the conflict until it is resolved, providing in a sense a constant reminder of the situation that needs to be addressed. Finally URET is able to provide a controller with sufficient warning time to allow for controller resolution before a violation occurs. Over the last several years, 37 operational errors occurring in Indianapolis and Memphis Centers have been analyzed. These analyses show that URET provides an average warning time of seven minutes before the loss of separation. Operational errors also occur when the controller successfully detects the conflict, but fails to take sufficient action to prevent a separation violation. Using the URET trial planning capability, the controller will be able to see if the proposed action will be sufficient and whether it will create any additional conflicts before issuing a clearance to the aircraft. There were also operational errors when the controller gave an ill-advised clearance that put an aircraft into an immediate conflict. By using the URET trial planning function, the controller will be warned of this possibility. The Department of Transportation Inspector General Office for auditing has recently examined the success of efforts to reduce operational errors (DOT, 2000). Much of this examination focused on the five air traffic control centers (Washington, Cleveland, New York, Indianapolis, and Chicago) that have traditionally had the highest number of operational errors, these centers are also five of the busiest six centers. Of the five centers, Indianapolis was the only one that succeeded in reducing its error rate between 1999 and 2000. In addition to the use of URET during this period, the center enhanced their quality assurance and training programs. Further analysis will be required to determine how URET contributed to this improvement. FAA Productivity The productivity gains and reduction in sector workload that URET will provide sector control teams are critical for the effective use of the tool. Relief from routine tasks and more efficient management of sector workload are essential aspects of URET that create the opportunity to carry out the strategic planning tasks that will achieve user benefits. URET is intended to be used as the primary source of flight data for the sector. The flight trajectory is a more accurate model of an aircraft's predicted flight path than what is presented on a paper strip. The trajectory is continually adjusted using Host track information, wind and temperature. These changes are automatically made to the displayed information. Conflict probe and trial plan results generated by URET provide new, highly accurate, continuously updated future situation awareness data. This relieves the controller from performing routine, recurring and often time-consuming manual calculations to predict and compare future positions of aircraft. Currently, all sectors in the Indianapolis and Memphis ARTCCs are using URET whenever URET is available. The conflicts detected by URET and displayed to controllers have proven to be accurate during the use of the URET prototype in the Indianapolis and Memphis ARTCCs. Controllers are encouraged to provide Discrepancy Reports (DRs) when they believe the system is not performing properly based on their training or when they have improvements to increase usability. Each DR that describes a possible false alert or missed alert is analyzed in detail for its probable cause. "False alerts" occur when the conflict probe warns a controller of a potential conflict unnecessarily. Operational experience at the Indianapolis and Memphis ARTCCs has demonstrated that very few, less than 3% of the DRs are "false alert" DRs and none have been filed since July 2000. "Missed alerts" occur when the conflict probe fails to warn a controller of a potential conflict. Approximately 14 percent of the DRs are missed alert DRs. When these missed alert reports were analyzed, in most instances, URET identified the conflict appropriately. However, URET notified a controller in another sector than was expected, or the displayed information was later than expected. In part, the issue of "missed alerts" is a training issue in the use of the tool. Supplemental training material on condlict notification logic were delivered to the Indianapolis and Memphis ARTCCs to assist in continuing training. However, each DR continues to be tracked and analyzed when reported. User Cost Savings Many route and altitude restrictions have been applied over the years as a means of providing structure to aid the controllers in manually handling growing levels of air traffic. Taken together these restrictions have caused users to burn more fuel and to fly longer than if they could fly unrestricted. Since URET provides an effective conflict probe that enables air traffic controllers to anticipate when and where conflicts will occur, the need to restrict traffic to predictable and potentially undesirable patterns by imposing restrictions should be minimized. Several recent studies have analyzed the potential impact of lifting restrictions nationwide. These studies estimated that the benefits of lifting these restrictions would amount to several hundred million dollars per year. Although the full level of benefits will not be achieved until URET is available nationwide, data collection and evaluations at the URET prototype sites have shown that significant changes can occur within an air traffic control center when URET is available. Controllers at Indianapolis and Memphis Centers have been conducting evaluations on relaxing altitude restrictions since October 1999. These altitude restrictions are imposed on arrival and departure aircraft and make it easier for the controller to detect conflicts. Restrictions on arrivals usually result in them descending to a less fuel-efficient altitude earlier than is necessary and spending some time in level flight at this less desirable altitude. The purpose of the evaluations is to understand the operational implications of relaxing restrictions and to determine whether the restrictions can be removed. As a result of these evaluations, 22 restrictions have been eliminated. It is estimated that eliminating these restrictions will result in annualized fuel savings of one million gallons. The horizontal paths of aircraft in the Indianapolis and Memphis Centers have also been examined to determine the effect of URET. Since URET started in daily-use in 1997, data analysis shows an increase of 50 percent in route shortening flight plan amendments. It should be noted that this data is for the heaviest traffic times in the center. The overall analysis to date shows an average savings of more than 0.5 mile per flight for every flight going through the Indianapolis and Memphis ARTCC airspace. This translates into a monthly economic benefit of approximately $1.5 million for both ARTCCs combined. Future Work and Current Related Activities In addition to supporting the FFP1 program, CAASD is incorporating lessons-learned from field evaluations into plans for the longer-term research and evolution of air traffic management capabilities. This work is being done within the United States and in other countries that face similar problems. There is clear recognition that many of the problems in air traffic today are global ones, and that a human-centered development model for en route decision-support tools is consistent with the strategic objective of achieving a more flexible, responsive class of service. Free Flight Phase Two (FFP2), which expands upon the successes of FFP1, extends the URET deployment to an additional 9 ARTCCs. CAASD is analyzing the operations in the additional facilities for unique operations that may require enhancements to URET. FFP2 also adds two new capabilities to improve the performance of the NAS ¿ a collaborative routing and coordination tool (CRCT) capability and digital controller-pilot data link communications (CPDLC). CAASD is serving in a major role in implementing CRCT capabilities into the Enhanced Traffic Management System and continuing field evaluations of CRCT concepts. For CPDLC, CAASD is working with the FAA and industry partners in overall systems integration and implementation planning, development of requirements, architecture recommendations, and certification. RTCA input on FFP2 included recommendations for priority research including two of which CAASD is actively engaged: - Problem Analysis, Resolution and Ranking (PARR), providing controllers with resolutions to URET detected conflicts
- Equitable allocation of limited resources, addressing the needs of balancing an ever-increasing demand for use of airspace
CAASD also is looking at integrating all of the FFP1 and FFP2 capabilities with URET. This includes integration of traffic management capabilities, such as TMA and CRCT, plus other sector controller tools, such as PARR and CPDLC. The priority research and integration of capabilities may be implemented during or after FFP2 depending on the maturity of the concepts and FAA funding.
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