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In today's National Airspace System (NAS), en route air traffic controllers must deal with a variety of operational situations to ensure the safe and efficient management of traffic flows. These situations include: possible conflicts between two or more aircraft; rerouting around severe weather; and implementing traffic flow initiatives such as meeting terminal arrival metering times. As en route air traffic increases in volume and complexity, the need for automated systems to assist en route air traffic controllers in dealing with such situations continues to grow. The FAA and the MITRE Corporation's Center for Advanced Aviation System Development (CAASD) are developing, in an evolutionary fashion, these needed automation capabilities.

Through the Free Flight Phase 1 (FFP1) program, the FAA is making the first significant strides in helping en route controllers deal with these difficult operational situations. The User Request Evaluation Tool (URET), developed by CAASD, is a key component of FFP1. This system detects and notifies controllers of potential aircraft conflicts with up to a 20-minute look-ahead time. URET also includes a Trial Planning capability that enables controllers to evaluate alternative routes, altitudes, or speeds to resolve problems and handle pilot requests. Since 1997, a prototype version of URET has been in daily use operation at two FAA en route control facilities, Indianapolis and Memphis Centers. By 2002, a production version of URET, Core Capability Limited Deployment (CCLD), built by Lockheed-Martin, will be installed at these two sites and five additional en route facilities. Deployment is planned to nine additional sites by 2005 under the Free Flight Phase 2 (FFP2) program.

In the future, en route controllers will be further assisted by the use of a capability called Problem Analysis, Resolution and Ranking (PARR), a URET enhancement that is currently under development at CAASD. While URET significantly improves the information and planning capabilities available to en route controllers, experience with the prototype has indicated an operational need for more advanced problem resolution capabilities; PARR addresses that need. Building upon URET's underlying functionality and computer-human interface (CHI) design, PARR generates candidate resolutions to solve various air traffic problems upon controller request. The resolutions are developed by PARR with the overarching objectives of solving problems safely, preserving user-preferred routes of flight, and rapidly and efficiently presenting results to controllers in an operationally useful manner. Because of its potential for enhancing NAS en route operations, PARR is designated as priority research by RTCA in its recommendations for advancing Free Flight.

PARR is partitioned into the following four increments that are being developed in parallel:

• Assisted Trial Planning (ATRP)
• Assisted Resolution Tool (ART)
• Severe Weather Resolutions (SWRL)
• Resolutions for Flow Initiative Execution (REFINE)

The first two increments, ATRP and ART, are more advanced in terms of research and development. Laboratory and field evaluations of ATRP and ART are on-going. It is envisioned that these capabilities will be implemented as an enhancement to URET CCLD in the FFP2 timeframe (2003-5). The last two increments are currently in the concept exploration stage of research, and laboratory evaluations are slated to begin in FY 2002.

Although there are some differences in the use of PARR for various types of en route problems, the general concept of use is essentially the same and is briefly described next. In using PARR to assist in resolving aircraft-to-aircraft or other air traffic problems, the en route controller invokes the capability to search for possible resolutions. The resolutions are generated and presented to the controller based on a number of operational factors (e.g. descending an aircraft already in descent is preferable to a climb). Resolutions are displayed in the form of Trial Plans that can be communicated to pilots and entered into the en route automation system as flight plan amendments. Controllers may choose to implement one of the resolutions or create one of his or her own solutions using the information from PARR to assist in deciding which solutions might work. Figure 1 is an example of a set of PARR resolutions resulting from a controller request to resolve an aircraft-to-aircraft conflict. The resolutions are preceded by a display of the maneuvered aircraft's current flight plan. In this example, the current flight plan is color-coded red, to denote a predicted conflict with another aircraft. The PARR resolutions presented here not only resolve the original aircraft-to-aircraft conflict but also do not introduce any new problems, as denoted by their green color-coding. The first displayed and highest ranked resolution, in this example, is a 10-degree left turn (see Figure 2). If operationally acceptable, the aircraft's flight plan can be easily updated to reflect the amendments provided in one of the resolutions. The presentation of the resolutions also facilitates communication to the pilot via voice, and will eventually support communications of flight plan amendments provided by PARR via air-ground data link.

Figure 1

Figure 2 - The red lines depict the paths of two aircraft converging into a potential conflict situation.. The green line shows a PARR generated alternative flight path that would ensure the aircraft remain safely separated.

URET and PARR are products of more than two decades of research and development of Air Traffic Management decision support capabilities at CAASD. These and other capabilities under development will contribute significantly to the safe and efficient flow of traffic in the increasingly crowded and complex NAS en route airspace.

History and Future of PARR

PARR is an integrated software component of a system called User Request Evaluation Tool (URET) which detects potential conflicts in aircraft flight paths and provides Trial Planning capabilities to assist in resolving those conflicts. PARR is invoked from URET displays and presents its results on URET displays. Behind the controller display, PARR builds directly on top of URET's underlying functionality; i.e., PARR uses the URET trajectory modeling and conflict detection functions.

As with URET, the key features of PARR were initially developed with extensive controller laboratory evaluations in the late 1980s and early 1990s as part of the AERA (Automated En Route ATC) program. The FAA/CAASD AERA laboratory was the site for the development and evaluation of the AERA prototype. Intensive operational evaluations were held three to four times a year with the Air Traffic AERA Concepts Team (ATACT), a team of approximately 14 active, Full Performance Level controllers selected to provide a complete system perspective.

Evaluation Workstation in FAA/CAASD AERA Laboratory.

A laboratory version of URET was developed in 1998. Since then, the insights and experience gained through URET field evaluations, together with the application of real-world recorded data and PARR laboratory evaluations, have resulted in significant development of the PARR algorithms and concept of use. One of the key elements of PARR is its concept of operations under which URET currently operates, e.g. as a tool for the Radar Associated (or D-controller).

Current development of PARR is mainly focused on an Assisted Resolution Tool (ART) that provides resolutions for aircraft-to-aircraft and aircraft-to-airspace problems. Research is also focused on an Assisted Trial Planning (ATRP) function which provides a range of Trial Plans on the Route, Altitude and Speed Trial Planning Menus in URET. ART and ATRP together are the initial PARR capabilities. In January and May 2001, field evaluations of initial PARR capabilities were conducted at Indianapolis Center. Additional evaluations are planned for later in 2001 and in 2002 at Indianapolis and Memphis Centers. It is anticipated that these initial capabilities will be deployed to selected facilities as a URET CCLD enhancement in the 2003-2005 time frame.

In parallel, CAASD is developing a series of more advanced PARR features. These advanced features include capabilities to assist controllers with severe weather situations and tools for assisting controllers in accommodating Traffic Flow Management Initiatives. Overarching these development efforts is attention to the integration of these capabilities into a common en route Sector Team Decision Support System that fosters an integrated sector team concept of operations.

A Research Management Plan has been prepared by the FAA and CAASD to support the incremental development process of PARR. It includes roles/responsibilities for the research, technical and operational issues that require attention, and detailed schedules for the research

Benefits of PARR

It is expected that PARR will make significant contributions to the attainment of key ATC system goals: safe, orderly, expeditious air traffic flow, and increased controller productivity. These expectations have been supported by on-going laboratory evaluations at CAASD and by initial field evaluations. Specific expected contributions are summarized below:

• Enhanced safety ¿ The expected increase in the use of airspace creates the potential for a higher number of conflict situations, and the relaxation of ATC restrictions can lead to more complex traffic patterns. The PARR enhancements are expected to increase safety by providing tools with which controllers can obtain an improved, strategic situational understanding, and more easily implement and coordinate strategic, problem-free resolutions in a complex environment.
• Controller workload reduction ¿ The PARR enhancements are expected to reduce controller workload by providing the automation to assist in the conflict resolution process. PARR will provide assistance by suggesting specific resolutions, by indicating successful and failed potential resolutions, and by providing enhanced visualization tools for situational awareness. Finally, the resolution enhancements will reduce downstream controller workload, as strategic, conflict-free resolutions will be designed to reduce downstream conflicts.
• Increased user benefits ¿ The reduction in controller workload described above will enable the accommodation of greater numbers of aircraft operating in less structured airspace. Controller workload reduction will also allow more time for controllers to address and accommodate requests from the airspace users. Furthermore, the efficiency of the PARR resolutions will reduce deviations from the user-preferred flight profile when maneuvers are required.
• Improved flight information for use in trajectory modeling ¿ PARR increases the potential for flight plan amendments to be entered into the Host computer (e.g. by providing route modifications that may be entered into the Host vs. incomplete vector clearances that cannot be entered). Because trajectory-modeling accuracy is in part a function of the degree to which flight plan data represents the intended path of the aircraft, this increased potential for better input data can improve modeling accuracy.
• Improved efficiency ¿ PARR is also expected to increase air traffic control efficiency by providing strategic resolutions. Strategic resolutions are more efficient than tactical operations in that relatively small perturbations to the user's preferred route can often resolve conflicts if started early enough. PARR will also improve efficiency by computing efficient maneuver parameters using detailed aircraft performance and atmospheric data.

Assisted Trial Planning

This section briefly describes Assisted Trial Planning (ATRP), a component of PARR that enhances the existing URET Altitude, Route and Speed Menus. In URET, the controller can

select a down-stream fix on the existing Route Menu, an altitude change on the Altitude Menu, or a speed change on the Speed Menu. The controller can check the results of the change via a Trial Plan where URET returns the conflict status of the selected entry. If conflict-free, the controller can use the Trial Plan to amend the current flight plan. If a conflict exists, the controller can try other menu entries until a conflict-free entry is found. ATRP automatically checks Trial Plans for the various alternatives on the menus (e.g. a range of altitudes on the altitude menu) and color-codes menu entries to reflect the conflict probe status of the corresponding action (see Figure 1). For example, if a climb from FL310 to FL350 would result in a "red" conflict, the FL350 menu entry would be colored red. Thus, the controller can determine which alternatives are conflict-free by simply viewing the menu items. This capability may be initiated for an aircraft whether or not it has an existing problem. For example, ATRP may be used to check a pilot's request to climb to a new altitude.

This function has been implemented in a laboratory version of the URET prototype, and laboratory and field evaluations with controllers indicate that it is an effective way to rapidly convey potential problem resolutions. Similar conclusions regarding probed altitude menus were previously found in McNally et al., 1999, A Controller Tool for Transition Airspace, AIAA 99-4298, AIAA Guidance, Navigation and Control Conference. Feedback from controllers indicates that implementation of ATRP should be expedited as it is a straight-forward extension to the current URET functions that could enable immediate benefits to both controllers and airspace users. The implementation of ATRP as a URET CCLD enhancement is anticipated in the FFP2 timeframe (2003-05)

Assisted Resolution Tool

This component of PARR, denoted Assisted Resolution Tool (ART), provides additional capabilities for dealing with aircraft-to-aircraft and aircraft-to-airspace problems. ART can be utilized when situations are too complex to be resolved by the options available with ATRP. Upon controller request, ART searches for resolutions to specified aircraft-to-aircraft or aircraft-to-airspace problems by applying various strategies. These strategies include the generation of aircraft maneuvers in the altitude, lateral, and longitudinal dimensions, and the evaluation and ranking of these maneuvers

Maneuver Generation

Altitude Maneuver Types (Figure 1)

Figure 1 illustrates examples of maneuvers generated by ART in the altitude dimension. The solid lines indicate the original flight paths. The dashed lines represent resolutions proposed by ART.

Figure 1

Lateral Maneuver Types (Figure 2)

Figure 2 illustrates examples of maneuvers generated by ART in the lateral dimension. Similar to the previous examples, the solid lines indicated the original flight paths and the dashed lines represent resolutions proposed by ART.

Figure 2

Longitudinal (Speed) Maneuver Types

ART also proposes maneuvers that increase or decrease speed. A smaller change of speed is preferred over a more extreme change.

Maneuver Evaluation and Ranking

In attempting to evaluate if a particular maneuver is feasible, ART iterates until it finds a path that meets the following constraints:

(a) The aircraft can be returned to its original route,
destination, or transition.
(b) The aircraft is capable of making the maneuver.
(c) New problems are not created by the maneuver.

Maneuvers are computed geometrically, and then an English language description of the maneuver, or clearance language, is formulated from the geometric model of the maneuver (see Figure 3). This language is stated in terms of heading changes, Very High Frequency (VHF) Omnidirectional Ranges (VORs), VOR radials, altitudes, and speeds in an abbreviated form as illustrated in Figure 3.

(Click on image for larger version) Figure 3

After a set of resolutions has been generated, the resolutions are assigned a score based on a variety of variables called ranking factors. Examples of ranking factors include:

• The kind of maneuver (e.g. direct routes are preferred).
• Change in time of arrival (e.g. minimize delay preferred).
• Change in altitude (e.g. a climb is generally preferred for an aircraft in cruise).
• Change in speed(speed changes are generally less-preferred).

Finally, after the resolutions are given scores, the highest-ranked resolutions are presented to the controller for consideration.

Field evaluations of the ART capability indicate that ART produces efficient and useful resolutions. Controllers confirm that ART resolutions can be used as flight plan amendments or to gain a better understanding of traffic situations. Resolutions can also be used as base plans for controllers to amend and create new solutions to meet specific objectives. Because ART is an extension of URET, based on the URET functionality and CHI, minimal training is necessary for URET users to operate ART effectively. This capability is being considered for implementation as a URET CCLD enhancement in the FFP2 time frame (2003-2005).

Severe Weather Resolutions

This section briefly describes a PARR component, denoted Severe Weather Resolutions (SWRL), that will assist the controller in avoiding severe weather when solving aircraft separation problems, and in granting pilot requests for avoidance of these areas. This enhancement will also assist the sector controller in implementing traffic management group-reroutes around large-scale severe weather areas.

The concept is similar to the detection and resolution of aircraft-to-airspace conflicts, with the exception that the airspace is defined by weather specialists, and multiple airspace definitions with different activation times may be defined to model moving weather patterns. The URET Automated Problem Detection (APD) function will be enhanced to include these airspace definitions and notify the controller when a Current or Trial Plan trajectory is predicted to enter a weather area.

SWRL will search for maneuvers that (within the operationally acceptable performance limits of aircraft) go left, right, above and below severe weather areas. Speed maneuvers will also be considered.

Prior to the introduction of the SWRL enhancements, severe weather data (e.g., Next Generation Weather Radar (NEXRAD) data) will be displayed on the URET Graphic Plan Display. URET Trial Planning capabilities along with heightened weather awareness may then be used independently of the SWRL resolution enhancements to check proposed paths for aircraft-to-aircraft and aircraft-to-airspace conflicts. As with ATRP and ART, the availability of SWRL resolutions for avoiding severe weather will not change the sector team's primary responsibilities. However, it is envisioned that controllers will use SWRL capabilities to determine operationally acceptable resolutions for separation problems that avoid vectoring aircraft through severe weather, and to assess pilot requests for the avoidance of these areas.

The pilot's responsibility for safe operation of the aircraft will also remain unchanged, including retention of pilots' option to fly through severe weather areas. To support this option, sector controllers will have the capability to activate and deactivate severe weather areas on an aircraft-specific basis. This will avoid the incorrect application of the severe weather area constraint when applying other PARR capabilities (e.g., for aircraft-to-aircraft problem resolution).

In the group reroute situation, SWRL will enable the sector controller to quickly and efficiently execute specific reroutes requested by the Traffic Management Unit, and will provide assistance for finding alternative, but similar routings when an originally defined reroute is not conflict-free.

SWRL is in the concept development stage. Its implementation as a URET CCLD enhancement is anticipated in the post-2005 timeframe.

Resolutions for Flow Initiative Execution

This section briefly describes the PARR enhancement, denoted Resolutions For Flow Initiative Execution (REFINE), to assist sector controllers with the implementation of Traffic Flow Management Initiatives, which are termed here Flow Initiatives (FIs). The concepts and algorithms developed in this research may also be used to support the development of applications to assist the Area and Traffic Management Coordinator in evaluating the impact of FIs before they are issued.

FIs are imposed by Traffic Management only when needed, typically due to impacts from weather, emergency conditions, or the temporary saturation of a given portion of airspace that a dynamic modification of sector and facility boundaries cannot resolve.

FIs usually follow a pre-defined format with a wide range of criteria: time, altitude, airspace entered, route, fix, arrival/departure airport, over-flight/arrival/departure status, aircraft ID, aircraft performance class, and aircraft user class (e.g., air carrier, general aviation, or military).

The types of constraints that FIs may impose on applicable aircraft for specified times and areas are as follows:

• On-airways/no directs: Restriction to established airways.
• Specified routing: Restriction to specified route(s).
• Flow-restricted area: Restriction from a specified airspace volume.
• Altitude: Restriction to a specified altitude range.
• Miles/minutes-in-trail: Miles or minutes between aircraft crossing a specified boundary.
• Speed limit: Restriction to speeds below a specified maximum.
• Metering: Assignment of a particular time at a specific TRACON boundary.

An example FI using criteria of time, arrival airport and area, and a constraint of a specified route is:

Criteria: 1500 - 1700Z LAX TRFC ENTERING SCTRS 71 OR 72 Constraint: RTE VIA CIM J96 DRK J10 TNP LAX

As a component of REFINE, the URET Automated Problem Detection (APD) function will be enhanced to detect non-compliance with FIs, and the results will be presented to the controller currently controlling the aircraft a parameter time before the predicted violation start.

REFINE will also provide Trial Plans that comply with FI constraints upon request. Specific maneuver dimensions that will be used by REFINE for each FI constraint are as follows:

• On airways/no directs: Lateral maneuvers.
• Specified routing: Lateral maneuvers.
• Flow-restricted area: Lateral and vertical maneuvers.
• Altitude: Vertical maneuvers.
• Miles/minutes-in-trail: Lateral and speed maneuvers (and holds where necessary).
• Speed limit: Speed maneuvers.
• Metering: Lateral and speed maneuvers (and holds where necessary).

Where necessary to avoid aircraft-to-aircraft or aircraft-to-airspace conflicts, when generating resolutions to comply with Flow initiatives, additional resolution dimensions may be used (e.g., a lateral maneuver to comply with a specified route may cause an aircraft-to-aircraft problem, which REFINE may resolve with an additional vertical maneuver).

The availability of resolutions for FIs will not change sector team operating guidelines and fundamental roles and responsibilities at the sector.

The REFINE component of PARR is in the concept development phase of research. Its implementation is expected as an enhancement to URET CCLD in the post-2005 timeframe.

Integration with the En Route Sector Team Decision Support System

PARR capabilities, when fully integrated into en route sector operations, will allow access to the full range of tactical and strategic tools and displays at both the R- and D-positions of the sector. All decision support system components, both tactical and strategic, will be integrated into a common, seamless CHI. With the integration of capabilities, the sector team members will be able to dynamically allocate ATC tasks within the sector team in a manner that attempts to maximize safety, productivity, and efficiency. Controller workload will be reduced, allowing the accommodation of greater numbers of aircraft operating in less structured airspace.

Previous research from AERA laboratory evaluations indicate that a fully available, integrated problem resolution capability such as PARR will allow significant procedural and system benefits to be realized. The procedural benefit will derive from the facilitation of resolving problems in a strategic manner, with the resolution being done by the sector controlling the aircraft rather than the sector where the problem will occur. With revised alert presentation to the controlling sector, problems may typically be resolved before handoff to a downstream sector. This will decrease the need for coordination between sectors. Solving conflicts strategically, before handoff, will also facilitate automated handoff in cases where trajectories are conflict-free. Furthermore, altitude restrictions arising from sector coordination needs may be removed as operations become more strategic.

In the future, tools such as data link, traffic management enhancements, and improved surveillance with ADS-B, may be added to the NAS as the capabilities mature. Potentially, each of these systems will be integrated with the suite of tools at the en route sector, providing a common and consistent presentation to the controllers. Examples of integration include:

• Integrated display of target, track, and trajectory data, with coding for improved aircraft-derived navigation information, as it becomes available.
• Access of information through the data block (formats, data sharing, pop-up menus).
• Display of severe weather.
• Display of tactical conflict alerts and strategic URET alerts.
• Functional integration of data link and PARR to allow digital delivery of PARR resolutions to aircraft.

Even with the new decision support capabilities coming to maturity over the next several years, the sector team responsibilities will remain very much the same: safe and expeditious movement of traffic. However, problem resolution capabilities, fully integrated into an en route sector tool chest and information display, will significantly enhance controllers' ability to safely and efficiently deal with the steadily rising volume and complexity of en route traffic.

FAA and Industry Consensus on Problem Resolution Enhancements

The need for problem resolution enhancements to achieve the principles of Free Flight has been identified by the FAA and the user community, e.g., as identified in the "ATS Concept of Operations for the National Airspace System in 2005" [1], the "National Airspace System Architecture, Version 4.0" [2], "Air Traffic Service Plan 1998 ¿ 2003" [3], the "Conflict Probe Operational Concept, Narrative for Initial and Future Capabilities' [4], the "Initial Requirements Document for En Route Domain Automation" [5], "En Route and Oceanic CHI Upgrade Plan" [6], the "En Route/Oceanic Domain Mission Need Statement MNS-309" [7], the "Final Report of RTCA Task Force 3 Free Flight Implementation" [8], and the "National Airspace System Concept of Operations" [9].

One of the key characteristics of the NAS, as defined in the FAA's "ATS Concept of Operations for the NAS 2005" [1], will be the increased usage of decision support tools that "... reduce the burden of routine tasks while increasing the controller's ability to evaluate traffic situations and plan the appropriate response." This document also calls for "Improved decision support tools for conflict detection, resolution, and flow management [to] allow increased accommodation of user-preferred trajectories, schedules, and flight sequences." and states that "structured routes (will be) the exception rather than the rule ".

The FAA's "Initial Requirements Document for En Route Domain Automation" [5] describes needed capabilities as including "... enhanced capabilities for automated detection of sector overloads and detection of conflicts between aircraft, and between aircraft and airspace with the ability to present the service provider with conflict resolutions."

The FAA's "Air Traffic Service Plan" [3] defines a future direction in support of user operational efficiency as "...an increased ability to accommodate user preferences for trajectories, scheduling, and flight sequencing with improved decision support tools for conflict detection, resolution, and flow management. These improved decision support tools will assist Air Traffic in granting user preferences on trajectories with minimum intervention and adjustment."

The RTCA's "National Airspace System Concept of Operations" [9] defines an En Route Operational Concept for the Year 2005 in which an increase in user-preferred routings "is underpinned by decision support systems to assist in conflict detection and the development of conflict resolutions. This reduces mental workload and gives the provider more time for other tasks such as responding to user requests."

In addition, the FAA and the aviation community have developed the NAS Operational Evolution Plan (OEP) that defines major planned enhancements to the NAS from now to 2010. This plan represents firm commitments to deploy and utilize specific, high-benefit technologies and procedures. PARR is one of those technologies.

1. ATS Concept of Operations for the National Airspace System in 2005, FAA/ATS-1, September 1997.
2. National Airspace System Architecture, Version 4.0, FAA/ASD, January 1999.
3. Air Traffic Service Plan, 1998 - 2003, FAA/AAT-1, May 1999.
4. Conflict Probe Operational Concept, Narrative for Initial and Future Capabilities; Revision 1.0, ATM, March 1997.
5. Initial Requirements Document for En Route Domain Automation, FAA/ARS-1, October 1998.
6. En Route and Oceanic CHI Upgrade Plan, FAA/ARS-1, March 1998.
7. En Route/Oceanic Domain Mission Need Statement MNS-309, FAA/ARX-200, November 6, 1997.
8. Final Report of RTCA Task Force 3 Free Flight Implementation, RTCA, October 1995.
9. National Airspace System Concept of Operations; RTCA, December 2000.

Date Posted: October 8, 2001