A Conflict Probe to Provide Early Benefits for Airspace Users and Controllers

Alvin L. McFarland

Center for Advanced Aviation System Development, The MITRE Corporation, USA

© 08/97 The MITRE Corporation

This is the copyright work of The MITRE Corporation, produced for the U. S. Government under Contract Number DTFA01-93-C-00001, and is subject to Federal Acquisition Regulation Clause 52.227-14, Rights in Data--General, Alt. III (JUN 1987) and Alt. IV (JUN 1987). No other use other than that granted to the U. S. Government, or to those acting on behalf of the U. S. Government, under that Clause is authorized without the express written permission of The MITRE Corporation. For further information, please contact The MITRE Corporation, Contracts Office, 7515 Colshire Dr., McLean, VA 22102, [703) 983-6000.

The contents of this material reflect the views of the author. Neither the Federal Aviation Administration nor the Department of Transportation makes any warranty or guarantee, or promise, expressed or implied, concerning the content or accuracy of the views expressed herein.

URET Photo

URET Display in use at the D-position of an Indianapolis Center Sector

Current Air Traffic Control (ATC) operations in many areas of the United States are highly structured and restrictive. Increasing structure imposed on flight operations over the years allowed controllers to safely manage rising traffic levels. This structure conflicts with users’ desires for more freedom to fly preferred routes and altitudes from origin to destination. Airspace users have made a unified call for a less structured environment they call Free Flight. Analyses performed to date indicate that airspace users would realize substantial savings in operating costs with free flight.

The automation support available to U.S. en route air traffic controllers today is provided by the Host computer system and has remained essentially unchanged for the last 25 years. It consists of processing that moves aircraft position symbols and data blocks along the radar display; processing that prints paper strips at appropriate sectors in advance of the aircraft’s arrival; and input/output capabilities. Detection and resolution of traffic conflicts is performed manually by the controller.

ATC service providers and users agree that, as soon as feasible, controllers should have a conflict probe to provide automatic and continuous detection, as well as earlier notice, of developing traffic problems. This capability will allow them to manage current and projected workloads more safely and efficiently. To achieve this goal, the conflict probe needs to work with the current Host computer system without requiring significant changes to it. The Federal Aviation Administration (FAA) typically operates en route sectors with two controllers - the Radar controller (R controller) who monitors the radar display for tactical problems and issues clearances to aircraft; and the Radar Associate (data controller or D controller) who manages the paper strips, looks for strategic problems, and manages coordination with other sectors. To expedite deployment, and to minimize changes to current procedures, FAA decided that the conflict probe would initially be introduced primarily for use by the D controller. The R controller’s operation would remain largely unchanged.

The FAA and The MITRE Corporation’s Center for Advanced Aviation System Development (CAASD) have been conducting extensive research, development, and evaluation of experimental systems meeting these needs. CAASD’s analyses to date estimate savings in user operating costs up to $620 million per year, if most route and arrival descent restrictions can be removed. Air traffic controllers participating in laboratory evaluations of conflict probe, and related decision aids for conflict resolution, have estimated that 60% of the restrictions in the system could be removed with the use of such tools by en route controllers.

In January 1995, FAA tasked CAASD to develop a prototype of conflict probe and related decision aids for field testing, install it at Indianapolis Center so that it could be operated from selected sectors on the control room floor, and conduct an evaluation of it using certified controllers in live operation. This prototype was named User Request Evaluation Tool (URET) to emphasize its potential value to controllers in providing a more flexible service to airspace users. The URET work described here is a derivative of earlier research performed by the FAA and CAASD under the program for Automated En Route Air Traffic Control (AERA). Many of the human factors insights related to the appropriate choice of decision aids and the preferred method of human/computer interaction evolved during this earlier research.

The URET algorithms are implemented in a single dedicated computer server that is networked to dedicated X-terminals at the D controller’s positions. The X-terminal provides a high resolution color display, a standard keyboard, and a mouse or trackball. URET has a one-way interface to the existing Host computer over which it receives all flight plans or amendments, and all radar track reports, in real time. It also obtains wind, temperature, and pressure data from the National Weather Service every three hours.

The flight plan and track data are combined with site adaptation data, aircraft performance characteristics, and wind and temperature data to build four-dimensional flight profiles, or trajectories, for all flights. These trajectories define the horizontal route, the altitude profile, the airspeeds and groundspeeds being flown, and the times that the aircraft will reach various points along the route. Each incoming track report is compared to the predicted trajectory position of the aircraft at the time of the report. If the track report differs from the trajectory position by more than a conformance distance in the lateral, longitudinal, or vertical direction, a new trajectory is built starting at that track report. The modeled speed, climb rate, or descent rate may be adjusted at this time based on the aircraft’s observed track history.

URET uses the trajectories of all aircraft to check continuously for conflicts. A potential conflict is declared when the trajectories of two aircraft indicate that the horizontal and vertical separation will both decrease below corresponding thresholds. Before deciding to notify a sector about the conflict, URET evaluates the time before conflict, as well as the conflict configuration, to estimate the probability that the current situation will actually develop into a close approach. It uses empirically derived statistics in this evaluation. If the estimated probability is low enough, and there is adequate time before the conflict, URET will defer notification to a future time. Otherwise, URET will notify one and only one sector immediately. URET will notify no earlier than 20 minutes, and will defer to no later than 10 minutes, before the start of conflict. In today’s ATC automation system, en route controllers receive approximately three minutes of warning about conflicts from the conflict alert function. The earlier 10-20 minute notification from URET will add a considerable margin of safety to the system.

To determine which sector to notify, URET takes account of which sectors currently control each of the aircraft in a conflict, and where the actual conflict is predicted to occur. In general, URET will notify the sector where the conflict occurs. Under current procedures for the evaluation of URET, the sector team is not required to take action for conflicts that are notified to their sector. It is, however, expected that the team will evaluate each notified conflict and make its own decision about whether and when to act. The sector team should be aware that if its sector is notified of a conflict, no other sector will be notified of the same conflict.

URET presents textual flight information on an Aircraft List where one row of information is presented for each aircraft. Aircraft are automatically added to a sector’s Aircraft List 20 minutes before projected entry into the sector, and they are automatically deleted after leaving the sector. Additional information for an individual aircraft can be called up as shown for AAL9138 in Figure 1. The controller can receive alert information on the Aircraft List. Each entry has four boxes at the beginning of the entry. The first is a bookkeeping box in which the controller can enter a mark to indicate that he has examined the alerts for that aircraft. The other three boxes provide areas to indicate red, yellow, and blue alerts, respectively. Conflicts with a predicted minimum horizontal separation of less than or equal to 5 nautical miles (nm) are coded in red. Those with a predicted minimum separation of greater than 5 nm, but still within close approach criteria, are coded in yellow. When URET detects that an aircraft’s trajectory will pass through a Special Use Airspace, it generates a blue alert.

For example, a yellow 2 in the yellow box for a given aircraft on the Aircraft List means that the aircraft has yellow conflicts with two different aircraft, and the subject sector is the notification sector for those conflicts. If an aircraft has at least one number in the red or yellow box, it may also have a plus sign in the red or yellow box. A plus sign means that that aircraft also has at least one other conflict for which some other sector has been notified. The controller can view the other alerts with URET’s Show All feature, but generally he will be interested in only those notified to his own sector. Aircraft ACA824 in Figure 1 has one red alert that has been notified to the subject sector and one yellow alert that has been notified to another sector.

In addition to the Aircraft List, the D controller can activate a Graphic Plan Display (GPD) on which he may graphically display aircraft trajectories and conflict situations. When the controller displays an aircraft’s current trajectory on the GPD, the trajectory is drawn with the color of the highest order alert for that aircraft that has been notified to the subject sector. The order from highest to lowest is red, yellow, blue. In addition, every alert on that trajectory that has been notified to the subject sector will be shown in the corresponding alert color and the trajectories for all aircraft causing those alerts will also be shown. The portions of the trajectories that satisfy the conflict criteria are displayed with heavier line segments. Colored numbers and plus signs on a line above the data block for each aircraft have the same meaning as those on the Aircraft List. An example of a displayed trajectory is shown for N6173J in Figure 2. This aircraft has one red conflict and one yellow conflict that have been notified to the subject sector, so the trajectory is shown in red.

Altitude information is shown on trajectories in the following way. If the trajectory indicates that the aircraft is currently climbing or descending, the altitude field in the data block (the second row) shows the target altitude followed by an up or down arrow and the trajectory altitude for the current time. N6173J in Figure 2 is an example. An asterisk with an accompanying 3-digit altitude is placed on the trajectory at the location where the aircraft will achieve the target altitude. Future points on the trajectory where URET calculates that the aircraft will begin a descent are indicated by an asterisk accompanied by a down arrow followed by the new target altitude. The point on the trajectory where the new altitude will be achieved is also shown with an asterisk and a 3-digit number. The trajectory for ASH5416 in Figure 2 shows where the aircraft is predicted to begin a descent to 300 feet and where it reaches that altitude.

URET also provides a trial planning function. Trial planning allows a controller to check a desired flight plan amendment for potential conflicts before a clearance is issued. Performing a trial plan causes no change to the aircraft’s current plan or trajectory. The controller can call up a menu of new altitude or speed assignments for an individual aircraft simply by clicking in the altitude or speed field of the Aircraft List entry or GPD data block. The trial plan is created and submitted by selecting a new assignment from one of these menus. The controller can also modify the horizontal route of an individual aircraft by clicking on either the destination field in the GPD data block, or clicking on the route field in the Aircraft List. The current route of flight will be shown in an edit field where it can be modified using mouse or keyboard and then submitted for trial planning. URET performs conflict detection for the trial plan trajectory against all other trajectories in the data base, and color codes the trial plan trajectory to reflect the result. The trajectories of any conflicting aircraft are also shown. If no conflicts are detected, the trial plan trajectory is shown in green.

After the controller enters a trial plan, URET presents an additional window called the Plans Display. This text window contains a label for the trial plan, the clearance that would be voiced to the aircraft, and text describing any conflicts found. The label is color coded to indicate the highest order conflict detected on the trial plan.

URET enables the controller to create trial plans graphically as well as from menus. This function is most useful for horizontal reroutes. The controller can click at one or more locations off the aircraft’s current trajectory and then click with the right mouse button to cause the new route to rejoin the current trajectory. An example of a graphical trial plan for UAL1040 is shown in Figure 3. The current trajectory has a red alert with UAL736. The graphical trial plan is conflict free as indicated by the green trajectory. The Plans Display for this trial plan is also shown.

When the controller submits a trial plan, URET evaluates the trajectory and color codes the trial plan to reflect its conflict status at that instant. URET will not update the conflict status of the trial plan as the situation changes, but it will automatically discard the trial plan after two minutes. URET has a feature that allows the controller to submit a trial plan to a function called Auto Reprobe. When this is done, URET periodically reprobes the trial plan and updates its color coding to reflect changes in conflict status. The utility of this feature is indicated with an example. Suppose a pilot requests a higher altitude in a situation where there is a conflicting aircraft above. The controller creates a trial plan to the higher altitude that shows a red conflict, but submits the trial plan to Auto Reprobe. When URET detects that the trial plan no longer has a conflict, the trial plan will turn from red to green.

URET provides a means for the controller to conveniently coordinate a trial plan with another sector. Because URET typically notifies a sector 10 to 15 minutes before the conflict, it often happens that the notification sector does not control one or both aircraft in the conflict at the time of notification. The D controller at the notification sector can assess the situation, choose one of the aircraft to be maneuvered, and submit a trial plan for the proposed resolution. If acceptable to the sector team, the controller can use a simple mouse action to coordinate it with the sector currently controlling the aircraft. URET will add the coordinated trial plan to the Plans Display of the sector currently controlling the aircraft and provide a color coded letter C above the GPD data block of the affected aircraft. The C above the data block will prompt the receiving controller to display the Plans Display or to display the coordinated trial plan on the GPD by clicking on the letter C. The receiving sector is expected to respond to the coordinated trial plan by either approving or disapproving it. The approval or disapproval is displayed to the initiating controller. URET’s trial plan coordination involves less work than current voice coordination; it is silent, non-interfering, and asynchronous, whereas voice coordination requires both the initiator and the receiver to be on the interphone at the same time.

Several other features of URET assist the controller in managing air traffic. On the GPD, there is a future time slider control with which the controller may display the air traffic situation at a time from 0 to 20 minutes in the future. When the controller selects a future time increment, URET advances all aircraft forward by that amount along their current trajectories and displays all aircraft at the future time. This capability could help in resolving conflicts.

URET builds trajectories for aircraft accounting for altitude restrictions imposed on all aircraft arriving at specified airports from certain directions. At many locations these restrictions are applied only during certain hours of the day. At other locations, the restrictions are imposed or lifted at random times. URET enables the controller at any time to activate or deactivate altitude restrictions that apply to his sector.

ATC facilities routinely combine and decombine sectors as traffic levels change. URET can readily accommodate these actions. A Configuration Control Display allows a controller to assign any sector to any workstation at any time. Multiple sectors may be displayed simultaneously on a single physical workstation. When this is done, all traffic from all of the sectors is presented in a single Aircraft List and all traffic for those same sectors is shown on a single GPD.

An interfacility coordination capability has been included in URET. Two-way communications are provided between the URET system at one center and the URET system at each of its neighboring centers. Interfacility coordination ensures that: conflicts occurring at boundaries are detected with the same warning times as those occurring entirely within a center; controllers can coordinate trial plans with sectors in other centers as well as their own; and both centers come to a consistent conclusion about which sector should be notified for conflicts occurring near center boundaries. Interfacility coordination was tested in live operation at Indianapolis and Memphis centers starting in October 1997.

URET has the capability to convert trial plans into Host flight plan amendment messages. Although the URET systems in the field have so far been connected to the Host computer system only through a one-way interface, URET has been operated in the laboratory with a two-way Host interface that permits URET-generated amendments to be entered into the Host. Real-time, human-in-the-loop experiments using URET-generated amendments are currently being conducted with controllers at the CAASD laboratory. Plans call for implementing this same capability at the field sites so that live operation can be evaluated with this capability by the end of 1998. Use of the two-way Host interface in URET allows the controller to create trial plans and submit them as amendments, all from the URET workstation without duplicating inputs. With the one-way interface he must enter the trial plan from the URET workstation, and then enter the amendment message from scratch using the Host keyboard, essentially duplicating the input actions.

Field evaluation of URET began at Indianapolis Center in February 1996. As of October 1997, over 200 hours of URET operation in an evaluation mode have been completed under live traffic conditions in the control rooms of the Indianapolis and Memphis Centers. In this mode, the sector team use the URET system at an active sector for several days while CAASD staff observe the operation. After each shift, the CAASD staff administer questionnaires and conduct debriefings of the controllers. To date, the Indianapolis and Memphis controllers and managers have been unanimous in their support of the tool, and they have expressed a strong desire to have a URET-based production version of conflict probe. They agree that the capabilities demonstrated by URET offer many new operational advantages for controllers. For example, consistently longer problem-identification lead times will enhance safety; URET capabilities do not degrade as traffic complexity and volume increase; and reliance on flight strip marking and manipulation will be reduced. In addition, there will be less uncertainty about potential conflict situations, and more confidence in long term effects of flight plan amendments. Finally, there will be a reduction in the need for clearance approval coordination between sectors. The evaluations showed not only the operational acceptability and desirability of the URET-type decision aids, but also clearly demonstrated the viability of implementing an initial conflict probe as an adjunct to the current Host computer system.

Following the initial successful evaluations, the FAA and CAASD planned a program to develop and implement a production version of conflict probe based on URET. The FAA’s Joint Resources Council (JRC), at a meeting on 30 April 1997, decided to proceed with a full-scale, extended use evaluation of the URET prototype and to initiate development of a production version of conflict probe. CAASD was asked to enhance the URET prototype so that it could be operated in extended use (up to 16 hours per day) by all sectors where it would be operationally useful (potentially all sectors in the center) at both Indianapolis and Memphis Centers. Indianapolis Center began extended use operation in early September 1997, and Memphis Center plans to do the same in mid-November 1997. URET has been used in several hundred hours of extended use live operation, in addition to the evaluation mode operation. URET equipment is being installed at additional sectors in both centers, and additional controllers are being trained in its use. Extended use operation should be underway at all planned sectors at both centers by mid-1998.

The FAA has a program called the Display System Replacement (DSR) to produce and install new controller workstations that will replace the existing equipment at every en route sector in the U.S. DSR will interface with the current Host computer system, which will continue largely unchanged. The JRC decided to develop a production version of conflict probe by integrating the URET capabilities into the DSR system. This integrated version of conflict probe would be deployed as a first functional enhancement to DSR following its initial deployment. A contract amendment to the current DSR contract that will develop and deploy the integrated conflict probe should be completed in the first half of 1998.