Abstract:
A method of generating aircraft tactical alerts includes receiving track positions for two aircraft; receiving trajectories and static conformance bounds for the two aircraft; receiving current position for the two aircraft; generating tactical check segments and variable conformance bounds for the two aircraft based on the current position, the static conformance bounds, trajectory, adapted data, and the track positions; and generating a tactical alert if the variable conformance bounds overlap within a specified lookahead time. The variable conformance bounds can be either symmetric or asymmetric about projected tracks. The variable conformance bounds can use step functions, or continuously widening bounds up to the static conformance bounds. The variable conformance bounds can be based on modifying the static conformance bounds in two or three spatial dimensions.

Description:
STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
   Statement under MPEP 310. The U.S. government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. DTFA01-01-C-00001, awarded by the Federal Aviation Administration. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to Air Traffic Control (ATC) automated aircraft conflict prediction, and, more particularly, to strategic, trajectory-based methods that utilize surveillance data (e.g., radar position reports) to monitor trajectory accuracy. 
   More particularly, the present invention relates to the reevaluation of variable conformance bounds and predicted aircraft positions over the first several minutes of lookahead time based on the observed aircraft track and navigational equipage. Further, it relates to a system and method for providing timely updates based on observed track positions. 
   2. Related Art 
   In conventional methods, accuracy monitoring is accomplished by comparing the position report with the position predicted from the trajectory for the given time. If the position difference is greater than some 3-dimensional allowance (termed “conformance bounds”), the trajectory may be regenerated to conform with the position data. However, if the position report is within the conformance bounds, no new trajectory is generated; this is done both for computational efficiency, and to maintain trajectory stability (e.g. for stable alert presentation). The present invention improves on these methods, by improving the accuracy of predicted aircraft conflicts over the first several minutes of lookahead time (tactical alerts) while maintaining both computational efficiency and the stability of predicted conflicts at longer lookahead times (strategic alerts) 
   An example of a conventional system that uses conformance bounds is the Federal Aviation Administration&#39;s User Request Evaluation Tool (URET). URET includes decision support capabilities to assist en route sector controllers to predict conflicts between aircraft (i.e., alerts due to proximity of two aircraft to each other), as well as between aircraft and special use or designated airspace. It also provides trial planning and enhanced flight data management capabilities. 
   Typically, the information about each aircraft includes its flight plan, current altitude, position, speed, direction, type of aircraft, etc. URET builds a trajectory for each aircraft using this information, atmospheric data, and adapted data (e.g., aircraft performance data, FAA adaptation data). A trajectory is a four dimensional (4-D) representation of the expected path of the aircraft. A trajectory includes a centerline modeled by a time-ordered sequence of cusps that describe nominal 4-D positions (X, Y, Z, t) and conformance bounds (lateral, longitudinal, and vertical distances) that define how far from a nominal position the track position can be before a trajectory is rebuilt. Trajectories are subdivided into segments that represent portions of a trajectory that can be modeled by constant speed, gradient, course, and conformance bounds. Each segment starts and ends at a cusp; the cusps contain the segment modeling parameters. 
   URET static conformance bounds are a constant magnitude at all lookahead times along a trajectory segment. The conformance bounds depend on the aircraft navigational equipment (e.g., area navigation). Lateral conformance bounds currently extend either 2.5 nautical miles (nm), or 3.5 nautical miles from the trajectory centerline along straight segments, depending on the aircraft navigational equipage. Note also that there is also a vertical tolerance (vertical conformance bounds) and longitudinal tolerance (longitudinal conformance bounds). Lateral and longitudinal conformance bounds are larger than the standard conformance bounds near large turns or for military formations. Vertical conformance bounds are increased near the start or end of altitude transitions. URET trajectories are updated to include observed speed, vertical rates, and course if a track position exceeds any conformance bound (lateral, longitudinal, or vertical) for more than a specified parameter number of consecutive reports. 
   Alerts are identified by determining if two aircraft trajectory conformance bounds have a loss of ATC separation standards (nominally 5 nm horizontally, and applicable vertical separation distance, e.g., 1000 feet at or below flight level (FL) 290, and 2000 feet vertically above FL290). If the conformance bounds have a simultaneous loss of horizontal and vertical ATC separation distances, the minimum separation distance between the trajectory centerlines is used to identify an alert color (red if the distance is less than or equal to a parameter distance (e.g., 5 nm); otherwise the alert is yellow). 
   The magnitude of the conformance bounds affects the number of trajectories, the number of correct alerts (alerts where the actual minimum separation distance—if no controller intervention occurred—would be less than or equal to a parameter distance e.g., 8 nm), the number of false alerts (alerts where an actual minimum separation distance would be greater than a parameter distance e.g., 8 nm), and the alert warning time before a predicted conflict start time. 
   For predicted alerts that start within a few minutes of the “current time,” constant magnitude conformance bounds identify some aircraft pairs with a predicted horizontal minimum separation distance (e.g., 10 nm) that is significantly larger than the Air Traffic Control horizontal separation requirement (e.g., 5 nm). Reducing the size of the conformance bounds will reduce the number of false alerts, but increase the number of missed alerts. Additionally, a reduction in these bounds would reduce trajectory stability. This stability is needed to ensure strategic conflicts do not change unless the aircraft positions are significantly different from the trajectory. 
   One desired improvement is to reduce the number of displayed alerts without significantly altering the strategic conflict probe notifications. In particular, alerts that have a predicted start time close to the current time (termed “short warning time alerts”) require improvements to better match a continuous track-based update. 
   Accordingly, there is a need to reduce the number of false alerts in the tactical timeframe, without a corresponding degradation in the number of missed alerts or trajectory stability. 
   SUMMARY OF THE INVENTION 
   The present invention relates to an aircraft Tactical Check (TC) algorithm, system and method that substantially obviates one or more of the disadvantages of the related art. 
   More particularly, in an exemplary embodiment of the present invention, a method of generating aircraft tactical alerts includes receiving tracks and trajectories for two aircraft including static conformance bounds for the two aircraft; receiving current trajectory position for the two aircraft; generating Variable Conformance Bounds (VCBs) and Tactical Check Segments (TCSs) for the two aircraft based on the current trajectory position, the static conformance bounds, adapted data, and the tracks; and generating a tactical alert if the VCBs have a loss of ATC separation standards within a specified lookahead time. The VCBs can be either symmetric or asymmetric about TCSs. The VCBs can use step functions, or continuously widening bounds up to the static conformance bounds. The VCBs can be applied in two or three spatial dimensions. 
   Further, this invention provides a computationally efficient method to periodically reevaluate displayed conflicts (alerts) with an objective to delete alerts in a more timely manner and consistent with a flight&#39;s track data (e.g., in response to an aircraft maneuver to increase separation distance). The reevaluation process also provides a more accurate estimate of the predicted minimum separation distance for tactical alerts that are not deleted by the reevaluation. 
   Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
       FIG. 1  illustrates how an alert is generated. 
       FIG. 2  shows lateral VCBs (variable conformance bounds) as a step function modeled from a track position on the trajectory centerline. 
       FIG. 3  shows lateral VCBs modeled from a track position not on the trajectory centerline. 
       FIG. 4  illustrates TCSs (tactical check segments) with VCBs. 
       FIG. 5  is a representation of a flow chart in the present invention for a TC (tactical check) process to re-evaluate strategic conflicts. 
       FIG. 6  is a representation of a flow chart to identify a list of aircraft that require TCSs due to a strategic conflict on a new trajectory. 
       FIG. 7  is a representation of a flow chart to identify eligible aircraft that may require new or updated TCSs due to the clock timer event. 
       FIG. 8  is a representation of a flow chart to identify a list of aircraft that require TCSs due to the clock timer event. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 1  illustrates how an alert may be generated. As shown in  FIG. 1 , two aircraft have trajectories centerlines  102 A,  102 B respectively, which represent their expected future tracks. Each of these trajectories is “bounded” by static conformance bounds  104 A,  104 B, which represent corridors within which the two aircraft may occupy for this trajectory without a trajectory update. These conformance bounds  104 A,  104 B typically depend on the type of aircraft and its navigational equipment, and are currently either 2.5 nautical miles (nm), or 3.5 nautical miles. Note also that there is also a vertical tolerance (vertical conformance bounds) and longitudinal tolerance (longitudinal conformance bounds), which are not shown in this figure. Note also that the conformance bounds are static (constant) entities along a trajectory segment. Also, lateral and longitudinal conformance bounds are larger than the standard conformance bounds near large turns or for military formations, and vertical conformance bounds are increased near the start or end of altitude transitions. 
   Normally, the alerts are generated for events that start several minutes forward into the future, for example, five minutes “lookahead time,” or 15 minutes lookahead time. In other words, in  FIG. 1 , the aircraft trajectory positions at t=t 1  are presumed to be at points A 1  and B 1 , respectively. At some future time t 2 , the nominal aircraft trajectory position will presumably be at points A 2  and B 2  respectively. 
   The interval  108  to  110  in  FIG. 1  is where the two aircraft traveling along the trajectory centerlines  102 A,  102 B would generate an alert. The predicted conflict start positions  108 A,  108 B at time t 2  are the earliest positions where the conformance bounds have a distance less than or equal to the required ATC minimum separation distance (horizontally nominally 5 nm). The predicted conflict end positions  110 A,  110 B at time t 3  are the latest positions where the conformance bounds have a distance less than or equal to the required ATC minimum separation distance. 
   However, the actual tracks usually differ from nominal trajectory centerline. In  FIG. 1 , actual tracks of the aircraft are shown as  106 A,  106 B. In other words, the two aircraft are in reality further apart than would be calculated from their nominal trajectory positions  102 A,  102 B. In this case, they may be far enough apart that no alert should be triggered. At time t 2  the actual aircraft positions, points Atk 2  and Btk 2 , are assumed to be left and right of the respective nominal positions A 2  and B 2 . If the actual positions occur along lines  106 A and  106 B, a false alert would be generated due to the loss of ATC separation standard between the conformance bounds in area  108  to  110 . 
   The present invention reduces the number of alerts, and improves the accuracy and stability of the displayed alerts. This is achieved by: 
   (1) Creating VCBs (variable conformance bounds) over the tactical lookahead time (first few (8) minutes of lookahead time) to more accurately represent predicted position uncertainty; 
   (2) Creating TCSs (tactical check segments) over the tactical lookahead time to account for the lateral track offset distance and VCBs, and 
   (3) Re-evaluating alerts (using the TCSs and VCBs) before notification, or periodically re-evaluating displayed alerts that have a time to predicted conflict start time that is less than the TC (tactical check) lookahead time from the current time. 
   Detailed methods for each of the above items (1-3) are provided in the following subsections. 
   Creating VCBs.  FIG. 2  illustrates symmetric lateral VCBs modeled as step functions at each minute of lookahead time, when the track position is on a trajectory centerline. VCBs model the growth of position predictions along a trajectory in the lateral, longitudinal, and vertical dimensions. Conformance bound growth rates may be determined from measured track-trajectory deviation data (using some number of standard errors). The inventors have found that lateral VCBs can reduce the number of alerts up to 8 minutes of lookahead time without introducing excessive instability in displayed alerts. VCBs may also apply to the longitudinal and vertical dimensions. 
   As shown in  FIG. 2 , the lateral VCBs can be represented by a number of step functions that project forward in time and in space, with the conformance bounds gradually widening left  220  and right  222  of the trajectory centerline  102  as the aircraft position is computed for some time further and further into the future. For example, in the example shown in  FIG. 2 , the VCBs expand symmetrically and do not move outside the static lateral conformance bound  104 , given that the track position of the aircraft Atk 1  at time t 1  is known and on the trajectory centerline  102 . 
     FIG. 3  illustrates the situation where the current aircraft position Atk 1  at t=t 1  is offset from the trajectory centerline  102 . In that case, an asymmetrical VCB step function is used, such that the VCB to the left ( 220 ) does not move outside the static lateral conformance bound  104 . The VCBs to the right ( 222 ) expand outwards to the right static lateral conformance bound  104  at a longer lookahead time. 
   TCS lateral VCBs from the trajectory centerline are built as follows. Default lateral VCBs are modeled as distances from the track offset distance, where track offset distance is the perpendicular projection from the track position to the trajectory segment associated with the current time on the trajectory  102 . Default lateral VCBs may be derived from either a table look-up or a continuous function that models the increase in lateral positional uncertainty as a function of lookahead time. For this description, it is assumed that track offset distance is negative for positions left of the trajectory and positive for positions right of the trajectory. Thus, a left VCB is the left default VCBs subtracted from the track offset distance; a right VCB is the right default VCB added to the track offset distance. VCBs are truncated if they exceed the static conformance bound, including special increments (e.g., turns). 
   For example, a table of default VCBs is illustrated in Table 1, where a left and right lateral VCB data structure is presented for lookahead times up to 8 minutes, three categories of track offset distance from a trajectory centerline, and one type of navigational equipment. This example assumes a static conformance bound distance of 1.5 nm. Left and right default VCBs may have different values depending on the track offset distance. As an example, if the track offset distance is −1.0 nm, then at one minute lookahead time, a left VCB would be −2.0 nm and a right VCB is 0.2 nm. If the static lateral conformance bound in this example is assumed to be 1.5 nm, the left VCB would be truncated to −1.5 nm from the trajectory centerline. Note, as illustrated, the lateral VCBs can apply to strategic conformance bounds smaller than the 2.5 nm and 3.5 nm discussed above. 
   
     
       
             
           
             
           
             
             
           
             
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Example default lateral VCB table structure 
             
           
        
         
             
               Track Offset 
             
           
        
         
             
               Distance from 
               Lookahead Time (minutes) 
             
           
        
         
             
               Trajectory 
               VCB Side 
               1 
               2 
               3 
               4 
               5 
               6 
               7 
               8 
             
             
                 
             
             
               −.9 to −1.5 nm 
               Left (nm) 
               1.0 
               1.0 
               1.0 
               1.0 
               1.0 
               1.5 
               1.5 
               1.5 
             
             
                 
               Right (nm) 
               1.2 
               1.2 
               1.4 
               1.8 
               2.1 
               2.1 
               2.1 
               3.0 
             
             
                .9 to −.9 nm 
               Left (nm) 
               0.6 
               0.7 
               0.8 
               0.8 
               1.0 
               1.2 
               1.3 
               1.5 
             
             
                 
               Right (nm) 
               0.6 
               0.7 
               0.8 
               0.8 
               1.0 
               1.2 
               1.3 
               1.5 
             
             
                .9 to 1.5 nm 
               Left (nm) 
               1.2 
               1.2 
               1.4 
               1.8 
               2.1 
               2.1 
               2.1 
               3.0 
             
             
                 
               Right (nm) 
               1.0 
               1.0 
               1.0 
               1.0 
               1.0 
               1.5 
               1.5 
               1.5 
             
             
                 
             
           
        
       
     
   
   Note that although in  FIGS. 2 and 3  the VCBs are shown as step functions, continuous functions may be employed as well. These may be, e.g., asymptotic functions, linear functions, etc. 
   It will be appreciated that the approach of using VCBs described above with reference to  FIGS. 2 and 3  may be combined with reducing the static conformance bounds with reference to  FIG. 1 . It will also be appreciated that although one embodiment of the invention uses VCBs in three spatial dimensions (X,Y,Z), the invention is also applicable to the use of VCBs in just two dimensions (e.g., X and Y). 
   Creating TCSs. To check for conflicts, a TCS structure is built that models the VCBs and centerline positions for a sufficient lookahead time to a time where the trajectory is rejoined. TC cusps (points where gradient, speed, course, or conformance bound magnitude change) are required at each time where the VCBs change or a cusp occurs in the trajectory. TCSs are modeled between consecutive TC cusps. 
     FIG. 4  illustrates lateral VCBs  220 ,  222  and TCSs  402  over the TC lookahead time to a cusp  432   d  where the TCSs rejoin the trajectory  102 . The lateral VCBs (left  220  and right  222 ) increase in magnitude based on the expected track-trajectory differences over the TC lookahead time (in this case as described in  FIG. 3 ). The lateral VCBs are bounded by the trajectory  102  static lateral conformance bounds  104  (e.g., in  FIG. 4 , left VCB is truncated at t 3  and right VCB is truncated at t 6 ). The lateral VCBs are contained within the static lateral conformance bounds since they model a smaller lateral uncertainty. 
   TCSs (see  FIG. 4 ) are built to determine positions that are centered within the lateral VCBs  220  and  222 , and are a parallel offset from the trajectory. Each TCS has a start and an end cusp (e.g.,  402   a ,  402   b ). The line from a TC cusp position (e.g.,  402   a ,  402   b ) to the trajectory position at the cusp time (e.g., t 1  or t 2 ) is perpendicular to the trajectory centerline segment. The time at the end cusp of one TCS and the time at the start cusp of the next TCS are equal. The horizontal position at the end cusp of one segment and the horizontal position at the start cusp of the next segment may be the same, or they may be different. 
   The first step to build TCSs is to determine an ordered list of TC cusp times that define the start and end of each TCS. TC cusp times are added to the list for each trajectory cusp time (e.g.,  432   a ,  432   b ) at or after the current time, and up to and including the first trajectory cusp (termed trajectory rejoin cusp time, e.g.,  432   d ) with a time that is greater than or equal to the current time plus a predefined time interval TC Maximum Lookahead Time. 
   Next, TC cusp times are also added to the TC cusp time list at the current time and every predefined interval (e.g., every 1 minute at t 2  through t 6 ) up to and including the TC Maximum Lookahead Time to model VCBs. If the cusp time at the TC Maximum Lookahead Time is not a trajectory cusp time, one additional TC cusp time is added to the TC cusp time list. The TC cusp times are unique with ascending time order. TCSs start and end times are derived from the time-ordered cusp times. Each TC cusp time in the list is the start time of a TCS (except for the last entry in the list); the end time of a TCS is the next TC cusp time in the list. The end time of the last TCS is the trajectory rejoin segment cusp time (e.g., t 7 ). 
   The TCS may include a pointer to the trajectory rejoin segment cusp. After the trajectory rejoin cusp, the TCSs and the trajectory segments are identical (e.g., trajectory segment defined by cusps  432   e  and  432   f ). Table 2 illustrates the start and end times for a TCS structure built from trajectory segments using cusps in  FIG. 4 . 
   
     
       
             
           
             
             
             
           
             
             
             
             
             
           
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Illustration of TCS time interval construction 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Trajectory Segments 
                 
             
           
        
         
             
                 
               Cusp 
               Start 
               Cusp 
               End 
             
             
                 
               Id 
               Time 
               Id 
               Time 
             
             
                 
                 
             
             
                 
               432a 
               10:00:00 
               432b 
               10:01:20 
             
             
                 
               432c 
               10:01:20 
               432d 
               10:07:00 
             
             
                 
               432e 
               10:07:00 
               432f 
               10:15:00 
             
             
                 
               432g 
               10:15:00 
               432h 
               10:22:00 
             
             
                 
                 
             
           
        
         
             
                 
               TCSs 
                 
             
           
        
         
             
                 
               Cusp 
               Start 
               Cusp 
               End 
             
             
                 
               Id 
               Time 
               Id 
               Time 
             
             
                 
                 
             
             
                 
               402a 
               10:00:00 
               402b 
               10:01:00 
             
             
                 
               402c 
               10:01:00 
               402d 
               10:01:20 
             
             
                 
               402e 
               10:01:20 
               402f 
               10:02:00 
             
             
                 
               402g 
               10:02:00 
               402h 
               10:03:00 
             
             
                 
               402i 
               10:03:00 
               402j 
               10:04:00 
             
             
                 
               402k 
               10:04:00 
               402l 
               10:05:00 
             
             
                 
               402m 
               10:05:00 
               402n 
               10:07:00 
             
             
                 
                 
               Pointer to 
             
             
                 
                 
               trajectory 
             
             
                 
                 
               cusp 
             
             
                 
                 
               432e 
             
             
                 
                 
             
           
        
       
     
   
   For each TCS cusp, additional variables needed for TC Automated Problem Detection (APD): such as speed, course, gradient, altitude, longitudinal conformance bound, and vertical conformance bound are derived from the associated trajectory segment. The associated trajectory segment is determined by the trajectory position at the TC cusp time. 
   Re-evaluating alerts. The TC procedure is initiated for the following events to determine aircraft that require new TCSs and strategic conflicts that require reevaluation:
         (1) A new strategic conflict is detected on a trajectory   (2) Due to the clock timer event, a strategic conflict becomes a tactical alert or the time since last re-evaluation of the TCS in an active tactical alert is greater than a specified parameter (e.g., 1 minute).   (3) New track reports are received, and an aircraft has a track position that is out of conformance with the TCS lateral conformance bound, or an aircraft in a tactical alert is no longer TC eligible       

   Referring to  FIG. 5 , which shows a flow chart diagram of the TC re-evaluation procedure, the TC procedure determines aircraft that require a new TCS without invoking the reconformance function and reevaluates each tactical alert that involves an aircraft with a new TCS. Alerts that are reevaluated can be deleted before notification, removed from an existing display, or have a changed severity level (color) or predicted conflict start time. In a few cases, a previously deleted alert may need to be redisplayed after reevaluation in TC APD. Tactical alerts deleted by TC APD are termed inactive alerts. A detailed discussion of  FIG. 5  is provided below for each of the initiation events. 
   New Strategic Conflict. Referring to  FIG. 5 , the flow chart diagram of the TC re-evaluation procedure starts with block  504 , where the data concerning new trajectories, new strategic conflicts on a trajectory, TCSs, and TC eligible aircraft are made available to the system. Trajectories are built for each aircraft after a flight plan is available, and whenever modifications are made to a trajectory, such as a flight plan amendment is received, or the system updates a trajectory for conformance with the track. Strategic APD (using trajectories and static conformance bounds) is run each time a new trajectory is built to determine new strategic conflicts. If an aircraft has TCSs, they are deleted whenever a new trajectory is built. 
   The TC process is started each time a new strategic conflict is found by strategic APD. In block  506 , if a strategic conflict has a predicted conflict start time that is less than or equal to the current time plus a predefined parameter for the Maximum TC Lookahead Time (e.g., 8 minutes), it is marked as an active tactical alert and requires further processing. Otherwise, if block  506  is “No”, the next new strategic conflict is processed by returning to block  504 . If a new tactical alert is found in block  506 , that is, block  506  is “Yes,” each aircraft in the tactical alert is processed to determine if it requires a new TCS and, if so, the aircraft identifier is added to the Build TCS Message block  508 . Additionally, since an aircraft can be involved in multiple tactical alerts, block  508  processing identifies aircraft in all interrelated tactical alerts that require TCSs, the details of which are shown in  FIG. 6 . 
   After all aircraft identifiers that require a new TCS are entered in the Build TCS Message, flow continues to block  510 . In block  510 , TCSs are built for each Build TCS Message entry and each tactical alert that involves an aircraft with a new TCS is marked for TC APD. TC APD block  512  is invoked for all strategic conflicts that include an aircraft with a new TCS. 
   Referring to  FIG. 6 , the procedure of block  508  starts in block  631 , where a Check TCS List is initialized by adding each TC eligible aircraft in the alert to the Check TCS List. Each TC eligible aircraft in the Check TCS List is sequentially processed block  632 . If the Check TCS List is not done, i.e., “No” in block  637 , processing flows to block  633  (“Existing TCS?”). If i.e., “No” occurs in block  633 , the aircraft is added to a Build New TCS Message in block  635 . 
   Next, processing in block  636  identifies each active tactical alert that includes this aircraft and, for each such alert, if the object aircraft is TC Eligible, the object is added to the Check TCS List. After all tactical alerts have been checked, flow returns to block  632 . Assuming the Check TCS List is not completed, that is block  637  is “No”, and “Existing TCS?” block  633  is “Yes,” flow continues to block  634  where an existing TCS is check to determine if it is to be updated due to the clock timer event. If “Current Time&gt;&gt;=Time Between TC Cusps plus First TCS Cusps Time?” is “Yes” in block  634 , the aircraft is added to a Build New TCS Message  635 . 
   Again, the flow continues to block  636  where each active tactical alert that includes this aircraft is identified and, for each such alert, if the object aircraft in the alert is TC Eligible, it is added to the Check TCS List. Flow returns to block  632  where processing continues as described above. If an aircraft has a TCS that does not need to be updated, block  634  is “No,” flow returns to block  632  where processing continues until the Check TCS List is found to be completed in block  637 . When all aircraft in the Check TCS List have been processed, “Check TCS List done?” block  637  is “Yes,” and processing flows to block  640  where it returns to block  510  of  FIG. 5 . 
   Thus, block  508  processing identified all aircraft that need a new TCS in the original new tactical alert and each interrelated tactical alert. Although it is possible that all tactical alerts are checked for reevaluation in response to one new tactical alert, in practice only a few tactical alerts are reevaluated when a new tactical alert occurs. 
   Clock Timer Event. A second event to initiate the TC procedure is the clock timer event. Referring to  FIG. 5  block  516 , the TC procedure is invoked for advancement of the clock, “Clock Timer Event”, where the strategic conflict list, TCSs, and TC Eligible aircraft are made available to the system. Due to the advancement of time, nominally every second, aircraft may require new TCSs if a strategic conflict becomes a tactical alert or existing TCS are older than a predefined time interval. As described above, each tactical alert that involves a new TCS is reevaluated in TC APD. Every second, new tactical alerts are identified and all TC eligible aircraft that are in any active tactical alert are added to Check TCS List in block  518 . The flow continues to block  520  where each aircraft in the Check TCS List is processed to determine if it needs a new TCS, and, if so, the aircraft is added to a Build TCS Message. Processing details of blocks  518  and  520  are described below in more detail. After all aircraft that require a new TCS are entered in the Build TCS message, TCSs are built for each Build TCS Message entry and each tactical alert that involves an aircraft with a new TCS is marked for TC APD block  510 . TC APD block  512  is invoked for all strategic conflicts that include an aircraft with a new TCS. 
   Referring to  FIG. 7 , the procedure of Block  518  starts in block  761  by starting a loop over the strategic conflict list. If a strategic conflict is a “Tactical Alert”, “Yes” is returned from block  762 . If “Yes”, flow continues to block  767  where each aircraft identifier in an active tactical alert that is TC Eligible is added to the Check TCS List. Flow continues to block  768 , where completion of the loop is checked. If in block  768  “All conflicts done?” is “No,” more strategic conflicts remain to be processed, the flow continues to block  761  to process the next strategic conflict. Flow continues to block  762  where the strategic conflict is checked if it a tactical alert. Assuming “Tactical Alert?” is “No,” flow continues to block  764  where the strategic conflict is checked, where it is now a tactical alert due to the clock timer event. In block  764 , if the predicted conflict start time is less than or equal to the current time plus a predefined time interval the Maximum TC Lookahead Time, a “Yes” occurs. If “Yes”, the strategic conflict is marked as an active tactical alert block  766 . Each aircraft identifier in the tactical alert that is TC Eligible is added to the Check TCS List  767 . If in block  764  a “No” occurs, that is a strategic conflict is not a new tactical alert, flow continues to block  768 . After all strategic conflicts have been processed, block  768  is “Yes” and processing returns to block  520 . 
   Referring to  FIG. 8 , the procedure of block  520  starts in block  881  where the Check TCS List is made available and each entry is processed to determine a list of aircraft that require a new TCS. Flow continues to block  883  where aircraft that do not have a TCS record are identified. If block  883  is “No”, processing continues to block  886  where the aircraft identifier is added to a Build TCS Message. Otherwise, assuming block  883  is “Yes”, the aircraft record has an existing TCS, and the flow continues to block  884  where the TCS is checked to determine if an update is needed due to the clock timer event. If the current time is greater than or equal to the TCS first cusp time plus a predefined parameter Time Between TCS Cusps  884 , the aircraft identifier is added to a Build TCS Message  886 . If TCS update is not needed, block  884  is “No” and the flow continues to block  888 . After all input records in the Check TCS List are processed, block  888  is “Yes”, and the flow branches to block  890  which returns to  FIG. 5  block  510 . 
   New Track Reports. TC stimuli include monitoring track reports to check TCS conformance and eligibility. Referring to  FIG. 5  block  502 , track report data, TCSs, and a list of TC Eligible aircraft are made available to a Track Management process. If an aircraft has a new track report and a TCS, the Track Management process checks if a track position is outside of the TCS lateral conformance bounds  514 . If an aircraft is found to be out of conformance with TCS segments, it is added to a Build TCS Message including the track offset distance from the trajectory. Secondly, the Track Management process checks if aircraft is no longer eligible for TC due to a loss of track data  514 . If an aircraft is not eligible for TC, it is added to a Build TCS Message. When all track reports are processed, flow continues to block  510 . Messages from Track Management include the aircraft identifier and the trajectory record index. If a flight has a simultaneous event that builds a new trajectory, the TC out of conformance along a prior trajectory is ignored. 
   In  FIG. 5  Block  512 , a new TCS is built for each entry in the Build TCS Message if the trajectory index in the message matches the trajectory index. Each strategic conflict that is a tactical alert and includes the aircraft identifier is marked for requiring TC APD. TC APD is invoked to reevaluate each marked Tactical Alert Block  512 . If a TC conflict is found, the alert is displayed using the TCS conflict data. New alert data can change the alert severity (color) based on the horizontal minimum separation distance, and the predicted conflict start time. If no conflict is found, the strategic conflict is marked as deleted (not displayed), and the tactical alert is inactive. The strategic conflict data is maintained until the strategic conflict end time or until a new trajectory is built since the deleted strategic conflict is reevaluated if an aircraft is TC out of conformance or TC ineligible. A previously deleted alert would be redisplayed if the TC APD determines a conflict exits. 
   Thus, the number of displayed alerts can be reduced by modeling VCBs over the TC lookahead time. Re-evaluation of alerts at short warning time thresholds improves the timeliness of deleting displayed alerts. The predicted minimum separation distance can be improved by modeling TCSs over the TC lookahead time. The number of display notifications deleted early by this approach, using variable lateral conformance bounds, is estimated to be approximately 5 to 9 percent, based on the 1) average horizontal separation distance compared to the standard lateral separation distance and 2) the magnitude of the trajectory lateral conformance bound. 
   It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.