PRIORITY CLAIM 
     This application is a divisional of U.S. application Ser. No. 11/364,066, filed on Feb. 28, 2006, all of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Modern aircraft are typically flown by a computerized autopilot (AP). The AP interfaces with Flight Control computers that are coupled both to actuators coupled to control surfaces and to engine computers such as a fully automated digital control (FADEC) computer. Together these cause the aircraft to follow a prescribed path and to maintain proper lift. A navigational computer or flight management system (FMS) receives pilot input regarding intended lateral path to a destination and either receives a vertical flight plan or develops the vertical flight plan based on pilot input, the present position and condition of the aircraft, and current flying conditions such as wind. The vertical and lateral flight paths are typically represented as a series of interconnected waypoints describing a path between points of departure and arrival. The FMS directs the AP to pilot the aircraft according to the flight plan. 
     In some instances, constraints are input to the AP based on instructions from ground based air traffic control (ATC) systems constraining the flight path of the aircraft. These constraints are typically an altitude ceiling above which the aircraft is not permitted to fly or an altitude floor above which an aircraft must fly. The constraints preempt control of the AP by the FMS. The FMS may nonetheless direct the AP to the extent a planned flight path does not conflict with AP constraints. 
     A surveillance system monitors hazards around the airplane and along a predicted flight path. Hazards include weather systems, turbulence, mountains, other aircraft, volcanic ash, and the like. The location of hazards is displayed to the operator of the aircraft (whether onboard or remote) by means of a screen or heads up display in the cockpit. Hazards may be displayed in a navigational, or plan, display illustrating the horizontal position of the aircraft and hazards. Hazards may also be displayed in a “vertical” display, showing the position of the aircraft and hazards in a vertical plane. 
     In the navigational display, it may not be immediately apparent that an aircraft&#39;s altitude carries it above or below a hazard such that the hazard does not require attention. Likewise, in the vertical display hazards are not apparent that are slightly to one side or the other horizontally from the aircraft&#39;s flight path. In some systems, the surveillance system visually distinguishes symbology representing hazards according to whether the hazards lie along a predicted flight path, or within a specific tolerance of a predicted flight path. Distinctive representation of hazards enables a pilot to focus attention on hazards likely to be encountered by the aircraft. For example, in  FIG. 1 , the aircraft  10  flying along the predicted flight path  12  is likely to encounter hazard  14   a  whereas hazard  14   b  does not lie on the predicted flight path. Accordingly, a navigational display  16  might appear as in  FIG. 2  having hazard  14   a  represented in a solid color whereas hazard  14   b  is shown with hash marks. Distinctive representation may be accomplished by other markings, fill patterns, colors, and the like. In some systems, a surveillance system is programmed to issue audible, pictoral, and/or textual alerts when a hazard is found to lie along a predicted flight path. Audible alerts may distinguish alerts for on-path hazards from off-path hazards by means of the volume of the alert, the gender of the speaker, words used in the alert, and the like. Accordingly, the surveillance system distinguishes between on- and off-path hazards when determining whether to issue an alert. 
     The AP, FMS, surveillance system, and various control panels are typically embodied as discrete autonomous units, interfacing with one another in precisely defined ways. The criticality of each of the components means that each must be carefully tested and certified by regulatory agencies before being approved for installation. Modification of the components requires similar testing and regulatory approval. Modification of the AP and associated control panels in particular is an extremely complicated and expensive process because its role in control of the aircraft is so vital. 
     In one system, the surveillance system receives the planned flight path determined by the FMS. The surveillance system may also be notified of any constraint that has been imposed, such as an altitude ceiling or floor, though in some systems no notice is given and imposition of the constraint is detected by other means. The surveillance system does not receive notice when the constraint ceases to be active. Accordingly, the surveillance system is unable to determine when the aircraft is no longer subject to the constraint and is therefore unable to determine whether the predicted flight path will follow the constrained flight path or the unconstrained planned flight path. 
     This problem arises in the scenario of  FIGS. 3A and 3B  illustrating a planned flight path  18  in the vertical view. An aircraft  10  may follow an actual path  20  passing through, or “sequencing,” a waypoint  22  forming part of the planned path  18  within an area in which a constraint  28 , such as an altitude ceiling ( FIG. 3A ) or an altitude floor ( FIG. 3B ) is in effect. At point  30 , the actual path  20  of the aircraft  10  transitions from following the planned flight path  18  to conform to the constraint  28 . At point  32  the aircraft  10 , the aircraft  10  begins to follow the planned path  18  and directs itself toward waypoint  34 . In  FIG. 3A , the aircraft  10  transitions to the planned path  18  because it lies below the constraint  28 . In  FIG. 3B , the aircraft  10  transitions because the constraint  28  is changed to an altitude lying below the planned path  18 . At points  30  and  32  the surveillance system is not notified which path will be followed as the aircraft  10  moves forward. Accordingly, it is not apparent for which of the hazards  14 a- 14 c to provide alerts. 
     Accordingly, it would be an advancement in the art to provide systems and methods for resolving which of the constrained flight path and unconstrained flight path will be followed by the aircraft. It would be a further advancement in the art to provide such systems that do not require modification of the AP or the FMS. 
     SUMMARY OF THE INVENTION 
     The present invention selects whether the constrained flight path or unconstrained flight path will be followed by an aircraft by evaluating whether the current location of the aircraft is within a predetermined tolerance of a constrained path, taking into account prior determinations, and predicting an unconstrained path will be followed if the current position is not within the tolerance. 
     Systems and methods for predicted path selection include a controller, such as an autopilot (AP), directly or indirectly actuating control surfaces and propulsion systems of an aircraft to cause the aircraft to follow an actual path. The controller receives a planned path from a flight planner, such as an FMS. The controller also occasionally receives a constraint from a control panel, such as a Flight Control Unit (FCU) or Mode Control Panel (MCP), constraining the actual path followed by the aircraft in at least one direction, such as the vertical direction. The control panel provides an output indicating what the current constraints are, and the controller or FMS may provide output indicating that a constraint has been imposed. One or more of these outputs are provided to a surveillance system operable to detect hazards and may provide a display visually distinguishing on- and off-path hazards. 
     In some embodiments, the controller, the FMS, or both, do not provide an output to the surveillance system indicating that a constraint has been imposed. In such embodiments the constraint may be detected by analyzing the altitude history of the aircraft  10  to determine if the aircraft is descending onto a floor or ascending from a floor. For example, if an aircraft  10  that was descending levels off at an altitude, the surveillance system may assume that a floor has been encountered. Likewise, if an aircraft that was ascending levels off at an altitude, the surveillance system may assume that a ceiling has been encountered. 
     The surveillance system compares the current location of the aircraft to the constraint. If the separation between current location and the constraint is outside a predetermined tolerance, the surveillance system displays symbols lying on the planned path as critical. If the separation between the current location and the constraint is within the predetermined tolerance and the surveillance system otherwise determines that a constraint was activated, and then the surveillance system displays symbols lying on the constrained path as critical. Distinguishing of symbols may be accomplished by representing critical and non-critical hazards with differing colors or line styles or fill patterns. Distinguishing hazards as critical or non-critical may also be used in alerting algorithms. 
     As the aircraft continues forward, selections of the predicted path are validated. In one embodiment, if the aircraft has deviated from the constraint in the direction opposite the flight plan, perhaps due to wind or fuel burn, the FMS will typically guide the aircraft back toward the original flight plan and back into the constraint. Accordingly, the surveillance system may continue to select the constrained path for strategic purposes (e.g. because the aircraft is not within tolerance of the flight plan), or may choose to switch to a tactical display, based on immediate actual flight path (speed and direction) for the period in which the aircraft deviates from the constraint. As the FMS returns the aircraft to within a certain tolerance of the constraint altitude and the aircraft deviates from the planned path to again follow the constrained path, the surveillance system will again select the constrained path as the future path as well as portions of the planned path that do not violate the constraint. Adequate timeguarding may be used to ensures a smooth and consistent presentation to the crew. 
     In instances where the aircraft has deviated from the constraint in the direction of the flight plan, perhaps again due to winds or fuel burn, either the AP will force the aircraft back to the constraint altitude, such that the constrained path continues to be used for distinguishing hazards, or else not, in which case the surveillance system will switch to either the unconstrained path or a tactical display, depending on proximity to the FMS flight plan and on timeguarding. 
     As will be readily appreciated from the foregoing summary, the invention provides a reliable method for selecting which of a planned path and a constrained path will be followed by an aircraft for hazard coding purposes. The above described system does not require modification of the AP or FMS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
         FIG. 1  is a side schematic view of an aircraft, flight path, and intervening hazards; 
         FIG. 2  is an exemplary on-screen representation of coded hazard information; 
         FIGS. 3A and 3B  are side schematic views of an aircraft following a flight path subject to a constraint; 
         FIG. 4  is a schematic block diagram of components of an avionic control and navigational system formed in accordance with an embodiment of the present invention; 
         FIG. 5  is a schematic block diagram of a surveillance system suitable for performing predictive flight path selection for hazard coding formed in accordance with an embodiment of the present invention; 
         FIG. 6  is a process flow diagram of a method for predictive flight path selection formed in accordance with an embodiment of the present invention; 
         FIG. 7  is a logic diagram for performing predictive flight path selection formed in accordance with an embodiment of the present invention; and 
         FIGS. 8A and 8B  are side schematic views of an aircraft and constrained and unconstrained flight paths formed in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 4 , in one embodiment an aircraft  10  includes an avionic control system  36 , which may include a controller  38 , such as an Autopilot (AP)  38 , an, FMS  40 , and a surveillance system  42 . The controller  38  is coupled to the propulsion system  44  and control surfaces  46  of the aircraft  10 . The controller  38  is programmed to control the aircraft propulsion systems  44  and control surfaces  46  to achieve a desired trajectory. Manual controls  48  and external controls  50  provide inputs to the controller  38  to provide a trajectory. External controls  50  include directives from systems external to the aircraft  10  such as air traffic control (ATC) or other remote “fly by wire” type systems as may be applicable to manned or unmanned aircraft. The FMS  40  calculates a planned flight path between the current location of the aircraft  10  and a destination and provides a trajectory to the controller  38  to cause the controller  38  to fly the aircraft  10  along the planned flight path. The surveillance system  42  detects hazardous conditions through means such as radar, uploaded weather data, topographical data, air traffic data, and the like. The FMS  40  provides data relating to a planned path to the surveillance system  42  to enable the surveillance system to provide alerts indicating hazards that are located along the planned path or to mark on-path hazards as critical in a strategic display provided to the pilot. 
     The controller  38  or one of the control panels  48  may provide an input to the FMS  40  and/or surveillance system  42  indicating what the current constraints are. Alternatively, the input is provided to the FMS  40  and the FMS  40  provides an indication that the constraint has become active to the surveillance system  42 . In one embodiment, this is accomplished by metadata associated with a waypoint defining a planned flight path provided to the surveillance system  42 . The metadata may include a single bit that is set or reset to indicate that a waypoint is a constraint waypoint. 
     In some embodiments, the surveillance system  42  is not provided notice that a constraint has become active. In such embodiments, the surveillance system  42  may analyze the actual path followed by the aircraft to determine whether a constraint has become active and where the constraint is. For example, the aircraft  10  may ascend according to the planned path  18  and then level off at an altitude not indicated in the planned path  18  as a level off point. The surveillance system  42  may therefore conclude that a constraint has been imposed at the constraint altitude. An altitude floor may be detected in a like manner during descent of the aircraft  10 . The surveillance system  42  may also detect imposition of the constraint by analyzing one or more of the actual path of the aircraft  10 , the path  18  calculated by the FMS  40 , and analysis of flight control laws followed by the FMS, controller  38 , and/or other systems within the aircraft  10 . 
     Referring to  FIG. 5 , the surveillance system  42  includes one or more detection modules  52 , a path selection module  54 , a coding module  56 , and a display module  58 . A detection module  52  may process radar, uploaded weather, terrain data, air traffic data, and the like in order to evaluate the location of potential hazards. A path selection module  54  determines which of the constrained path and planned path will be used for hazard coding purposes. In one embodiment, the path selection module  54  evaluates the separation between the current position of the aircraft  10  and the constrained path. If the separation exceeds a certain tolerance, the path selection module  54  selects the planned flight as the future path purpose of distinguishing between on- and off-path hazards. If a constraint has been initiated and the separation is less than the tolerance, then the path selection module  54  selects the constrained path and portions of the planned path  18  that do not violate the constraint  28  as the future path for purposes of distinguishing between on- and off-path hazards. A coding module  56  determines which of the detected hazards lies along the path selected by the path selection module  54  in order to code symbols as on- or off-path in a symbolic display provided to the pilot. The display module  58  displays coded symbols representing the hazards on a screen or heads-up display. Alternatively, the display module  58  provides visible or audible alerts when a hazard is detected along the selected path. 
     Referring to  FIG. 6 , in one embodiment, the path selection module  54  executes a method  60  for determining which of the constrained path and planned path to use for hazard coding purposes. The method  60  includes determining  62  the current location of the aircraft  10 . Determining  62  the current location includes evaluating the altitude of the aircraft in instances where the constraint is an altitude constraint. The difference between the current location and the constraint is then evaluated  64  to determine whether the current location is within a predetermined tolerance of the constraint. Differences between the current location and the constraint may be caused by changes in aircraft position or changes in the value of the constraint. The tolerance may be a navigational tolerance substantially equal to the distance an aircraft  10  can deviate from an intended flight path and still be deemed to be following the flight path. Alternatively, the tolerance may be half or some other proportion, of the required vertical separation between aircraft under FAA regulations such as the Reduced Vertical Separation Minimum (RVSM) standards. Vertical separations under the RVSM currently range from 500 feet to 1000 feet depending on the altitude. 
     If not within tolerance, the path selection module  54  selects  66  the planned path as the future path that will be followed by the aircraft  10  for purposes of distinguishing on- and off-path hazards. If the aircraft&#39;s current location is within the tolerance, the method  60  includes evaluating  68  whether a constraint was initiated. Step  68  may therefore include evaluating whether a waypoint, such as the most recently sequenced waypoint, or “from point,” is a constraint waypoint. Alternatively, step  68  may include detecting initiation of constraint by other means, such as by detecting leveling off of the airplane at an altitude not on the flight path. If a constraint has not been initiated, the path selection module  54  selects  66  the planned path as the path to be followed by the aircraft  10 . If the waypoint is a constraint waypoint, the surveillance system  42  selects  70  the constrained path as the future path for purposes of providing alerts or distinguishing between on- and off-path hazards. 
       FIG. 7  illustrates a logic diagram implementing a method for selecting which of a constrained path and planned path will be followed by an aircraft  10 . Inputs to the logical circuit include the current altitude  80  of the aircraft  10 , such as a corrected barometric altitude from an air data computer (ADC); the constraint altitude  82 ; a tolerance  84 ; and the value  86 , or state, of a variable within the flight path generated by the FMS indicating whether the previously sequenced waypoint, or “from” point was a constraint waypoint. 
     The constraint  82  is subtracted  88  from the current altitude  80  to determine the difference therebetween. The absolute value of the difference is then calculated  90 . The tolerance  84  is subtracted  92  from the absolute value and the result is compared  94  to zero. If the absolute value is greater than zero, a status indicator  96  is set to indicate that the planned path is to be used for hazard coding. The status indicator  96  may be a set/reset flip flop having the comparison step  94  resetting the flip flop when the absolute value is greater than zero. 
     The value  86  indicating the status of the “from” waypoint is evaluated  98  to determine whether the value  86  indicates that the “from” waypoint is a constraint waypoint. If so, the status indicator  96  is updated to indicate that the constrained path is to be used for hazard coding purposes. Where the status indicator  96  is embodied as a set-reset flip flop, the result of the evaluation  98  is input to the set terminal of the flip flop. The status indicator  96  is coupled to the coding module  54  to indicate which of the constrained path and planned path to use for hazard coding. For status indicators  96  embodied as a set/reset flip-flop, an output of a logical one (1) indicates that the constrained path will be used whereas an output of a logical zero (0) indicates that the planned path will be used. 
     Referring to  FIGS. 8A and 8B , in one scenario an aircraft  10  has a planned flight path  110  at point  112 . However, an altitude ceiling  114  ( FIG. 5A ) or altitude floor  116  ( FIG. 5B ) constrains the aircraft  10  to follow a constrained path  124 . The FMS  40  may generate an updated planned path  120  based on the current location of the aircraft  10  at points  122  along the constrained path  124 . As the aircraft  10  passes through the boundary  126  of an area subject to a ceiling  114  or floor  116 , the surveillance system  42  in some systems is not notified that the ceiling  114  or floor  116  is no longer active. 
     To resolve this situation, where the current location of the aircraft  10  is separated from the constrained path  124  by a distance greater than a tolerance  128 , the path selection module  54  selects the updated planned path  120  as the future path for purposes of distinguishing between on- and off-path hazards. If a constraint has been initiated and the current location of the aircraft  10  is within the tolerance  128 , then the path selection module  54  selects the constrained path  124  and portions of the updated planned path  120  that do not violate the constraint  28  as the future path. 
     The above described novel method for selecting which of the constrained path  124  and updated planned path  120  will be followed by the aircraft  10  is effective to provide accurate hazard coding and hazard alerts. The FMS  40  is typically programmed to update the flight plan during ascent and descent such that the updated planned path  120  originates from the aircraft&#39;s current position, which is on or near the constrained path  124  when a constraint is active. Accordingly, differences in short-range hazard coding and alerts will not differ substantially between the constrained path  124  and updated planned path  120 . Long and medium range predictions may differ. However, where an aircraft deviates from a constrained path  124  while a constraint should be active, external or pilot input commands will reinstate the constraint, which may result in explicit notice to the surveillance system  42  that the constraint has become active as described above. The surveillance system  42  may also detect reinstating of the constraint by other means such as by detecting leveling off of the airplane at an altitude not on the planned path  120 . Until the constraint is reinstated, the assumption that the updated planned path  120  will remain accurate for short range hazard coding and other predictions inasmuch as the updated planned path  120  is constantly updated to reflect the current position of the aircraft. Where the constraint is no longer active, the assumption that the updated planned path  120  will be followed will also be accurate. 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.
A surveillance system detects potential hazards and alerts the pilot to them. The alerts can be modified to indicate proximity to the predicted path of the aircraft. An autopilot receives instructions from a flight management system (FMS) regarding a planned path and is subject to constraints preempting the planned path. The surveillance system selects which of the planned and a constrained path will be followed for alerting and hazard coding purposes. Means are disclosed to determine when the constrained path will be followed by comparing the current position of an aircraft, the planned path, and the constraint data. Current positions exceeding the tolerance cause the surveillance system to select the planned path as the future path to be followed. If initiation of a constraint has been detected and the current position is within the tolerance, the surveillance system selects the constrained path as the future path.