Abstract:
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 evaluating the current position of an aircraft, the planned path, and the constraint data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is related to Nonprovisional application Ser. No. 11/364,066 filed Feb. 28, 2006 and entitled PREDICTED PATH SELECTION SYSTEM AND METHOD FOR HAZARD CODING IN SELECTIVELY CONSTRAINED AIRCRAFT CONTROL SYSTEMS, which is hereby incorporated by reference. 
   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, pictorial, and/or textual alerts when a hazard is found to lie along a predicted flight path. 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. 
   BRIEF 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  10  is descending onto a floor or ascending up to a ceiling. 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. 
   In one embodiment, 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 selects either the planned path or a tactical path as the future path for purposes of distinguishing between on- and off-path hazards. The tactical path is a projection based on the current trajectory of the aircraft. 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. 
   In one embodiment, the constrained path is selected only if the planned path and the aircraft current location are both either above or below the constraint. In another embodiment, the surveillance system examines whether the aircraft is within a tolerance of either the planned path or the constrained path and selects the planned path or the constrained path if the aircraft lies within tolerance of either. In such embodiments if the aircraft does not lie within tolerance of either, then the surveillance systems selects the tactical path. 
   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 planned path, 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 planned 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 SEVERAL VIEWS OF THE DRAWING 
     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 logic diagram of a method for performing predictive flight path selection, in accordance with an embodiment of the present invention; 
       FIG. 7  is a side schematic view of an aircraft and constrained and unconstrained flight paths illustrating alternative methods for future path selection; and 
       FIG. 8  is a logic diagram of a method for performing predictive flight path selection, 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. 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. 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. 
     FIG. 6  is a logic flow diagram implementing a method for selecting a future path for purposes of identifying on- and off-path hazards. Inputs to the logic diagram include a tolerance  60 , a current altitude  62 , a constraint altitude  64 , and an altitude  66  for a waypoint forming part of the planned path, such as the waypoint  34  of  FIG. 2 . The tolerance  60  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. 
   The difference between the current altitude  80  and the constraint altitude  82  is calculated  68  and the difference between the constraint altitude and the waypoint altitude  66  is calculated  70 . The absolute value of the result of the calculation  68  is taken at step  72  and subtracted  74  from the tolerance  60 . The result of the calculations  68 ,  70 ,  74  are compared to zero at steps  76 ,  78 , and  80 , respectively, with values greater than zero resulting in a logical one and values less than zero resulting in a logical zero. 
   The result of the comparison step  76  is inverted  82  and input to the reset terminal  84  of a set/reset flip-flop  86 . The result of the comparison step  76  is also input to an AND gate  88 . The result of the comparison step  78  is inverted  90  and input into an AND gate  92 . The result of the comparison step  78  is also input to an AND gate  94 . The result of the comparison step  80  is input into the AND gate  92 . The result of the comparison step  80  is also inverted  96  and input into the AND gate  94 . The output of the two AND gates  92 ,  94  is input to an OR gate  98 . The output of the OR gate  98  is input into a time guard  100 . The time guard  100  outputs a logical one upon the input of a logical one. However, the time guard  100  outputs a logical zero only upon receiving a logical zero input for a predetermined timer period, such as a five second period. The output of the time guard  100  is input into the AND gate  88  and the output of the AND gate  88  is connected to the set terminal  102  of the set/reset flip-flop  86 . A logical one at the output of the set/reset flip-flop  86  indicates to the surveillance system that the constrained path will be followed by the aircraft  10 . 
   In operation, the logic diagram of  FIG. 6  causes the surveillance system to use the constrained path and portions of the planned path  18  that do not violate the constraint  28  as the future path if the aircraft  10  has not deviated from the constraint by an amount exceeding the tolerance  60  and if both the next waypoint and the aircraft are above the constraint or if both the next waypoint and the aircraft are below the constraint. The time guard  100  ensures that deviations of short duration will not cause the selected future path to change. 
   Referring to  FIG. 7 , in an alternative embodiment, the surveillance system  42  selects a future path according to the position of the aircraft  10  relative to the planned path  18  and the constraint  28 . At point  168  the aircraft is within a tolerance  106  of the constraint  28 , in such instances the constraint altitude is selected by the surveillance system  42  for distinguishing between on- and off-path hazards. At point  108  the aircraft  10  is outside the tolerance of the constraint  28  and has deviated from the constraint  28  away from the planned path  18 . At points such as point  108 , the FMS  40  typically generates an updated planned path  110  and guides the aircraft  10  back toward the planned path  18  and the constraint  28 . In such instances the surveillance system  42  uses a future path for distinguishing between on- and off-path hazards that includes the updated planned path  110  until the point where updated planned path  110  intersects the constraint altitude  28 . The future path in such instances may also includes the constraint altitude  28  after the point of intersection of the planned path  18  with the constraint  28  and portions of the planned path  18  below the constraint  28 . 
   At points  112 ,  114  the aircraft deviates from the constraint  28  in the direction of the planned path  18 . In such instances, the surveillance system  42  selects the planned path  18  as the future path for purposes of distinguishing hazards. If in fact, the deviation is due to factors such as fuel burn or wind rather than pilot input or the like, such as at point  112  the controller  38  will force the aircraft  10  back to within the tolerance distance  106  of the constraint  28  such and the surveillance system  42  will select the constrained path as the future path. If deviation toward the planned path  18  is intentional, such as at point  114  where the aircraft  10  is directed toward the waypoint  34 , the controller  38  will not cause the aircraft  10  to return to the constraint and the selection of the planned path  18  is validated. Selection of future paths and re-selection in response to changes in aircraft position take place in about the same amount of time it takes to update a display, so adequate time guarding ensures consistent presentation. 
   In some embodiments, the surveillance system  42  determines whether the aircraft is within a tolerance distance  106  of the constraint  28  or within a tolerance distance  116  of the planned path  18 . The surveillance system  42  selects the constrained path if the aircraft  10  is within the tolerance distance  116 . The surveillance system  42  selects the planned path  18  as the future path if the aircraft  10  is within the tolerance distance  116  of the planned path  18 . In instances where the aircraft  10  is within neither tolerance  106 ,  116 , the surveillance system  42  uses the current trajectory of the aircraft as the future path. 
   Referring to  FIG. 8 , in one embodiment a method  118  may be executed by the surveillance system  42 , or other system within the avionic control system  26  to determine which of the planned path  18  and the constraint  28  will be used for purposes of distinguishing hazards. At block  120  the method  118  evaluates whether the controller  38 , such as an autopilot (AP) provides an indication of when a constraint  28  has become active. Block  120  may be executed manually or automatically and may be executed at each iteration of the method  118  or only on an initial iteration. For example, a surveillance system  42  installed in an aircraft  10  that does not provide an indication that constraints have become active may be programmed or hardwired such that it is not necessary to execute block  120 . If indication is provided, then at block  122  the method  118  includes using whichever path is indicated by the controller  38 , whether the constraint  28 , planned path  18 , or a tactical path. 
   If the controller  38  does not provide an indication, at block  124 , the method  118  includes evaluating whether the aircraft  10  is within a tolerance of a constraint  28  determined by another means, such as by analysis of the flight path followed by the aircraft  10  in view of flight control laws followed by the controller  38 . 
   If the aircraft is not within tolerance of the constraint  28 , the method  118  includes evaluating at block  126  whether the aircraft  10  is within a predetermined tolerance of a planned path  18 , such as a flight plan generated by the FMS  40 . If the aircraft is within the predetermined tolerance of the planned path  18 , the constraint  28  is designated not active at block  128  and the planned path  18  is designated as the future path for purposes of distinguishing between on- and off-path hazards. If the aircraft  10  is not within the predetermined tolerance of the planned path  18 , then the method  118  includes concluding at block  130  that the constraint  28  is not active and that a, tactical path based on the current trajectory of the aircraft  10  will be used for distinguishing hazards. 
   If the aircraft  10  is found to lie within the constraint  28  at block  124 , then the method  118  includes evaluating at block  132  whether a constraint is already active (e.g. whether in the previous iteration of the method  118  it was determined that a constraint  28  was active). If not, at block  134 , the method  118  includes evaluating whether the aircraft  10  is within a predetermined tolerance of the planned path  18 , such as a flight plan generated by the FMS  40 . If the aircraft  10  is within the predetermined tolerance, then at block  136  whichever path currently being used for distinguishing hazards is deemed the future path for hazard coding, whether it is the planned path  18 , constraint  28 , or the like. 
   If the aircraft  10  is found to not lie within the predetermined tolerance of the planned path  18  at block  198 , then, whether the aircraft  10  is above the constraint  28  (or outside a horizontal constraint) is evaluated at block  138 . Evaluating whether the aircraft  10  is above the constraint  28  may include evaluating from what direction the aircraft approached the constraint  28 . Thus the aircraft  10  is deemed to be above the constraint  28  if the aircraft  10  descended to the constraint  28  and is deemed to be below the constraint  28  if the aircraft  10  ascended to the constraint  28 . If the aircraft  10  is not above the constraint  28 , then the constraint  28  is designated a ceiling (or a keep-in constraint if it is a horizontal constraint) at block  140 . If it is above the constraint  28 , then the constraint  28  is designated a floor (or a keep-out if it is a horizontal constraint) at block  142 . In either case, at block  144  the constraint  28  is designated as active such that it will be used as the future path for purposes of distinguishing between on- and off- path hazards. 
   Where the planned path  18 , such as a flight plan generated by the FMS  42 , is selected by the method  118  as the future path it is typically used as the future path until the constraint  28  is selected by the method  118 . In either case the planned path  18  will be used as the future path when distinguishing between on- and off-path hazards in the horizontal view. 
   The method  82  may also include selecting a constraint  28  as the future path in instances where the constraint  28  is not currently active. A constraint  28  will typically also be selected as the future path, or to define a portion of the future path, where a waypoint within the FMS flight path is designated as a constraint waypoint indicating that a constraint  28  will be active starting at that waypoint. 
   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 reference to the claims that follow.