Patent Publication Number: US-8977411-B2

Title: Providing a description of aircraft intent

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to and incorporates herein by reference in its entirety, co-pending U.S. patent application Ser. No. 13/901,603, concurrently filed and entitled “Providing a Description of Aircraft Intent” which claims priority to EP Patent Application No. 12382194.4, filed on May 24, 2012 in the Spanish Patent Office. 
     This application is related to and incorporates herein by reference in its entirety, co-pending U.S. patent application Ser. No. 13/901,606, concurrently filed and entitled “Providing a Description of Aircraft Intent” which claims priority to EP Patent Application No. 12382196.9, filed on May 24, 2012 in the Spanish Patent Office. 
     PRIORITY STATEMENT 
     This application claims the benefit of EP Patent Application No. 12382195.1, filed on May 24, 2012 in the Spanish Patent Office, the disclosure of which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present disclosure relates to providing a method of forming a description of aircraft intent expressed using a formal language. Such a description allows the path of an aircraft to be predicted unambiguously. 
     BACKGROUND 
     The ability to predict an aircraft&#39;s trajectory is useful for several reasons. By trajectory, a four-dimensional description of the aircraft&#39;s path is meant. The description may be the evolution of the aircraft&#39;s state with time, where the state may include the position of the aircraft&#39;s centre of mass and other aspects of its motion such as velocity, attitude and weight. 
     For example, air traffic management (ATM) would benefit from an improved ability to predict an aircraft&#39;s four-dimensional trajectory. Air traffic management is responsible for the safe separation of aircraft, a particularly demanding task in congested airspace such as around airports. ATM decision-support tools based on accurate four-dimensional trajectory predictions could allow a greater volume of aircraft to be handled while maintaining safety. 
     The ability to predict an aircraft&#39;s four-dimensional trajectory will also be of benefit to the management of autonomous vehicles such as unmanned air vehicles (UAVs), for example in programming flight plans for UAVs as well as in commanding and de-conflicting their trajectories. 
     In order to predict an aircraft&#39;s four-dimensional trajectory unambiguously, one must solve a set of differential equations that model both aircraft behaviour and atmospheric conditions. Different sets of differential equations are available for use, some treating the aircraft as a six degrees of freedom of movement system and others treating the aircraft as a point mass with three degrees of freedom of movement. In addition, to solve the equations of motion, information concerning the aircraft&#39;s configuration is required as it will respond differently to control commands depending upon its configuration. Hence, further degrees of freedom of configuration may require definition that describe the configuration of the aircraft. For example, three degrees of freedom of configuration may be used to define landing gear configuration, speed brake configuration and lift devices configuration. Accordingly, aircraft intent may need to close six degrees of freedom to define an unambiguous trajectory, three degrees corresponding to motion of the aircraft in three axes and the other three degrees corresponding to aircraft configuration. 
     The computation process requires inputs corresponding to the aircraft intent, for example a description of aircraft intent expressed using a formal language. The aircraft intent provides enough information to predict unambiguously the trajectory that will be flown by the aircraft. The aircraft intent is usually derived from a description of flight intent, that is more-basic information that does not allow an unambiguous determination of aircraft trajectory. Aircraft intent may comprise information that captures basic commands, guidance modes and control inputs at the disposal of the pilot and/or the flight management system. 
     Aircraft intent must be distinguished from flight intent. Flight intent may be thought of as a generalisation of the concept of a flight plan, and so will reflect operational constraints and objectives such as an intended or required route and operator preferences. Generally, flight intent will not unambiguously define an aircraft&#39;s trajectory, as it is likely to contain only some of the information necessary to close all degrees of freedom. Put another way, there are likely to be many aircraft trajectories that could be calculated that would satisfy a given flight intent. Thus, flight intent may be regarded as a basic blueprint for a flight, but that lacks the specific details required to compute unambiguously a trajectory. Thus additional information must be combined with the flight intent to derive the aircraft intent that does allow an unambiguous prediction of the four-dimensional trajectory to be flown. 
     Aircraft intent is expressed using a set of parameters presented so as to allow equations of motion to be solved. The parameters may be left open (e.g. specifying a range of allowable parameters) or may be specified as a particular value. The former is referred to as an instance of parametric aircraft intent to distinguish it from the latter that is referred to as complete aircraft intent or just as aircraft intent. The theory of formal languages may be used to implement this formulation: an aircraft intent description language provides the set of instructions and the rules that govern the allowable combinations that express the aircraft intent, and so allow a prediction of the aircraft trajectory. 
     Co-pending U.S. patent application Ser. No. 12/679,275 published as US 2010-0305781 A1, also in the name of The Boeing Company, describes aircraft intent in more detail, and the disclosure of this application is incorporated herein in its entirety by reference. Co-pending U.S. patent application Ser. No. 13/360,318 published as US 2012-0290154 A1, also in the name of The Boeing Company, describes flight intent in more detail, and the disclosure of this application is incorporated herein in its entirety by reference. 
     SUMMARY 
     Against this background and according to a first aspect, the present disclosure resides in a computer-implemented method of generating a description of aircraft intent expressed in a formal language that provides an unambiguous description of an aircraft&#39;s intended motion and configuration during a period of flight. The period of flight may be all or part of a flight from takeoff to landing, and may also include taxiing on the ground. 
     The method comprises obtaining a description of flight intent corresponding to a flight plan spanning the period of flight. This flight intent may be generated by a pilot or automatically generated by flight management software in the aircraft. 
     The method further comprises ensuring that the flight intent description is parsed to provide instances of flight intent, each instance of flight intent spanning a flight segment with the flight segments together spanning the period of flight. Thus the flight intent descriptions contained in the flight intent are reviewed and used to define flight segments that correspond to the time intervals for which the flight intent description is active. Thus, the period of flight is divided into a series of flight segments with the boundaries between flight segments corresponding to a flight intent description becoming active or expiring. The descriptions of flight intent are generally referred to herein as instances of flight intent and the period of time for which they are active is generally referred to herein as their execution interval. Ensuring that the parsing has been done may correspond to checking that the received description of flight intent has been parsed in this way, or it may correspond to performing the parsing. 
     For each flight segment, the method comprises generating an associated flight segment description that comprises one or more instances of flight intent. Each instance of flight intent provides a description of the aircraft&#39;s motion in at least one degree of freedom of motion thereby closing the associated degrees of freedom of motion. A flight segment has a flight segment description that may comprise one or more instances of flight intent. To generate aircraft intent that will unambiguously define a trajectory, all degrees of freedom of motion must be closed. This may be achieved using only a single instance of flight intent if that instance affects all degrees of freedom of motion. Generally, this is not the case and a flight segment description will have multiple instances of flight intent associated with it. Optionally, the method may also require one or more degrees of freedom of aircraft configuration to be closed. 
     The flight intent obtained may contain a complete description, that is all flight segments have all degrees of freedom of motion defined. However, in general there will be some incomplete flight segment descriptions requiring further instances of flight intent to close still-open degrees of freedom of motion. Consequently, the method further comprises identifying flight segments where not all degrees of freedom of motion are closed and completing the identified flight segments by adding one or more instances of flight intent to close all degrees of freedom of motion. 
     Once all flight segment descriptions are completed by adding instances of flight intent to close all degrees of freedom of motion, the aircraft intent may be formed by collating the flight segment descriptions. The instances of flight intent may be expressed using a formal language thereby reducing the complexity of collating the instance of flight intent into a description of aircraft intent expressed in a formal language. Where degrees of freedom of configuration are used, the method may further comprise identifying flight segments where not all degrees of freedom of configuration are closed and completing the identified flight segments by adding one or more instances of flight intent to close all degrees of freedom of configuration, and collating the flight segment descriptions thereby providing a description of aircraft intent for the period of flight expressed in a formal language. 
     The task of adding instances of flight intent to complete the flight segment descriptions may be assisted by using pre-defined strategies. These strategies may be stored. The strategies correspond to templates upon which instances of flight intent may be based. A suitable strategy may be selected, and an instance of flight intent corresponding to that strategy may be added to the flight segment description. In order to aid selection of a suitable strategy, the strategies may be characterised and/or identified. For example, different strategies may be formulated to close different degrees of freedom. In this way, a class of strategies may be formed that relate to the vertical motion of the aircraft. This class may be further divided into groups corresponding to climbing, descending and maintaining altitude. Different strategies may also apply to different phases of flight, such as take off, climb out, cruise, descent, final approach, landing and taxiing. Strategies may also be identified according to the phase of flight to which they relate. Then, a flight segment may be reviewed to determine the current phase of flight, and an appropriate strategy selected. For example, a flight segment may apply to final approach, in which case strategies may be selected applying to this phase of flight and that may be based on templates that propose decreases in altitude and decreases in speed. Strategies may also be prioritised. 
     The step of adding an instance of flight intent may include adding a composite describing the flight intent, for example using a flight intent description language. The composite may correspond to a combination of flight intent descriptions, such as a combination of descriptions of flight intent closing more than one degree of freedom. For example, a composite may define motion aspects and also configuration aspects. In addition, the composite may define a parameter range thereby forming a parametric aircraft intent. Parametric aircraft intent contains information to close all degrees of freedom of motion, but as ranges of parameters are specified rather than specific values, the parametric aircraft intent does not define a unique trajectory. In a sense, parametric aircraft intent bridges flight intent that may not contain information relating to all degrees of freedom and aircraft intent that closes all degree of freedom with specific parameter values that allows a unique trajectory to be calculated. 
     The method may further comprise optimising the parametric aircraft intent by determining an optimal value for the parameter of each parameter range. This may be done in a variety of ways, although evolutionary techniques such as genetic evolution offer a good choice. For example, the method may comprise generating initial parameter values thereby forming a model aircraft intent. These initial parameter values may then be optimised, but a way of determining whether changes to the parameter values result in an improvement or not must be used. To this end, the method may comprise defining a merit function against which the aircraft intent may be measured. That is, the method may comprise calculating a trajectory from the model aircraft intent and calculating a merit function value for the trajectory using the merit function. In order to optimise the parameter values, the method may comprise repeated iterations of amending the parameter values, calculating the resulting trajectory and calculating the resulting merit function value. Changes in the merit function value may be used to determine whether the aircraft intent has improved. 
     The merit function may be based upon objectives pertaining to the period of flight. For example, the objectives may be pre-defined by operators of the aircraft, such as airlines or pilots. These objectives may be stored in a user preferences model. Objectives relating to user preferences are usually directed to safety and efficiency. The objectives may define functions to be optimised like: operational revenue such as maximising payload weight, minimising fuel consumption, minimising over-flight fees, minimising landing fees, minimising maintenance costs; environmental impact such as minimising COx and NOx emissions, minimising noise emissions; and quality of service such as increasing passengers&#39; comfort (e.g. avoiding sudden and extreme manoeuvres) and reducing delays. 
     In general, a description of a set of initial conditions of the aircraft at the start of the period of flight will be needed. This description of the initial conditions may be part of the description of flight intent obtained. Alternatively, the method may further comprise obtaining a description of a set of initial conditions of the aircraft at the start of the period of flight and ensuring that the flight intent description and the initial conditions are parsed to provide the instances of flight intent. 
     As noted above, the instances of flight intent may include descriptions of aircraft configuration. The aircraft configuration may be grouped into degrees of freedom that require definition in the aircraft intent. For example, three degrees of freedom of configuration may be required, one degree defining the configuration of the landing gear, one degree defining the configuration of high lift devices such as flaps, and one degree defining the configuration of the speed brakes. Landing gear may be defined as either stowed or deployed, and the speed brakes may also be defined as stowed and deployed. High lift configurations may have many more states, for example corresponding to stowed and several extended positions. 
     Consequently, an aircraft may be defined by aircraft intent having six degrees of freedom, namely three degrees of freedom of motion, and three degrees of freedom of configuration corresponding to landing gear, high lift devices and speed brakes. 
     The three degrees of freedom of motion may comprise one degree corresponding to the lateral profile and two degrees corresponding to the vertical profile. To close the two degrees relating to the vertical profile, flight intent may be required that provides a description of two out of the following three aspects of aircraft motion: vertical path, speed and propulsion. 
     The method may further comprise identifying flight segments where not all degrees of freedom of aircraft configuration are defined and completing the identified flight segments by adding one or more instances of flight intent to define all degrees of freedom of aircraft configuration. For example, an instance of flight intent may be added that defines extending the flaps during the final approach phase of a flight. 
     Constraints may apply to the aircraft during the flight period, for example air traffic constraints that govern no fly zones or other restricted areas. Air traffic constraints may limit the trajectory followed by an aircraft in one or more dimensions. They may include altitude constraints, speed constraints, climb/descend constraints, heading/vectoring/route constraints, standard procedures constraints, route structures constraints, SID constraints, STAR constraints, and coordination and transfer constraints (e.g. speed and altitude ranges and the location of entrance and exit points which should be respected by any flight when it is moving from one sector to the next one). 
     The method may comprise checking that the aircraft intent generate as described above meets one or more constraints. If any constraints are not met, the method may comprise returning to the parsed flight intent description derived from the obtained flight intent. That is, all additional information added to the flight intent originally obtained is effectively removed and a fresh start made. Then the method may comprise identifying flight segments where not all degrees of freedom are closed and completing the identified flight segments selecting an alternative strategy from the plurality of stored strategies and adding an instance of flight intent corresponding to that strategy to close all degrees of freedom. That is, the strategy used previously to augment the flight segment description is not used in the next iteration. Then, the flight segment descriptions are collated thereby providing an alternative description of aircraft intent for the period of flight expressed in a formal language. The step of checking that the aircraft intent meets one or more constraints pertaining to the aircraft is then repeated, including a further iteration of returning to the parsed flight intent and trying further alternative strategies. Thus the method loops until an aircraft intent satisfying all constraints is generated or until the method otherwise terminates (e.g. self-checking suggests a valid aircraft intent cannot be generated or because of a time out). 
     Alternatively, a more targeted approach may be made to improving the aircraft intent. For example, if checking the aircraft intent indicates that one or more constraints pertaining to the aircraft are not met, the method may continue by returning to the flight description derived by parsing the obtained flight intent for the flight segments where constraints are not met. For these flight segments, the flight segment may be completed by selecting an alternative strategy from the plurality of stored strategies and adding an instance of flight intent corresponding to that strategy to close all degrees of freedom. 
     All flight segment descriptions may then be collated thereby providing an alternative description of aircraft intent for the period of flight expressed in a formal language. The step of checking that the aircraft intent meets one or more constraints pertaining to the aircraft is repeated as necessary, such that the method again loops until an aircraft intent is generated that meets all the constraints. 
     Any of the above methods may further comprise calculating a trajectory for the period of flight from the aircraft intent for use in a variety of applications. For example, the trajectory may be made available to a pilot for inspection. Alternatively, the aircraft may be made to fly the trajectory either manually by a pilot or automatically by an autopilot. The aircraft intent and trajectory may be used by air traffic control. For example, air traffic control may compare trajectories found in this way to identify conflicts between aircraft. 
     As will be appreciated from the above, computers and computer processors are suitable for implementing the present disclosure. The terms “computer” and “processor” are meant in their most general forms. For example, the computer may correspond to a personal computer, a mainframe computer, a network of individual computers, laptop computers, tablets, handheld computers like PDAs, or any other programmable device. Moreover, alternatives to computers and computer processors are possible. Programmed electronic components may be used, such as programmable logic controllers. Thus, the present disclosure may be implemented in hardware, software, firmware, and any combination of these three elements. Further, the present disclosure may be implemented in the computer infrastructure of an aircraft, or on a computer readable storage medium having recorded thereon a computer program comprising computer code instructions, when executed on a computer, cause the computer to perform one or more methods of the invention. All references above to computer and processor should be construed accordingly, and with a mind to the alternatives described herein. 
     Other aspects of the invention, along with preferred features, are set out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present disclosure may be more readily understood, preferred embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  shows a system for computing an aircraft&#39;s trajectory using flight intent and aircraft intent according to an embodiment of the invention; 
         FIG. 2  shows the system of  FIG. 1  in greater detail according to an embodiment of the invention; 
         FIG. 3  shows elements of the flight intent description language according to an embodiment of the invention; 
         FIG. 4  is a diagram showing the different types of trigger conditions according to an embodiment of the invention; 
         FIG. 5  is a method of deriving aircraft intent according to an embodiment of the invention; 
         FIG. 6  shows how flight intent may be completed to form parametric aircraft intent according to an embodiment of the invention; 
         FIG. 7  shows how parametric aircraft intent may be optimised to provide a complete aircraft intent according to an embodiment of the invention; 
         FIG. 8  shows a lateral flight profile to be followed when approaching an airport; 
         FIG. 9  shows vertical flight profile restrictions that apply to the approach shown in  FIG. 8 ; and 
         FIG. 10  shows two vertical flight profiles that meet the restrictions shown in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
     A system for computing an aircraft&#39;s trajectory  100  from a description of aircraft intent that is in turn derived from flight intent is shown in  FIGS. 1 and 2 . 
       FIG. 1  shows a basic structure of how flight intent  101  may be used to derive aircraft intent  114 , and how aircraft intent  114  may be used to derive a description of an aircraft&#39;s trajectory  122 . In essence, flight intent  101  is provided as an input to an intent generation infrastructure  103 . The intent generation infrastructure  103  determines aircraft intent  114  using the instructions provided by the flight intent  101  and other inputs to ensure a set of instructions is provided as aircraft intent  114  that will allow an unambiguous trajectory  122  to be calculated. This process may comprise intermediate steps of enriching the flight intent  101  and augmenting the enriched flight intent to provide parametric aircraft intent, before finally optimising the parametric aircraft intent to produce the aircraft intent  114 . 
     The aircraft intent  114  output by the intent generation infrastructure  103  may then be used as an input to a trajectory computation infrastructure  110 . The trajectory computation infrastructure  110  calculates an unambiguous trajectory  122  using the aircraft intent  114  and other inputs that are required to solve the equations of motion of the aircraft. 
       FIG. 2  shows the system of  FIG. 1  in further detail. As can be seen, the intent generation infrastructure  103  receives a description of the flight intent  101  as an input along with a description of the initial state  102  of the aircraft (the initial state  102  of the aircraft may be defined as part of the flight intent  101 , in which case these two inputs are effectively one and the same). The intent generation infrastructure  103  comprises an intent generation engine  104  and a pair of databases, one storing a user preferences model  105  and one storing an operational context model  106 . 
     The user preferences model  105  embodies the preferred operational strategies governing the aircraft and may correspond to both constraints and objectives, e.g. the preferences of an airline with respect to loads (both payload and fuel); how to react to meteorological conditions such as temperature, wind speeds, altitude, jet stream, thunderstorms and turbulence as this will affect the horizontal and vertical path of the aircraft as well as its speed profile; cost structure such as minimising time of flight or cost of flight, maintenance costs, environmental impact; communication capabilities; and security considerations. The user preferences model  105  may be used when converting the flight intent input  101  to the aircraft intent output  114 —in any of the steps of enriching the flight intent  101 , augmenting the enriched flight intent to provide parametric aircraft intent, completing the aircraft intent—by providing further detail, as will be described in more detail below. 
     The operational context model  106  embodies constraints on use of airspace. For example, the operational context model  106  may contain details of restricted airspace and of standard terminal arrival routes (STARS) and standard instrument departures (SIDS) to be followed into and out from an airport. The operational context model  106  is also used when converting the flight intent input  101  to the aircraft intent output  114 —in any of the steps of enriching the flight intent  101 , augmenting the enriched flight intent to provide parametric aircraft intent, completing the aircraft intent—by providing further detail, as will be described in more detail below. 
     The intent generation engine  104  uses the flight intent  101 , initial state  102 , user preferences model  105  and operational context model  106  to provide the aircraft intent  114  as its output. 
       FIG. 2  shows that the trajectory computation infrastructure  110  comprises a trajectory engine  112 . The trajectory engine  112  requires as inputs both the aircraft intent  114  described above and also the initial state  116  of the aircraft. The initial state  116  of the aircraft may be defined as part of the aircraft intent  114  in which case these two inputs are effectively one and the same. For the trajectory engine  112  to provide a description of the computed trajectory  122  for the aircraft, the trajectory engine  112  uses two models: an aircraft performance model  118  and an Earth model  120 . 
     The aircraft performance model  118  provides the values of the aircraft performance aspects required by the trajectory engine  112  to integrate the equations of motion. These values depend on the aircraft type for which the trajectory is being computed, the aircraft&#39;s current motion state (position, velocity, weight, etc) and the current local atmospheric conditions. 
     In addition, the performance values may depend on the intended operation of the aircraft, i.e. on the aircraft intent  114 . For example, a trajectory engine  112  may use the aircraft performance model  118  to provide a value of the instantaneous rate of descent corresponding to a certain aircraft weight, atmospheric conditions (pressure altitude and temperature) and intended speed schedule (e.g. constant calibrated airspeed). The trajectory engine  112  will also request from the aircraft performance model  118  the values of the applicable limitations so as to ensure that the aircraft motion remains within the flight envelope. The aircraft performance model  118  is also responsible for providing the trajectory engine  112  with other performance-related aspects that are intrinsic to the aircraft, such as flap and landing gear deployment times. 
     The Earth model  120  provides information relating to environmental conditions, such as the state of the atmosphere, weather conditions, gravity and magnetic variation. 
     The trajectory engine  112  uses the inputs  114  and  116 , the aircraft performance model  118  and the Earth model  120  to solve a set of equations of motion. Many different sets of equations of motion are available that vary in complexity, and that may reduce the aircraft&#39;s motion to fewer degrees of freedom by means of a certain set of simplifying assumptions. For example, equations of motion describing aircraft motion in six degrees of freedom of motion may be used. A simplified set of equations of motion may use only three degrees of freedom of motion. 
     The trajectory computation infrastructure  110  may be air-based or land-based. For example, the trajectory computation infrastructure  110  may be associated with an aircraft&#39;s flight management system that controls the aircraft on the basis of a predicted trajectory that captures the airline operating preferences and business objectives. The primary role for land-based trajectory computation infrastructures  120  is for air traffic management. 
     Using a standardised approach to describing an aircraft&#39;s trajectory allows greater interoperability between airspace users and managers. It also allows greater compatibility between many of the legacy software packages that currently predict trajectories, even if interpreters are required to convert information from the standard format into a proprietary format. 
     Moreover, a standardised approach also works to the benefit of flight intent  101  and aircraft intent  114 . For example, flight intent  101  may be expressed using the instructions and other structures of the formal language implementation used to express aircraft intent  114 . In addition, flight intent  114  provides a user with an extension to the aircraft intent language that allows flight intent  114  to be formulated where only certain aspects of aircraft&#39;s motion are known. By using a common expression format, these instances of flight intent may be easily augmented to form instances of aircraft intent. 
     As flight intent  101  may be thought of as a broader and generalised form of aircraft intent  114 , it is useful to start with a consideration of aircraft intent  114  such that key concepts also used in generating flight intent  114  may be introduced. 
     Aircraft Intent 
     The aircraft intent description is an expression of a set of instructions in a formal language, an aircraft intent description language, which defines unambiguously the trajectory  122  of the aircraft. This expression is used by the trajectory computation engine  112  to solve the equations of motion that govern the aircraft&#39;s motion. To solve the equations, the configuration of the aircraft must be specified also. For example, configuration information may be required to resolve the settings of the landing gear, speed brakes and high lift devices. Hence, the aircraft intent  114  comprises a set of instructions including both configuration instructions that describe completely the aerodynamic configuration of the aircraft and motion instructions that describe unambiguously how the aircraft is to be flown and hence the resulting motion of the aircraft. As the motion instructions and the configuration instructions are both required to define uniquely the aircraft&#39;s motion, they are together referred to herein as the defining the degrees of freedom: motion instructions relate to the degrees of freedom of motion and configuration instructions relate to the degrees of freedom of configuration. For example, six degrees of freedom may be used to describe the aircraft such as lateral path (motion), vertical path (motion), speed (motion), landing gear (configuration), high lift devices (configuration) and speed brakes (configuration). 
     There exist in the art many different sets of equations of motion that may be used to describe an aircraft&#39;s motion. The sets of equations generally differ due to their complexity. In principle, any of these sets of equations may be used with the present disclosure. The actual form of the equations of motion may influence how the aircraft intent description language is formulated because variables that appear in the equations of motion also appear in the instructions defining the aircraft intent  114 . However, the flight intent  101  is not constrained in this way in that it may express flight intent  114  generally. Any detail specific to the particular equations of motion to be used need not be specified in the flight intent  101 , and may be added when forming the aircraft intent  114 . 
     The aircraft intent description language is a formal language whose primitives are the instructions. The grammar of the formal language provides the framework that allows individual instructions to be combined into composites and then into sentences that describe flight segments. Each flight segment contains a complete set of instructions that close the degrees of freedom of motion and so unambiguously defines the aircraft trajectory  122  over its associated flight segment. 
     Instructions may be thought of as indivisible pieces of information that capture basic commands, guidance modes and control inputs at the disposal of the pilot and/or the flight management system. Each instruction may be characterised by three main features: effect, meaning and execution interval. The effect is defined by a mathematical description of its influence on the aircraft&#39;s motion. The meaning is given by its intrinsic purpose and is related to the operational purpose of the command, guidance mode or control input captured by the instruction. The execution interval is the period during which the instruction is affecting the aircraft&#39;s motion. The execution of compatible instructions may overlap, while incompatible instructions cannot have overlapping execution intervals (e.g. instructions that cause a conflicting requirement for the aircraft to ascend and descend would be incompatible). 
     Lexical rules capture all the possible ways of combining instructions into aircraft intent  114  such that overlapping incompatible instructions are avoided and so that the aircraft trajectory is unambiguously defined. 
     Flight Intent 
     The definition of a specific aircraft trajectory is the result of a compromise between a given set of objectives to be met and a given set of constraints to be followed. These constraints and objectives are included as part of the flight intent  101  that could be considered as a flight blueprint. Importantly, flight intent  101  does not have to determine the aircraft motion unambiguously: in principle, there may be many trajectories that fulfil the set of objectives and constraints encompassed by a given flight intent  101 . An instance of flight intent  101  will generally give rise to a family of aircraft intents  114 , each instance of aircraft intent  114  resulting in a different unambiguous trajectory. For example, flight intent may define a lateral path to be followed but may not specify a vertical path to be followed over the same execution interval: many aircraft intents could be generated from this flight intent, each aircraft intent corresponding to a different vertical profile. 
     Thus, flight intent  101  must normally be supplemented with enough information to allow a unique aircraft intent  114  to be determined and thus a unique trajectory. Adding to the flight intent  101  to complete an aircraft intent  114  is the responsibility of the intent generation engine  104 , whereas the trajectory engine  112  assumes responsibility for determining the corresponding trajectory  122  from the aircraft intent  114 . 
     As explained above, each instance of flight intent  101  contains trajectory-related information that does not necessarily univocally determine the aircraft motion, but instead usually incorporates a set of high-level conditions that define certain aspects that the aircraft should respect during its motion (e.g. following a certain route, keeping a fixed speed in a certain area). The flight intent  101  is augmented to capture key operational objectives and constraints that must be fulfilled by the trajectory (e.g. intended route, operator preferences, standard operational procedures, air traffic management constraints, etc.) by reference to the user preferences model  105  and the operational context model  106 . 
     Considering the information that is used directly to generate and augment the flight intent  101 , it is possible to group similar elements into three separate structures: flight segments, operational context and user preferences. 
     The flight segments combine to form the flight path to be followed by the aircraft during the flight, i.e. the four-dimensional trajectory is made up of a series of successive flight segments. As explained above with respect to the operational context model  106 , the operational context may include the set of air traffic management constraints that may limit the trajectory followed by an aircraft in one or more dimensions. They may include altitude constraints, speed constraints, climb/descend constraints, heading/vectoring/route constraints, standard procedures constraints, route structures constraints, SID constraints, STAR constraints, and coordination and transfer constraints (e.g. speed and altitude ranges and the location of entrance and exit points which should be respected by any flight when it is moving from one sector to the next). These constraints may be retrieved from the operational context model  106  and used to enrich the flight intent  101 . 
     As explained above with respect to the user preferences model  105 , user preferences are usually directed to safety and efficiency, and generally differ from one user (such as an airline or pilot) to another. The most common user preferences relate to: increasing operational revenue such as maximising payload weight to be flown, minimising fuel consumption, minimising over-flight fees, minimising landing fees, and minimising maintenance costs; environmental impact such as minimising COx and NOx emissions, minimising noise emissions; and quality of service such as increasing passengers&#39; comfort (e.g. avoiding sudden and extreme manoeuvres, avoiding turbulence) and reducing delays. These preferences may correspond to constraints or objectives. These constraints and objectives may be retrieved from the user preferences context model  105  and used to enrich the flight intent  101 . 
     Flight Intent Description Language (FIDL) 
     It is proposed to represent flight intent using a formal language, composed of a non-empty finite set of symbols or letters, known as an alphabet, which are used to generate a set of strings or words. A grammar is also required, namely a set of rules governing the allowable concatenation of the alphabet into strings and the strings into sentences. 
     The alphabet comprises three types of letters, as shown in  FIG. 3 : flight segment descriptions, constraints and objectives. A sentence is formed by the proper combination of these elements following grammatical rules that will be described below. A sentence is an ordered sequence of flight segment descriptions, i.e. ordered according to when they occur, in which different constraints and objectives are active to influence the aircraft motion. 
     Flight segment descriptions, within the alphabet, represent the intent of changing the aircraft motion state from one state into another (e.g. a translation from one 3D point to another 3D point, a turning between two courses, an acceleration between two speeds or an altitude change). A flight segment may be characterised in its flight segment description by two aircraft motion states identified by a condition or event that establishes certain requirements for the trajectory to be flown between these states. These conditions represent the execution interval of the flight segment. The conditions may close one or more degrees of freedom during the flight segment, including both degrees of freedom of motion and of configuration. 
     Constraints represent restrictions on the trajectory, as described above, and the constraints may be achieved by making use of the open degrees of freedom that are available during the applicable flight segment(s). 
     Objectives, as described above, represent a desire relating to the trajectory to maximize or minimize a certain functional (e.g. cruise to minimise cost). The objectives may be achieved by making use of the open degrees of freedom that are available during the applicable flight segment(s), excluding those that are used to respect the constraints affecting that flight segment(s). 
     Combining these three elements it is possible to build words as valid FIDL strings. For example, the flight intent information “fly from waypoint RUSIK to waypoint FTV” can be expressed by an FIDL word containing a flight segment description whose initial state is defined by the coordinates of waypoint RUSIK and whose final state is defined by the coordinates of waypoint FTV. This flight intent description could be enriched by a constraint such as “maintain flight level above 300 (FL300)”. In the same way, it would be possible to add information to this FIDL word regarding some objectives over the trajectory such as maximise speed. To ensure that any constraint or objective is compatible with a flight segment description, the affected aspect of aircraft motion or configuration, expressed as a degree of freedom, should not have been previously closed. In the previous example, the flight level constraint is compatible with the flight segment description because the flight segment description does not define any vertical behaviour. Often constraints and objectives will extend over a sequence of flight segments and so are added to multiple flight segment descriptions. 
     The attributes of a flight segment description are effect, execution interval and a flight segment code. The effect provides information about the aircraft behaviour during the flight segment, and could range from no information to a complete description of how the aircraft is flown during that flight segment. The effect is characterised by a composite which is an aggregated element formed by groups of aircraft intent description language (AIDL) instructions or is a combination of other composites, but need not meet the requirement for all degrees of freedom to be closed. 
     The execution interval defines the interval during which the flight segment description is active, fixed by means of the begin and end triggers. The begin and end triggers may take different forms, as indicated in  FIG. 4 . Explicit triggers  310  are divided into fixed  312  and floating  314  triggers. Fixed triggers  312  correspond to a specified time instant for starting or ending an execution interval such as to set an airspeed at a fixed time. Floating triggers  314  depend upon an aircraft state variable reaching a certain value to cause an execution interval to start or end, such as keep airspeed below 250 knots until altitude exceeds 10,000 feet. Implicit triggers  320  are divided into linked  322 , auto  324  and default  326  triggers. A linked trigger  322  is specified by reference to another flight segment, for example by starting when triggered by the end trigger of a previous flight segment. Auto triggers delegate responsibility for determining whether the conditions have been met to the trajectory computation engine  112 , for example when conditions are not known at the intent generation time, and will only become apparent at the trajectory computation time. Default triggers represent conditions that are not known at intent generation, but are determined at trajectory computation because they rely upon reference to the aircraft performance model. 
     Constraints could be self-imposed by the aircraft operator such as avoid over-flight fees (in which case information relating to the constraints are stored in the user preferences model  105 ), or by the operational context or by air traffic management such as follow a STAR flight path (in which case information relating to the constraints are stored in the operational context model  106 ). In any case, the final effect over the aircraft motion will be a limitation on the possible aircraft behaviour during a certain interval. Constraints may be classified according to the degree(s) of freedom affected by the constraint which is useful when determining whether it can be applied to a flight segment description (i.e. when determining whether that degree of freedom is open and so available). 
     Objectives are defined as a functional that can be combined into a merit function whose optimisation drives the process of finding the most appropriate trajectory. The functional may define explicitly the variable or variables used for the optimisation (e.g. altitude, climb rate, turn radius), and may return the value for them that minimises or maximises the functional. The variables of control are related to the degrees of freedom used to achieve the functional. Therefore, they specify the intention of using one or more degrees of freedom to achieve the optimisation. When no variable of control is defined, the aircraft intent generation process will use any remaining open degree of freedom to achieve the optimisation. Objectives may be classified considering the degree of freedom that can be affected by the objective effect. 
     The FIDL grammar is divided in lexical and syntactical rules. The former contains a set of rules that governs the creation of valid words using flight segment descriptions, constraints and objectives. The latter contains a set of rules for the generation of valid FIDL sentences. 
     The lexical rules consider the flight segment descriptions as the FIDL lexemes, i.e. the minimal and indivisible element that is meaningful by itself. Constraints and objectives are considered as FIDL prefixes (or suffixes) which complement and enhance the meaning of the lexemes but do not have any sense individually. Therefore the lexical rules describe how to combine the lexemes with the prefixes in order to ensure the generation of a valid FIDL string. They also determine whether a string formed by lexemes and prefixes is valid in the FIDL. 
     The lexical rules are based on the open and closed degrees of freedom that characterise a flight segment. If the flight segment has no open degree of freedom, it means that the associated lexemes are totally meaningful and their meaning cannot be complemented by any prefix (constraint or objective). For lexemes whose flight segments have one or more open degrees of freedom, as many prefixes as open degrees of freedom may be added. 
     The FIDL syntactical rules are used to identify if a sentence formed by FIDL words is valid or not. A well-formed FIDL sentence is defined by a sequence of concatenated flight segment descriptions, enriched with constraints and objectives, that represent a chronological succession of aircraft motion states during a period of flight. 
     Generation of Aircraft Intent 
     A method of generating aircraft intent according to an embodiment of the present disclosure will now be described with reference to  FIG. 5 . 
     As step  510 , the intent generation infrastructure  103  is initialised to create a flight intent instance to be used in a specific operational context, for a specific user and for a specific aircraft model. 
     At step  520 , the flight intent  101  and initial conditions  102  are received by the intent generation infrastructure  103 , and are parsed to create flight segments and corresponding flight intent objects to span each flight segment. Each flight object will contain a flight segment description. In some embodiments, the parsed flight intent will contain flight objects already augmented by constraints or objectives, for example as already provided by an operator when defining the original flight intent  101  as part of a mission plan or the like. 
     The parsed flight intent is provided to the intent generation engine  104  so that it may be converted to aircraft intent  114 . The intent generation engine  104  has at its disposal a set of strategies and heuristics to allow it to convert the flight intent  101  into aircraft intent  114  by adding information to the parsed flight intent objects to close all degrees of freedom. This process comprises steps  530  to  560  shown in summary in  FIG. 5 , and as shown in more detail in  FIGS. 6 and 7 . 
     At step  530 , the intent generation engine  104  passes the flight intent objects to the user preferences model  105  and operational context model  106  so that the flight intent  101  may be enriched. The intent generation engine  104  identifies constraints and objectives from the models  105  and  106  that are relevant to each flight intent object (e.g. not all the constraints included in the operational context are likely to apply to a specific route or to all flight segments on a particular flight path). How relevant constraints and objectives are identified is described in more detail below. The intent generation engine  104  enriches the flight intent  101  by expanding the flight intent objects to add the relevant constraints and objectives to the associated flight segment descriptions according to the syntactical and lexical rules imposed by the flight intent description language. The output of step  530  is an enriched flight intent. 
     At step  540 , the intent generation engine  104  identifies flight intent objects of the enriched flight intent having open degrees of freedom. The intent generation engine  104  fills these flight intent objects with composites to close all degrees of freedom. This process is driven by several completion strategies based on the sequence and type of any constraints included in the enriched flight intent object. In general, constraints will not cause a particular parameter to be uniquely specified, but instead usually set a range of parameters. For example, a constraint added to a flight intent object may specify a maximum airspeed to be flown leaving open a range of airspeed parameters. 
     As step  550 , the intent generation engine  104  optimises the parametric aircraft intent. This optimisation process takes all the parameter ranges specified in the parametric aircraft intent, and calculates optimal values for each parameter by optimising an overall merit function that is calculated from all the objectives present in the enriched flight intent. The parametric ranges specified in each flight intent object are then replaced by the optimal values. 
     At the end of the optimisation step  550 , the method proceeds to step  560  where the intent generation engine  104  checks that the predicted trajectory for the aircraft intent fulfils all constraints defined by the operational context model  106 , user preferences model  105  and the flight intent  101 . 
     If all constraints are fulfilled, the method ends at step  570  where completed aircraft intent  114  is provided and/or a description of the corresponding trajectory  122  is provided. If any constraints are found not to be fulfilled, the method returns to step  540  where the original set of enriched flight intent provided at step  530  is retrieved and the intent generation engine  104  uses an alternative strategy to complete the flight intent by inserting composites. The method then continues as before through steps  540 ,  550  and  560 . A number of iterations of the loop may be performed in an attempt to find a solution. For example, strategies may be ranked such that the intent generation engine  104  selects strategies in turn according to rank until an aircraft intent  114  is formed that is found to meet all constraints at step  560 . Self-checking is performed such that the intent generation engine  104  will return an exception declaring the impossibility of generating an aircraft intent based on the initial flight intent  101  in the defined operational context. The declaration of an exception may be triggered after a set number of iterations or after a pre-defined time delay. 
     Flight Intent Enrichment 
     At step  530  in  FIG. 5 , the intent generation engine  104  enriches the flight intent objects with constraints and objectives retrieved from the user preferences model  105  and/or the operational context model  106 . To do this, the intent generation engine  104  identifies constraints and objectives from the models  105  and  106  that are relevant to each flight intent object (e.g. not all the constraints included in the operational context are likely to apply to a specific route or to all flight segments on a particular flight path). 
     Relevance of constraints and objectives to flight intent objects may be determined using descriptions associated with the data stored in the user preferences model  105  and the operational context model  106 . For example, data may be identified by the geographical region to which it applies and/or by the phase of flight to which it applies. For example, the operational context model  106  may contain a topographical description of several regions within an airspace. Each region may have a description of hazards to be avoided such as mountains and densely populated areas. A flight intent object that will apply within that region may be augmented with the associated constraints for that region. As a further example, the operational context model  106  may contain descriptions of STARs to be followed when arriving at an airport. The flight intent  101  may indicate a preferred arrival waypoint into the terminal area, and so only the STAR description relating to that arrival point would be relevant, and so its constraints may be added to the flight intent objects of the corresponding flight segments. 
     Turning to the user preferences model  105 , this may contain an airline&#39;s preferences relating to different phases of flight or to different aircraft types. For example, it might define that during take off and climb out, the aircraft is flown to minimise fuel consumption. Alternatively, the user preferences model  105  might define that during descent, the aircraft is maintained at the maximum altitude possible for as long as possible. It will be appreciated that flight segments relating to the descent phase of a flight may then have an associated objective to maintain maximum altitude. 
     The intent generation engine  104  enriches the flight intent  101  by expanding the flight intent objects to add relevant constraints and objectives to the associated flight segment descriptions according to the syntactical and lexical rules imposed by the flight intent description language. The output of step  530  is an enriched flight intent that has flight intent objects comprising flight segment descriptions that may or may not be enriched with constraints and objectives. 
     Generating a Parametric Aircraft Intent 
     At step  540 , the intent generation engine  104  closes any open degrees of freedom within flight intent objects. Thus, the enriched flight intent that may still contain open degrees of freedom is completed to ensure all degrees of freedom of motion and configuration are closed for all flight intent objects. At this stage, parametric ranges may be used to close degrees of freedom, such that parametric aircraft intent is formed. This contains information on all degrees of freedom, but does not contain specific values for parameters such that the parametric aircraft intent does not define a unique trajectory. 
       FIG. 6  shows how the enriched flight intent may be completed to form a parametric aircraft intent. The process starts at  610  where the first flight segment is selected. The flight segments may be ordered in any way, although ordering the flight segments chronologically is the obvious example. The ordering merely needs to provide a list of flight segments that may be processed sequentially. 
     After the first flight segment has been selected at  610 , the process continues to a routine indicated at  620  in  FIG. 6 . The routine  620  is repeated for each flight segment in turn, as will be now be described. 
     At step  630 , the selected flight segment is checked to see whether it has any open degrees of freedom. If not, the method continues to step  615  where the next flight segment is selected and the process enters routine  620  once more. If one or more open degrees of freedom are found at step  630 , that flight segment continues through procedure  620  for further processing. 
     Next, at step  640 , the flight segment description and any constraints pertaining to the current flight segment are retrieved. This data is used at step  650  to select an appropriate strategy for completing the open degrees of freedom. This may be done by looking at which degree or degrees of freedom must be closed. For example, the open degrees may relate to the vertical flight profile or may relate to landing gear configuration. The intent generation engine  104  has at its disposal strategies corresponding to templates for closing particular degrees of freedom. These strategies are tagged to identify to which degrees of freedom they relate. Composites may also be stored and associated with a strategy, ready for selection by the intent generation engine  104  and insertion into the flight segment description. 
     The following are examples of strategies and associates composites: geometric paths providing different lateral path composites to define different path shapes (e.g. right turn, left turn, sequence of turns), level flight, constant path angle ascend/descend, constant speed ascend/descend, general ascend/descend, CAS-MACH climb, MACH-CAS descend, level trust acceleration/deceleration, clean configuration (e.g. of landing gear, high lift devices and speed brakes), and scheduled configuration settings (e.g. landing gear deployed and high lift device extension for landing). 
     The strategies may also be tagged to indicate to which phase of flight they apply (e.g. take off, climb out, cruise, descent, final approach, landing, taxiing). The constraints are also used in determining which strategy should be selected. Returning to the example above, a constraint may specify a region of restricted airspace that is nearby, thus guiding the strategy chosen to ensure that the turn is made at an appropriate point to avoid the restricted airspace. 
     Heuristics may also be used when selecting a strategy. For example, a flight segment may not close the vertical profile. The intent generation engine  104  may revert back to the earlier flight segments to find the last altitude specified and may then scan ahead to find the next flight segment that specifies an altitude. Comparison of the two altitudes may then guide selection of a suitable strategy. For example, if two flight segments specify the same altitude, intervening flight segments that do not specify an altitude may be amended using a strategy that maintains level flight. 
     Once a suitable strategy has been selected at step  650 , the procedure  620  continues to step  660  where an aircraft intent primitive corresponding to the selected strategy is generated and added to the flight segment description of the flight intent object being processed. The primitive may be added as part of a composite where two or more primitives are to be combined, i.e. a strategy may require a primitive or a composite of primitives to describe the required instructions depending upon the complexity of the strategy. 
     Steps  650  and  660  are performed as necessary to ensure all open degrees of freedom are closed. With this processing finished, at step  670  a check is made to see whether the flight segment being processed is the final flight segment. If not, the process loops back to step  615  where the next flight segment is selected and procedure  620  is entered once more. 
     When all flight segments have been processed, as determined at step  670 , the process continues to step  680  where all the completed flight objects are collated to form the parametric aircraft intent, expressed using a formal language (the aircraft intent description language). This completes step  540  of  FIG. 5 . The parametric aircraft intent is then processed according to step  550  where the parametric ranges are resolved into specific parameter values through an optimisation process that will now be described with respect to  FIG. 7 . 
     Optimising the Parametric Aircraft Intent 
     The optimisation process of step  550  takes all the parameter ranges specified in the parametric aircraft intent and calculates optimal values for each parameter by optimising an overall merit function that reflects the objectives defined in the flight intent objects. 
     As shown in  FIG. 7 , the process starts at step  710  where the first flight segment is selected. As described above, the flight segments may be ordered in any way that provides a list of flight segments for processing sequentially. 
     At step  720 , the fight segment is reviewed to determine whether it contains any parameter ranges than need resolving. If not, the method proceeds to step  725  where the next flight segment is selected for processing. When a flight segment is found at step  710  to define one or more parameter ranges, that parameter range and any associated objectives are retrieved and stored in respective lists, as shown at step  730 . Then, at step  740 , a check is made to see if the flight segment being processed currently is the final flight segment. If not, the process loops back to step  725  so that the next flight segment may be selected for processing at step  720  once more. In this way, all flight segments are checked for parameter ranges, and lists are compiled that collate the parameter ranges to be resolved along with associated objectives. 
     At step  750 , the objectives stored in the associated list are mathematically combined into a merit function that reflects all the objectives. The objectives may be stored in the user preferences model  105  as a mathematical function expressing the objective to be targeted. Then, forming the merit function may correspond to combining the individual mathematical functions describing each objective. The mathematical functions may be combined in any straightforward manner. For example, a weighted combination may be formed, where weights are assigned to each objective according to its importance. Data may be stored in the user preferences model  105  to indicate the relative importance of the objectives. 
     If a parameter range is found that does not have an associated objective, a library of pre-defined mathematical functions may be used to provide a mathematical function for inclusion in the merit function. For example, a mathematical function may be associated with the parameter range that assigns a constant value irrespective of the parameter value chosen, such that the parameter value may be chosen as any within the parameter range, but optimised to lead to an overall improvement of the merit function value. For example, selection of a particular value for the parameter may contribute to achieving an objective relating to the preceding flight segment. 
     Consequently, the merit function rewards how well the objectives are met and penalises how badly the objectives are not met. 
     At step  760 , each parameter range in the associated list is read, and the associated flight intent object that appears in the parametric aircraft intent is amended such that the parameter range is replaced by a value falling within the range. Different schemes may be used to select a value, for example by selecting the maximum value, the minimum value, the mean value or by randomly generating a value. At the end of step  760 , an aircraft intent results that has all parameters defined and with no parameter ranges remaining. This model aircraft intent is then tested by using the trajectory engine  112  to calculate the corresponding trajectory, from which the intent generation engine  104  can calculate the merit function value for the model aircraft intent. 
     The process then proceeds to step  780  where the model aircraft intent is optimised. This optimisation process improves the parameter values iteratively. That is, intent generation engine  104  goes through iterations of randomly changing some or all the parameter values, then calling the trajectory engine  112  to compute the new trajectory, and computing the new merit function value and determining whether it has been improved. In this way, the parameter values are evolved in a way that optimises the merit function. This may be done using any well known technique, such as using evolutionary algorithms like genetic algorithms or through linear optimisation. These techniques provide an optimised complete aircraft intent, and this is provided as an output at step  790 . 
     Example of Approach to Airport 
     An example of the above methods will now be described with reference to  FIGS. 8 to 10 . In this example, an aircraft  810  is approaching an airport to land on a runway  820 . The flight intent may merely specify that the aircraft is to land on runway  820  after arrival at waypoint ALPHA. 
     In order to provide a complete aircraft intent, the intent generation engine  104  may augment this basic flight intent with information retrieved from the operational context model  106  describing a STAR procedure to be followed when approaching the airport. For example, intent generation engine  104  may establish the wind direction, determine the direction for a headwind approach to the runway  820 , and retrieve the STAR procedure for such a landing for aircraft arriving at waypoint ALPHA. 
     The STAR procedure will correspond to a set of restrictions. In this example, the lateral path to be followed routes the aircraft through waypoints ALPHA, BETA, GAMMA and DELTA, ready for a final straight approach to runway  820 . These waypoints are shown in  FIG. 9 . The STAR procedure may also contain restrictions on speeds along the route as well as altitudes to be maintained at each waypoint. These altitudes are shown in  FIG. 10 . 
     At waypoint ALPHA, a broad permissible altitude range is defined, as indicated at  910 . Smaller altitude ranges are defined for waypoints BETA and GAMMA, as shown at  920  and  930  respectively. A specific altitude is defined for waypoint DELTA as shown at  940 , corresponding to a starting altitude for final approach from which a glide slope may be intercepted. 
     The intent generation engine  104  may use these restrictions to augment the flight intent. For example, additional flight segments may be created corresponding to the segments between the waypoints to be followed. Moreover, parametric intent may be created where the altitude ranges at each waypoint are defined without a specific altitude being provided. Objectives may be used to specify altitudes to be met, as follows. 
       FIG. 10  shows two alternative vertical profiles,  810   a  and  810   b . Profile  810   a  corresponds to aircraft  810  being operated by an airline that prefers to fly as high as possible for as long as possible. This objective will be recorded in the user preferences model  105 . Accordingly, the intent generation engine  104  sets altitudes at each waypoint as the maximum specified, then calculates the maximum rate of descent possible for the aircraft  810  to establish when each descent phase must begin, and creates segments that define level flight between each descent phase, along with defining the top of descent point (TOD 2 ). Thus, by using the objective, intent generation engine  104  generates aircraft intent that will produce the stepped-down vertical profile shown at  810   a . This profile sees the aircraft  810  fly as high as possible for as long as possible before making a steep descent just in time to meet the maximum altitude prescribed for each waypoint. 
     Another airline may not like such an approach that sees the aircraft accelerate between level flight and descents a number of times. This second airline may prefer to fly a steady continuous descent with minimal changes in flight path angle. This approach may be reflected as an objective stored in the user preferences model  105 . Intent generation engine  104  may retrieve this objective, and determine the vertical profile shown as  810   b  in  FIG. 10 . This vertical profile sees a steady descent with constant flight path angle from a calculated top of descent point TOD 1  that passes through all the required altitude ranges. 
     As can be seen from  FIG. 10 , some variation in flight path angle may be made while still ensuring the altitude restrictions are met. Further objectives may guide the final selection of vertical profile. For example, the airline may have a further objective of flying continuous descent approaches with the throttles set to idle and with minimal deployment of speed brakes. This objective may then be used by the intent generation engine  104  to set an appropriate flight path angle. 
     Objectives to fly a constant flight path angle during descent and to fly continuous descent approaches at idle complement each other in that they both affect the vertical profile. At times, these objectives will cause a conflict in that both cannot be met. To avoid this, objectives may be prioritised such that the intent generation engine  104  can determine which objective is to be met where conflicts arise. 
     The airline may store restrictions in the user preferences model  105  as well as objectives. For example, as explained above, the lateral profile is defined in part by the waypoints specified in the STAR description in the operational context model  106 . However, these restrictions leave open how the aircraft  810  makes the turns to meet the lateral position of each of waypoints ALPHA, BETA, GAMMA and DELTA. The airline may also set restrictions, for example not to exceed a certain bank angle for the benefit of passenger comfort. The intent generation engine  104  may retrieve this objective from the user preferences model  105  during the flight intent enrichment step  530 . At step  550 , this restriction may be used to set a parameter range for the bank angle which is then optimised at step  560 . 
     Contemplated Applications 
     The present disclosure may find utility on any application that requires prediction of an aircraft&#39;s trajectory. For example, the trajectory computation infrastructure  110  may be provided as part of a flight management system of an aircraft. The flight management system may make use of the trajectory prediction facility when determining how the aircraft is to be flown. 
     A trajectory predicted as described in the preceding paragraph may be provided to air traffic management, akin to the provision of a detailed flight plan. 
     For an air-based trajectory computation infrastructure, the flight management system may have access to some of the information required to generate the aircraft intent. For example, airline preferences may be stored locally for retrieval and use. Moreover, the aircraft performance model  118  and Earth model  120  may be stored locally and updated as necessary. Further information may be input by the pilot, for example the particular SID, navigation route and STAR to be followed, as well as other preferences like when to deploy landing gear, change flap settings, engine ratings, etc. Some missing information may be assumed, e.g. flap and landing gear deployment times based on recommended airspeed. 
     All this required information may be acquired before a flight, such that the trajectory of the whole flight may be predicted. Alternatively, only some of the information may be acquired before the flight and the rest of the information may be acquired en route. This information may be acquired (or updated, if necessary) following a pilot input, for example in response to a change in engine rating or flight level. The trajectory computation infrastructure  110  may also update the predicted trajectory, and hence the aircraft intent as expressed in the aircraft intent description language, due to changes in the prevailing atmospheric conditions, as updated through the Earth model  120 . Updates may be communicated via any of the types of well-known communication link  230  between the aircraft and the ground: the latest atmospheric conditions may be sent to the aircraft and the revised aircraft intent or predicted trajectory may be sent from the aircraft. 
     Air traffic management applications will be similar to the above described air-based system. Air traffic management may have information necessary to determine aircraft intent, such as flight procedures (SIDs, STARs, etc), information relating to aircraft performance (as an aircraft performance model), atmospheric conditions (as an Earth model), and possibly even airline preferences. Some information, such as pilot preferences relating to for example when to change the aircraft configuration, may be collected in advance of a flight or during a flight. Where information is not available, air traffic management may make assumptions in order for the aircraft intent to be generated and the trajectory to be predicted. For example, an assumption may be made that all pilots will deploy their landing gear ten nautical miles from a runway threshold or at a particular airspeed. 
     Air traffic management may use the predicted trajectories of aircraft to identify potential conflicts. Any potential conflicts may be resolved by advising one or more of the aircraft of necessary changes to their flight/aircraft intent. 
     The person skilled in the art will appreciate that variations may be made to the above described embodiments without departing from the scope of the invention defined by the appended claims.