Patent Publication Number: US-2009234519-A1

Title: Joining a civil trajectory and a military trajectory

Description:
RELATED APPLICATIONS 
     The present application is based on, and claims priority from, French Application No. 08/01323, filed Mar. 11, 2008, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a trajectory calculation method making it possible to join a so-called military trajectory from a so-called civil trajectory and, vice versa, to join a so-called civil trajectory from a so-called military trajectory. 
     BACKGROUND OF THE INVENTION 
     Specifically, flight management systems, commonly referred to by the acronym FMS, or mission preparation systems, generally make a distinction between “civil” trajectories and “military” trajectories. Thus, the constraints related to the following of a civil trajectory are not the same as those related to the following of a military trajectory. 
     Civil standards, which apply to civil trajectories, impose safety constraints on the speed, the ground height or the turning radius for example. Conversely, during a mission in a theatre of operations, constraints of a tactical nature are imposed on aircraft. For example, it may be obligatory to fly at very low altitude, at very high speed, or to perform very tight turns. 
     Now, aircraft frequently take off and perform part of their mission on a civil trajectory before reaching the theatre of operations and switching to a military trajectory for a tactical mission, then finally rejoining the civil trajectory for the return flight. 
     In this case, the switch from the civil trajectory to the military trajectory and then from the military trajectory to the civil trajectory exhibits discontinuities at the trajectory calculation systems level and at the FMS level. 
     Currently, no method allows automated or systematic calculation of the transition between civil and military trajectories. 
     Specifically, today, during flight preparation or during in-flight rerouting, the crew record their flight plan on the FMS of the aircraft. This FMS comprises various modules allowing it to calculate the trajectories corresponding to the flight plan provided. The functions of a standard FMS are described in the ARINC 702 standard and comprise:
         a location module allowing geo-location of the aircraft;   a flight plan;   a navigation database making it possible to construct geographical routes;   a performance database, containing the aerodynamic characteristics and the parameters of the engine of the aircraft;   a lateral trajectory calculation module making it possible to construct a continuous trajectory on the basis of the points of the flight plan, and complying with the performance of the aircraft as well as any confinement constraints;   a prediction module making it possible to construct a vertical profile optimized on the lateral trajectory;   a guidance module, so as to guide the aircraft in the lateral and vertical planes;   a data link making it possible to communicate with the control centres and the other aircraft.       

     Within the framework of a tactical mission for example, there may be a flight plan section in which civil constraints and tactical constraints overlap. 
     Generally, in this case a point of the military trajectory, from which the aircraft will have to follow the military trajectory, and a point from which the aircraft will have to join the civil trajectory, are chosen. 
     Currently, within prior state FMSs, no method of calculating a transition trajectory between civil and military trajectories exists. The transitions are therefore discontinuous. 
     It is in order to alleviate this drawback that the invention proposes a trajectory calculation method aimed at allowing an aircraft to join a military trajectory from a civil trajectory, and vice versa, based on the positioning of a capture point and the determination of transition “legs”. The term “leg” refers to an object particular to the FMS domain, consisting of a path and of a termination. 
     SUMMARY OF THE INVENTION 
     For this purpose, the subject of the invention is a trajectory calculation method aimed at allowing an aircraft to join a secondary trajectory exhibiting secondary characteristics from a primary trajectory exhibiting primary characteristics, the primary and secondary characteristics possibly being termed “civil” or “military”, and exhibiting different constraints in terms at least of ranges of values permitted for the speed, the said primary and secondary characteristics being subject to the said different constraints, the secondary trajectory exhibiting an entry point starting from which the aircraft absolutely must follow the secondary trajectory according to the secondary characteristics, characterized in that the said trajectory calculation method comprises at least the following steps:
         the choice of a capture point at which the aircraft must have captured the secondary characteristics of the secondary trajectory so that the said aircraft can follow the secondary trajectory starting from the entry point according to the secondary characteristics,   the calculation of a trajectory for joining the secondary trajectory from the primary trajectory comprising at least one first transition leg.       

     The primary trajectory can for example be a civil trajectory, exhibiting civil characteristics. 
     The secondary trajectory can for example be a military trajectory, exhibiting military characteristics. 
     Advantageously, the military trajectory can comprise a low-altitude flight phase. 
     Advantageously, the first transition leg is one of the legs defined by the ARINC 424 standard: IF; CF; DF; TF; AF; RF; VI; CI; VA; CA; FA; VD; CD; VR; CR; FC; FD; VM; FM; HA; HA; HF; HM; PI. 
     Advantageously, the first transition leg is a CF leg. 
     In an exemplary implementation, the trajectory calculation method according to the invention comprises the following steps:
         the choice of the capture point on the secondary trajectory backwards from the entry point,   the definition of the first transition leg having the capture point as termination point and the course of the secondary trajectory at the capture point as arrival course,   the calculation of a trajectory for joining the first transition leg from the primary trajectory and according to the primary characteristics.       

     In another exemplary implementation, the trajectory calculation method according to the invention can furthermore comprise a phase of joining a tertiary trajectory, that may possibly be identical to the primary trajectory, from the secondary trajectory, the tertiary trajectory exhibiting tertiary characteristics and a return point, starting from which the aircraft absolutely must follow the said tertiary trajectory according to the tertiary characteristics, characterized in that the said method comprises the following steps:
         the determination of an exit point, situated on the secondary trajectory, at which the aircraft must have captured the tertiary characteristics of the tertiary trajectory,   the definition of a second transition leg having the exit point as termination point,   the calculation of a trajectory for joining the second transition leg from the secondary trajectory and according to the secondary characteristics,   the definition of a third transition leg having the return point as termination point and the course of the tertiary trajectory at the return point as arrival course,   the calculation of a trajectory for joining the third transition leg from the exit point and according to the tertiary characteristics.       

     The tertiary trajectory can for example be a civil trajectory, exhibiting civil characteristics. 
     Advantageously, the second transition leg is one of the legs defined by the ARINC 424 standard: IF; CF; DF; TF; AF; RF; VI; CI; VA; CA; FA; VD; CD; VR; CR; FC; FD; VM; FM; HA; HA; HF; HM; PI. 
     Advantageously, the third transition leg is one of the legs defined by the ARINC 424 standard: IF; CF; DF; TF; AF; RF; VI; CI; VA; CA; FA; VD; CD; VR; CR; FC; FD; VM; FM; HA; HA; HF; HM; PI. 
     Advantageously, the second transition leg is a DF leg. 
     Advantageously, the third transition leg is a CF leg. 
     Advantageously, the third transition leg is a TF leg between the exit point and the return point. 
     Advantageously, a flight management system can comprise means suitable for executing the trajectory calculation method according to the invention. 
     Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious aspects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       The present invention is illustrated by way of example, and not by limitation, in the figures of accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein: 
         FIG. 1 : an example of a section of a flight plan including a primary civil trajectory and a secondary military trajectory; 
         FIG. 2 : the illustration of the positioning of a capture point at which the characteristics of the military trajectory must be captured, with a view to joining this trajectory, in accordance with the method according to the invention; 
         FIG. 3 : the diagram of a transition trajectory making it possible to join the military trajectory from the civil trajectory via the capture point in accordance with the method according to the invention; 
         FIG. 4 : the illustration of the positioning of an exit point at which the characteristics of the civil trajectory must be captured, with a view to joining this trajectory, in compliance with the method according to the invention; 
         FIG. 5 : the diagram of a transition trajectory making it possible to join the civil trajectory from the military trajectory via the exit point in accordance with the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  presents a diagram with two trajectories with the different characteristics. Of interest is the case where an aircraft must join the secondary trajectory Tm 1  from the primary trajectory Tc 1 . In this example, it is considered that the primary trajectory Tc 1  is civil while the secondary trajectory Tm 1  is military. 
     In a basic manner, a flight plan can be considered to be a succession of waypoints Wo, We . . . with which are associated characteristics such as the speed, the altitude and the heading of the aircraft at the said waypoint. These waypoints Wo, We . . . are generally linked by legs L 1 , L 2  . . . , that the aircraft is presumed to follow as closely as possible. The flight management system FMS is charged with formulating the trajectories Tc 1 , Tm 1  . . . . which will allow the aircraft to comply with its flight plan. These trajectories are subject to certain constraints, in terms of ranges of values permitted for the altitude, speed, roll, etc. These constraints depend on the type of mission, the environment, etc. They may be so-called civil or military. In the first case, the constraints are essentially related to safety and significant margins are taken with respect to the risks related to the topology of the terrain or to the performance of the aeroplane notably. Civil standards defined by State bodies govern these constraints. In the second case, the tactical constraints are essential. The safety margins are generally reduced so as to be able to accomplish the mission. 
     Thus, in the illustration of  FIG. 1 , it is noted that to travel from the waypoint Wo to the waypoint We, the civil trajectory Tc 1  and the military trajectory Tm 1  are very different. On the military trajectory Tm 1 , it is notably possible to perform tighter turns. 
     In the example considered here, the aircraft absolutely must have travelled on the military trajectory Tm 1  at the entry point We. The last waypoint of the flight plan overflown on the civil trajectory Tc 1  is the point Wo, the end of the leg L 1 . The transition is therefore performed at the level of the leg L 2 . 
     The idea is to ensure continuous guidance of the aircraft. For this purpose, a single and continuous trajectory must be defined. However, the construction of the join between the trajectories Tc 1  and Tm 1  is in no way obvious a priori. This is the subject of the invention. 
       FIG. 2  illustrates the first phase of the method of calculating a transient trajectory between trajectories with different characteristics according to the invention. 
     This first phase consists in positioning a capture point PC 1  starting from which the aircraft A must have captured the characteristics of the military trajectory Tm 1 , in terms of speed, altitude, etc., so as to be able to ideally follow the said military trajectory Tm 1  starting from the entry point We. 
     A point PC 1  must therefore be chosen on the military trajectory Tm 1 , backwards from the entry point We, where it is necessary to capture the flight characteristics complying with the military framework of the trajectory Tm 1 . To position this point PC 1 , a criterion for joining the military trajectory Tm 1  is chosen. For example, it may be desired to capture the military trajectory Tm 1  at a certain altitude, typically, in the case where the military trajectory Tm 1  were to consist of a tactical flight at very low altitude. 
     The criterion for choosing the point PC 1  can also be a speed to be reached on the military trajectory Tm 1 , etc. 
     When this capture point PC 1  is positioned, the method continues with the calculation of a transition trajectory making it possible to join the capture point PC 1 , and then the military trajectory Tm 1 . 
       FIG. 3  represents the process of constructing this transition trajectory. 
     For this purpose, a leg aimed at bringing the aircraft A to the point PC 1  is firstly defined. Various types of legs exist. Thus, the ARINC 424 standard catalogues 23 types of legs, as a function of their characteristics. Among the principal legs may be cited the legs:
         CF, signifying Course to a Fix, characterized by a fixed termination point, that is to say a waypoint constituting the end of the said leg, and an arrival course, which corresponds to the course of the aircraft A at the termination point, the course of the aircraft A being the angle that the aircraft A makes with respect to North;   TF, signifying Track between two Fixes, a leg consisting of a direct route between two fixed points, therefore exhibiting an origin point and a termination point;   DF, signifying Direct to a Fix, consisting in joining up, in a direct line, with a fixed point constituting the termination point of the said leg.       

     The other legs of the ARINC 424 standard are presented briefly in the following table: 
     
       
         
           
               
               
               
             
               
                   
               
               
                   
                 Name in the ARINC 424 
                   
               
               
                 Leg 
                 standard 
                 Meaning 
               
               
                   
               
             
            
               
                 IF 
                 Initial Fix 
                 Fixed initial point on the ground 
               
               
                 AF 
                 Arc DME to Fix 
                 Defines a circular arc around a specified 
               
               
                   
                   
                 remote DME beacon, with an aperture limit 
               
               
                 RF 
                 Radius to a Fix 
                 Defines a circular arc between two fixed points 
               
               
                   
                   
                 (the 1 st  point being the fixed point of the 
               
               
                   
                   
                 previous leg), on a centre of the fixed circle 
               
               
                 VI 
                 Heading to Intercept 
                 Defines a heading to be followed up to 
               
               
                   
                   
                 interception of the next leg 
               
               
                 CI 
                 Course to Intercept 
                 Defines a route to be followed up to 
               
               
                   
                   
                 interception of the next leg 
               
               
                 VA 
                 Heading to Altitude 
                 Defines a heading to be followed up to a given 
               
               
                   
                   
                 altitude 
               
               
                 CA 
                 Course to Altitude 
                 Defines a route to be followed up to a given 
               
               
                   
                   
                 altitude 
               
               
                 FA 
                 Fix to Altitude 
                 Defines a route to be followed, starting from a 
               
               
                   
                   
                 fixed point, up to a given altitude 
               
               
                 VD 
                 Heading to DME Distance 
                 Defines a heading to be followed up to 
               
               
                   
                   
                 interception of a specified DME arc 
               
               
                 CD 
                 Course to DME Distance 
                 Defines a route to be followed up to 
               
               
                   
                   
                 interception of a specified DME arc 
               
               
                 VR 
                 Heading to Radial 
                 Defines a heading to be followed up to 
               
               
                   
                   
                 interception of a specified radial 
               
               
                 CR 
                 Course to Radial 
                 Defines a route to be followed up to 
               
               
                   
                   
                 interception of a specified radial 
               
               
                 FC 
                 Track from Fix to Distance 
                 Defines a route to be followed, starting from a 
               
               
                   
                   
                 fixed point, over a specified distance 
               
               
                 FD 
                 Track from Fix to DME 
                 Defines a route to be followed, starting from a 
               
               
                   
                 Distance 
                 fixed point, until it intercepts a DME arc 
               
               
                   
                   
                 (specified DME distance) 
               
               
                 VM 
                 Heading to Manual 
                 Defines a heading without termination (infinite 
               
               
                   
                   
                 half-line) 
               
               
                 FM 
                 Fix to Manual 
                 Defines a route, starting from a fixed point, 
               
               
                   
                   
                 without termination (infinite half-line) 
               
               
                 HA 
                 Hippodrome to Altitude 
                 Hippodrome circuit, with altitude exit condition 
               
               
                   
                 Termination 
               
               
                 HF 
                 Hippodrome to Fix Termination 
                 Hippodrome circuit, with a single lap 
               
               
                 HM 
                 Hippodrome to Manual 
                 Manual hippodrome circuit, without exit 
               
               
                   
                 Termination 
                 condition 
               
               
                 PI 
                 Fix to Manual 
                 Outbound procedure defined by an outbound 
               
               
                   
                   
                 route starting from a fixed point, followed by a 
               
               
                   
                   
                 half-lap, and interception of the initial outbound 
               
               
                   
                   
                 route for the return 
               
               
                   
               
            
           
         
       
     
     In the example illustrated in  FIG. 3 , a CF leg is constructed, denoted CF 1 , having the capture point PC 1  as termination point and the course of the military trajectory Tm 1  at the capture point PC 1  as arrival course. 
     The trajectory is thereafter recalculated by using civil algorithms to join the leg CF 1 . Having reached the leg CF 1 , the aircraft A has joined the military trajectory Tm 1  that it will definitely follow starting from the waypoint We. 
     The same problem arises when the aircraft A gets ready to leave the military trajectory Tm 1  so as to return to the civil trajectory Tc 1  or join another civil trajectory Tc 2 , and the construction of the transition from the trajectory Tm 1  to the trajectory Tc 1  or Tc 2  is similar to the transition from the trajectory Tc 1  to the trajectory Tm 1 , described with the aid of  FIGS. 1 to 3 . 
     Thus,  FIG. 4  presents by way of example the first phase of joining the tertiary trajectory Tc 2 , the civil trajectory, from the secondary trajectory Tm 1 , the military trajectory. It should be noted that the tertiary trajectory Tc 2  can actually be in reality the primary trajectory Tc 1 . 
     The last point overflown on the military trajectory Tm 1  is the waypoint Ws, the end of the leg L 3 ; the transition is performed at the level of the leg L 4  so that the aircraft A has joined the civil trajectory Tc 2  at the point Wr, the origin of the leg L 5 . 
     This therefore involves positioning an exit point PS 2 , at which the aircraft A must absolutely have captured the civil characteristics of the civil trajectory Tc 2 , so that the aircraft A is able to follow the civil trajectory Tc 2  as from the waypoint Wr. The point PS 2  is chosen on the military trajectory Tm 1  and therefore indeed constitutes the exit point of the said trajectory Tm 1 . 
       FIG. 5  illustrates the next step, which consists in joining up with the exit point PS 2  and then the trajectory Tc 2 . To join the point PS 2 , a leg having the point PS 2  as termination point is defined, for example a DF leg, denoted DF in the figure. 
     A CF leg for example, denoted CF 2 , is thereafter defined having the waypoint Wr as termination point at which the aircraft A must absolutely have joined the trajectory Tc 2 , and the course of the original leg L 4  as arrival course, the latter generally being a TF leg, plotted between the waypoints Ws and Wr. 
     Finally, a transition trajectory is recalculated complying with the characteristics of the tertiary trajectory Tc 2 , that is to say here using the civil algorithms, so as to join the leg CF 2 , after passing through the exit point PS 2 . 
     The aircraft A is then able to follow the tertiary trajectory Tc 2  from the waypoint Wr. 
     It should be noted that the procedure for joining the tertiary trajectory Tc 2  from the secondary trajectory Tm 1  can be transposed identically for joining a secondary trajectory from a primary trajectory. The examples described through the appended figures are illustrative. 
     To summarize, the principal advantage of the invention is to propose an original trajectory calculation method aimed at allowing the joining of trajectories exhibiting distinct constraints. For example, if the flight of an aircraft A must comply with civil standards over part of its flight plan and then perform a mission comprising tactical constraints before returning to a civil trajectory, the method described in the present patent application is entirely suitable. 
     It will be readily seen by one of ordinary skill in the art that the present invention fulfils all of the objects set forth above. After reading the foregoing specification, one of ordinary skill in the art will be able to affect various changes, substitutions of equivalents and various aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by definition contained in the appended claims and equivalents thereof.