Patent Publication Number: US-10761326-B2

Title: Dual harmonization method and system for a head-worn display system for making the display of piloting information of an aircraft conform with the outside real world

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to foreign French patent application No. FR 1701342, filed on Dec. 21, 2017, the disclosure of which is incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for dual harmonization of a head-worn display system for making the display of piloting information of an aircraft conform with the outside real world. 
     The invention lies in the technical field of the piloting human-system interface (HSI) for aircraft, such as, for example, helicopters or aeroplanes, equipped with a head-worn or helmet-mounted display system (HWD or HMD) and a head posture detection device DDP. 
     BACKGROUND 
     The head-up display systems, whether worn or not, make it possible to display in particular a “symbology” conforming to the outside world, that is to say a set of symbols whose position in front of the eye of the pilot allows for a superimposition with the corresponding elements in the outside world. It may be for example a speed vector, a target on the ground or in the air, a synthetic representation of the terrain or even a sensor image. 
     This conformal display requires knowledge of the position and the attitude of the aircraft and, for the head-worn display devices, the attitude of the display relative to a fixed reference frame linked to the aircraft. These various positions and attitudes are supplied by the avionics systems for those of the aircraft, and by the posture detection device DDP for those of the display. 
     For example and in particular, the avionics systems for supplying the position and attitude of an aircraft can be, respectively:
         a global positioning device of GPS (global positioning system) type; and   an inertial reference system IRS based on gyroscopes and accelerometers of MEMS (micro electro mechanical systems) type or laser gyroscope type, or an attitude and heading reference system AHRS.       

     As is known, a harmonization of the head-worn display system is performed on installation of the display system, in a cockpit, in order to compute the corrections of angles to be made to switch from the display reference frame to the aircraft reference frame, and in order to obtain a conformal head-up display. 
     Now, some head-worn display devices these days have a certain mobility between the display device or display and the worn part of the posture detection system DDP, because of an absence of mechanical rigidity between these two elements, i.e. the display and the mobile worn part of the DDP, for example when there is a device for tilting the display alone outside of the field of view of the operator. There is then a need, when the display is once again tilted into the field of view of the operator, to once again proceed with a harmonization in order to compute new corrections of angle to be made to the head once the head-up display is installed and thus be able to display a conformal symbology in the display device worn on the head. 
     In order to make it possible and to facilitate this relatively frequent need for reharmonization, it is known practice to install a dedicated instrument on board the aircraft, called boresight reference unit or boresight reticle unit BRU. 
     The boresight reference unit BRU, installed in the cockpit facing the head of the operator displays a collimated symbol with an orientation that is fixed and known to the head-up system. 
     Each time there is a need to realign the conformal symbology, i.e. for reharmonization, the operator aligns a symbol displayed in his or her head-up display with the collimated symbol of the boresight reference unit BRU. 
     When the symbol displayed in the head-up display, i.e. the display is aligned on the collimated symbol, the detection-device output harmonization system then computes a rotation matrix from three correction angles, in order to reharmonize the attitude of the reference frame of the display relative to the reference frame of the aircraft. 
     The main fault with this harmonization system based on the use of a boresight reference unit BRU is the inclusion of an additional item of equipment dedicated to just this realignment or harmonization function and a cost in terms of installation complexity, an additional bulk and weight that can be restrictive, in particular for small civilian aircraft. This BRU equipment item has to be powered through electrical wiring and installed in a robust manner. This BRU equipment item requires a lengthy harmonization procedure when it is installed with an additional error entry. A risk of misalignment through movement is possible for example upon installation or during a maintenance operation. 
     Furthermore, the exact parameters of orientation of this boresight reference unit BRU on the bearer, i.e. the bearing structure of the aircraft, has to be also introduced into the helmet-mounted display system HMD, and the BRU unit has to then always remain perfectly fixed relative to the bearer. Now, the current mechanical technologies do not make it possible to guarantee a mounting of the BRU unit in the cockpit without a risk of variations over time. Indeed, the vibratory environment, the interventions of the pilot and of the maintenance operators in particular can provoke slight rotations or movements of the boresight reference unit BRU, which results in the introduction of an error on the line of sight that cannot be compensated and, in many cases, that cannot be detected, and therefore the prevention of any subsequent reharmonization. 
     A first technical problem is how to provide a head-worn display system and a harmonization method which makes it possible to realign the symbology on the outside world when the head-up display or viewing system HWD/HMD has a mechanism for releasing and re-engaging the display in the field of view of the pilot, a source of misalignment, and to avoid the use of a calibration landmark installed inside the cockpit, also a source of error. 
     A second technical problem is how to more accurately determine the relative orientation M 01  between the display D 0  and the mobile tracking element D 2  of the head posture detection subsystem DDP when the head-up display system HWD/HMD has a mechanism for releasing and re-engaging the display in the field of view of the pilot. 
     A third technical problem is how to correct the orientation of the aircraft supplied by its inertial station relative to the Earth, in particular for the heading whose value is generally not known with sufficient accuracy for a conformal display. 
     SUMMARY OF THE INVENTION 
     To this end, the subject of the invention is a dual harmonization method for a head-worn display system for making the display of piloting information of an aircraft conform with the outside real world, the head-worn display system comprising: a transparent head-worn display D 0 ; a head posture detection subsystem DDP having a mobile tracking first element D 1  securely attached to the transparent display D 0 , a fixed second element D 2  securely linked to the platform of the aircraft, and a means for measuring and determining the relative orientation M 12  of the mobile tracking first element D 1  relative to a reference frame of the fixed second element D 2  linked to the platform; an attitude inertial device D 3  for supplying the relative attitude M 3   t  of the platform relative to a terrestrial reference frame linked to the Earth; a harmonization subsystem for the head-worn display system for making the display of piloting information on the display D 0  conform with the outside real world, the harmonization subsystem having a dual harmonization computer and a human-system interface for managing and performing the implementation of the dual harmonization method. 
     The dual harmonization method is characterized in that it comprises the steps consisting in:
         performing a series of an integer number N greater than or equal to 3 of different sightings Vi, i varying from 1 to N, performed through the display D 0  by aligning a centred sighting visual pattern on any same fixed target of the outside real world, each sighting Vi corresponding to a different fixed position Pi of the centre of the sighting pattern on the display D 0  and having a sighting vector {right arrow over (xi)} determined as a function of the position Pi, and, for each sighting Vi, acquiring the corresponding measurement {circumflex over (K)} l  of the relative angular orientation of the tracking element relative to a DDP reference direction, that is fixed relative to the platform of the aircraft, then   computing the matrix of relative orientation M 01  between the display D 0  in the tilted position and the tracking first element D 1  as the right matrix {circumflex over (D)}, the solution of the system of equations: Ĝ·{circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (y)} 0  for i=1 to N, the vector {right arrow over (y 0 )} denoting the vector in the inertial reference frame of the platform corresponding to the target point targeted in the outside real world and being unknown, and the left matrix Ĝ being the matrix M 23  of relative orientation between the reference frame of the fixed second element D 2 , linked to the platform of the aircraft, and the reference frame of the attitude inertial device D 3 , which is potentially incorrect but assumed constant as a function of time, and which, when it is unknown, requires at least four measurements {circumflex over (K)} l .       

     According to particular embodiments, the dual harmonization method comprises one or more of the following features, taken alone or in combination:
         the number N of measurements is greater than or equal to three and the left matrix Ĝ of relative orientation between the reference frame of the fixed second element D 2 , linked to the platform of the aircraft, and the reference frame of the attitude inertial device D 3  is known, and the solving of the system of equations Ĝ·{circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (y)} 0 , i varying from 1 to N, uses an iterative process and a rectifying operator π(·) which transforms any matrix A into a 3×3 square matrix of rotation π(A) that is as close as possible to the least squares directions over all of the terms of the matrix π(A)-A over all of the 3×3 rotation matrices, to determine the matrix {circumflex over (D)} and the vector {right arrow over (y 0 )};   the step of solving of the system of equations comprises a first set of substeps consistings in: in a first, initialization substep, initializing a first series of right matrices {{circumflex over (D)} [s] }, [s] denoting the integer rank of progress through the series {circumflex over (D)}{ [s] }, by setting {circumflex over (D)}[ 0 ] equal to I 3 , I 3  denoting the identity matrix; then repeating a second, iterative substep for passing from the iteration [s] to [s+1] by computing {right arrow over (y)} [s+1]  then {circumflex over (D)} [s+1]  using the following equations:       

     
       
         
           
             
               
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             the series {{right arrow over (y)} [s] } denoting a second series of external direction vectors, the series {{right arrow over (y)} [s] } and {{circumflex over (D)} [s] } converging respectively towards {right arrow over (y 0 )} and  D ; then, in a third, stopping substep, stopping the iterative process performed through the second substep when the limits  1  and C are approximated with a sufficient accuracy defined by one or two predetermined threshold values; 
             the number N of measurements is equal to three, and the centred sighting visual pattern is set fixed on the display by the harmonization computer at three different positions P 1 , P 2 , P 3  corresponding respectively to the three sightings V 1 , V 2 , V 3 : a first position P 1  in the left part of the display and vertically to the centre, and a second position P 2  in the right part of the display and vertically to the centre, and a third position P 3  horizontally to the centre and upwards; 
             the number N of measurements is greater than or equal to four and the left matrix Ĝ of relative orientation between the reference frame D 2  linked to the platform of the aircraft and the reference frame linked to the inertial unit D 3  is unknown, and not seeking to determine the vector {right arrow over (y 0 )}, the solving of the system of equations: Ĝ·{circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (y)} 0 , i varying from 1 to N, amounts to the solving of the system of equations: {circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (z)} 0  for i varying from 1 to 4, by denoting {right arrow over (z)} 0 =Ĝ T ·{right arrow over (y)} 0 , which solving uses an iterative process and a rectifying operator π(·) which transforms any matrix A into a 3×3 square matrix of rotation π(A) that is as close as possible to the least squares direction over all of the terms of the matrix π(A)-A over all of the 3×3 rotation matrices, to determine the matrix D; 
             the step of solving of the system of equations comprises a second set of substeps consisting in: in a fourth, initialization substep, initializing a first series of right matrices {circumflex over (D)}{ [s] }, [s] denoting the integer rank of progress through the series {circumflex over (D)}{ [s] }, by setting {circumflex over (D)} [0]  equal to I 3 , I 3  denoting the identity matrix; then repeating a fifth, iterative substep for passing from the iteration [s] to [s+1] by computing the value {right arrow over (z)} [s+1] , then the value {circumflex over (D)} [s+1]  of the first matrix series using the following equations: 
           
         
       
    
     
       
         
           
             
               
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             the series {{right arrow over (z)} [s] } denoting an auxiliary second series of vectors and the series {{circumflex over (D)} [s] } converging towards {circumflex over (D)}; then, in a sixth, stopping substep, stopping the iterative process performed through the fifth substep when the limit {circumflex over (D)} is approximated with a sufficient accuracy defined by a predetermined threshold value; 
             the number N of measurements is equal to four, and the centred sighting visual pattern is set fixed on the display by the harmonization computer at four different positions P 1 , P 2 , P 3 , P 4  corresponding respectively to the four sightings V 1 , V 2 , V 3 , V 4 : a first position P 1  in the left part of the display and vertically to the centre, and a second position P 2  in the right part of the display and vertically to the centre, and a third position P 3  horizontally to the centre and upwards, and a fourth position P 4  horizontally to the centre and downwards; 
             the knowledge of the matrix M 01  determined is used to realign the symbology by correcting the alignment error between the display and the tracking element of the posture detection subsystem DDP; 
             the visual pattern provided with a central point is a reticle of the symbology. 
           
         
       
    
     Another subject of the invention is a head-worn display system for making the display of piloting information of an aircraft on a display conform with the outside real world comprising: a transparent head-worn display D 0 ; a head posture detection subsystem DDP having a mobile tracking first element D 1  securely attached to the transparent display D 0 , a fixed second element D 2  securely linked to the platform of the aircraft, and a means for measuring and determining the relative orientation M 12  of the mobile tracking first element D 1  relative to a reference frame of the fixed second element D 2  linked to the platform; an attitude inertial device D 3  for supplying the relative attitude M 3   t  of the platform relative to a terrestrial reference frame linked to the Earth, securely fixed to the platform; a harmonization subsystem for the head-worn display system for making the display of piloting information on the display D 0  conform with the outside real world, the harmonization subsystem having a dual harmonization computer and a human-system interface for managing and performing the implementation of the dual harmonization method. 
     The head-worn display system is characterized in that the harmonization subsystem is configured to: perform a series of an integer number N greater than or equal to 3 of different sightings Vi, i varying from 1 to N, performed through the display D 0  by aligning a centred sighting visual pattern on any same fixed target of the outside real world, each sighting Vi corresponding to a different fixed position Pi of the centre of the sighting pattern on the display D 0  and having a sighting vector {right arrow over (x i  )} determined as a function of the position Pi, and, for each sighting Vi, acquiring the corresponding measurement {circumflex over (K)} l  of the relative angular orientation of the tracking element relative to a DDP reference direction, that is fixed relative to the platform of the aircraft; then computing the matrix of relative orientation M 01  between the display D 0  in the tilted position and the tracking first element D 1  as the right matrix {circumflex over (D)}, the solution of the system of equations:
         Ĝ·{circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (y)} 0  for i=1 to N, the vector {right arrow over (y 0 )} denoting the vector in the inertial reference frame of the platform corresponding to the target point targeted in the outside real world and being unknown, and the left matrix Ĝ being the matrix M 23  of relative orientation between the reference frame of the fixed second element D 2 , linked to the platform of the aircraft, and the reference frame of the attitude inertial device D 3 , which is potentially incorrect but assumed constant as a function of time, and which, when it is unknown, requires at least four measurements {circumflex over (K)} i .       

     According to particular embodiments, the head-up display system comprises one or more of the following features, taken alone or in combination:
         the number N of measurements is greater than or equal to three and the left matrix Ĝ of relative orientation between the reference frame D 2  linked to the platform of the aircraft and the reference frame linked to the inertial unit D 3  is known; and the solving of the system of equations Ĝ·{circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (y)} 0 , i varying from 1 to N, uses an iterative process and a rectifying operator π(·) which transforms any matrix A into a 3×3 square matrix of rotation π(A) that is as close as possible to the least squares direction over all of the terms of the matrix π(A)-A over all of the 3×3 rotation matrices, to determine the matrix D and the vector {right arrow over (y 0 )};   the step of solving of the system of equations comprises a first set of substeps consisting in: in a first, initialization substep, initializing a first series of right matrices {{circumflex over (D)} [s] }, [s] denoting the integer rank of progress through the series {{circumflex over (D)} [s] }, by setting {circumflex over (D)} [0] , equal to I 3 , I 3  denoting the identity matrix; then repeating a second, iterative substep for passing from the iteration [s] to [s+1] by computing {right arrow over (y)} [s+1]  then {circumflex over (D)} [s+1]  using the following equations:       

     
       
         
           
             
               
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             the series {{right arrow over (y)} [s] } denoting a second series of external direction vectors, the series {{right arrow over (y)} [s] } and {{circumflex over (D)} [s] } converging respectively towards {right arrow over (y 0 )} and {circumflex over (D)}; then, in a third, stopping substep, stopping the iterative process performed through the second substep when the limits {circumflex over (D)} and Ĝ are approximated with a sufficient accuracy defined by one or two predetermined threshold values; 
             the number N of measurements is greater than or equal to four and the left matrix Ĝ of relative orientation between the reference frame D 2  linked to the platform of the aircraft and the reference frame linked to the inertial unit D 3  is unknown, and not seeking to determine the vector {right arrow over (y 0 )}, the solving of the system of equations: Ĝ·{circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (y)} 0 , i varying from 1 to N, amounts to the solving of the system of equations: {circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (z)} 0  for i varying from 1 to 4, by denoting {right arrow over (z)} 0 =Ĝ T ·{right arrow over (y)} 0 , which solving uses an iterative process and a rectifying operator π(·) which transforms any matrix A into a 3×3 matrix of rotation π(A) that is as close as possible to the least squares direction over all of the terms of the matrix π(A)-A over all of the 3×3 rotation matrixes, to determine the matrix {circumflex over (D)}; 
             the step of solving of the system of equations comprises a second set of substeps consisting in: in a fourth, initialization substep, initializing a first series of right matrices {{circumflex over (D)} [s] }, [s] denoting the integer rank of progress through the series {{circumflex over (D)} [s] }, by setting {circumflex over (D)} [0] , equal to I 3 , I 3  denoting the identity matrix; then repeating a fifth, iterative substep for passing from the iteration [s] to [s+1] by computing the value {right arrow over (z)} [s+1] , then the value {circumflex over (D)} [s+]  of the first matrix series using the following equations: 
           
         
       
    
     
       
         
           
             
               
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             the series {{right arrow over (z)} [s] } denoting an auxiliary second series of vectors and the series {{circumflex over (D)} [s] } converging towards {circumflex over (D)}; then, in a sixth, stopping substep, stopping the iterative process performed through the fifth substep when the limit {circumflex over (D)} is approximated with a sufficient accuracy defined by a predetermined threshold value. 
           
         
       
    
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on reading the following description of several embodiments, given purely by way of example and by referring to the drawings in which: 
         FIG. 1  is a view of a head-worn display system according to the invention for making the display of piloting information of an aircraft conform, which makes it possible to harmonize all the components of the head-up display system without having to use a bore site reference unit or a bore site reticle unit BRU, serving as calibration landmark inside the cockpit; 
         FIG. 2  is a flow diagram of a first embodiment of a harmonization method according to the invention for the head-worn display system of  FIG. 1 ; 
         FIG. 3  is a flow diagram of a second embodiment of a harmonization method according to the invention for the head-worn display system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     According to  FIG. 1 , a head-up display system  2  according to the invention for making the display of piloting information of an aircraft  4  on a display conform with the outside real world  6  comprises the following devices and means:
         a transparent head-worn display device or display  12 , denoted D 0 , positioned in front of the eye  14  of a pilot and being able to be used by him or her as viewfinder, for example a lens;   a posture detection subsystem  16  DDP, having a mobile tracking first element  18 , denoted D 1 , rigidly attached to the head  19  or to the helmet  20  of the pilot and rigidly attached to the display D 0  when the display D 0  is placed in the field of view of the pilot, a fixed second element  22  D 2 , securely linked to the platform  24  (denoted also by “pl”) of the aircraft  4  and serving as reference frame with respect to the posture detection subsystem  16  DDP, and a means  26  for measuring and determining the relative orientation M 12  of the mobile tracking first element  18  D 1  relative to a reference frame of the fixed second element  22  D 2  linked to the platform,   an attitude inertial device  30  D 3 , for example an AHRS inertial unit, for supplying the relative attitude M 3   t  of the platform relative to a terrestrial reference frame “t” linked to the Earth, that is fixed to the platform,   a device  32  Dp for supplying the position of the aircraft relative to the terrestrial reference frame linked to the Earth, for example a satellite positioning system of GPS type or a radio navigation system;   a dual harmonization subsystem  34  for the head-up display system  2  for making the display of piloting information on the display D 0  conform with the outside real world, the harmonization subsystem  34  having a dual harmonization computer  36  and a human-system interface  38  for managing and performing the implementation of the dual harmonization method.       

     The dual harmonization computer  36  can be an electronic computer dedicated specifically to the implementation of the dual harmonization method or a more general-purpose electronic computer provided to also implement other functions of the head-up display system  2 . 
     Likewise, the human-system interface  38  can be a human-system interface dedicated only to performing the harmonization method or a more general human-system interface sharing other functions of the head-up display system  2 . 
     The display system also comprises a means  42  for defining, measuring or determining the relative angular orientation M 2   t  of the fixed second element  22  D 2  relative to the Earth, and a means  44  making it possible to know the relative orientation M 23  of the fixed second element  22  D 2 , linked to the platform  24 , relative to the attitude inertial device  30  D 3 . 
     The means  44  is implemented in the form of a procedure performed on installation of the head-worn display system  2  and the orientation M 23  is assumed constant over time. 
     The means  42  uses the data of the attitude inertial device D 3 , attached to the platform of the aircraft and configured to measure its own orientation M 3   t  relative to the Earth, and the angular orientation M 23  supplied by the means  44 . 
     The conformal piloting information comprises, for example, a speed vector, a target on the ground, a synthetic representation of the terrain or even an image from an electromagnetic sensor, for example an infrared sensor. 
     It is noteworthy that, in the current state of the art of head-up display systems, the posture detection subsystem  16  DDP is relatively complex in practice because it implements two measurements:
         an inertial measurement of the relative angular orientation M 2   t  of the fixed second element D 2  relative to the Earth, and   a direct measurement of the relative orientation of the mobile tracking first element D 1  relative to the fixed second element D 2 , often in the form of image processing,   and uses the knowledge of the relative orientation M 2   t  of the fixed second element D 2  relative to the Earth. However, while this particular feature makes the algorithms more complex, this particular feature has no impact on the head-up display system and the dual harmonization method of the present invention, and it will be possible to then consider that the relative orientation M 12  of the tracking first element D 1  relative to the fixed second element D 2  is simply supplied by a direct measurement of the posture detection subsystem DDP.       

     Here, and according to a subsequently preferred embodiment, the posture detection subsystem  16  DDP is configured to supply raw DDP output data deriving as a priority from the direct optical measurements of the relative orientation between the tracking first element D 1  relative to the fixed second element D 2 . 
     It is noteworthy also that, here, for simplification reasons, the platform and the attitude inertial device D 3  are related. Generally, the means  44  for supplying the relative orientation M 23  is configured to perform this function in two steps: a first step of transition by the platform in which the tri-axial reference frame of the attitude inertial device D 3  is “aligned” on the tri-axial reference frame of the platform, then a second step in which the orientation of the fixed second element D 2  is harmonized on the reference frame of the platform. 
     These comments have no impact here on the content of the present invention. 
     Subsequently, the means Mij making it possible to know the relative orientation of one reference frame “i” to another “j” are likened hereinbelow in this document to the matrix describing this orientation. Indeed, the orientation Mij of one reference frame relative to another can be described equally by:
         three angles called Euler angles, which, conventionally in aeronautics, correspond to the order of the rotations for these following angles:   Bearing: rotation about the axis z which is oriented downwards (or towards the Earth);   Pitch: rotation about the axis y which is oriented to the right (or towards the east of the Earth);   Roll: rotation about the axis x which is oriented towards the front (or towards the north of the Earth),   a 3×3 matrix describing this rotation.       

     Subsequently, the matrix Mij will be able to also be denoted M(i/j), the matrix Mij or M(i/j) describing the relative orientation of the reference frame “i” relative to “j” (or from “i” to “j”). If vi is the expression of a vector in the reference frame “i” and vj is the expression of this vector in the reference frame “j”, then the relationship applies. Consequently, there is the relationship: vi=M(i/j)*vj and the relationship of transition between reference frames: M(i/k) (from i to k)=M(j/k)*M(ij). 
     The basic principle of the harmonization method for the head-up display system according to the invention rests on the use of a predetermined element of the outside terrestrial landscape used as landmark and a certain number of sightings consisting in aligning or superimposing a reticle of the symbology, fixed with respect to the display, on this outside element according to a number of positions of the reticle on the display which depends on the degrees of freedom affected by error of the relative angular orientation of the display D 0  with respect to the mobile tracking first element D 1  of the posture detection subsystem secured to the head. 
     According to  FIG. 2 , a dual harmonization method  202  for the head-worn display system for making the display of piloting information of an aircraft on the display conform with the outside terrestrial world comprises a set of steps. 
     In a first, launching step  204 , a triggering of the procedure for harmonizing the display of information conforming with the outside terrestrial world is actuated by the user of the head-worn display system, for example by pressing and holding a button situated in the cockpit and dedicated to this realignment. The display system is then set in a harmonization mode. 
     Then, in a second step  206  of acquisitions of sighting measurements, a centred sighting visual patter, for example a reticle of the symbology, is set fixed on the display by the computer at different positions, for example the following three different positions: (P 1 ) left of the display and vertically to the centre, (P 2 ) right of the display and vertically to the centre, then (P 3 ) horizontally to the centre and upwards. In the same second step  206 , the corresponding sightings, respectively denoted V 1 , V 2 , V 3 , are performed by aligning or superimposing the reticle, placed at the different positions P 1 , P 2 , P 3  on the display, on a predetermined element of the real outside terrestrial landscape serving as landmark. These sightings V 1 , V 2 , V 3  must be performed by taking the head roll: once to the right about the roll axis, that is to say the sighting axis, once to the left about the roll axis. For an optimal performance, each position of the reticle can give rise to two sightings: head inclined to the left then to the right, but this condition is not necessary to perform a harmonization of quality. 
     For each sighting Vi, i varying from 1 to 3, a corresponding measurement matrix Ki, i varying from 1 to 3, of relative orientation of the mobile part D 1  of the posture detection subsystem DDP relative to the device D 2  forming the fixed part of the subsystem is measured by the posture detection subsystem DDP and computed by the subsystem itself or the electronic harmonization computer which is connected to it. 
     Then, in a third step  208 , the harmonization computer solves the following dual harmonization equation: Ĝ·{circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (y)} 0 , in which:
         the left correction matrix G is none other than the matrix M 23 , of transition from the device D 2  (fiducial reference frame) to the inertial unit of the device D 3 ;   the matrices Ki are matrices of measurements corresponding to the data from the posture detection subsystem DDP for each sighting V 1 , V 2 , V 3  and the same matrices M 12   i  corresponding to each sighting;   the right correction matrix D is the matrix M 01  that is sought here that makes it possible to switch from the display D 0  to the angular tracking element D 1 ;   the vectors xi are the vectors corresponding to the position Pi of the sighting reticle for each sighting Vi, i varying from 1 to 3. If the reticle located at the position Pi is displayed with the bearing xiº and pitch yiº, the vector xi expressed in the reference frame of the display is a column vector [cos(xiº)*cos(yiº); sin(xiº)*cos(yiº); −sin(yiº)];   the vector y 0  is the vector in the reference frame of the inertial unit corresponding to the point targeted in the outside world serving as landmark. For example, a targeted point serving as landmark situated exactly in the axis of the inertial unit D 3  will have the coordinates (1; 0; 0).       

     To solve the harmonization equation, the third step  208  uses the algorithms of the fifth and sixth embodiments described in the French patent application entitled “Global dual harmonization method and system for a posture detection system” and filed on the same date as the present French patent application, depending on whether the computation of the direction y 0  is desired or not. 
     When the computation of the direction y 0  is desired, the matrix G has to be known and the coordinates have to be expressed in the reference frame of G, the third step  208  uses the fifth embodiment of the dual harmonization algorithm described in the patent application entitled “Dual harmonization method for a posture detection subsystem incorporated in a head-worn display system” and implements a first set  212  of first, second, third computation substeps  214 ,  216 ,  218 . 
     The fifth embodiment of the dual harmonization algorithm solves, through the first, second, third computation substeps  214 ,  216 ,  218 , the system of equations: Ĝ·{circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (y)} 0  for i varying from 1 to 3. 
     In the first, initialization substep  214 , a first series of right matrices {{circumflex over (D)} [s] } is initialized by setting {circumflex over (D)} [0]  equal to I 3 , I 3  denoting the identity matrix. 
     Then, the second, iterative substep  216  is repeated for passing from the iteration [s] to [s+1] by computing {right arrow over (y)} [s+1]  then {circumflex over (D)} [s+1]  using the following equations: 
     
       
         
           
             
               
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             the series {{right arrow over (y)} [s] } denoting a second series of external direction vectors. 
           
         
       
    
     The series {{right arrow over (y)} [s] } and {{circumflex over (D)} [s] } converge respectively towards {right arrow over (y 0 )} and {circumflex over (D)}. 
     In the third, stopping substep  218 , the iterative process performed through the second substep  216  is stopped when the limits {circumflex over (D)} and Ĝ are approximated with a sufficient accuracy. 
     It is noteworthy that the fifth configuration mode of the dual harmonization computation demands, as usage constraints, that the minimum number of measurements N is greater than or equal to 3 and that the vector family {{right arrow over (x)} i } is free. That means that, as a variant of the harmonization method of the dual harmonization method for the display described in  FIG. 2  in which the number of measurements is equal to 3, the dual harmonization method according to the first embodiment can also acquire a number strictly greater than 3 of measurements Ki, that is to say of sightings Vi, provided that the vector family {{right arrow over (x)} i } is free, and process the measurements Ki by using the sixth embodiment of the dual harmonization algorithm. 
     According to  FIG. 3  and a second embodiment, a method  302  for harmonizing the display of piloting information on the display conforming with the outside terrestrial world comprises a second set  303  of fourth, fifth, sixth steps  304 ,  306 ,  308 . 
     In the fourth, launching step  304 , identical to the first step  204 , a triggering of the procedure  302  for harmonizing the display of information conforming with the outside terrestrial world is actuated by the user of the head-worn display system, for example by maintained pressure on a button situated in the cockpit and dedicated to this realignment. The display system is then set in a harmonization mode. 
     Then, in the fifth step  306  of acquisitions of sighting measurements, a reticle of the symbology is set fixed on the display by the harmonization computer at four different positions, for example the following four different positions: (P 1 ) left of the display and vertically to the centre, (P 2 ) right of the display and vertically to the centre, (P 3 ) horizontally to the centre and upwards, then (P 4 ) horizontally to the centre and downwards. In the same fifth step  206 , the corresponding sightings, respectively denoted V 1 , V 2 , V 3 , V 4 , are performed by aligning or superimposing the reticle, set at the different positions P 1 , P 2 , P 3 , P 4  on the display, on a predetermined element of the outside real terrestrial landscape serving as landmark. These sightings V 1 , V 2 , V 3 , V 4  have to be performed by taking the head roll: once to the right about the roll axis, that is to say the sighting axis, once to the left about the roll axis. For optimal performance, each position of the reticle can give rise to two sightings: head tilted to the left then to the right, but this condition is not necessary to perform a harmonization of quality. 
     For each sighting Vi, i varying from 1 to 3, a matrix of corresponding measurement Ki, i varying from 1 to 4, of relative orientation of the mobile part D 1  of the posture detection subsystem DDP relative to the device D 2  forming the fixed part of the subsystem is measured by the posture detection subsystem DDP and computed by the subsystem itself or the electronic harmonization computer which is connected to it. 
     Then, in the sixth step  308 , the harmonization computer solves the following dual harmonization system of equations: Ĝ·{circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (y)} 0 , in which:
         the left correction matrix G is none other than the matrix M 23 , of transition from the device D 2  (fiducial reference frame) to the inertial unit of the device D 3 ;   the matrices Ki are the matrices of measurements corresponding to the data from the posture detection subsystem DDP for each sighting V 1 , V 2 , V 3 , V 4  and the same matrices M 12   i  corresponding to each sighting;   the right correction matrix D is the matrix M 01  that is sought here that makes it possible to switch from the display D 0  to the angular tracking element D 1 ;   the vectors xi are the vectors corresponding to the position Pi of the sighting reticle for each sighting Vi, i varying from 1 to 4. If the reticle located at the position Pi is displayed with the bearing xiº and pitch yiº, the vector xi expressed in the reference frame of the display is a column vector [cos(xiº)*cos(yiº); sin(xiº)*cos(yiº); −sin(yiº)];   the vector y 0  is the vector in the reference frame of the inertial unit corresponding to the point targeted in the outside world serving as landmark. For example, a targeted point serving as landmark situated exactly in the axis of the inertial unit D 3  will have the coordinates (1; 0; 0).       

     Here, the dual harmonization system of equations to be solved differs from that of the second embodiment of the harmonization method in that:
         the number of sightings Vi is equal to 4, the family of the vectors {{right arrow over (x)} i } being free,   the matrix G is not known and any matrix G can be taken; and   there is no attempt to compute the direction of the landmark y 0 .       

     In this case, to solve the harmonization equation, the sixth step  308  uses the algorithm of the sixth configuration described in the French patent application entitled “Global dual harmonization method and system for a posture detection system” and filed on the same date as the present French patent application, and implements one of the second set  312  of fourth, fifth, sixth computation substeps  314 ,  316 ,  318 . 
     The dual solving harmonization algorithm of the sixth configuration amounts to the solving of the system of equations: Ĝ·{circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (y)} 0  for i varying from 1 to 4, to the solving of the system of equations: {circumflex over (K)} i ·{circumflex over (D)}·{right arrow over (x)} i ={right arrow over (z)} 0  for i varying from 1 to 4, by denoting {right arrow over (z)} 0 =Ĝ T ·{right arrow over (y)} 0 . 
     In the fourth substep  314 , a first series of right matrices {{circumflex over (D)} [s] } is initialized by setting {circumflex over (D)} [0]  equal to I 3 , I 3  denoting the identity matrix. 
     Then, the fifth, iterative substep  316  for passing from the iteration [s] to [s+1] is repeated by computing {right arrow over (z)} [s+1]  then {circumflex over (D)} [s+1]  using the following equations: 
     
       
         
           
             
               
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             the series {{right arrow over (z)} [s] } denoting a second series of external direction vectors. 
           
         
       
    
     The series {{circumflex over (D)} [s] } converges towards {circumflex over (D)}. 
     In the sixth, stopping substep  318 , the iterative process performed through the fifth substep  316  is stopped when the limit {circumflex over (D)} is approximated with a sufficient accuracy. 
     It is noteworthy that the dual harmonization algorithm of the sixth configuration demands, as usage constraints, that the minimum number of measurements N is greater than or equal to 4 and that the vector family {{right arrow over (x)} i } is free. That means that, as a variant of the dual harmonization method  302  for the display described in  FIG. 3  in which the number of measurements is equal to 4, a dual harmonization method according to the invention can also acquire a number strictly greater than 4 of measurements Ki, that is to say of sightings Vi, provided that the vector family {{right arrow over (x)} i } is free, and process the measurements Ki by using the sixth embodiment of the dual harmonization algorithm. 
     Thus, the knowledge of the orientation of a BRU unit relative to the bearer has been replaced by the assumption of identity of the fiducial direction with different sightings. Thus, the rotation matrix M 03  of orientation of the display relative to the fiducial reference frame D 2 , if it is unknown in the absolute, is identical in the different sightings. By using:
         the measurement of the matrix M 12  of the relative orientation of the mobile tracking element D 1  relative to the fixed element D 2  of the posture detection subsystem DDP, and   the knowledge of the matrix M 23 , i.e. the left matrix G, potentially incorrect but assumed constant,   it is possible to determine the matrix M 01  of relative orientation of the display D 0  with respect to the mobile tracking element of the posture detection subsystem DDP such that, for each measurement: M 12 *M 01 *sighting vector=constant vector. Here, the sighting vectors are the vectors forming the family {{right arrow over (x)} i }.       

     Advantageously, in addition to the saving of a calibration instrument such as the BRU and above all the complex installation thereof, the dual harmonization method according to the invention makes it possible to obtain a better alignment accuracy than that provided through the use of a BRU, particularly on the harmonization in terms of roll. 
     The dual harmonization method described above also makes it possible to dispense with the errors and drifts of relative orientation between a BRU and the inertial device D 3 .