DEVICE FOR DEPLOYING AND AIMING STRUCTURAL ELEMENTS IN A SPACE ENVIRONMENT The present invention relates to tlie field of articulated devices designed to be placed on a payload placed in Earth orbit. More particularly, the invention relates to the field of satellites comprising structures to be deployed such as solar generators. Finally the invention relates to devices having two life phases in which the first phase comprises the deployment of structural elements in space and in which the second life phase comprises the aiming of these elements at a target during the flight in orbit. Currently, when satellites are placed in orbit around the Earth, they require a phase for the deployment of the structural elements carrying solar generators or antennas depending on the mission. This phase is particularly critical because the success of the mission depends on it. The structural elements assembly is a structure which forms an articulated arm. Notably, amongst these structural elements there are yokes and/or solar panels and/or antennas. These elements are joined together by articulations from the root section to the end of the articulated arm. In order to obtain a fully deployed posture of the structural elements and to minimize on the one hand the distribution of the motive force on the structural elements and on the other hand the kinematic interferences between the elements, it is often necessary to use a coupling system making it possible to coordinate and articulate the deployment of the structural elements in a uniform manner. The initially-folded structure is deployed according to a movement sequence and at a determined speed. All the angles between two consecutive structural elements are opened according to a determined law by the coupling mechanism. A known coupling system uses pulleys positioned on each of the articulations and cables connecting the pulleys making it possible to transfer the forces between the structural elements at the time of deployment. 2 In order to activate the first articulation which comprises a first pulley, a motor is usually situated at the root section, that is to say at the first articulation which joins together the payload of the satellite and the first structural element. At the time of deployment, certain structures have, in addition to the coupling system, a system for locking the articulations together. The locking system makes it possible to immobilize the articulations and hence the couplings linked to the articulations for the rest of the mission of the satellite which will no longer need to deploy or retract these structural elements. The locking makes it possible to pass from a first life phase of the satellite corresponding to placement in orbit and deployment of its structural elements to a second life phase corresponding to the mission itself, in which the structural elements will have a specific mobility. Once the structure is deployed, it may be necessary, throughout the lifetime of the satellite, to aim the structural elements at a target. In this second life phase, the deployment of the structure being completed, it is necessary, depending on the mission, to aim the structure in a direction of a target. For example, this may be the sun, if the structure comprises solar generators such as solar panels, or else a geographic region on the planet in the case of an antenna or else a precise point in space as required. In order to protect the deployment phase, it is usually separated from the phase corresponding to the mission. Currently, the drive means used in each life phase are partitioned for security reasons. Notably, the motor which is used to articulate the structural elements for aiming at a target is different from the motor which is used to deploy the structure. The drive means are different depending on the life phase because the mechanisms of articulations with and without coupling of the two phases are different. Usually, the pivoting connection of the root section is fixed after the deployment; it is said to be locked. In the prior art, there is at the root section of the payload, a pivoting connection making it possible to orient the whole of the deployed structure, the structure comprising all the attached structural elements when they are locked together. 3 One drawback of the solutions of the prior art is the space requirement and the weight of a second motor for aiming the deployed structure at a target. A second drawback is that the locking system immobilizes the coupling of the articulations and locks the structure, the pulley mechanism making it possible to deploy the structural elements then no longer being used. The second motor then activates the articulations of the structure in order to cause all or some of it to pivot. This solution means that it is necessary to couple the locked coupling system with another system making it possible to rotate a portion of the structure so as to make it pivot towards a target. One object of the invention is to alleviate the aforementioned drawbacks. One object of the invention is to share the drive means in the two life phases. In particular, the invention proposes to define two modes of motor function of the coupling for each of the life phases. The invention comprises a locking/unlocking system making it possible to pass from a first coupling mode comprising a first law of articulation of the structure for the deployment to a second coupling mode comprising a second law of articulation of the structure for the aiming of the structure. Advantageously, the device for deploying and aiming structural elements designed to be placed in Earth orbit comprises: • a plurality of structural elements linked together by articulations, the assembly forming an articulated arm linked to a payload via a root section; • the articulations comprising at least one pivoting connection making it possible to have two consecutive structural elements pivot relative to one another, • the root section comprising a pivoting connection making it possible to have the first structural element pivot relative to the payload; 4 • a locking/unlocking system comprising at least one locking device placed on an articulation making it possible to lock at least one pivoting connection; • a coupling system comprising a first coupling mode allowing each of the structural elements to pivot according to a predefined law of rotation. • at least one motor making it possible to activate the coupling system, the said motor being positioned on an articulation, called a driving articulation. Advantageously, the locking system makes it possible to deactivate the first coupling mode and to activate a second coupling mode, the second coupling mode making it possible to retain the unlocking of a pivoting connection of at least one articulation, called the aiming articulation, the rotation of the motor causing the rotation of the aiming articulation. Advantageously, the coupling system comprises: • a plurality of pulleys linked by cables, one pulley of which is activated by the motor, called the driving pulley and; • means for attaching the pulleys to the structural elements, each of the articulations comprising at least two pulleys one of which is attached to an upstream structural element, called the upstream pulley, and the other is attached to a downstream structural element, called the downstream pulley, the root section comprising a pulley attached to the first structural element. Advantageously, the locking/unlocking system comprises at least one device for locking/unlocking a coupling pulley relative to the articulation. Advantageously, the articulation comprising the motor, and all the articulations situated between the articulation comprising the motor and the aiming articulation comprise a device for locking/unlocking a pulley, the aiming articulation comprising a device for locking/unlocking a pulley only on the downstream pulley. 5 Advantageously, all the articulations have a device for locking/unlocking a pivoting connection of an articulation except the aiming articulation. Advantageously, the engagement of the second coupling mode causes: • a plurality of pivoting connections to be locked; • the pulleys of each articulation included between the articulation comprising the motor, called the driving articulation, and the aiming articulation to be unlocked; • the upstream pulley of the aiming articulation to be kept locked; • the downstream pulley of the aiming articulation to be unlocked; • the pivoting connection of the aiming articulation to be kept unlocked. Advantageously, the engagement of the second coupling mode simultaneously causes: • the pivoting connection of the driving articulation to be locked; • the driving pulley to be unlocked relative to the articulation. Advantageously, the device for locking/unlocking a pivoting connection of an articulation comprises at least one ratchet and a return spigot. Advantageously, the device for locking the pivoting connection of the driving articulation comprises at least one ratchet, a return spigot and a retaining arm. Advantageously, the device for locking the pivoting connection of the aiming articulation comprises a locking finger making it possible to release the pulley at a determined angle. Advantageously, the structural elements comprise yokes and solar generators. Advantageously, the motor and the driving pulley are placed at the root section. 6 Other features and advantages of the invention will appear with the aid of the following description made with respect to the appended drawings which represent: • Figure 1: a coupling system; • Figure 2: an articulated structure deployed according to the device of the invention; • Figures 3A, 3B, 3C, 3D: the structure according to the device of the invention in its life phase of aiming according to various locking modes; • Figure 4: a device for locking/unlocking the articulations and for coupling motor functions of the invention; • Figure 5: a first 3D view of an articulation at the root section of a satellite arm according to the invention; • Figure 6: a second 3D view of an articulation at the root section of a satellite arm according to the invention; • Figure 7: a 3D view of an aiming articulation of a satellite arm according to the invention. "Structure" in the rest of the description means the articulated arm comprising a plurality of structural elements comprising yokes, solar generators, articulations, a locking/unlocking system and a coupling system. In the rest of the description, the system that comprises a plurality of locking/unlocking devices positioned at the articulations of the articulated arms is called the locking/unlocking system. Figure 1 represents a coupling system between three structural elements 1, 2, 3 connected two by two by articulations 4, 5. Each articulation 4, 5 comprises a pivoting connection making it possible to have two structural elements pivot relative to one another. Moreover, the articulations 4, 5 each comprise a pulley. A cable 8 is wound round each of the two pulleys of each articulation 4, 5. Under the impulsion of a motive force rotating the first pulley, the cable drives the second pulley of the articulation 5. 7 During the structure-deployment phase, the coupling systenn allows, under the impulsion of the motive force, a coordinated deployment of the structural elements. The deployment takes place until the structural elements are positioned on the same axis. The first pulley of the articulation 4 is secured to the pivoting connection of the structural element of this same articulation; the rotation of the first pulley rotates the structural element 2 relative to the structural element 1. The second pulley is secured to the pivoting connection of the articulation 5 of the structural element 3. The rotation of the second pulley rotates the structural element 3 relative to the structural element 2. The absolute value of the angle a between the structural element 2 and the structural element 1 in the anticlockwise direction is equal to the absolute value of the angle between the structural element 2 and the structural element 3 in the anticlockwise direction. The cable 8 operates the pivoting connection of the articulation 5 and causes the structural element 3 to pivot at an identical angle a. The system of coupling by pulleys and by cable reverses the directions of the angles of two consecutive articulations. In the rest of the description this coupling is called the first coupling mode. It corresponds to the coupling of the angles connected together between the various structural elements to deploy the structure. It is used in a first life phase of the satellite which corresponds to the deployment of its arms. The coupling system of Figure 1 requires a locking device between the pivoting connection of the articulation 4 of the structural element 1 and the pulley of this same articulation. When the deployment is complete, a second life phase then begins; it involves the aiming phase which corresponds to another operating mode of the device of the invention. A second coupling mode is then initiated as soon as the locking/unlocking system is engaged after the deployment. The locking/unlocking device actuates so as to release the pivoting connection of the articulation 4 of the structural element 1 and the pulley of the same articulation. As a result, the pulley does not operate the pivoting connection. The unlocking of the pulley relative to the pivoting connection 8 causes a locking of the pivoting connection connecting the structural element 1 and the structural element 2 which are then fixed relative to one another and the pulley is free. The rotation of the pulley of the articulation 4 then operates the pulley of the articulation 5. If the pulley of the articulation 5 is connected to the pivoting connection of the articulation 5, the structural element 3 is rotated about the pivoting connection of the articulation 5. The first life phase between the structural elements is stopped by the locking/unlocking system which breaks the coupling principle of the first phase. The latter locking/unlocking system begins a second life phase which makes it possible to have a structural element 3 pivot about the articulation 5, the rotation being engaged by means of the same motive force as that of the deployment applied to the pivoting connection of the articulation 4. In this case, the device of the invention with an activatable locking/unlocking system makes it possible to pass from a first life phase of deployment to a second life phase of aiming by using the same motive force during a deployment, the same structural elements and the same coupling elements. Figure 2 represents a device for deploying and aiming structural elements of a satellite according to the invention. In one embodiment, so as to share the motor for the various life phases, the device of the invention comprises one of the locking/unlocking devices positioned at the articulations of the arm. On the one hand there are locking/unlocking devices for the pulleys and on the other hand locking/unlocking devices for the pivoting connections making it possible to deploy the whole structure in a first life phase and to aim all or some of the deployed structure in a second life phase which corresponds to the mission of the satellite. In the first life phase, the structure is initially retracted. It is deployed under the impulsion of the second motor and of the coupling system in its first mode. The pulley 22 is initially secured to the satellite fixed portion. The 9 motor is secured to one side of the pulley 22 and secured to the structural element 26 for the other side. The motor has the same rotation axis 221 as the pivoting connection 21. The motor rotates the structural element 26. The motor 21 is secured to the pulley 22 except for the reduction gears. When the motor 21 is running, it makes it possible to rotate the pulley 22 relative to the structural body 26. Since the pulley 22 is secured to the portion attached to the satellite 1, the body 26 is deployed about the axis 221. The rotation of the pulley 22 relative to the structural element 26 causes, with the aid of the cable 223, the rotation of the pulley 23 which itself rotates the structural element 27 securely connected to the pulley 23. The rotation of the structural element 26 securely connected to the pulley 23' causes the rotation of the pulley 23. The rotation of the pulley 23' drives the cable 234 which itself drives the pulley 24 securely connected to the structural element 28. The structure 28 is therefore rotated under the impulsion of the rotation of the pulley 24. The rotation of the pulley 24' then rotates under the impulsion of the rotation of the structure 28 and in turn rotates by means of the cable 245 the pulley 25 which is securely connected to the structural element 29 which pivots about the axis 251. The structural elements 26, 27, 28 and 29, each being secured to a pulley, are deployed by action of the motor in a uniform manner under the stress of the coupling system. Each of the structural elements pivots about the axis of the articulation that separates it from the previous or next structural element. The structural element 26 pivots about the axis 221, the structural element 27 pivots about the axis 231, the structural element 28 pivots about the axis 241 and the structural element 29 pivots about the axis 251. The absolute values of the angles of rotation 220, 230, 240 and 250 are equal by virtue of the coupling mechanism in its first mode provided by the pulleys and the cables and the motive force. On the other hand, the angles are in opposite directions in twos. The angle 220 is backwards, the 10 angle 230 is anticlockwise, the angle 240 is clockwise and finally the angle 250 is anticlockwise. When the deployment is complete, the first life phase is complete. Once the structural elements are deployed, they are in the same axis. The angles between each structural element is 180° in the example of Figure 2. The invention comprises locking/unlocking devices at the articulations making it possible to lock a portion of the articulations when the deployment is complete, thus closing the first life phase of the satellite. The second motor is also used for the second life phase. In each articulation, the pulley and the pivoting connection can be independently locked or unlocked. One embodiment of the invention makes it possible to link the locking of a pulley in rotation to the unlocking of a pivoting connection. Several variant embodiments are possible depending on the mission and depending on which portion of the structure it is desired to cause to pivot towards a target. This involves configuring the locking/unlocking system so that one articulation unlocks its pivoting connection which will be operated by the force of the second motor. The locking/unlocking system therefore allows a second coupling mode in which a rotation of the pulley allows the rotation of a predefined pivoting connection. The second motor 21 makes it possible to pivot the structure comprising a plurality of structural elements about an axis 221, or about an axis 231 or else about an axis 241 when it is necessary to orient a portion of the structure 200 and in particular the solar generators 28 and 29 towards a designated target. The invention makes it possible to have a locking/unlocking system making it possible to pivot a portion, situated at the end of the structure, not necessarily at the root section but at an articulation distant from the payload. This configuration makes it possible for example to get away from the shadow of the satellite which could cover the solar generators. Another example makes it possible to limit the specific inertia variations associated with the aiming of the solar generators. 11 Figures 3A, 3B, 3C and 3D represent various possible aiming configurations depending on tlie configuration of the locl 11. Device for deploying and aiming structural elements designed to be placed in Earth orbit according to any one of Claims 1 to 10, characterized in that the structural elements comprise yokes and solar generators. 12.Device for deploying and aiming structural elements designed to be placed in Earth orbit according to any one of Claims 1 to 11, characterized in that the motor and the driving pulley are placed at the root section.