Abstract:
An antenna including a reflector with coverage and frequency flexibility is provided. The antenna comprises a reversible reflector having two separate reflecting surfaces shaped geometrically so as to cover respectively a first and a second geographical zone which are different and have predetermined shapes, in which the two reflecting surfaces are fastened back to back on a common support, and at least two independent sources arranged in a fixed configuration and connected to separate radiofrequency supply chains defining different and predefined operating frequency planes, the reflector having a first deployment position, in which the focal point of the first reflecting surface is located at the phase center of the first source, and a second deployment position, in which the focal point of the second reflecting surface is located at the phase center of the second source. Application notably to the field of satellite telecommunication antennae.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to foreign Patent Application FR 09 02995, filed on Jun. 19, 2009, the disclosure of which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to an antenna having a reflector with coverage and frequency flexibility and to a satellite comprising such an antenna. It applies notably to the field of satellite telecommunication antennae. 
     BACKGROUND OF THE INVENTION 
     The increasing service life of telecommunications satellites and the change in the requirements linked to the various missions which may be entrusted to them make it necessary for the payloads, and particularly the antennae, of future generations of satellites to be flexible. This flexibility may be implemented in terms of the geographical coverage zone of the antenna, and/or in terms of the polarization and/or the operating frequency band. This flexibility is not intended to cover all the geographical coverage zones simultaneously, but, instead, to have a choice between a plurality of geographical coverages capable of being generated by the same antenna and to make it possible to modify the satellite&#39;s mission in orbit. 
     The antennae placed on board satellites typically comprise a geometrically shaped reflector illuminated by a single source in order to cover a coverage zone aimed at the Earth. A satellite generally comprises a transmission and reception antenna or a transmission antenna and a reception antenna per coverage zone. The geometrical shape of the reflector may, where appropriate, be defined so as to be optimized for a plurality of orbital positions of the satellite, but, in general, so as to cover a single geographical coverage. 
     Frequency flexibility over a broad-band spectrum, for example the frequency plane Ku, Ku+ covering the frequencies between 10.7 GHz and 18.4 GHz and a single coverage zone, cannot be obtained by means of a single source, since, at the present time, no source has a sufficiently broad band. Furthermore, there is a critical point regarding the diplexing between the transmission and reception bands, and it is necessary to preserve an allowance of the order of 250 MHz between the high frequency of the transmission band and the low frequency of the reception band. 
     A first known solution is to use two separate antennae in order to cover the same geographical zone, but this solution presents problems of mass, of bulk and of cost. 
     A second known solution involves placing two sources side by side in front of an over-dimensioned reflector, so as to minimize the defocusing of the two sources. The phase centers of the two sources are located in the focal plane of the reflector, and their radiation axes are parallel. The two sources are positioned as near as possible to the focal point of the reflector in order to reduce the defocusing of the sources and the directivity losses of the antenna which arise as a result. However, this solution is not optimal. 
     As an example, one reference discloses an antenna device comprising two sources and a pivotable auxiliary reflector provided with two reflecting surfaces. On the one hand, this device has the abovementioned defocusing disadvantages, thus impairing the performances of the antenna, and, on the other hand, the number of degrees of freedom accessible on an auxiliary reflector is relatively low, which amounts to limiting the deformation possibilities of the coverage obtained by means of the antenna beam. 
     Another possibility involves using a single source located at the focal point of a reflector, the source being connected to a complex electrical architecture combining two radiofrequency chains, the first chain operating in a first frequency plane and the second chain operating in a second frequency plane. However, this architecture entails a complexity which gives rise to appreciable ohmic losses and a high implementation cost. 
     Moreover, in order to produce two separate coverage zones, the present solutions make it necessary to use two separate and independent antennae, each comprising a deployable reflector, and the reflector has to be linked to two different sources in order to cover a selected frequency band completely, thus making it necessary to have a total of four sources placed on a side face of a satellite, and a double stacking system for deploying or stowing the two reflectors of the two antennae. 
     Another reference describes another solution involving using a reversible reflector comprising two reflecting surfaces covering two different coverage zones, the reflector being linked to a single source. Positioning one of the reflecting surfaces in front of the source makes it possible to select one of the coverage zones, but this solution does not have any frequency flexibility and does not make it possible to operate in a broad-band frequency plane. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention advantageously produce an optimal antenna making it possible to satisfy the coverage and frequency flexibility requirements, making it possible to eliminate the aberrations and losses attributable to defocusing, which are simple to implement and the geometry of which does not result from a compromise in terms of performances and makes it possible to reduce the ohmic losses, as compared with prior solutions. 
     In this regard, one embodiment of the present invention provides an antenna, with coverage and frequency flexibility, that comprises a reversible reflector having two separate reflecting surfaces shaped geometrically so as to cover respectively a first and a second geographical zone which are different and have predetermined shapes, in which the two reflecting surfaces are fastened back to back on a common support, and at least two independent sources arranged in a fixed configuration and connected to separate radiofrequency supply chains defining different and predefined operating frequency planes, the reflector having a first deployment position, in which the focal point of the first reflecting surface is located at the phase centre of the first source S 1 , and a second deployment position, in which the focal point of the second reflecting surface is located at the phase centre of the second source S 2 . 
     Thus, whatever the configuration in which the antenna according to the invention is used, the active source S 1  or S 2  is focused, since its phase centre is positioned at the focal point of the reflector. 
     Advantageously, the antenna comprises means for deploying the reflector, comprising at least one first motor, and means for reversing the reflector, comprising at least one second motor, the two motors having axes of rotation perpendicular to one another, the second motor actuating the reversal of the reflector from the first position to the second position by means of a rotation of the common support through a predetermined angle. 
     Advantageously, the reflector comprises a third deployment position, in which the focal point of the first reflecting surface is located at the phase centre of the second source, and a fourth deployment position, in which the focal point of the second reflecting surface is located at the phase centre of the first source. 
     Advantageously, the antenna comprises, furthermore, means for the translation of the reflector, comprising a third motor connected to the first motor and to the second motor by means of lever arms, the third motor having an axis of rotation parallel to the axis of rotation of the first motor, the first and the third motor which actuate the reflector in translation making it possible to change the position of the focal point of the first reflecting surface or of the second reflecting surface from the first source to the second source. The antenna according to the invention thus benefits from specific kinematics, notably by virtue of the expediently placed three motors, and makes it possible to achieve optimal RF performances on two separate coverages and on two different frequency planes. 
     According to one embodiment, the antenna comprises a single reflector, this reflector being the reversible reflector. A large number of different coverages can thus be produced (although, ultimately, only two coverages are accessible on the reflector), for example highly deformed and highly elongated geographical coverages. 
     According to another embodiment, the antenna comprises a main reflector associated with an auxiliary reflector (for example, an antenna with a Cassegrain-type set-up). In this case, preferably, the main reflector comprises two reversible reflecting surfaces, so as to profit from maximum degrees of freedom in producing the coverages. 
     Advantageously, the sources may be fastened side by side or one above the other. 
     The invention also relates to a telecommunications satellite comprising such an antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other particulars and advantages of the invention will become clearly apparent from the rest of the description given by way of purely illustrative and non-limiting example, with reference to the accompanying diagrammatic drawings in which: 
         FIG. 1   a  shows a perspective diagram of an antenna having a reflector with coverage flexibility, mounted on the platform of a satellite, the reflector being in a stowed position, according to an embodiment of the present invention; 
         FIG. 1   b  shows a perspective diagram of the reflector in the deployed position, showing the two reflecting surfaces of the reflector which are mounted on a common support, according to an embodiment of the present invention; 
         FIGS. 2   a  and  2   b  show two diagrams of the same antenna for two different aiming directions, according to an embodiment of the present invention; 
         FIGS. 3   a  and  3   b  show two diagrams of the same antenna in a second and a third position in which respectively the source S 1  and the source S 2  are at the focal point of the reflector aimed in the same direction, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the example illustrated in  FIG. 1   a , the offset simple passive antenna comprises a reflector  10  in the stowed position on the platform  11  of a satellite, for example on a side face parallel to a plane YZ, and two independent sources S 1 , S 2  of radiofrequency signals. A deployment mechanism  12 , which can be seen in the following figures, makes it possible to deploy the reflector  10  so that, in a deployed position, the two sources S 1 , S 2  are arranged in front of the reflector in the focal plane of the latter. The reflector  10  comprises two separate reflecting surfaces R 1 , R 2  having different shapes and fastened back to back on a common support  15 , as illustrated, for example, in  FIG. 1   b . Each reflecting surface is shaped geometrically and optimized for a given mission so as to illuminate a ground coverage zone having predetermined dimensions when a single source is located at its focal point. This shape has approximately the configuration of a parabola and differs from this only slightly. The sources S 1 , S 2 , for example of the horn type, are fastened on an inclined plane  16  formed on the platform  11  and are arranged in a predetermined fixed configuration, for example one beside the other. The sources S 1  and S 2  may in some cases be placed one above the other or in any other configuration. 
     In the deployed position, one of the reflecting surfaces R 1 , R 2  is positioned opposite the two sources S 1 , S 2  and is oriented in a predetermined aiming direction  17 . The reflector  10  is reversible with respect to the plane of the support  15  as a result of a rotation of the assembly consisting of the support  15  and of the two reflecting surfaces R 1 , R 2 , thus making it possible to be able to change the reflecting surface and therefore the desired coverage zone. The invention therefore involves positioning the two reflecting surfaces R 1 , R 2  on the common support  15  in such a way that, in a first position of the reflector  10  corresponding to a first mission of the satellite, the phase centre of the source S 1  is located at the focal point of the first reflecting surface R 1 , and in such a way that, in a second position of the reflector obtained by means of a rotation of the reflector and corresponding to a second mission of the satellite, the phase centre of the second source S 2  is located at the focal point of the second reflecting surface R 2 . The rotation making it possible to reverse the reflector from the first position to the second position is carried out about an axis  22  parallel to the plane of the support  15  and through a predetermined angle depending on the relative positioning of the reflecting surfaces R 1 , R 2  on the support  15 . As a non-limiting example, the angle of rotation for reversing the reflector is adjustable within a predetermined value range, for example between 175° and 195°. 
     The mechanism for deploying the reflector comprises, for example, a motor M 1  having an axis of rotation parallel to the plane YZ and a deployment arm  13  capable of being actuated in rotation by the motor M 1  between a position in which the reflector  10  is stowed against the wall of the platform  11 , parallel to the plane YZ of the satellite, and a deployment position. The mechanism for reversing the reflector  10  comprises, for example, a second motor M 2  having an axis perpendicular to the axis of the motor M 1  and connected to the deployment arm  13  and to the reflector  10 . The second motor M 2  actuates the reversal of the reflector  10  from the first position to the second position by means of a rotation of the common support  15  through a predetermined angle. 
     The two sources S 1 , S 2  are supplied respectively by means of two different chains  2 ,  3  for the supply of radiofrequency signals RF, which are preferably integrated in a housing  14 . Since each RF chain  2 ,  3  is dedicated to telecommunication functions, the two sources  51 , S 2  can be supplied in different frequency planes F 1  and F 2 , each frequency plane being capable of comprising one or more transmission and/or reception frequency sub-bands. 
     In  FIG. 2   a , the phase centre  5  of the source  51  is positioned in the focal point of the first reflecting surface R 1  which is aimed in a first aiming direction  17  located on a first ground coverage zone corresponding to a first predetermined mission. In  FIG. 2   b , the phase centre  6  of the source S 2  is positioned at the focal point of the second reflecting surface R 2  which is aimed in a second aiming direction  18  located on a second ground coverage zone different from the first coverage zone and corresponding to a second predetermined mission. The change from the first mission to the second mission is carried out by means of a rotation of the reversible reflector  10  through a predetermined angle, for example of 180°, with respect to the plane of the support  15 . The drive of the reflector  10  in rotation is carried out by means of the second motor M 2 . The desired change in aiming direction between mission  1  and mission  2  determines the relative position of the two reflecting surfaces R 1 , R 2  with respect to one another on the support  15 . 
     In addition to the coverage flexibility obtained by the reversal of the reflector  10 , it is possible to obtain frequency flexibility on the same coverage zone and therefore for the same position and same aiming direction of the reflector, without losses or aberrations attributable to defocusing. For this purpose, the invention involves selecting one of the sources S 1  or S 2  as a function of the desired frequency, then displacing and orienting the reflector  10  in such a way that the selected source is positioned at the focal point of the reflector and that the reflector illuminates the selected coverage zone. 
     In the initial configuration illustrated in  FIG. 3   a , the phase centre  5  of the source S 1  is positioned at the focal point of the first reflecting surface R 1  of the reflector  10  which is aimed in an aiming direction  17  located, for example, on the terrestrial equator. If the source S 1  is supplied, for example, in a frequency plane F 1  by means of a first RF chain, and the source S 2  is connected to a second RF chain optimized for operating in a frequency plane F 2 , in order to change from the frequency plane F 1  to the frequency plane F 2  without any change in the aim of the antenna, the invention involves switching the supply of the source S 1  to the source S 2  and displacing the reflector in translation from the source S 1  towards the source S 2  in order to position the focal point of the first reflecting surface R 1  at the phase centre  6  of the source S 2 , as illustrated in  FIG. 3   b . The displacement and orientation of the reflector  10  in front of the source S 2  without any change in the aiming direction  17  of the antenna can be carried out, for example, by means of two motors M 1 , M 3 , the motor M 3  being connected to the motor M 1  and to the motor M 2  by means of corresponding lever arms  20 ,  21 . The two motors M 1 , M 3  have axes of rotation parallel or virtually parallel to one another and virtually parallel to the plane YZ of the side face of the platform  11  of the satellite which supports the reflector  10 . The actuation of the motor M 1  in anti-clockwise rotation and at an angle of rotation depending on the spacing between the sources S 1  and S 2  drives the first lever arm  20  in rotation in the same direction, the effect of which is to displace the motor M 3  and reflector  10  and to bring them closer to the platform  11  of the satellite, as shown in  FIG. 3   b , and thus to displace the reflector from the source S 1  towards the source S 2 . The actuation of the motor M 3  in clockwise rotation at the same angle of rotation as the motor M 1  then makes it possible, by means of the lever arm  21 , to swing the reflector  10  in rotation in the other direction, until it is in a position parallel to its initial position and until the phase centre  6  of the source S 2  is thus positioned at the focal point of the reflector  10  and illuminates the same ground coverage zone. The successive rotations of the various motors M 1  and M 3  thus cause the reflector  10  to undergo a translation such that the focal point of the reflecting surface R 1  changes from the source S 1  to the source S 2 . Using a number of sources greater than two, the same operations can be reproduced with one or more additional sources, for example in order to carry out one or more other missions in other frequency planes, if each of the additional sources is connected to a dedicated RF chain optimized through a frequency plane other than that of the sources S 1  and S 2 . 
     With a single antenna comprising two interchangeable reflecting surfaces, three motors and two sources S 1 , S 2 , it is thus possible to deploy the antenna in orbit, and to position it so as to perform selectively one of four possible missions. Since the two reflecting surfaces are interchangeable and fixed together mechanically, the four different missions are performed, using a single mechanical support and deployment structure. The first mission is carried out by placing the focal point of the reflective surface R 1  at the phase centre  5  of the first source S 1 , for the second mission the reflector is moved in translation and the phase centre  6  of the second source S 2  is at the focal point of the reflecting surface R 1 , for the third mission the reflector is rotated through a predetermined adjustable angle, for example of between 175° and 195° in the exemplary embodiments, and the first source S 1  is at the focal point of the reflecting surface R 2 , and, for the fourth mission, the second source S 2  is at the focal point of the reflecting surface R 2 . 
     By virtue of the three motors, it is thus possible to focus the sources S 1  or S 2  at the focal point of one of the reflecting surfaces R 1 , R 2  of the reflector  10 , thus making it possible to obtain optimal performances over the entire frequency plane. By additional sources being added, additional missions in different frequency planes likewise become possible. 
     Although the invention has been described in connection with particular embodiments, it is quite clear that it is in no way limited to these and that it comprises all the technical equivalents of the means described and also their combinations if these come within the scope of the invention.