Abstract:
This invention relates to a system for emitting electromagnetic beams, comprising a network of elements for the far-field emission of electromagnetic beams, the signals coming from and/or arriving towards each element weighted by excitation coefficients digitally determined by calculation means. According to the invention, the system comprises: a second separate network of sensors arranged close to the network of radiating elements in order to measure the near field radiated by the elements, means for calculating the far field radiated by the network from the near field actually measured by the sensors, and means for calculating the correction of the excitation coefficients of the elements from the difference between the far field calculated from the measurement of the near field and a pre-determined nominal far field.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a national phase entry under 35 USC §371 of International Application No. PCT/EP2010/050583, filed Jan. 19, 2010, published in French, which claims the benefit of and priority to French Patent Application No. 0950320, filed Jan. 20, 2009, the entire disclosures of which are incorporated herein by reference. 
     The invention relates to a large size antenna emission and/or reception system including a network of radiating elements. 
     BACKGROUND OF THE INVENTION 
     The field of application of the invention is satellite antennas, radar antennas, aircraft antennas, generally ground-based or on-board antennas integrating networks of radiating elements. 
     In emission, the radiating elements of the network antenna are fed with electromagnetic signals which are digitally phase-and-amplitude-weighted beforehand with excitation coefficients determined by computing means. In reception, the electromagnetic signals received by elements of the network antenna are then phase-and amplitude-weighted digitally with excitation coefficients determined by these same computing means. These excitation coefficients are used in reception for transforming the signals received by the elements of the network antenna and stemming from one or several directions into a useful coherent signal, and in emission for transforming a useful signal into different signals feeding the elements of the network and forming one or more given illumination beams, in both cases for observing a certain intended illumination law for the network. One skilled in the art will recognize in the digital generation of the excitation coefficients and in the digital weighting of the signals of the elements of the network antenna, a digital network for forming beams (Digital Beamforming Network or DBFN). 
     One of the problems of large size network antennas is the fact that the arrangement and the orientation of the elements of the network may vary over time. 
     For example, an orbiting satellite may be subject to sudden changes in temperature according to whether it is illuminated by the sun or not. 
     The result is deformations of the antenna due to the existence of significant thermal gradients. 
     Generally, the antenna may be subject to significant thermal and mechanical stresses generating deformations of the latter. 
     These deformations perturb the illumination law of the elements of the network. 
     Presently, in order to limit these deformations, it is resorted to mechanical structures supporting the network antenna, the design of which should allow the rigidity, the flatness and the shape of the antenna to be maintained under very severe thermal and mechanical stresses. Consequently, these mechanical supporting structures generally have significant mass, cost and bulkiness. 
     Presently, the functions for calibrating the elements of the network are generally ensured by using couplers inserted in the emission circuit in order to pick up a portion of the signal sent to the emitting elements. 
     Another calibration solution consists of conducting remote measurements. For example, on an orbiting satellite, the measurements are carried out from an earth station. 
     These means are burdensome and costly to apply and the corrections cannot always be made in real time for reasons of logistics and/or cost-effectiveness. Further, many approximations are made during these measurements (mutual couplings between elements not taken into account, behavior of the radiating elements not taken into account, non-exhaustive tests, etc.) This is detrimental to optimum operation of the antennas since the environmental conditions under which the latter are found (high and rapid temperature gradients for example for space antennas, winds for radar, ground antennas, etc.) cause variations in the shape of the network, in the performances of the radiating elements and in the resulting radiation diagram of the antenna. The consequences are designs of antennas with complex and often heavy and bulky mechanical structures. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the invention is to overcome these drawbacks by proposing a network antenna system allowing a desired illumination law and radiation diagram to be observed as much as possible. 
     Another object of the invention is to obtain a network antenna system which is less burdensome to apply. 
     Another object of the invention is to allow real-time control of each of the elements of the antenna and of the far field radiation diagram. 
     A first subject matter of the invention is a system for emitting electromagnetic beams, including a network of elements for emitting far field electromagnetic beams, the signals stemming from and/or arriving at each of the elements being weighted by excitation coefficients digitally determined by computing means, 
     characterized in that the system includes:
         a second distinct network of sensors laid out in proximity to the network of radiating elements in order to measure the existing near field radiated by the elements,   means for computing the far field radiated by the network from the near field actually measured by the sensors,   means for computing corrections of the excitation coefficients of the elements from the difference existing between the far field, computed from the measurement of the near field and a predetermined set far field.       

     By means of the invention, the illumination law of the network is controlled in real time from local measurements of the near field radiated by the latter, thereby allowing rapid reconfiguration of the beams. The system thereby includes on-board monitoring means with which the radiation diagram of the network antenna may be checked in real time. This allows adjustment and real-time compensation of the radiation diagram of the antenna in the case of deformation of the network or else of a failure of one or more elements of the network. The emission and reception radiation diagrams of the antenna are corrected in real time by acting on the values of the excitation coefficients of each of the elements of the network. The system allows the mechanical and thermal deformations to which the antenna may be subject, to be taken into account and which may be non-negligible with respect to the Ku or Ka band wavelength for an orbiting satellite, for example. 
     This will subsequently allow relaxation of certain manufacturing constraints for large size network antennas and their supporting means, notably in the space medium, and reduction in the mass and the cost of the antennas and of the system. Thus, some deformability of the network antenna and of its supporting means may be accepted under the effect of external conditions, while being aware that the on-board control of the illumination law of the antenna and the calculations for correcting the excitation coefficients will allow compensation of this real-time deformation. 
     Thus, according to an embodiment, the radiating elements of the network are attached to a first support, the second network of sensors being attached to a second support distinct from the first support, the first support and the second support being attached to a common base with a space between the first support and the second support allowing deformation of the first support. 
     According to other embodiments of the invention:
         The first support comprises a common plate for supporting the radiating elements of the network, and a second support is provided for each sensor, this support for each sensor including a holding rod, one end of which is attached to the sensor and the other end of which is attached to a base, to which the first support is also attached via spacers, the plate including holes for the crossing of the rods with said space present between the edge of the hole and the rod.   The sensors are positioned in the free space and distributed above the plane of the network of radiating elements.   The height between the sensors and the radiating elements of the network is greater than a fraction of the working wavelength of the elements.   The excitation coefficients comprise a phase shift and an amplitude, the system includes for each element of the network an associated reception channel and/or an associated emission channel, the computing means being provided for calculating the phase shift adjustments of the excitation coefficients and the amplitude adjustments of the excitation coefficients so that the radiation diagram measured from the sensors is as close as possible to radiation diagram of a set instruction.   The system includes means for addressing the sensors in order to collect the near field measurement locally at the location of each sensor by using a modulated broadcasting technique for example.       

     The invention will be better understood upon reading the following description, only given as a non-limiting example with reference to the appended drawings, wherein: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a modular block diagram of an exemplary emission and reception antenna system according to the invention, 
         FIG. 2  illustrates a modular block diagram of a regulation portion of the antenna system according to  FIG. 1 , 
         FIG. 3  illustrates a side view of an exemplary portion of the network of elements of the antenna system according to  FIG. 1 , 
         FIG. 4  illustrates a top view of another exemplary portion of the network of elements of the antenna system according to  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The invention is described below in the example of a satellite network antenna, responsible for retransmitting to Earth a signal received from an earth base station.
         The emission and reception system  1  includes a network  2  of a plurality of radiating elements  2   1 ,  2   2 , . . .  2   i , . . .  2   N . This network  2  is for example arranged on a plane. Each element  2   i  is for example in the form of a horn or a printed element having an aperture oriented towards a direction DIR common to all the antennas  2   i .       

     The network  2  of antennas is connected to a computer  3  via a reception circuit  4  on the one hand and through an emission circuit  5  on the other hand. The separation between the reception and emission channels is achieved by means of a set  7  of frequency diplexers placed close to the radiating elements. 
     The reception circuit  4  includes a reception channel  4   i  for processing each signal s i  received on each antenna  2   i  and for bringing it onto an input  6   i  of the computer  3 . The processing of each reception channel  4   i  comprises, as this is known, a frequency diplexing stage  7 , a low noise amplification stage  8 , a variable gain amplification stage  9 , a base band stage  10  and an analog/digital conversion stage  110 . 
     The emission circuit  5  includes an emission channel  5   i  for each element  2   i  of the network  2  and allows forwarding of a signal s′ i  to be emitted by the elements  2   i  of the network  2 . The processing of each emission channel  5   i  comprises, as this is known, a digital/analog conversion stage  12 , a carrier frequency switching stage  13 , a stage  14  for distribution through Buttler matrices, an amplification stage  15 , a filtering stage  16 , a stage  17  for recombination through Buttler matrices and a frequency diplexing stage  18 . 
     The computer  3  includes means  30   a  for computing the complex excitation coefficients of the antennas  2   i  in reception and means  30   b  for computing the complex excitation coefficients of the antennas  2   i  in emission. 
     Therefore there is a complex excitation coefficient Ki for each antenna  2   i  in reception and a complex excitation coefficient Lk for each antenna  2   i  in emission. The excitation coefficients Ki and Lk respectively allow reconstruction from signals s i  received by the antenna  2   i , of a useful coherent signal S, and this useful signal S may be sent back as the signal s′ k  to each emission channel  5   k  by forming the desired emission beams. The excitation coefficients Ki and Lk provide a gain and a phase shift, i.e. a complex multiplicative factor or complex weighting, respectively to each reception channel  4   i  with respect to the other reception channels  4   i , and to each emission channel  5   k  with respect to the other emission channels  5   k . In a way known to one skilled in the art, the complex values of the reception coefficients Ki are optimized and digitally computed by the computing means  35  of the computer  3  in order to maximize the coherent signal stemming from the sum weighted by the Ki coefficients of the received signals s i . 
     The means  35  of the computing means  30   a , depending on the reception signals s i  of the antennas  2   i , produce a signal S equal to the weighted sum of the signals s i , weighted by the excitation coefficients Ki according to the equation: 
     
       
         
           
             S 
             = 
             
               
                 ∑ 
                 
                   i 
                   = 
                   1 
                 
                 N 
               
               ⁢ 
               
                 Ki 
                 · 
                 
                   s 
                   i 
                 
               
             
           
         
       
     
     According to the invention, sensors  10   1 ,  10   2 , . . . ,  10   j , . . .  10   M  measuring the near field radiated by the elements  2   i , M being able to be different from N and being generally greater than the number N of elements  2   i , are provided in proximity to the network  2  of radiated elements  2   i . 
     The network of the sensors  10  is connected through addressing, collecting and receiving means  11  to the computing means  30   b  of the computer  3 . 
     The means  30   b  for computing the complex excitation coefficient Lk in emission are illustrated in  FIG. 2 . 
     The computing means  30   b  includes a module  31  for determining the excitation coefficients Lk from the near field Epj measured by the sensors  10   j . 
     Each sensor  10   j  is used for measuring the near field Epj radiated by the network  2  of radiating elements  2   i . An addressing, collecting and receiving means  11  is provided between each sensor  10   j  and a module  32  for computing the far field. The module  32  computes the existing far field El from the near field Epj measured by the sensors  10   j . The module  32  for example has for this purpose advanced algorithms for computing the far field from data in planar near fields, tables of pre-recorded values of the radiation diagram of the sensors  10   j  and elements  2   i  and/or other pre-recorded correspondence rules, a memory being provided for this purpose. 
     A comparator  33  compares this computed existing far field E 1  with a pre-determined and pre-recorded set far field Elc, for example in a module  34 . The comparator  33  thus computes a far field error signal Err depending on the difference between the computed existing far field El and the set far field Elc. The computing module  31  determines by means of advanced optimization algorithms the excitation coefficients Lk of the elements  2   i  from this error signal Err in the far field. The signal S is sent from the module  35  of the portion  30   a  when it is provided or from a generator of a signal S to be emitted to the computing module  31 . The excitation coefficients Lk are applied to the signal S to be emitted over the different emission channels  5   k  by the module  31  in order to form the signals s′ k .
 
 s′   k   =L   k   ·S  
 
     The module  31  modifies the emission field radiated by the elements  2   i , which will again be measured by the sensors  10 . Thus, the far field radiated by the elements  2   i  is optimized by acting on the coefficient Lk in order to be closer to the ideal field Elc or to be equal to the latter. The far field radiated by the elements  2   i  is therefore regulated so as to be closer or equal to the ideal far field Elc. 
     Of course, there may be more or less radiating elements used in emission as compared with reception, the number of emitting elements used may be different from the number of receiving elements used. Of course, the system may only operate in emission. In the foregoing, the index i relates to the elements used in reception, is less than or equal to the number N of elements of the network  2 , and k relates to the elements used in emission, less than or equal to the number N of elements of the network  2 . In a satellite, the system operates in reception and in emission, i.e. as a transponder, where the received signal is retransmitted. If the system does not operate as a satellite transponder, but mainly in emission, such as for example for a radar, in which the signal is emitted, an echo signal is received which is processed separately, while the signal S stems from a signal generator and the block  30   a  becomes a source of a digital signal S. 
     In  FIG. 3 , the plurality of radiating elements  2   i , symbolized by two lines in  FIG. 3 , is attached on a same first support  20 , while the plurality of sensors  10   j  is attached to another second support  100 , different from the first support  20 . The first support  20  is for example formed by a same planar plate. For example a second support  100  is provided for each sensor  10 . This support  100  is for example formed by a holding rod, one end of which is attached to the sensor  10   j  and the other end of which is attached to a stable and rigid base  40  which may be the platform of the satellite, to which the first support  20  is also attached via spacers  21 . The sensors  10   j  are positioned in the free space in front of the plane of the network of radiating elements  2   i , for example by being located in a same geometrical plane parallel to the plane in which are arranged the elements  2   i  of the network  2 . The height H between the sensors  10  and the elements  2   i , for example perpendicularly to the plane on which the elements  2   i  are arranged, is for example greater than one fifth of the working wavelength λ of the elements  2   i . 
       FIG. 3  shows that the sensors  10   i  are provided on the side and between elements  2   i . There exists a space  22  between the first support  20  of the elements  2   i  and each second support  100  of the sensors  10   j . In  FIG. 3 , the plate forming the first support  20  includes holes  23  for letting through the second supports  100  therein. Therefore, each second support  100  passes through a hole  23  of the plate forming the first support  20  with the space  22  present between the edge of the hole  23  and the support  100 . The space  22  therefore allows clearance between the support  20  and the support  100 . This clearance permitted by the spaces  22  allows the first support  20  to deform to a certain extent because of thermal or mechanical strengths for example. The deformation of the support  20  will be taken into account by the sensors  10   j  because these sensors  10   j  will measure the near field Epj radiated by the elements  2   i . Therefore this deformation may be corrected in real time. It will therefore be possible to impose much less strict requirements to the first support  20  and accept to a certain extent deformation of the latter, which will allow lightening of this support  20  and of the means  21  for connecting to the base  40 . 
       FIG. 4  shows that several sensors  10   j  may be provided around and between each radiating element  2   i , such as for example 6 in number per elements  2   i  in the illustrated hexagonal configuration. Further, a sensor  10   j  may be provided above each element  2   i , as this is also illustrated in  FIG. 4 . In this case, the support  100  of the sensor  10  located above the element  2   i  passes through both the first support  20  and this element  2   i . 
     The sensors  10  are very discreet because of their small size and because they do not perturb the field radiated by the network antennas  2 . Modulated broadcasting techniques may be applied to the sensors  10  for locally measuring the near field radiated by the network antennae  2 . 
       FIG. 1  illustrates an embodiment of a system of sensors  10  using the modulated broadcasting technique for conducting measurements of the near field Epj locally at the location of the sensors. For this purpose, the system includes a bus  11   j  for addressing the sensors  10   j  from the computer  3  and another channel  19  for collecting measurements of the near field Epj from the sensors towards a measurement reception module  36 . Because, in order to address one of the sensors  10   j , the addressing signal sent by the computer  3  on the bus  11   j  is modulated for this sensor  11   j , with for example a modulation different from one sensor to the other in order to identify the responses of the sensors to this modulation over the collecting channel  19 . The measurement signal Epj collected by the module  36  over the collecting channel  19  and having the modulation sent to the sensor  11   j , will be the one provided by this sensor  11   j . After having been digitized beforehand, the module  36  will provide different near field measurements Epj to the means  30   b.    
     The sensors  10  may be calibrated by receiving a far field calibration signal in the direction DIR, for example from the Earth for a satellite. This calibration may be periodic, for example once a month or a week or other. In the case of a satellite, an earth base station illuminates the satellite with plane waves. By this means, the complex correction coefficients of each sensor  10  are determined so that the amplitude and phase responses of the sensors are uniformized, and also the radioelectric axes of each sensor are orthogonal per sensor and parallel with each other.