Abstract:
A method for characterizing a transmitting antenna of a satellite in orbit comprising a payload comprising signal amplification means, includes: configuring the said amplification means to generate thermal noise at the input of the transmitting antenna; acquiring, by means of a ground station, the signal transmitted by the transmitting antenna on the downlink of the satellite over a predetermined time period; during the said predetermined time period, controlling the said satellite in orbit in order to impart on it an angular bias of predetermined variation and recording this variation; correlating the measurement of the signal transmitted on the downlink and the variation of angular bias of the satellite in order to derive therefrom the gain variations of the transmitting antenna as a function of the angular bias of the satellite.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to foreign French patent application No. FR 1202393, filed on Sep. 7, 2012, the disclosure of which is incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a method and a system for testing the payload of a satellite in orbit. More precisely, the invention relates to the characterizing of the transmitting antenna of a satellite in orbit. 
         [0003]    The field of the invention is that of methods and systems making it possible to test the nominal operation of a transmitting antenna of a satellite in orbit, that is to say to characterize the radiation pattern of the antenna and to compare it with expected operational specifications. 
         [0004]    The invention is notably applied for testing a telecommunications satellite in orbit but also any satellite whose payload is composed by at least one repeater and an antenna for transmitting in the direction of a downlink. 
       BACKGROUND 
       [0005]    The known methods of testing the payload of a satellite in orbit are most often based on the use of a test signal on an unmodulated carrier which is generated and transmitted on the uplink of the satellite in orbit by a ground station coupled to the test device. The satellite receives this test signal via a receiving antenna; the signal is propagated through a repeater and retransmitted to the ground station via a transmitting antenna. It is possible to characterize the response of the transmitting antenna of the satellite from measurements carried out on the signal transmitted on the downlink of the satellite. 
         [0006]    There are many disadvantages of test methods based on the use of specific test signals. 
         [0007]    The transmitting antenna test is limited to a part of the cover area of the receiving antenna. In fact, in order that the test method may be used, the ground station, which operates both the transmission of the test signal on the uplink and the acquisition of the signal retransmitted by the satellite on the downlink, must be positioned within the area of intersection of the cover areas of the receiving antenna and of the transmitting antenna of the satellite. Thus, it is not possible to test the transmitting antenna over the whole of its angular cover. 
         [0008]    Moreover, the extraction of the contribution of the transmitting antenna alone necessitates the use of an automatic gain loop in order to compensate for the gain variations of the receiving antenna. This loop provides a constant level at the input interface of the transmitting antenna, which makes it possible to measure, on the ground, solely the variations of the transmitting antenna. However, as the radiation pattern of the receiving antenna is generally more directive than that of the transmitting antenna, the result of this is that the dynamic measuring range of the transmitting antenna under test is limited to the dynamic range available in the automatic gain control loop. 
         [0009]    Moreover, the isolation measurements, that is to say the characterizing of the transmitting antenna for cover areas in which the gain of the antenna is very low, are also limited by the weak reception level on the receiving antenna. 
         [0010]    Another problem associated with the use of test signals is that when it is desired to test the operation of a multi-beam transmitting antenna or of an antenna transmitting at several frequencies, this necessitates the generation of multi-carrier test signals and therefore a device for generating such signals, which increases the complexity of the test system. 
         [0011]    Finally, the generation of a test carrier can interfere with other adjacent satellites and this necessitates overall frequency coordination. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention proposes a method and a system which make it possible to characterize the transmitting antenna of a satellite in orbit over the whole of its dynamic range and over the whole of its cover area, independently with respect to the characteristics of the receiving antenna. 
         [0013]    The invention does not necessitate the use of test signals on an unmodulated carrier and therefore makes it possible to eliminate the disadvantages associated with their use. 
         [0014]    Moreover, the invention makes it possible to test the transmitting antenna of a satellite having several beams and/or several separate frequencies without necessitating the modification of test equipment and thereby increasing its complexity. 
         [0015]    The invention thus relates to a method for characterizing a transmitting antenna of a satellite in orbit comprising a payload comprising signal amplification means, the said method consisting in:
       Configuring the said amplification means to generate thermal noise at the input of the transmitting antenna,   Acquiring, by means of a ground station, the signal transmitted by the transmitting antenna on the downlink of the satellite over a predetermined time period,   During the said predetermined time period, controlling the said satellite in orbit in order to impart on it an angular bias of predetermined variation and recording this variation,   Correlating the measurement of the signal transmitted on the downlink and the variation of angular bias of the satellite in order to derive therefrom the gain variations of the transmitting antenna as a function of the angular bias of the satellite.       
 
         [0020]    Advantageously, the amplification means are configured to operate at saturation. 
         [0021]    The amplification means can also comprise a first intermediate amplification means and a second high power amplification means, the said intermediate amplification means being configured to amplify the signal to a predetermined level at the input of the second high power amplification means, the said level being determined so that the level of the output signal of the second high power amplification means is higher than the noise level generated by the antenna of the ground station. 
         [0022]    According to a particular embodiment of the invention, the payload comprises at least one receiving antenna and a reception channel and wherein the reception channel is disconnected from the receiving antenna in order to limit the influence of the noise coming from the uplink of the satellite. 
         [0023]    The invention also relates to a system for characterizing a transmitting antenna of a satellite in orbit comprising a payload comprising signal amplification means, the said system comprising:
       Means of configuring the said amplification means in order to generate thermal noise at the input of the transmitting antenna,   Means of acquiring the signal transmitted by the transmitting antenna on the downlink of the satellite over a predetermined time period,   Means for controlling the satellite in orbit, during the said predetermined time period, in order to impart on it an angular bias of predetermined variation and means of recording this variation, Means for correlating the measurement of the signal transmitted on the downlink and the variation of angular bias of the satellite in order to derive therefrom the gain variations of the transmitting antenna as a function of the angular bias of the satellite       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Other features and advantages of the present invention will become more apparent on reading the following description with reference to the appended drawings in which: 
           [0028]      FIG. 1  is a synoptic diagram showing a method of testing the transmitting antenna of a satellite in orbit according to the prior art, 
           [0029]      FIG. 2  is a graph showing the limitation, induced by the test method shown in  FIG. 1 , on the dynamic angular range of mispointing of the transmitting antenna which can be tested, 
           [0030]      FIG. 3  is a synoptic diagram showing certain disadvantages of the test method shown in  FIG. 1 , 
           [0031]      FIG. 4  is a synoptic diagram showing the test method according to the invention, 
           [0032]      FIG. 5  is a diagram of the payload of a satellite to be tested and in particular of the on-board amplification system, 
           [0033]      FIG. 6   a  is a graph of the gain of the signal measured on the downlink of the satellite as a function of time, 
           [0034]      FIG. 6   b  is a graph of the angular bias imparted by the satellite during the acquisition of the signal on the downlink as a function of the same interval of time as in  FIG. 6   a,    
           [0035]      FIG. 6   c  shows an example of the antenna pattern obtained by combining the measurements shown in  FIGS. 6   a  and  6   b,    
           [0036]      FIG. 6   d  shows the comparison of the antenna pattern obtained with the specifications of the transmitting antenna of the satellite. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIGS. 1 ,  2  and  3  show, in several diagrams and graphs, the principle of a known method of testing the transmitting antenna of a satellite in orbit and its disadvantages as already mentioned in the introduction section of the present application. 
         [0038]      FIG. 1  shows, in a diagram, the principle of a known method of testing the transmitting antenna  102  of a satellite  100  in orbit. 
         [0039]    A test station  104 , coupled to an antenna  103 , is used for generating a test signal on an unmodulated carrier and for transmitting it on the uplink of the satellite  100 . The test signal is received by the receiving antenna  101  of the satellite  100  and then retransmitted by the transmitting antenna  102  of the satellite  100  via the downlink. The signal is received by the antenna  103  of the test station  104  which carries out measurements on it making it possible to characterize the antenna pattern of the transmitting antenna  102 . 
         [0040]    As explained above, a disadvantage of this method is that the testable cover area is limited to the area of intersection  130  between the cover area  110  of the receiving antenna  101  and the cover area  120  of the transmitting antenna  102 . 
         [0041]      FIG. 2  shows, in a graph, the relative antenna gain  201 , expressed in decibels, as a function of the angular bias  202  imparted by the satellite with respect to a reference pointing direction. The respective gain curves of the receiving antenna  210  and of the transmitting antenna  220  of the satellite are shown in the graph of  FIG. 2 . The receiving antenna is generally more directive than the transmitting antenna and this generates a faster attenuation of the antenna gain as the angular bias increases. 
         [0042]    In order to compensate for the variations of the gain of the receiving antenna, an automatic gain control loop is actuated in the payload of the satellite in order to bring the signal at the input of the transmitting antenna back to a constant level  211 . In this way, the ground station can measure, on the signal received via the downlink, only the variations of the gain of the transmitting antenna. 
         [0043]    The automatic gain control loop has a limited dynamic input operating range. In the example shown in  FIG. 2 , it operates only up to an input level of the order of −20 dB. Thus, for angular biases greater than a given value, of the order of 1.5° in the example shown in  FIG. 2 , the variations of the gain of the receiving antenna are no longer compensated for and this means that the angular excursions  230  beyond a certain threshold cannot be tested. 
         [0044]      FIG. 3  illustrates the necessity of a frequency coordination plan for the uplink of the satellite under test  100 . A test signal on an unmodulated carrier  310  is transmitted by the antenna  103  of the test station  104  on the uplink of the satellite  100 . On reception, the test station  104  retrieves, by filtering, the unmodulated carrier retransmitted on the downlink of the satellite  100 . 
         [0045]    The transmission of an unmodulated carrier  310  on the uplink can generate interference  301  on one or more adjacent satellites  300 . Thus, a frequency coordination plan is necessary in order to avoid disturbing the frequencies elsewhere used by other satellites. 
         [0046]      FIG. 4  shows the use of the method according to the invention making it possible to characterize the transmitting antenna  102  of a satellite in orbit  100  without generating or transmitting a test signal on the uplink of the satellite. 
         [0047]    The method according to the invention consists in generating, on board the payload of the satellite, thermal noise of sufficient power to ensure, at the input interface of the transmitting antenna  102 , the presence of wide band noise of level that is substantially constant and independent with respect to the angular excursions of the satellite. In this way, the signal transmitted by the transmitting antenna  102  on the downlink is acquired by the antenna  103  of the test station  104  and then analyzed in order to characterize the antenna gain over the whole of the cover area  120  which is no longer limited to the intersection with the cover area of the receiving antenna  101 . 
         [0048]    The noise  400  generated at the input interface of the antenna  102  is filtered by the test station  104  within a predetermined frequency analysis band. 
         [0049]      FIG. 5  shows, in a simplified block diagram, an example of a payload  500  of a satellite which comprises a receiving antenna  501 , a reception channel  502 , a channel filter  503 , an intermediate amplification system  504 , a high power amplifier  505  and a transmitting antenna  506 . The payload  500  thus constituted carries out the function of a repeater, that is to say that the signal received on the reception channel  502  is retransmitted on the downlink of the satellite via the transmitting antenna  506 . In the example shown in  FIG. 5 , a single repeater is shown, but a payload can contain several repeaters associated with several different receiving and/or transmitting frequencies. 
         [0050]    When no signal is received by the receiving antenna  501 , the payload  500  however generates noise which has two main components: a first noise component resulting from the Earth&#39;s radiation transmitted to the payload by the receiving antenna  501  and potentially containing interference coming from adjacent satellite systems, and a second thermal noise component generated by the repeater itself, in particular by the receiving system  502 . 
         [0051]    By configuring  510  the intermediate amplification system  504  in order to make it operate at saturation and thus to cause the saturation of the high power amplifier  505 , it is possible to obtain a sufficient noise level at the input interface of the transmitting antenna  506  to be able to characterize its antenna pattern. The noise level generated at the output of the high power amplifier  505  must be higher than the noise level generated by the antenna  103  of the test station on the ground. 
         [0052]    An example embodiment of the intermediate amplification system  504  and the associated configuration for obtaining the desired effect to allow the characterization of the transmitting antenna  506  will now be described in greater detail. 
         [0053]    The amplification system  504  comprises at least a first amplifier  541  which makes it possible to adjust the signal level at the output of the channel filter  503  so that it is compatible with the dynamic range of the automatic gain control loop  542  placed at the output of the first amplifier  541 . The automatic gain control loop  542  makes it possible to obtain a signal of constant level at the input of a second amplifier  543  which is capable of amplifying the signal in order to reach the desired operating point at the input of the high power amplifier  505 . A third amplifier  544  is present at the output of the second amplifier  543  in order to compensate for the non-linearities of the high power amplifier  505 . 
         [0054]    The amplification system  504  example shown in  FIG. 5  is given by way of illustration and is non-limiting. In particular, other amplifiers arranged in cascade can be included. For example, the automatic gain control loop  542  can be optional. 
         [0055]    In order to obtain a sufficient noise level at the input of the transmitting antenna  506 , the first and second amplifiers  541 ,  543  are configured to amplify the input signal with a maximum gain for the purpose of achieving operation at saturation. The third amplifier  544  is configured in order to obtain the saturation of the high power amplifier or in order to obtain a sufficient operational level which depends on the link budget of the transmitting antenna and on the link budget of the antenna of the ground station  103 . The noise level thus generated also has high temporal stability which makes it possible to acquire the signal on the downlink over a time period that is sufficient for carrying out the measurements necessary for the characterization of the transmitting antenna  506 . 
         [0056]    In a variant embodiment of the invention, the reception channel  502  of the payload can be disconnected from the receiving antenna  501  of the satellite, for example by connecting its input to the input of another reception channel (not shown). In this way, the noise received by the receiving antenna  501  is eliminated and only the thermal noise generated by the repeater itself is used. 
         [0057]    The use of the amplification system of the payload whilst operating at saturation makes it possible to replace the test signal usually generated on the uplink of the satellite. In this way, the test method does not necessitate any specific signal and does not depend on the characteristics and operation of the receiving antenna  501  of the satellite. The test method according to the invention consists in the execution of the following steps. 
         [0058]    The test station  104 , which comprises means of acquisition of the signal transmitted on the downlink of the satellite and means of measuring and of spectral analysis of that signal, is connected to the downlink of the satellite. 
         [0059]    The intermediate amplification system  504  on board the payload of the satellite is configured to amplify the noise of the repeater in order to reach saturation of the high power amplifier  505  or a sufficient operational noise level at the input interface of the transmitting antenna  506 . The configuration of the payload of the satellite is carried out by a control centre of the satellite remote from the test station. 
         [0060]    The spectral analysis of the signal is for example carried out by a spectrum analyzer configured to carry out a low-pass filtering of the received signal in order to smooth the level of the signal by eliminating the high frequency components. Several successive spot measurements are carried out during a time sweep. 
         [0061]    Moreover, an angular bias is imparted on the satellite with respect to its reference pointing direction in order to allow the characterization of the antenna over the whole of its cover area and to observe the variations of the gain of the antenna as a function of time. The control of the pointing of the satellite is also carried out from a remote control centre via a remote control and telemetry link. 
         [0062]    Means of processing the signal acquired by the test station  104 , which can be included in the test station or which can be remote, are then executed in order to produce a measurement of the pattern of the transmitting antenna of the satellite.  FIGS. 6   a ,  6   b ,  6   c ,  6   d  illustrate the processings carried out on the acquired signal. 
         [0063]      FIG. 6   a  shows, in a graph, the measurement of the level of the signal acquired by the test station, in decibels of milliwatt (dBm), as a function of time t. 
         [0064]      FIG. 6   b  shows, over the same time scale as  FIG. 6   a , the variation of the angular bias, expressed in degrees, of the pointing direction of the satellite. 
         [0065]    By correlating the graphs of  FIGS. 6   a  and  6   b , the graph of  FIG. 6   c  is obtained which gives the level of the acquired signal as a function of the angular bias imparted on the satellite. 
         [0066]      FIG. 6   d  shows the last step of the method according to the invention consisting in comparing the variations of the antenna gain  603 , reconstituted over a given range of angular variation, with a specified envelope of the pattern of the transmitting antenna of the satellite. Such an envelope consists, for example, of a maximum gain curve  601  and a minimum gain curve  602  between which the measured gain must vary.