Abstract:
A method is provided for demodulating a signal carrying a message transmitted by a terrestrial beacon, executed by a system comprising a constellation of satellites suitable for detecting said signal and for repeating it towards receiving stations on the ground, and an analysis module suitable for receiving signals from said stations. Each receiving station transmits the signals that it receives from the satellite to the analysis module, said module realigning said signals in frequency and/or in time relative to one another, combining the realigned signals to generate a synthetic signal having an enhanced signal-to-noise ratio, and determining the content of said message and/or the modulation parameters of said synthetic signal. The method applies notably to the accurate and reliable location of distress beacons by a satellite system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to foreign French patent application No. FR 1103284, filed on Oct. 26, 2011, the disclosure of which is incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for processing signals transmitted by beacons received by different independent sub-elements of a collection module. It applies notably to the accurate and reliable location of distress beacons by a satellite system. 
     BACKGROUND 
     One known distress beacon location system is the MEOSAR (Middle Earth Orbit Search and Rescue) system, which is a satellite system in middle earth orbit used for search and rescue. This comprises:
         signal-transmitting beacons to be located;   relay satellites in middle earth orbit (which can be satellites used in a geolocation and navigation system, a system that is often designated by the acronym GNSS, standing for Global Navigation Satellite System);   independent processing modules on the ground, or stations, sometimes also qualified as MEOLUT (Middle Earth Orbit Local User Terminals);   a centre for coordinating the processing modules on the ground, sometimes qualified as MTCF, standing for MEOLUT Tracking Coordination Facility.       

     The centre for coordinating the processing modules on the ground makes it possible to assist in the programming of the antennas of the different modules in order to improve the coverage and the overall efficiency of the network of modules, in particular when the modules are equipped with a number of antennas (as is the case with some MEOLUT stations). 
     An SAR (Search and Rescue) distress beacon is seen simultaneously by a number of satellites which pick up the signals that it transmits and retransmit them to stations on the ground. These stations on the ground demodulate the beacon messages coded in the signals, then measure the times of arrival (TOA) of the signals as well as the frequency shift that these signals (FOA, standing for Frequency of Arrival) have undergone by Doppler effect in order to determine the position of the beacon. 
     However, the signal-to-noise ratio of the received signals is not always sufficient to enable the stations to analyse them in order to locate the beacon, or even in order to determine the content of its message. Thus, even when a sufficient number of satellites—generally, four satellites—has been able to transmit beacon signals, the TOA and FOA cannot be determined for all these signals, which prevents the position of the beacon from being determined. 
     The degradation of the signal-to-noise ratio can be inherently heterogeneous, and caused notably by the environment of the beacon during transmission, by the atmospheric propagation, by the relay satellite used, and by the environment of the MEOLUT station on reception. 
     SUMMARY OF THE INVENTION 
     One aim of the invention is to propose means for detecting signals transmitted by a beacon in a more reliable and more accurate manner than with the known systems. To this aim, the subject of the invention is a method for demodulating a signal carrying a message transmitted by a terrestrial beacon, executed by a system comprising a constellation of satellites suitable for detecting said signal and for repeating it towards receiving stations on the ground, and an analysis module suitable for receiving signals from said stations, wherein each receiving station transmits the signals that it receives from the satellite to the analysis module, said module realigning said signals in frequency and/or in time relative to one another, combining the realigned signals to generate a synthetic signal having an enhanced signal-to-noise ratio, and determining the content of said message and/or the modulation parameters of said synthetic signal. 
     According to one implementation of the demodulation method according to the invention, the signal transmitted by the beacon begins with a pure carrier, the step of mutually realigning the signals comprising a phase of searching for this pure carrier by searching for the frequency difference between the signals received by the stations for which the result of a correlation between said signals comes closest to a pure carrier signal. 
     According to one implementation of the demodulation method according to the invention, the signal comprises a synchronization word, the step of mutually realigning the signals comprising a phase of searching for the time and frequency differences of the signals between them by performing a correlation calculation on the synchronization word. 
     According to one implementation of the demodulation method according to the invention, a replica signal is generated from the determined modulation parameters, and said replica signal is compared with the signals received by the stations on the ground in order to determine the measurements of time and frequency of arrival of the signal. 
     According to one implementation of the demodulation method according to the invention, the analysis module broadcasts to at least one station the optimum replica generated. 
     According to one implementation of the demodulation method according to the invention, at least one receiving station receives a signal transmitted by one and the same satellite via a number of different antenna channels, the receiving station selecting from said channels the signal provided with the best signal-to-noise ratio before transmitting it to the analysis module. 
     According to one implementation of the demodulation method according to the invention, the analysis module broadcasts to at least one station parameters characteristic of the beacon signal (binary content, modulation index, bit rate, bit rise time, bit phase model) instead of the complete signal. 
     Another subject of the invention is a method for locating a signal-transmitting terrestrial beacon, the steps of the demodulation method as described above are executed, the location method also comprising a step of analysing said parameters to determine the location of the beacon. 
     Another subject of the invention is a system for locating a beacon transmitting a signal transmitted by a terrestrial beacon, wherein said system comprises an analysis module suitable for executing the steps of the abovementioned location method. 
     According to one embodiment of the location system according to the invention, the system comprises means for coordinating the stations suitable for programming and for coordinating orientations of the antennas of the modules, and the analysis module is colocated with said coordination means, the analysis module being configured to share the same antennas for transmitting to the receiving stations as the coordination means. This embodiment makes it possible to reduce the transmission means used to communicate with the receiving modules. The coordination centre is sometimes designated by the acronym “MTCF”, standing for MEOLUT Tracking Coordination Facility. 
     According to another embodiment of the location system according to the invention, the system comprises communication means between the stations, and the analysis module is colocated with one of said stations. 
     According to another embodiment of the location system according to the invention, the system comprises communication means between the stations, and an analysis module is colocated in each of said stations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features will become apparent from reading the following detailed description given as a nonlimiting example, given in light of the appended drawings which represent: 
         FIG. 1 , an illustration of a system according to the invention; 
         FIG. 2 , a diagram illustrating the steps of a method according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a system according to the invention. The system  100  comprises a constellation of satellites  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107 ,  108  moving in middle altitude orbit (of the order of 22 000 km altitude), receiving stations  111 ,  112  distributed at different points on the earth&#39;s surface—in the example, MEOLUT (Middle Earth Orbit Local User Terminal) stations and a coordination centre  120  for the stations. Furthermore, unlike the systems of the prior art, the system according to the invention comprises an analysis module  150 . 
     When a beacon on the ground, for example a beacon of SAR (Search and Rescue) type, transmits signals, these are received by satellites of the constellation which act as transponders by retransmitting the received signals to the ground. As an example, an SAR beacon takes the form of a module transmitting a periodic signal every 50 seconds on a carrier with a frequency approximately equal to 406 MHz. 
     Each receiving station  111 ,  112  on the ground comprises one or more antennas  131 ,  132 ,  133 ,  134 ,  141 ,  142 ,  143 ,  144  which are configured to pick up signals transmitted by the satellites of the constellation which, at a given instant, are within the field of visibility of the station. Thus, the signals from the beacon on the ground are transmitted to the stations  111 ,  112  via the satellites of the constellation. Preferably, at least four satellites that have received the signals transmitted by the beacon are within the field of visibility of one and the same station  111 ,  112 , so that beacon location measurements can be performed by analysing the times of arrival of the signals and the Doppler frequency shift undergone by the signal. 
     In the example of  FIG. 1 , if a beacon is seen by the first satellite  101 , the second satellite  102 , the third satellite  103  and the fifth satellite  105 , this beacon can be located by the first station  111 . Similarly, if the beacon is seen by the first satellite  101 , the fifth satellite  105 , the seventh satellite  107  and the eighth satellite  108 , this beacon can be located by the second station  112 . 
     The coordination centre  120  makes it possible to configure the antennas of the stations  111 ,  112  so as to optimize the chances of reception of the beacon signals on these stations  111 ,  112 . To transmit commands to the stations  111 ,  112 , this coordination centre  120  also comprises communication means enabling it to communicate with the stations  111 ,  112 . 
     The analysis module  150  comprises computation means which are not represented in the figure and means of communication with the stations  111 ,  112 , also not represented. The analysis module  150  is capable of receiving signals and of transmitting signals to the stations  111 ,  112 . It plays a central role in the implementation of the method according to the invention because it makes it possible, from a number of beacon signals received by the stations  111 ,  112 , to combine these signals in order to produce an accurate model of the signal transmitted by the beacon. Advantageously, the analysis module  150  shares the communication means such as the antennas with the coordination centre  120 , so as to reduce the volume of equipment needed to implement the method according to the invention. 
       FIG. 2  illustrates the steps of a method according to the invention. In a first step  201 , the signals from the beacon obtained from the different satellites  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107 ,  108  acting as relay are detected by the station or stations  111 ,  112 . 
     Each of the signals is affected by a specific time shift which depends notably on the distance between the station  111 ,  112  which received it and the satellite which transmitted it, and by a frequency shift, also called Doppler shift, which depends notably on the speed of displacement of the satellite that transmitted the signal in relation to the station  111 ,  112  which received it. Since the position of the relay satellites and the position of the stations  111 ,  112  are known, these delays and frequency shifts are known. However, the delays and frequency shifts which likewise occur on the uplink path between the beacon and the satellite are not known since the position of the beacon is not known. 
     In a second step  202 , these stations  111 ,  112  correct the signal of the delay and of the Doppler shift created by the downlink channel to reconstruct a signal as it was received on board the satellite, with only the shifts due to the uplink channel. These stations then transmit these signals to the analysis module  150 . 
     In a third step  203 , the analysis module  150  mutually realigns the signals. 
     As an example, the wave form of the chosen signal is that of the signals transmitted by an SAR beacon. This type of signal begins with a signal in the form of a pure frequency which is followed by a synchronization word, which is followed by the content of the message carried by the signal. According to a first realignment step  231 , the pure frequency of the transmitted signal is sought by correlating the received signals together, in the search domain of the frequency shifts. The frequency shift corresponding to the correlation for which a pure frequency appears most clearly indicates the Doppler difference between the signals. 
     The addition of the signals retransmitted by a number of satellites and/or a number of stations makes it possible to reduce the noise in as much as the latter are in fact at least partially independent. In fact, if the intermediate satellite is different, the direction of transmission and the uplink atmospheric propagation are different, and therefore decorrelated; if the satellite is identical but the receiving station is different, the same decorrelation will this time appear during the downlink phase of the signal. If both the satellite and the station are different, these two gains are added together. 
     A test  232  is then executed to estimate whether the abovementioned method has made it possible to reliably determine the pure frequency used in the signal transmitted by the beacon. For example, if the level of the correlation peak is below a predetermined threshold, it is considered that the frequency difference between the signals has not been able to be determined. If this pure frequency has not been able to be determined, a second step  233  is executed to search again for the time and frequency differences between the signals from the synchronization word. On completion of this step  233 , the mutual time and frequency differences of the signals are determined, so that it becomes possible to realign them. 
     In a fourth step  204 , the realigned signals are aggregated in a coherent manner in time and in frequency to produce a resultant signal for which the signal-to-noise ratio is augmented. According to one implementation of the method according to the invention, only the part corresponding to the content of the message carried by the signal is taken into account in this aggregation step (notably, by not taking into account the synchronization word). On completion of this step, there is thus a signal carrying a message, for which the signal-to-noise ratio is augmented. 
     In a fifth step  205 , the abovementioned resultant signal is demodulated, or at least its modulation parameters are determined, these parameters being, for example, the bit rise time (or more generally the characteristic form of the bits), the modulation index and the bit rate. 
     In a sixth step  206 , a replica of the demodulated signal is generated. This replica is a form close to the signal transmitted by the beacon; ideally, it corresponds to the exact form of the signal transmitted by the beacon. 
     In a seventh step  207 , the replica of the demodulated signal is compared to the signals received by the stations  111 ,  112 , so as to determine by correlation (early-late) the time of arrival (TOA) and the frequency shift (or FOA, standing for Frequency of Arrival) of the signals received on each of the stations, for each satellite in the field of visibility of this station. Thus, on completion of this step, a pair (TOA, FOA) is obtained for each pair (station, visible satellite). 
     In an eighth step  208 , the abovementioned times of arrival and the Doppler shifts are analysed to determine the position of the beacon, according to techniques known to the person skilled in the art. 
     According to the exemplary method described in  FIG. 2 , all of the steps from the realignment step  203  are performed in the analysis module  150 . According to another implementation of the method according to the invention, the processing operations can be distributed differently between the analysis module  150  and the stations. In particular, if a first station  111  has succeeded in demodulating the signal without requiring the step  203  of the analysis module  150 , the signal can nevertheless be transmitted to the analysis module  150  which, by adding to it the corresponding signal received from a second station  112 , will obtain a better quality replica and will therefore be able to obtain a better accuracy in the subsequent steps. 
     Similarly, according to one implementation of the method according to the invention, it is possible, in order to minimize the bandwidth, to arrange for the stations to no longer transmit signals to the analysis module, but only parameters—demodulated message or characteristics of the replica—characteristic of the modulation, bit rate, index, phase model (for example in the form of a series of phase samples, or of a characteristic polynomial such as Fourier, Chebyshev, Lagrange). In this case, the station uses the parameters received from a number of stations to establish an ideal set of parameters enabling the stations to themselves reconstruct the replica from these parameters. 
     It should be noted that one and the same station  111 ,  112  may receive the same signals from one and the same satellite on a number of different antennas, if these antennas are configured (for example correctly oriented) to be able to pick up signals from this satellite. In this case, a selection of the signal that has the best signal-to-noise ratio to construct the replica is advantageously performed. 
     According to one implementation of the method according to the invention, the processing may also be differentiated according to the stations  111 ,  112 . For example, in the case where the analysis module  150  is closely linked with two stations  111 ,  112  (that is to say, connected with a link enabling signals to be exchanged), but less closely linked with a third station (not represented) (that is to say, connected with a link that allows only parameters to be exchanged), it can transmit to this third station the parameters of the replica obtained by virtue of the measurements on the first two stations  111 ,  112  (or at least of the best of the replicas obtained on the first two stations). 
     According to one implementation of the method according to the invention, the analysis module  150  is directly integrated in one of the receiving stations  111 ,  112 , or in each of the stations  111 ,  112 , so that each station  111 ,  112  can implement the steps  203  to  208  described above by combining the signals that it has received itself with the signals received by the other stations that they have retransmitted to it. 
     The method according to the invention makes it possible to improve the detection and improve the accuracy of the location of beacons such as SAR (Search and Rescue) beacons by virtue of the processing operations performed by an analysis module collecting the signals received by a number of stations, notably by using the redundancy of signals obtained by the visibility of one and the same beacon by a number of receiving stations on the ground. 
     The system according to the invention offers a number of advantages. In a system according to the prior art, if the signal-to-noise ratio on the stations is insufficient, the beacon is not located by any station whereas, with the system according to the invention, it is possible, by combining the replicas, to nevertheless obtain a successful detection. Also, regardless of the solution used for the detection (whether individual processing in the station or combinational processing), the combination of the signals in any case makes it possible to improve the quality of the replicas of the signals and the quality of the TOA and FOA measurements, and therefore the location.