Abstract:
The invention concerns satellite radionavigation, especially GPS (Global Positioning System), Galileo, GLONASS (Global Navigation Satellite System, Russian definition) type satellite radionavigation, etc.  
     This invention makes the receiver for Code Only mode in tracking more robust not only with BOC modulation but with any modulation involving one or more subcarriers.  
     The invention proposes a method to compute the discriminant function of signals modulated by modulation with one or more subcarriers, wherein it comprises elimination of said subcarrier(s).  
     In addition, the invention concerns a device to track BOC modulated satellite radionavigation signals. It comprises a discriminant function computation device implementing the discriminant function computation method eliminating said subcarrier(s). The discriminant function is used by the code loop of said signal tracking device.

Description:
START  
         [0001]    The invention concerns satellite radionavigation, especially GPS (Global Positioning System), Galileo, GLONASS (Global Navigation Satellite System, Russian definition) type satellite radionavigation, etc.  
         STATE OF THE ART  
         [0002]    Satellite radionavigation is used to obtain the position of the receiver by a method similar to triangulation. The distances are measured using signals sent by satellites.  
           [0003]    The signals transmitted by the satellites are formed by modulation of the signal carrier with a spreading code. Thus, the satellite signals provide two types of measurement in order to localise the receiver. In addition, carrier modulation by a spreading code extends the spectrum in the spectral band, which makes the system more resistant to jamming. And, moreover, this provides a means of dissociating the satellites (by using a different code for each satellite).  
           [0004]    The first type of distance measurement by satellite radionavigation is a traditional measurement based on the carrier of the received signal. Measurements based on the carrier phase are accurate but ambiguous. The receiver can in fact evaluate the number of wavelengths between the satellite and the receiver to an accuracy of one wavelength.  
           [0005]    The second type of distance measurement uses the code of the received signal. Measurements based on the code, unlike those based on the carrier, are not ambiguous since the receiver can evaluate the integer number of code periods between the satellite and the receiver. However, measurements based on the code are much less accurate than those based on the phase.  
           [0006]    To perform these two types of measurement, the receiver acquires and tracks the received signal. To do this, it generates replicas of the code and the carrier, called local, which it correlates with the received signal. Since the code information and the carrier information are not coherent, the generations of code and carrier replicas are slaved by two separate loops.  
           [0007]    The carrier loop is generally a PLL (Phase Lock Loop), for example the Costa loop. The code loop generally includes a double correlation in order to evaluate the shift between the local code and received code which corresponds to a measurable energy difference, as shown on FIG. 1 a  for BPSK modulation. First, code phase correlation is used to slave the carrier loop. The difference of the I code correlations in advance and in lag is used to slave the code loop. This difference, called the discriminant function, is represented by FIG. 1 b  for BPSK modulation.  
           [0008]    The receiver uses these two loops to obtain accurate, unambiguous measurements. In an initial phase, called the acquisition phase, the receiver operates in open loop to seek the received signal by testing several assumptions regarding the position and speed of the local code and the local carrier. Once the code loop has removed the possibility of ambiguity, the receiver operates in closed loop. The carrier loop provides its accurate measurements and the code loop is used for tracking.  
           [0009]    If the signal to noise ratio is low, for example in the event of jamming, the carrier loop is the first to disconnect. If the receiver has an external speed aid, it can continue to operate in Code Only, i.e. with the code loop only, unaided by the carrier loop.  
           [0010]    Generally, the modulation used in the satellite radionavigation systems is BPSK (Binary Phase Shift Keying) modulation. Another modulation: BOC (Binary Offset Chip) modulation, may be preferred since it offers a different use of the available band. For example, in military applications, it can be used to save energy when the band used by BPSK modulation is jammed. For civilian applications, it makes the system compatible with American systems which use different bands. In addition, with BOC modulation, the receiver performance is better since the spectrum is wider.  
           [0011]    [0011]FIGS. 2 a  and  2   b  represent respectively the self-correlation function and the discriminant function for BOC modulation.  
           [0012]    The disadvantage of BOC modulation is that the tracking receiver is less robust in Code Only mode than when BPSK modulation is used. The code self-correlation function which determines the stable equilibrium (or capture) areas of the code loop is also modulated by the subcarrier. This modulation reduces the central capture area and, consequently, increases the likelihood of disconnecting from the slaving on leaving this area. Disconnection is due to noise or dynamic trailing.  
         PURPOSE OF THE INVENTION  
         [0013]    This invention makes the receiver for Code Only mode in tracking more robust not only with BOC modulation but with any modulation involving one or more subcarriers.  
           [0014]    The invention consists in the fact that the method used to track satellite radionavigation signals modulated by modulation with one or more subcarriers in reception comprises the elimination of the subcarrier(s). 
       
    
    
     DESCRIPTION  
       [0015]    The advantages and features of the invention will be clearer on reading the following description, given as an example, illustrated by the attached figures representing in:  
         [0016]    [0016]FIGS. 1 a  and  1   b  respectively the self-correlation function and the discriminant function for BPSK modulation,  
         [0017]    [0017]FIGS. 2 a  and  2   b  respectively the self-correlation function and the discriminant function for BOC modulation.  
         [0018]    [0018]FIG. 3, device to track BOC modulated radionavigation signals according to the invention,  
         [0019]    [0019]FIG. 4, representation of the code at various points of the device represented on FIG. 3, 
     
    
       [0020]    [0020]FIG. 3 shows an example of the signal tracking device according to the invention. This device uses elimination of the BOC modulation subcarrier to make the tracking of the received radionavigation signal more robust in Code Only mode.  
         [0021]    It includes a discriminant function computation device  10 . This device  10  has seven inputs E 1  to E 7  and one output S. It receives the signal r from the satellite on the first input E 1 . The first input E 1  forms the primary channel V 1   1  which is divided into two parallel identical secondary channels V 2   1  and V 2   2 .  
         [0022]    The first secondary channel V 2   1  is multiplied by the carrier in phase c i  of the second input E 2 . The second secondary channel V 2   2  is multiplied by the carrier in quadrature c q  of the third input E 3 . Each of these secondary channels V 2   1  and V 2   2  is divided into two parallel ternary channels V 3   1  to V 3   4 .  
         [0023]    The first ternary channel V 3   1  resulting from the first secondary channel V 2   1  is multiplied by the subcarrier in phase si of the sixth input E 6 . The second ternary channel V 3   2  resulting from the first secondary channel V 2   1  is multiplied by the subcarrier in quadrature s q . The third ternary channel V 3   3  resulting from the second secondary channel V 2   2  is multiplied by the subcarrier in phase s I  of the sixth input E 6 . The fourth ternary channel V 3   4  resulting from the second secondary channel V 2   2  is multiplied by the subcarrier in quadrature s q  of the seventh input E 7 . Each of the ternary channels V 3   1  to V 3   4  is divided into two parallel quaternary channels V 4   1  to V 4   8 .  
         [0024]    The first quaternary channel V 4   1  resulting from the first ternary channel V 3   1  is multiplied by the phase advance code c a  of the fourth input E 4 . The second quaternary channel V 4   2  resulting from the first ternary channel V 3   1  is multiplied by the phase lag code c, of the fifth input E 5 . The third quaternary channel V 4   3  resulting from the second ternary channel V 3   2  is multiplied by the phase advance code c a  of the fourth input E 4 . The fourth quaternary channel V 4   4  resulting from the second ternary channel V 3   2  is multiplied by the phase lag code c r  of the fifth input E 5 . The fifth quaternary channel i V 4   5  resulting from the third ternary channel V 3   3  is multiplied by the phase advance code c a  of the fourth input E 4 . The sixth quaternary channel V 4   6  resulting from the third ternary channel V 3   3  is multiplied by the phase lag code c r  of the fifth input E 5 . The seventh quaternary channel V 4   7  resulting from the fourth ternary channel V 3   4  is multiplied by the phase advance code c a  of the fourth input E 4 . The eighth quaternary channel V 4   8  resulting from the fourth ternary channel V 3   4  is multiplied by the phase lag code c r  of the fifth input E 5 .  
         [0025]    On each quaternary channel V 4   1  to V 4   8 , the signals so obtained are processed by an integrate and dump device  11   1  to  11   8  which produces non-spread and cumulated samples. The signal I IA  of the first quaternary channel V 4   1  is formed by the cumulated samples in phase for the carrier, in phase for the subcarrier and in phase advance for the code. The signal, I IR  of the second quaternary channel V 4   2  is formed by the cumulated samples in phase for the carrier, in phase for the subcarrier and in phase lag for the code. The signal I QA  of the third quaternary channel V 4   3  is formed by the cumulated samples in phase for the carrier, in quadrature for the subcarrier and in phase advance for the code. The signal I QR  of the fourth quaternary channel V 4   4  is formed by the cumulated samples in phase for the carrier, in quadrature for the subcarrier and in phase lag for the code. The signal Q IA  of the fifth quaternary channel V 4   5  is formed by the cumulated samples in quadrature for the carrier, in phase for the subcarrier and in phase advance for the code. The signal Q IR  of the sixth quaternary channel V 4   6  is formed by the cumulated samples in quadrature for the carrier, in phase for the subcarrier and in phase lag for the code. The signal Q QA  of the seventh quaternary channel V 4   7  is formed by the cumulated samples in quadrature for the carrier, in quadrature for the subcarrier and in phase advance for the code. The signal Q QR  of the eighth quaternary channel V 4   8  is formed by the cumulated samples in quadrature for the carrier, in quadrature for the subcarrier and in phase lag for the code.  
         [0026]    Thus, all the signals in phase advance for the code are standardised and summed by device  13   A  to form I IA   2 +I QA   2 +Q IA   2 +Q QA   2  on one channel and all the signals in phase lag for the code are standardised and summed by device  13   R  to form I IR   2 +I QR   2 +Q IR   2 +Q QR   2  on another channel. A code discriminator  14  receives two energies and divides the difference of the advance energy and the lag energy by their sum ε=(I IA   2 +IA 2 +Q IA   2 +Q QA   2 −(I IR   2 +I QR   2 +Q IR   2 +Q QR   2 ))/(I IA   2 +I QA   2 +Q IA   2 +Q QA   2 +I IR   2 +Q IR   2 +Q QR   2 ).  
         [0027]    This discrimination information c is used by the code corrector  21 . The code correction information produced by this corrector  21  is added using the external speed a ve  and used by the code oscillator  22  of the code loop  20 , for example a numerical controlled oscillator (NCO). This oscillator  22  controls the code replica generator  23  and the BOC subcarrier replica generator  24 .  
         [0028]    The code replica generator  23  provides the phase advance code c a  replica coupled on the fourth input E 4  of the discriminant function computation device  10  and the phase lag code c r  replica coupled on the fifth input E 5  of the discriminant function computation device  10 . The subcarrier replica generator  24  provides the subcarrier in phase s i  replica coupled on the sixth input E 6  of the discriminant function computation device  10  and the subcarrier in quadrature S q  replica coupled on the seventh input E 7  of the discriminant function computation device  10 .  
         [0029]    In the carrier loop  30 , the carrier oscillator  31 , e.g. an NCO, receives the external speed aid a ve  in Code Only mode. It checks the carrier replica generator  32 . This carrier replica generator  32  may, for example, include a sine function  32   S  and a cosine function  32   C . One of these functions generates the carrier in phase c i  replica coupled to the input E 2  of device  10  and the other the carrier in quadrature c q  replica coupled to the input E 3  of device  10 .  
         [0030]    [0030]FIG. 4 shows firstly the code received without BOC modulation on the first line, and the code received with BOC modulation r by the tracking device of FIG. 3 on the second line. The third and fourth lines illustrate the code replicas, respectively in phase advance c a  and phase lag c r , generated by the device  23  and coupled to inputs E 4  and E 5  of the discriminant function computation device  10 . The fifth and sixth lines illustrate the BOC subcarrier replicas, respectively in phase s i  and quadrature s q , generated by the device  24  and coupled to inputs E 6  and E 7  of the discriminant function computation device  10 .  
         [0031]    This method of eliminating the subcarrier to compute the discriminant function can be used for any modulation with a subcarrier and for any type of application involving the computation of this discriminant function. The use in the context of BOC modulation and for radionavigation signal tracking is only an example of how the invention can be used.  
         [0032]    In addition, this method of eliminating the subcarrier by multiplying by the subcarrier replica in phase and in quadrature can be used more than once when the particular modulation is modulation with several subcarriers and not just one.