Abstract:
This acquisition method is provided for acquiring a satellite signal emitted by a radio navigation satellite, the satellite signal containing a cyclic pseudo-random identification code specific to the satellite. The acquisition method includes the generation of a local code producing a replica of the identification code, and the production of a combined correlation (EDDC) of a received signal with the local code, the combined correlation corresponding to the linear combination of a first double delta correlation and of a second narrow correlation.

Description:
[0001]    This claims the benefit of French Patent Application ER 13 02219, filed Sep. 24, 2013 and hereby incorporated by reference herein. 
         [0002]    The present invention relates to the field of receiving satellite radio navigation signals. 
       BACKGROUND 
       [0003]    A satellite navigation system allows a user to determine his/her geographical position in three dimensions (longitude, latitude and altitude), his/her speed and the time. Satellite navigation systems are known under the names of GPS, GALILEO or GLONASS. 
         [0004]    Such a satellite navigation system comprises a constellation of satellites in orbit. Each satellite emits a satellite signal consisting of a carrier with a determined frequency, modulated by a navigation message and an identification code called          pseudo-random code         . The navigation message notably contains data on the ephemeris of the satellite. The pseudo-random code is a pseudo-random cyclic binary signal specific to the satellite. Each satellite has its own pseudo-random code and its own carrier frequency. The pseudo-random code spreads out the satellite signal over a wide frequency band and embeds the satellite signal in the background noise. This limits the interferences between the satellite signals and the sensitivity to external perturbations. 
         [0005]    In order to detect a satellite signal, the receiver generates a local code by means of a code generator driven by a digitally controlled oscillator, the local code reproducing the pseudo-random code of the satellite signal, and correlates the received signal with the local code. 
         [0006]    In an initial acquisition phase, in which the receiver tries to detect the satellite signal, it is necessary to synchronize the locally generated local code with the received pseudo-random code of the satellite. To do this, the receiver comprises a code loop (or DLL, “Delay Lock Loop”) for subordinating the code oscillator. The code loop comprises a suitable code discriminator for receiving the result of the correlation, in order to determine a code error depending on the result of the correlation and for sending a corresponding code correction signal to the code oscillator. 
         [0007]    The satellite navigation receiver receives the satellite signal in a direct line-of-sight and optionally, depending on the environment, in an indirect line-of-sight, for example after reflection on the ground, the sea or on buildings. These multiple paths perturb the detection of the satellite signal and are sources of error in calculating the positioning of the receiver. 
         [0008]    To overcome this drawback, in the acquisition phase it is known to correlate the satellite signal with a combination of punctual, advanced and/or delayed local codes, so as to obtain a correlation function with which the pseudo-random code received in direct line-of-sight may be better discriminated. 
         [0009]    FR 2 739 695 discloses a Double Delta correlator which will be described in more detail hereafter. The theoretical correlation function of the Double Delta correlator has a narrow capture area surrounded by two so-called          false lock-on         areas, in which the correlation coefficient is zero. 
         [0010]    The Double Delta correlator is efficient for an initial code error located in the capture area. Nevertheless, if the initial code error is located in a false lock-on area, the code loop operates in an open loop, which leads to a stationary measurement error. 
         [0011]    Further, in a practical embodiment, the band pass of the receiver is not infinite so that the actual correlation function has secondary zeros in the false lock-on areas, over which the code loop may be locked on. 
         [0012]    In order to overcome this drawback, FR 2 974 914 proposes a receiver comprising a Double Delta correlator associated with a false lock-on detector. 
         [0013]    Nevertheless, this requires adaptation of the receiver for implementing the false lock-on detector, which is not necessarily possible upon updating a receiver. 
       SUMMARY OF THE INVENTION 
       [0014]    It is an object of the present invention to provide a method for receiving satellite navigation signals which are not very sensitive to false lock-ons. 
         [0015]    The present invention provides method for acquiring a satellite signal emitted by a radio navigation satellite, the satellite signal containing a cyclic pseudo-random identification code specific to the satellite, the acquisition method comprising the generation of a local code as a replica of the identification code, and producing a combined correlation of a received signal with a local code, the combined correlation corresponding to the linear combination of a first Double Delta correlation and a second narrow correlation. 
         [0016]    The method optionally comprises one or several of the following features, taken alone or according to all the technically possible combinations: 
         [0017]    the first Double Delta correlation is produced by correlation of the received signal with an arithmetic combination of two advanced local codes and of two delayed codes obtained by a time shift of the local point code with delays − 2   d,  −d, d and  2   d,  wherein d is a delay increment, and respectively assigned to the coefficients  1 , − 2 , + 2  and − 1 ; 
         [0018]    the second narrow correlation is based on the difference between an advanced code with the delay −D relatively to the local code and a local code delayed by the delay D relatively to the local point code; 
         [0019]    the increment delay d of the first double delta correlation is equal to the delay increment D of the second narrow correlation; 
         [0020]    the linear combination is produced by assigning the first double delta correlation and the second narrow correlation of positive coefficients with a sum equal to 1; 
         [0021]    the linear combination is obtained by alternating the first correlation on a first fraction of a time cycle and the second correlation of a second fraction of the time cycle complementary of the first fraction, in a cyclic manner; 
         [0022]    the linear combination is obtained by correlation and calculation from advanced and delayed local codes; 
         [0023]    the acquisition method comprises the synchronization of the local code with the identification code so as to minimize a determined code error depending on the result of the combined correlation; 
         [0024]    the local code is generated by means of a local code generator driven by a code oscillator subordinated by a code loop to the result of the combined correlation. 
         [0025]    The invention also relates to a satellite radio navigation receiver, for receiving a satellite signal containing a cyclic pseudo random identification code specific to the satellite, the receiver comprising a code generator and a correlation module for applying an acquisition method as defined above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The invention and advantages thereof will be better understood upon reading the description which follows, only given as an example and made with reference to the appended drawings, wherein: 
           [0027]      FIG. 1  is a functional diagram of a satellite navigation receiver; 
           [0028]      FIGS. 2 and 3  are functional diagrams of code correlation modules of the satellite navigation receiver; and 
           [0029]      FIGS. 4 ,  5  and  6  are graphs illustrating correlation functions. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    The satellite navigation receiver  2  illustrated in  FIG. 1  is able to determine its geographical position, its speed and time, from satellite signals emitted by satellites of a satellite navigation system and received by the receiver  2 . 
         [0031]    A satellite  4  is illustrated in  FIG. 1 . In practice, the satellite navigation system comprises a constellation of satellites. The satellite signals of four different satellites are required for determining the geographical position. 
         [0032]    The receiver  2  comprises an antenna  6 , a conditioning stage  8 , a receiving stage  10  and a processing stage  12 . 
         [0033]    The conditioning stage  8  receives the signal provided by the antenna  6 . In a known way, the conditioning stage  8  filters the received signal, changes the frequency of the received signal, amplifies the received signal and digitizes the received signal. 
         [0034]    The receiving stage  10  comprises receiving channels, each receiving channel being associated with a respective satellite and able to detect the signal of the satellite. The receiving channels are similar. A single receiving channel  14 , associated with a satellite  4  is illustrated in  FIG. 1  and will be described in detail subsequently. 
         [0035]    The receiving channel  14  comprises a carrier generator  16  for generating a local carrier forming a replica of the carrier of the satellite signal, a carrier oscillator  18  driving the carrier generator  16  and a carrier correlator  20  for correlating the received signal with the local carrier. 
         [0036]    The receiving channel  14  comprises a code generator  22  for generating local codes forming a replica of the pseudo-random code of the satellite, a code oscillator  24  for driving the code generator  22  and a code correlation module  26  for correlating the received signal with the local codes. 
         [0037]    The code generator  22  is able to generate a punctual local code P, advanced local codes E, each advanced local code E being advanced by a delay relatively to the punctual local code P, and delayed local codes L, each delayed local code L being delayed by a delay relatively to the punctual local code P. 
         [0038]    The carrier oscillator  18  and the code oscillator  24  are Numerical Controlled Oscillators or “NCO”. 
         [0039]    The receiving channel  14  comprises an integrator  28  receiving the correlated signals and able to deliver corresponding integrated signals. 
         [0040]    The receiving channel  14  has a carrier loop  30  for subordinating the carrier oscillator  18  so as to depend on the output of the integrator  28 , so as to minimize a carrier error between the carrier of the satellite signal and the local carrier. 
         [0041]    The carrier loop  30  comprises a carrier discriminator  32  connected to the output of the integrator  28  and able to determine a carrier error and to send to the carrier oscillator  18  a corresponding carrier correction signal. The carrier discriminator  32  receives the correlated received signal with the local carrier and with the punctual local code P. 
         [0042]    The carrier error is a frequency or phase error for example due to the relative speed of the satellite and of the receiver  2  which modifies the apparent frequency or phase of the carrier of the satellite signal (Doppler effect). 
         [0043]    The receiving channel  14  comprises a code loop  34  for subordinating the code oscillator  24  to the output of the integrator  38 , so as to minimize a code error between the pseudo-random code of the satellite and the punctual local code P. 
         [0044]    The code loop  34  comprises a code discriminator  36  connected to the output of the integrator  28  and adapted for determining code error depending on the result of the correlation produced between the received signal and the local codes, and for sending a corresponding correction signal to the code oscillator  24  for reducing the code error. 
         [0045]    The code error corresponds to a time shift between the punctual local code P and the pseudo-random code of the satellite. The code loop  34  allows synchronization of the punctual local code P with the pseudo-random code of the satellite signal. 
         [0046]    The receiving channel  14  provides at the output the local carrier and the punctual local code P to the processing stage  12  which uses them for extracting the data signal of the navigation message of the satellite by demodulation. 
         [0047]    The code correlation module  26  is able to produce a combined correlation corresponding to a linear combination of a Double Delta correlation and of a narrow correlation. 
         [0048]    The Double Delta correlation and the narrow correlation are each obtained by correlation of the received signal with an arithmetic combination of advanced E and delayed L local codes. 
         [0049]    The combined correlation corresponds to the sum of a narrow correlation weighted with a weighting coefficient α and a Double Delta correlation weighted with a weighting coefficient ( 1 −α). The weighting coefficient α is less than 1. The weighting coefficients are therefore positive and with a sum equal to 1. 
         [0050]    The linear correlation is produced by calculation or by time partitioning between the Double Delta correlation and the narrow correlation. 
         [0051]    The code correlation module  26  of  FIG. 2  comprises a Double Delta correlator  38  for producing a Double Delta correlation between the received signal and the punctual local code, a narrow correlator  40  for producing a narrow correlation between the received signal and the punctual local code, and an adder  42  for adding both correlations weighted by their respective weighting coefficients. 
         [0052]    The Double Delta correlator is designated by the expression “Double Delta Correlator” (DCC). The narrow correlator is designated by the expression “Narrow Correlator” (NC). 
         [0053]    The Double Delta correlator  38  and the narrow correlator  40  each receive the advanced and delayed local codes generated by the code generator  22  and which are necessary to them. 
         [0054]    The combined correlation is produced by individual correlation of each advanced or delayed local code with the received signal and then by combination of the results, or by combination of the advanced and delayed codes followed by correlation of the combinatorial code obtained with the received signal. 
         [0055]    The code correlation module  26  of  FIG. 3  differs from that of  FIG. 2  in that the linear combination is not calculated but obtained by time partitioning between the Double Delta correlation and the narrow correlation. 
         [0056]    The Double Delta correlation and the narrow correlation are cyclically alternated in time, the Double Delta correlation being produced on a first fraction of a time cycle and the narrow correlation being produced on a second fraction of the time cycle, the second fraction being complementary to the first fraction. 
         [0057]    The code correlation module  26  of  FIG. 3  differs from that of  FIG. 2  in that the adder is replaced with a selector  44  adapted for selectively selecting the Double Delta correlator or the narrow correlator cyclically in time. 
         [0058]    The selector  44  is controlled in order to select the Double Delta correlator on the first fraction of the time cycle and for selecting the narrow correlator on the second fraction of the time cycle, cyclically. 
         [0059]    The integrator located downstream from the correlation multiplier comprises a lowpass filter. Consequently, the integrator produces an average over the time cycle and therefore produces the desired linear combination on the basis of the respective time period fractions allocated to the Double Delta correlation and to the narrow correlation. 
         [0060]    A Double Delta correlation corresponds to a correlation of the received signal with an arithmetic combination of two advanced local codes E 2 , E 1  and of two delayed local codes L 1 , L 2  obtained by a time shift of the punctual local code P with delays − 2   d,  −d, d and  2   d,  wherein d is a delay increment, and respectively assigned to coefficients  1 , − 2 , + 2  and − 1 , according to the relationship: 
         [0000]      C DDC =E 2 − 2 .E 1 + 2 .L 1 −L 2 
 
         [0061]    wherein 
         [0062]    E 2  is an advanced local code by a delay of  2   d  relatively to the punctual local code, 
         [0063]    E 1  is an advanced local code by a delay of d relatively to the punctual local code, 
         [0064]    L 1  is a delayed local code by a delay of d relatively to the punctual local code; and 
         [0065]    L 2  is a delayed local code by a delay of  2   d  relatively to the punctual local code. 
         [0066]    The delay increment d is less than ¼ chip. 
         [0067]    A narrow correlation corresponds to a correlation of the received signal with an arithmetic combination of an advanced local code E 3  and of a delayed local code L 3  shifted relatively to the punctual local code P with delays −D and +D and respectively modified with a coefficient − 1  and + 1 , according to the relationship: 
         [0000]      C NC =−E 3 +L 3 
 
         [0068]    wherein 
         [0069]    E 3  is an advanced local code by a delay D; and 
         [0070]    L 3  is a delayed local code by a delay D. 
         [0071]    The delay increment D is less than 0.5 chip. 
         [0072]    Preferably, the receiver  2  produces a combination of a Double Delta correlation and of a narrow correlation based on the same delay increment (D=d). 
         [0073]    The narrow correlation is then written according to the relationship: 
         [0000]      C NC=−E1+L1    
         [0074]    The combined correlation obtained by linear combination of the narrow correlation and of the Double Delta correlation then corresponds to a combination according to the following relationship: 
         [0000]      C EDDC =( 1 −α).E 2 −( 2 +α).L 1 −( 1 −α).L 2 
 
         [0075]      FIGS. 4 to 6  are graphs illustrating normalized correlation functions, and each representing the correlation coefficient (abscissa) depending on the code error (ordinate) expressed in “chip(s)”. 
         [0076]      FIG. 4  illustrates in solid lines the theoretical correlation function DDC(t) of a Double Delta correlator and in dotted lines the theoretical auto-correlation function R(t) of the pseudo-random code. 
         [0077]    The pseudo-random code is designed so that its auto-correlation function R(t) is even and has a triangular shape on the interval [− 1 ; + 1 ]. 
         [0078]    The normalized correlation function of the Double Delta correlation DCC is expressed according to the following relationship: 
         [0000]      DDC(t)=(R(t− 2 d) − 2 .R(t−d)+2.R(t+d)+R(t+ 2 d))/R(t)
 
         [0079]    wherein 
         [0080]    t is the code error; 
         [0081]    R is the auto-correlation function of the pseudo-random code; 
         [0082]    d is the delay increment of the Double Delta correlation. 
         [0083]    The correlation function of the Double Delta correlation has a capture area  50  on the interval [− 2   d;  + 2 d] in which the correlation function is odd. The capture area is surrounded by two false lock-in areas  52 ,  54  on the intervals [− 1 + 2   d;  − 2 d] and [ 2   d;    1 - 2 d], wherein the correlation function is zero. 
         [0084]      FIG. 5  illustrates the correlation function of a narrow correlator NC. 
         [0085]    The normalized correlation function of the narrow correlation NC is expressed, for a delay increment d, according to the following relationship: 
         [0000]      NC(t)=(−R(t−d)+R(t+d))/R(t)
 
         [0086]    wherein 
         [0087]    T is the code error; 
         [0088]    R is the auto-correlation function of the pseudo-random code; 
         [0089]    d is the delay increment. 
         [0090]    The narrow correlation function NC is odd over the interval [− 1 −d;  1 +d] and zero outside this interval. It has plateaus  56 ,  58  on the intervals [− 1 +d;  1 −d] and [d;  1 −d]. 
         [0091]      FIG. 6  illustrates the theoretical normalized correlation function of the combined correlation EDDC obtained by linear combination of the Double Delta correlation DCC and of the narrow correlation NC based on a time increment d, according to the relationship: 
         [0000]      EDDC(t)=( 1 −α).DDC(t)+α.NC(t)
 
         [0092]    As this is visible in  FIGS. 4 to 6 , the plateaus of the correlation function of the narrow correlation coincide with the false lock-in areas of the correlation function of the Double Delta correlation. The linear combination of the narrow correlation with the Double Delta correlation gives the possibility of widening the capture area of the Double Delta correlation function by suppressing the false lock-in areas. 
         [0093]    In the case of a lock-in in the capture area of the Double Delta correlator, the measurement performances are unchanged and the immunity to multiple paths is degraded very little. 
         [0094]    In the case of a lock in a false lock-in area of the Double Delta correlator, the code discriminator continues to provide an error correction signal to the code oscillator, which resorbs the code error slowly but certainly until it returns into the capture area of the Double Delta correlator. 
         [0095]    Generally, the selection of the coefficient α depends: 
         [0096]    on the bandwidth of the receiver which determines the defects of the actual correlation function of the Double Delta correlator, in particular the secondary areas; 
         [0097]    on the desired reaction rapidity in the case of an initial code error located in a false lock-in area of the Double Delta correlator, and 
         [0098]    on the desired immunity to multiple paths, which decreases when the coefficient α increases. 
         [0099]    The coefficient α is practically selected so as to be as small as possible while suppressing the secondary zeros of the false lock-in areas of the actual correlation function of the Double Delta correlator. 
         [0100]    Preferably, the linear combination is produced by combining a larger fraction of the Double Delta correlator than the narrow correlator. The coefficient α is preferably less than 0.5. 
         [0101]    The acquisition method is easily applied. In particular, a receiver designed for receiving a Double Delta correlation has a code generator able to generate the advanced E 1  and delayed L 1  local codes required for producing a narrow correlation based on the same delay increment. It may therefore be easily adapted for producing a linear combination of a Double Delta correlation and of a narrow correlation.