Patent Publication Number: US-9891323-B2

Title: GNSS signal processing method, positioning method, GNSS signal processing program, positioning program, GNSS signal processing device, positioning apparatus and mobile terminal

Description:
TECHNICAL FIELD 
     The present invention relates to a GNSS signal processing method, with which a code phase of a GNSS signal code-modulated with a diffusion code is locked and tracked. 
     BACKGROUND ART 
     Conventionally, various kinds of devices have been proposed, which capture and track GNSS (Global Navigation Satellite System) signals, such as GPS (Global Positioning System) signals, and perform positioning. A GNSS signal is a signal obtained by code-modulating a carrier wave of a predetermined frequency with a diffusion code. The diffusion code is set for every GNSS satellite (GNSS signal) individually. 
     A positioning apparatus generally performs tracking of the GNSS signal with the following method. The positioning apparatus generates a replica signal having a replica code of the diffusion code set for the aimed GNSS satellite. The positioning apparatus correlates the received GNSS signal with the replica signal. The positioning apparatus calculates an error detection value based on the correlation value. The positioning apparatus controls a code phase of the replica signal by using the error detection value, locks the code phase of the aimed GNSS signal, and thus, tracks the aimed GNSS signal. 
     Meanwhile, with only direct wave signals which are the GNSS signals received directly by the positioning apparatus from the GNSS satellites, the tracking can easily and accurately be performed. If a multipath signal which is the GNSS signal reflected on a tall building or the like and then received by the positioning apparatus is included, the tracking accuracy may degrade. 
     As a method of avoiding the influence of this multipath signal, in Non-patent Document 1 and Patent Document 1, a calculation equation of the error detection value is set so that the correlation value becomes “0” within a specific code phase range. Specifically, with the code phase of the aimed GNSS signal as a reference phase, an insensible range where the correlation value becomes “0” is set within a predetermined code phase range which is away from the reference phase by a predetermined code phase. When the code phase of the multipath signal enters this insensible range, the code phase of the aimed GNSS signal is locked without receiving the influence of the multipath signal. 
     REFERENCE DOCUMENTS OF CONVENTIONAL ART 
     Patent Document(s) 
     
         
         Patent Document 1: JP1999-142502A 
       
    
     Non-Patent Document(s) 
     
         
         Non-patent Document 1: “A Practical Approach to the Reduction of Pseudorange Multipath Errors in a L1 GPS Receiver”, Bryan R. Townsend and Patrick C. Fenton, NovAtel Communications Ltd., ION GPS-94, Salt Lake City, Sep. 20-23, 1994 
       
    
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, with the methods described in Patent Document 1 and Non-patent Document 1 described above, if the code phase of the aimed GNSS signal of the direct wave signal enters the insensible range, the code phase cannot be locked. In this case, the aimed GNSS signal can no longer be tracked. 
     Then, in the capturing processing, a plurality of replica signals for capturing are generated with a predetermined code phase resolution and a code phase to be given in the initial stage of the tracking is set based on correlation values of the respective replica signals with the GNSS signal. Therefore, the detected code phase is not necessarily the one which is extremely close to the code phase of the aimed GNSS signal. When shifting from the capturing to the tracking, the code phase difference to a certain extent remains between the code phases of the aimed GNSS signal and the replica signal, and the code phase of the aimed GNSS signal may enter the insensible range. Particularly when a code phase interval of the replica signals for capturing is large, it is easier to enter the insensible range. 
     Therefore, the present invention aims to provide a GNSS signal processing method, with which a code phase of an aimed GNSS signal can surely locked with a high accuracy. 
     SUMMARY OF THE INVENTION 
     A GNSS signal processing method of this invention includes a correlating process, a differential value calculating process, an error detection value calculating process, and a code phase controlling process. 
     In the correlating process, the GNSS signal is correlated with each of a first early replica signal that is advanced from a prompt replica signal by a first code phase, a first late replica signal that is retarded from the prompt replica signal by the first code phase, a second early replica signal that is advanced from the prompt replica signal by a second code phase, and a second late replica signal that is retarded from the prompt replica signal by the second code phase. 
     In the differential value calculating process, an early differential value is calculated by subtracting a second early correlation value from a first early correlation value. The first early correlation value is obtained based on the correlation result between the GNSS signal and the first early replica signal. The second early correlation value is obtained based on the correlation result between the GNSS signal and the second early replica signal. In the differential value calculating process, a late differential value is calculated by subtracting a second late correlation value from a first late correlation value. The first late correlation value is obtained based on the correlation result between the GNSS signal and the first late replica signal. The second late correlation value is obtained based on the correlation result between the GNSS signal and the second late replica signal. 
     In the error detection value calculating process, an error calculating method is set based on signs of the early differential value and the late differential value, and an error detection value is calculated by using the set error calculating method. 
     In the code phase controlling process, a code phase of the prompt replica signal is controlled based on the error detection value, and a code phase of the GNSS signal is tracked. 
     This method utilizes that the signs of the early differential value and the late differential value change according to a phase difference between a code phase of the received GNSS signal and a code phase of the prompt replica signal. By suitably setting the error detecting method based on the signs of the early differential value and the late differential value, a suitable code phase control according to the code phase difference is possible. Thus, tracking performance of the GNSS signal improves. 
     Moreover, in the error detection calculating process of the GNSS signal processing method of this invention, when the signs of the early differential value and the late differential value are different from each other, the error detection value may be calculated with a first error detecting method using a first calculation equation by which a code phase range where the error detection value takes a value other than 0 becomes wide. In the error detection calculating process, when the signs of the early differential value and the late differential value are the same as each other, the error detection value may be calculated with a second error detecting method using a second calculation equation by which the code phase range where the error detection value takes a value other than 0 becomes narrow. 
     This method shows specific examples of the error detecting method to be selected. When the signs of the early differential value and the late differential value are different from each other, as described in the embodiment and the respective drawings below, the code phase difference between the prompt replica signal and the GNSS signal is large. Therefore, by using the first error detecting method with the wide code phase range where the error detection value does not become 0, the GNSS signal is not easily lost and sure tracking thereof becomes possible. When the signs of the early differential value and the late differential value are the same as each other, as described in the embodiment and the respective drawings below, the code phase difference between the prompt replica signal and the GNSS signal is small. Therefore, by using the second error detecting method with the narrow code phase range where the error detection value takes a value other than 0, influence of a multipath is not easily received, the code phase of the GNSS signal can be kept locked highly accurately, and highly accurate tracking becomes possible. 
     Moreover, with the GNSS signal processing method of this invention, the first calculation equation may use the first early correlation value and the first late correlation value, or the second early correlation value and the second late correlation value. With the GNSS signal processing method of this invention, the second calculation equation may use the first and second early correlation values and the first and second late correlation values. 
     With this method, combinations of the correlation values to be used in the first calculation equation and the second calculation equation are shown. Although specific calculation equations are described in the embodiment below, by using such combinations of the correlation values, properties of the error detection value as described above can easily be achieved. 
     Moreover, a positioning method of this invention includes a process for acquiring a navigation message based on the correlation result between the GNSS signal tracked with the GNSS signal processing method of any of the description above and the prompt replica signal. This positioning method includes a process for calculating a pseudorange based on the error detection value of the tracked GNSS signal. This positioning method includes a process for performing positioning by using the navigation message and the pseudorange. 
     With this method, by using the GNSS signal surely and highly accurately tracked as described above, the demodulation of the navigation message can surely be performed, and the pseudorange can be calculated highly accurately. Thus, highly accurate positioning becomes possible. 
     Effect(s) of the Invention 
     According to this invention, a code phase of an aimed GNSS signal can be tracked surely and highly accurately. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a flowchart of a GNSS signal processing method according to an embodiment of the present invention. 
         FIG. 2  is a chart illustrating a relation of timings of code phases of respective replica signals with the GNSS signal processing method according to the embodiment of the present invention. 
         FIG. 3  is a chart illustrating a property of an error detection value Δτ A  with respect to a code phase difference, which is calculated with a first error detecting method. 
         FIG. 4  is a chart illustrating a property of an error detection value Δτ B  with respect to the code phase difference, which is calculated with a second error detecting method. 
         FIG. 5  shows charts illustrating a first situation where a code phase of a prompt replica signal is advanced compared to that of an aimed GNSS signal. 
         FIG. 6  shows charts illustrating a second situation where the code phase of the prompt replica signal is advanced compared to that of the aimed GNSS signal. 
         FIG. 7  shows charts illustrating a third situation where the code phase of the prompt replica signal is advanced compared to that of the aimed GNSS signal. 
         FIG. 8  shows charts illustrating a fourth situation where the code phase of the prompt replica signal is retarded than that of the aimed GNSS signal. 
         FIG. 9  shows charts illustrating a fifth situation where the code phase of the prompt replica signal is retarded than that of the aimed GNSS signal. 
         FIG. 10  is a block diagram illustrating a configuration of a positioning apparatus  1  according to the embodiment of the present invention. 
         FIG. 11  is a block diagram illustrating a configuration of a demodulation unit  13  of the positioning apparatus  1  according to the embodiment of the present invention. 
         FIG. 12  is a flowchart of a positioning method according to the embodiment of the present invention. 
         FIG. 13  is a block diagram illustrating a main configuration of a mobile terminal  100  including the positioning apparatus  1  according to the embodiment of the present invention. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     A GNSS signal processing method according to an embodiment of the present invention is described with reference to the drawings.  FIG. 1  is a flowchart of the GNSS signal processing method according to the embodiment of the present invention. 
     With the GNSS signal processing method of this embodiment, an aimed GNSS signal is tracked by repeating the flow illustrated in  FIG. 1 . 
     As Step S 101 , the GNSS signal is correlated with respective replica signals to calculate respective correlation values. A replica signal is a signal having a replica code of a diffusion code signal of the aimed GNSS signal. As the replica signals, a prompt replica signal S RP , a first early replica signal S RE , a second early replica signal S RVE , a first late replica signal S RL , and a second late replica signal S RVL  are used. Code phases of these replica signals are set as illustrated in  FIG. 2 .  FIG. 2  is a chart illustrating a relation of timings of code phases of the respective replica signals with the GNSS signal processing method according to the embodiment of the present invention. 
     As illustrated in  FIG. 2 , the code phase of the replica code is set to the prompt replica signal S RP  so as to match with that of the received GNSS signal, based on a previously calculated error detection value Δτ. In other words, the code phase of the prompt replica signal S RP  is set so as to obtain the highest correlation value with the GNSS signal. 
     As illustrated in  FIG. 2 , the first early replica signal S RE  is a signal of which code phase is advanced compared to the prompt replica signal S RP  by a code phase difference τ 1 /2. The second early replica signal S RVE  is a signal of which code phase is advanced compared to the prompt replica signal S RP  by a code phase difference τ 2 /2. The code phase difference τ 2 /2 is set to be larger than the code phase difference τ 1 /2. For example, the code phase difference τ 1 /2 is 0.05 chip, and the code phase difference τ 2 /2 is 0.075 chip. 
     As illustrated in  FIG. 2 , the first late replica signal S RL  is a signal of which code phase is retarded than the prompt replica signal S RP  by the code phase difference τ 1 /2. The second late replica signal S RVE  is a signal of which code phase is retarded than the prompt replica signal S RP  by the code phase difference τ 2 /2. 
     By setting such code phases, the code phase difference (spacing) between the first early replica signal S RE  and the first late replica signal S RL  becomes τ 1 . For example, in the example above, the spacing is 0.1 chip. Moreover, the code phase difference (spacing) between the second early replica signal S RVE  and the second late replica signal S RVL  becomes τ 2 . For example, in the example above, the spacing is 0.15 chip. 
     By correlating the GNSS signal with the prompt replica signal S RP , a prompt correlation value CV P  is calculated. By correlating the GNSS signal with the first early replica signal S RE , a first early correlation value CV E  is calculated. By correlating the GNSS signal with the second early replica signal S RVE , a second early correlation value CV VE  is calculated. By correlating the GNSS signal with the first late replica signal S RL , a first late correlation value CV L  is calculated. By correlating the GNSS signal with the second late replica signal S RVL , a second late correlation value CV VE  is calculated. 
     Next, an early differential value ΔCV E  and a late differential value ΔCV L  are calculated (S 102 ). The early differential value ΔCV E  is calculated by subtracting the second early correlation value CV VE  from the first early correlation value CV E . Specifically, the early differential value ΔCV E  is calculated by using a calculation equation of the early differential value ΔCV E =CV E −CV VE . The late differential value ΔCV L  is calculated by subtracting the second late correlation value CV VL  from the first late correlation value CV L . Specifically, the late differential value ΔCV L  is calculated by using a calculation equation of the late differential value ΔCV L =CV L −CV VL . 
     Next, signs of the early differential value ΔCV E  and the late differential value ΔCV L  are compared to each other. If the sign of the early differential value ΔCV E  is different from the sign of the late differential value ΔCV L , (S 103 : NO), an error detection value Δτ (Δτ A ) is calculated with a first error detecting method. With the first error detecting method, the error detection value Δτ (Δτ A ) is calculated by substituting the first early correlation value CV E , the first late correlation value CV L , and the prompt correlation value CV P , into the following first calculation equation. 
     
       
         
           
             
               
                 
                   
                     Δτ 
                     A 
                   
                   = 
                   
                     
                       
                         CV 
                         E 
                       
                       - 
                       
                         CV 
                         L 
                       
                     
                     
                       2 
                       ⁢ 
                       
                         CV 
                         P 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
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     If the signs of the early differential value ΔCV E  and the late differential value ΔCV L  are the same (S 103 : YES), an error detection value Δτ (Δτ B ) is calculated with a second error detecting method. With the second error detecting method, the error detection value Δτ (Δτ B ) is calculated by substituting the first and second early correlation values CV E  and CV VE , the first and second late correlation values CV L  and CV VL , and the prompt correlation value CV P , into the following second calculation equation. 
     
       
         
           
             
               
                 
                   
                     Δτ 
                     B 
                   
                   = 
                   
                     
                       
                         
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                           ( 
                           
                             
                               CV 
                               E 
                             
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                               CV 
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                       - 
                       
                         
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                           ( 
                           
                             
                               CV 
                               VE 
                             
                             - 
                             
                               CV 
                               VL 
                             
                           
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                         c 
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                         CV 
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     Note that, in Equation 2, c 1 , c 2  and c 3  are suitably set invariables. 
     Next, by using the calculated error detection value Δτ (either one of Δτ A  and Δτ B ), a code phase control of the replica signal is performed. Here, the code phase of the prompt replica signal S RP  is advanced or retarded so that the error detection value Δτ becomes 0. Further, due to the code phase of the prompt replica signal S RP  being set as above, the code phases of the first and second early replica signals S RE  and S RVE , and the first and second late replica signals S RL  and S RVL  are also set as described above. 
     By repeating such calculation of the error detection value Δτ and such code phase control, the code phase of the GNSS signal is locked and the tracking of the GNSS signal is performed. Here, the locking of the code phase indicates that the code phase control is performed so that the code phases of the prompt replica signal S RP  and the code phase of the GNSS signal substantially match with each other continuously. 
     Further, in the present invention, as described above, the calculation equation is selected depending on the situation and the error detection values are calculated. Next, operations and effects obtained by such selection of the calculation equations of the error detection values Δτ is described. 
     First, code phase difference properties of the error detection values Δτ when the first error detecting method (Equation 1) is used and when the second error detecting method (Equation 2) is used are described, respectively.  FIG. 3  is a chart illustrating a property (900NW) of the error detection value Δτ A  with respect to the code phase difference, which is calculated with the first error detecting method.  FIG. 4  is a chart illustrating a property (900ELS) of the error detection value Δτ B  with respect to the code phase difference, which is calculated with the second error detecting method. Note that,  FIGS. 3 and 4  are illustrated schematically so that only a difference of the properties can be understood clearly. 
     When the first calculation equation (Equation 1) of the first error detecting method is used (in the case with the property as in  FIG. 3 ), the error detection value Δτ (Δτ A ) does not become 0 until the absolute value of the code phase difference corresponds to 1.0 chip, except for the case where the code phase difference is 0. Therefore, the error detection value Δτ that is not 0 can be obtained in a wide range of the code phase difference. Thus, even if the code phase difference between the aimed GNSS signal and the prompt replica signal S RP  is comparatively large, the code phase control of the prompt replica signal S RP  can be performed so that these code phases surely match with each other. 
     By such properties, the first error detecting method is particularly effective while shifting from the capturing to the tracking. In the capturing processing of the GNSS signal, normally a plurality of replica signals are generated at predetermined code phase intervals and correlated with the GNSS signal. Then, for example, the code phase of the replica signal with the highest correlation value is set to be an initial code phase in tracking the GNSS signal. Therefore, it is because, depending on the code phase interval to be used in the capturing and the reception situation, the code phase in the initial stage of the tracking may be away from the true code phase of the GNSS signal. Note that, with the code phase property as in the first error detecting method, since the code phase range where the error detection value is not 0 is wide, influence of a multipath signal is easily received. 
     When the second calculation equation (Equation 2) of the second error detecting method is used (in the case with the property as in  FIG. 4 ), the code phase ranges where the error detection value Δτ (Δτ B ) becomes 0 exist before the absolute value of the code phase difference corresponds to 1.0 chip, except for the case where the code phase difference is 0. When this property is described more specifically, as illustrated in  FIG. 4 , the error detection value Δτ (Δτ B ) does not become 0 from a predetermined chip (negative value) at which the code phase difference is on 0.0 side than −1.0 chip, to a predetermined chip (positive value) at which the code phase difference is on 0.0 side than +1.0 chip, except for the case where the code phase difference is 0. Further, the insensible ranges where the error detection value Δτ (Δτ B ) becomes 0 are provided, each over a predetermined code phase range on a side of the code phase difference away from 0.0 than such a range where the error detection value Δτ does not become 0. Thus, even if the multipath signal is received, the code phase of the multipath signal becomes easier to enter the insensible ranges. By the code phase of the multipath signal entering the insensible range, the code phase control can be performed accurately without receiving the influence of the multipath signal. 
     By such properties, the second error detecting method is particularly effective in the state where the code phase difference between the prompt replica signal S RP  and the GNSS signal becomes small and is driven close to 0. In this case, the code phase is controlled so that the code phase difference between the prompt replica signal S RP  and the GNSS signal becomes 0, and even if the multipath signal is received, the influence of the multipath signal does not appear on the error detection value Δτ (Δτ B ). Therefore, the code phase can be controlled accurately. 
     By using either one of the first error detecting method and the second error detecting method according to the situation as described above, the code phase of the aimed GNSS signal can surely be locked with a high accurately, and the aimed GNSS signal can be tracked. Further, even if the multipath signal is received during the tracking, the aimed GNSS signal can accurately be kept tracked without receiving the influence of the multipath signal. 
     Next, the determining method of selecting between the first and second error detecting methods is described.  FIG. 5  shows charts illustrating a first situation where the code phase of the prompt replica signal S RP  is advanced compared to that of the aimed GNSS signal.  FIG. 6  shows charts illustrating a second situation where the code phase of the prompt replica signal S RP  is advanced compared to that of the aimed GNSS signal.  FIG. 7  shows charts illustrating a third situation where the code phase of the prompt replica signal S RP  is advanced compared to that of the aimed GNSS signal. Here, with the first situation, the code phase difference between the prompt replica signal S RP  and the aimed GNSS signal is larger than the second and third situations. In the second situation, the code phase difference between the prompt replica signal S RP  and the aimed GNSS signal is larger than the third situation. 
       FIG. 8  shows charts illustrating a fourth situation where the code phase of the prompt replica signal S RP  is retarded than that of the aimed GNSS signal.  FIG. 9  shows charts illustrating a fifth situation where the code phase of the prompt replica signal S RP  is retarded than that of the aimed GNSS signal. Here, in the fourth situation, the code phase difference between the prompt replica signal S RP  and the aimed GNSS signal is larger than the fifth situation. 
     In  FIGS. 5, 6, 7, 8 and 9 , (A) illustrates the correlation value property based on the code phase difference between the replica signal and the GNSS signal, and 900P indicates a correlation curve. (B) illustrates the code phase difference property of the error detection value in the case of using the second error detecting method, and 900ELS indicates a second error detection value property curve. (C) illustrates the code phase difference property of the error detection value in the case of using the first error detecting method, and 900NW indicates a first error detection value property curve. 
     (1) A Case where the Code Phase of the Prompt Replica Signal S RP  is Advanced Compared to that of the Aimed GNSS Signal. 
     As illustrated in  FIG. 5 , as the first situation, when the code phase of the prompt replica signal S RP  is advanced greatly from the aimed GNSS signal, the first and second early correlation value CV E  and CV VE , the first and second late correlation values CV L  and CV VL , and the prompt correlation value CV P  appear in line on the correlation curve 900P within a range where the code phase difference is a negative value. 
     In this case, the first early correlation value CV E  becomes higher than the second early correlation value CV VE . The prompt correlation value CV P  becomes higher than the first early correlation value CV E . The first late correlation value CV L  becomes higher than the prompt correlation value CV P . The second late correlation value CV VL  becomes higher than the first late correlation value CV L . That is, CV E &lt;CV VE &lt;CV P &lt;CV L &lt;CV VL . 
     Therefore, the early differential value ΔCV E =CV E −CV VE  becomes a positive value. The late differential value ΔCV L =CV L −CV VL  becomes a negative value. Therefore, the signs of the early differential value ΔCV E  and the late differential value ΔCV L  are different from each other. 
     Here, the position of the code phase of the prompt replica signal S RP  is an A point, and as illustrated in  FIG. 5  at (B), the error detection value Δτ B  obtained with the second error detecting method becomes 0. As illustrated in  FIG. 5  at (C), the error detection value Δτ A  obtained with the first error detecting method becomes a negative value. Therefore, the code phase control is not possible with the second error detecting method, but is possible with the first error detecting method. 
     Next, as illustrated in  FIG. 6 , as the second situation, when the code phase of the prompt replica signal S RP  is advanced greatly from the aimed GNSS signal (when it is not as advanced as the first situation), similarly to the first situation, the first and second early correlation value CV E  and CV VE , the first and second late correlation values CV L  and CV VL , and the prompt correlation value CV P  appear in line on the correlation curve 900P within the range where the code phase difference is a negative value. 
     Therefore, similarly to the first situation, the early differential value ΔCV E =CV E −CV −VE  becomes a positive value. The late differential value ΔCV L =CV L −CV VL  becomes a negative value. Therefore, the signs of the early differential value ΔCV E  and the late differential value ΔCV L  are different from each other. 
     In this second situation, the position of the code phase of the prompt replica signal S RP  is a B point, and as illustrated in  FIG. 6  at (B), the error detection value Δτ B  obtained with the second error detecting method becomes a border of switching from 0 to a negative value. As illustrated in  FIG. 6  at (C), the error detection value Δτ A  obtained with the first error detecting method becomes a negative value. Therefore, if the code phase difference of the prompt replica signal S RP  from the aimed GNSS signal is smaller than this second situation, there is a possibility that the code phase control can be performed even with the second error detecting method. However, actually, it is better to take an observation error into consideration, but by taking the observation error into consideration, it is difficult to perform the code phase control with the second error detecting method. And, the code phase control is possible with the first error detecting method. 
     Next, as illustrated in  FIG. 7 , as the third situation, in the state where the code phase of the prompt replica signal S RP  is advanced and when the code phase difference therebetween is small (in the case where it is not as advanced as the first or second situation), the first and second early correlation values CV E , CV VE , the first late correlation value CV L , and the prompt correlation value CV P  appear in line on the correlation curve 900P within the range where the code phase difference is a negative value. However, the second late correlation value CV VL  appears on the correlation curve 900P within a range where the code phase difference is a positive value. Then, when the code phase difference between the aimed GNSS signal and the first late correlation value CV L  becomes smaller than the code phase difference between the aimed GNSS signal and the second late correlation value CV L , the late differential value ΔCV L =CV L −CV VL  becomes a positive value. 
     Therefore, both the early differential value ΔCV E , and the late differential value ΔCV L =CV L −CV VL  become positive values. Therefore, the signs of the early differential value ΔCV E  and the late differential value ΔCV L  become the same. 
     In this case, the position of the code phase of the prompt replica signal S RP  is a C point, and as illustrated in  FIG. 7  at (B) and  FIG. 7  at (C), both of the error detection value Δτ B  obtained with the second error detecting method and the error detection value Δτ A  obtained with the first error detecting method are negative values. Therefore, the code phase control is possible even with the second error detecting method or the first error detecting method. However, as described above, since the influence of the multipath signal is easily received with the first error detecting method, the method is switched to the second error detecting method. Thus, after this switch, the influence of the multipath signal is not easily received and the code phase control can be performed accurately so that that code phase of the aimed GNSS signal is locked. 
     (2) a Case where the Code Phase of the Prompt Replica Signal S RP  is Retarded than the Aimed GNSS Signal. 
     As illustrated in  FIG. 8 , as the fourth situation, when the code phase of the prompt replica signal S RP  is greatly retarded than the aimed GNSS signal, the first and second early correlation value CV E  and CV VE , the first and second late correlation values CV L  and CV VL , and the prompt correlation value CV P  appear in line on the correlation curve 900P within the range where the code phase difference is a positive value. 
     In this case, the first early correlation value CV E  becomes lower than the second early correlation value CV VE . The prompt correlation value CV P  becomes lower than the first early correlation value CV E . The first late correlation value CV L  becomes smaller than the prompt correlation value CV P . The second late correlation value CV VE  becomes lower than the first late correlation value CV L . That is, CV VE &gt;CV V &gt;CV P &gt;CV VL &gt;CV L . 
     Therefore, the early differential value ΔCV E =CV E −CV VE  becomes a negative value. The late differential value ΔCV E =CV E −CV VL  becomes a positive value. Therefore, the signs of the early differential value ΔCV E  and the late differential value ΔCV L  are different from each other. 
     Here, the position of the code phase of the prompt replica signal S RP  is a D point, and as illustrated in  FIG. 8  at (B), the error detection value Δτ B  obtained with the second error detecting method becomes 0. As illustrated in  FIG. 8  at (C), the error detection value Δτ A  obtained with the first error detecting method becomes a positive value. Therefore, the code phase control is not possible with the second error detecting method, but is possible with the first error detecting method. 
     Next, as illustrated in  FIG. 9 , as the fifth situation, in the state where the code phase of the prompt replica signal S RP  is retarded and when the code phase difference therebetween is small (in the case where it is not as retarded as the fourth situation), the first early correlation value CV E , the first and second late correlation values CV L  and CV VL , and the prompt correlation value CV P  appear in line on the correlation curve 900P within the range where the code phase difference is a negative value. However, the second early correlation value CV VE  appears on the correlation curve 900P within the range where the code phase difference is a positive value. Then, when the code phase difference between the aimed GNSS signal and the first early correlation value CV E  becomes smaller than the code phase difference between the aimed GNSS signal and the second early correlation value CV VE , the early differential value ΔCV E =CV E −CV VE  becomes a positive value. 
     Therefore, both of the early differential value ΔCV E  and the late differential value ΔCV E =CV L −CV VE  become positive values. Therefore, the signs of the early differential value ΔCV E  and the late differential value ΔCV L  become the same. 
     In this case, the position of the code phase of the prompt replica signal S RP  is an E point and, as illustrated in  FIG. 9  at (B) and  FIG. 9  at (C), both the error detection value Δτ B  obtained with the second error detecting method and the error detection value Δτ A  obtained with the first error detecting method become positive values. Therefore, the code phase control is possible either with the second error detecting method or the first error detecting method. However, since the influence of the multipath signal is easily received with the first error detecting method as described above, the method if switched to the second error detecting method. Thus, after this switch, the influence of the multipath signal is not easily received and the code phase control can be performed accurately so that that code phase of the aimed GNSS signal is locked. 
     As above, by using the GNSS signal processing method of this embodiment, the error detecting method can be switched at a suitable timing. Thus, the aimed GNSS signal can be surely and accurately tracked and the influence on the tracking of the aimed GNSS signal due to the multipath signal can also be suppressed. 
     Note that, when capturing and tracking the GNSS signal, the processing described above can be used specifically with the method described as follows. When starting the tracking of the GNSS signal, the code phases of the GNSS signal and the prompt replica signal S RP  are not necessarily close to each other. Therefore, at a timing of starting the tracking of the GNSS signal, by using the first error detecting method, the code phase control is performed so that the code phases of the GNSS signal and the prompt replica signal S RP  match with each other. 
     During this driving processing of the code tracking, the signs of the early differential value ΔCV E  and the late differential value ΔCV L  are detected. Then, when it is detected that the combination of the signs is changed, that is, when it is determined that the code phase difference between the GNSS signal and the prompt replica signal S RP  is smaller than a predetermined value, the method is switched to the second error detecting method and the tracking of the GNSS signal is continued. 
     Hereinafter, for example, while the GNSS signal is tracked with the second error detecting method, by using the early differential value ΔCV E , the late differential value ΔCV L , and an early late differential value ΔCV EL  corresponding to the error detection value Δτ A  in the first error detecting method, the code phase difference between the GNSS signal and the prompt replica signal S RP  is monitored. Further, when it is determined that the code phase difference between the GNSS signal and the prompt replica signal S RP  becomes larger than the predetermined value based on these differential values, the method is switched to the first error detecting method and the tracking of the GNSS signal is continued. 
     The GNSS signal processing method of this embodiment as described above can be achieved by the following configuration of a function part.  FIG. 10  is a block diagram illustrating a configuration of a positioning apparatus  1  according to the embodiment of the present invention.  FIG. 11  is a block diagram illustrating a configuration of a demodulation unit  13 . 
     The positioning apparatus  1  includes a GNSS reception antenna  11 , an RF processor  12 , the demodulation unit  13  corresponding to the GNSS signal processing device of the present invention, a navigation message analyzer  14 , and a positioning unit  15 . 
     The GNSS reception antenna  11  receives the GNSS signals transmitted from GNSS satellites (e.g., GPS satellites) and outputs them to the RF processor  12 . The down-converter  12  converts each GNSS signal into a predetermined intermediate-frequency signal (hereinafter, referred to as the IF signal), and outputs it to the demodulation unit  13 . 
     Although the detailed configuration is described later by using  FIG. 11 , the demodulation unit  13  performs the code phase control of the replica signal with the error detection value Δτ such as that described above, and captures and tracks the GNSS signal formed of the IF signal. The demodulation unit  13  locks the code phase of the GNSS signal and, when succeeding in the tracking, outputs the correlation value between the GNSS signal and the prompt replica signal S RP  (prompt correlation value CV P ) to the navigation analyzer  14 . Moreover, the demodulation unit  13  calculates a pseudorange by integrating the error detection values Δτ for a predetermined period of time in the tracking state, and outputs it to the positioning unit  15 . 
     The navigation analyzer  14  demodulates and analyzes a navigation message based on the prompt correlation value CV P  transmitted from the demodulation unit  13 , and gives contents of the navigation message to the positioning unit  15 . The positioning unit  15  performs positioning based on the contents of the navigation message from the navigation message analyzer  14  and the pseudorange from the demodulation unit  13 , and estimatedly calculates a position of the positioning apparatus  1 . 
     As illustrated in  FIG. 11 , the demodulation unit  13  includes a replica signal generator  31 , correlators  32 P,  32 VE,  32 E,  32 L and  32 VL, and an operator  33 . 
     The replica code generator  31  generates the prompt replica signal S RP , the first early replica signal S RE , the second early replica signal S RVE , the first late replica signal S RL , and the second late replica signal S RVL  described above, based on the code phase control signal given from the operator  33 . The replica code generator  31  outputs the prompt replica signal S RP  to the correlator  32 P. The replica code generator  31  outputs the first early replica signal S RE  to the correlator  32 E. The replica code generator  31  outputs the second early replica signal S RVE  to the correlator  32 VE. The replica code generator  31  outputs the first late replica signal S RL  to the correlator  32 L. The replica code generator  31  outputs the second late replica signal S RVL  to the correlator  32 VL. 
     The correlator  32 P correlates the GNSS signal with the prompt replica signal S RP  and outputs the prompt correlation value CV P . The prompt correlation value CV P  is outputted to the operator  33  as well as the navigation message analyzer  14 . The correlator  32 E correlates the GNSS signal with the first early replica signal S RE  and outputs the first early correlation value CV E . The first early correlation value CV E  is outputted to the operator  33 . The correlator  32 VE correlates the GNSS signal with the second early replica signal S RVE  and outputs the second early correlation value CV VE . The second early correlation value CV VE  is outputted to the operator  33 . The correlator  32 L correlates the GNSS signal with the first late replica signal S RL  and outputs the first late correlation value CV L . The first late correlation value CV L  is outputted to the operator  33 . The correlator  32 VL correlates the GNSS signal with the second late replica signal S RVL , and outputs the second late correlation value CV VL . The second late correlation value CV VL  is outputted to the operator  33 . 
     The operator  33  is comprised of, for example, a CPU. The operator  33  stores a program which achieves the error detection value calculation and the code phase control described above, and it reads and executes the program. 
     The operator  33  selects the error detecting method as described above, by using the prompt correlation value CV P , the first early correlation value CV E , the second early correlation value CV VE , the first late correlation value CV L , and the second late correlation value CV VL . The operator  33  calculates the error detection value Δτ with the selected error detecting method. The operator  33  generates a code phase control signal based on the calculated error detection value Δτ so that the code phase difference between the prompt replica signal and the GNSS signal becomes closer to 0. The operator  33  gives the code phase control signal to the replica signal generator  31 . 
     By using such a configuration, as described above, the GNSS signal can surely and accurately be tracked. Further, since the accurate tracking can be performed, the code phase of the GNSS signal can be acquired highly accurately, and the demodulation of the navigation message and the calculation of the pseudorange can be performed highly accurately. Thus, highly accurate positioning can be performed. 
     Note that, in the above description, the example in which the positioning apparatus  1  is divided into the respective functional components to perform the positioning is shown; however, the RF processor  12 , the demodulation unit  13 , the navigation message analyzer  14 , and the positioning unit  15  may be integrated in an information processing device, such as a computer. In this case, specifically, a flowchart of the positioning illustrated in  FIG. 12  including the respective processing described above is programmed and stored. Then, the program of positioning is read and executed by the information processing device.  FIG. 12  is the flowchart of a positioning method according to the embodiment of the present invention. 
     The GNSS signal is received and the capturing is performed (S 201 ). As the capturing method, as described above, the plurality of replica signals are generated at the predetermined code phase intervals. Each of the plurality of the replica signals is correlated with the GNSS signal. The code phase of the replica signal with the highest correlation value is set to be the code phase of the GNSS signal. 
     The tracking is started by having the code phase set by the capturing as the initial phase (S 202 ). Here, the tracking of the GNSS signal is performed while selecting the calculation method of the error detection value Δτ, based on the signs of the early differential value ΔCV E  and the late differential value ΔCV E . 
     The pseudorange is calculated by integrating the error detection values Δτ every predetermined period of time (S 203 ). The navigation message is demodulated and acquired by integrating the prompt correlation values CV P  (S 204 ). Note that, the performance order of the calculation of the pseudorange and the demodulation and the acquisition of the navigation message is not limited to this, and both may be performed simultaneously. 
     The positioning is performed by using the acquired pseudorange and the navigation message (S 205 ). 
     The positioning apparatus  1  and the positioning function as described above are utilized in a mobile terminal  100  as illustrated in  FIG. 13 .  FIG. 13  is a block diagram illustrating a main configuration of the mobile terminal  100  including the positioning apparatus  1  according to the embodiment of the present invention. 
     The mobile terminal  100  as illustrated in  FIG. 13  is, for example, a mobile phone, a car navigation device, a PND, a camera, or a clock, and includes a GNSS reception antenna  11 , an RF processor  12 , a demodulation unit  13 , a navigation message analyzer  14 , a positioning unit  15 , and an application processor  120 . The GNSS reception antenna  11 , the RF processor  12 , the demodulation unit  13 , the navigation message analyzer  14 , and the positioning unit  15  are configured as described above, and the positioning apparatus  1  is configured by these components as described above. 
     The application processor  120  displays a position and a speed thereof based on the positioning result outputted from the positioning apparatus  1 , and performs the processing to be utilized in navigation and the like. 
     With such a configuration, since the highly accurate positioning result described above can be obtained, highly accurate positional display, navigation and the like can be achieved. 
     Note that, in the above description, as the first error detecting method, the error detection value Δτ A  is calculated based on the first early correlation value CV E  and the first late correlation value CV L . As the first error detecting method, an error detection value Δτ AA  may be calculated based on the second early correlation value CV VE  and the second late correlation value CV VL . 
     In this case, the following equation may be used. 
     
       
         
           
             
               
                 
                   
                     Δτ 
                     AA 
                   
                   = 
                   
                     
                       
                         CV 
                         VE 
                       
                       - 
                       
                         CV 
                         VL 
                       
                     
                     
                       2 
                       ⁢ 
                       
                         CV 
                         P 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Moreover, the spacing for the calculation of the early differential value ΔCV E  and the late differential value ΔCV E  and the spacing for the calculation of the error detection value may be different from each other. 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
           1 : Positioning Apparatus 
           11 : GNSS Reception Antenna  11   
           12 : RF Processor 
           13 : Demodulation Unit 
           14 : Navigation Message Analyzer 
           15 : Positioning Unit 
           31 : Replica Signal Generator 
           32 P,  32 VE,  32 E,  32 L,  32 VL: Correlator 
           33 : Operator 
           100 : Mobile Terminal 
           120 : Application Processor