Abstract:
A code phase error estimating method includes calculating correlation for I and Q components of a reception signal, judging code phase based on the result of the correlation calculation; calculating a ratio of first difference to a second difference, where the first difference is a first correlation value minus a third correlation value and the second difference is a second correlation value minus the third correlation value, the first correlation value being a correlation value at the code phase, the second correlation value being a correlation value at a phase shifted from the code phase by X chip (0&lt;X&lt;1), and the third correlation value being a correlation value at a phase shifted from the code phase by Y chip (1&lt;Y); and estimating the code phase error when the reception signal is a multipath signal.

Description:
Japanese Patent Application No. 2007-334583 filed on Dec. 26, 2007, is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a code phase error estimating method to estimate code phase error when a reception signal is a multipath signal. 
     2. Related Art 
     GPS (global positioning system) is widely known as positioning system using positioning signals, and applied to positioning device contained in cellular phone, car navigation system and the like. According to the GPS, the current position of the positioning device is determined through positioning calculation which obtains four parameters of three-dimensional coordinates indicating a position of the positioning device and a clock error based on information about positions of plural GPS satellites and pseudo ranges between the respective GPS satellites and the positioning device. 
     Multipath is one of the main factors which causes errors in positioning using positioning signals. An environment where multipath occurs is referred to as a multipath environment. In a multipath environment, multipath signals produced by superimposing indirect waves such as reflection waves reflected by buildings, ground or the like, transmissive waves transmitted through obstacles, and diffraction waves diffracted through obstacles on direct waves transmitted from a signal generating source of the positioning signal (GPS satellites in case of GPS) are received. Under this environment, indirect waves appear as error signals, making it difficult to decode codes. 
     Various technologies for reducing the effect of multipath have been proposed. For example, a method disclosed in JP-A-2000-312163 performs correlation calculation between a reception signal and plural replica codes of C/A codes having different phases for each chip. Then, the phase range where the correlation value is expected to become the peak value is calculated. Subsequently, correlation calculation for the phase range between the reception signal and plural replica codes having different phases for each 0.1 chip is performed to detect code phase. 
     However, it is generally difficult to judge whether the reception signal is a multipath signal or not. Thus, a preventive method is usually employed which reduces the effect on the positioning calculation without judging whether the reception signal is multipath signal or not even when the signal is in fact a multipath signal. For example, the technology disclosed in JP-A-2000-312163 is a method for detecting appropriate code phase even when the reception signal is a multipath signal. Such preventive methods are imposing more calculation and a larger scale of the circuit. 
     SUMMARY 
     It is an advantage of some aspects of the invention to provide a code phase error estimating method and others capable of solving the problems described above. 
     A code phase error estimating method according to a first aspect of the invention includes: calculating correlation for I and Q components of a reception signal as a positioning signal modulated by spread codes which is different for each of positioning satellites and a replica signal of the spread codes; judging code phase based on the result of the correlation calculation; calculating a ratio between a first correlation value for a third correlation value and a second correlation value for the third correlation value, wherein the first correlation value being a correlation value at the code phase, the second correlation value being a correlation value at a phase shifted from the code phase by X chip (0&lt;X&lt;1), and the third correlation value being a correlation value at a phase shifted from the code phase by Y chip (1&lt;Y); and estimating the code phase error by using the ratio and an offset value of the ratio determined in advance for each of the spread codes when the reception signal is a multipath signal. 
     A code phase error estimating device according to a second aspect of the invention includes: a correlation calculating unit which calculates correlation for I and Q components of a reception signal as a positioning signal modulated in spread codes which is different for each of positioning satellites and a replica signal in the spread codes; a code phase judging unit which judges code phase based on the result of the correlation calculation; a ratio calculating unit which calculates a ratio between a first correlation value for a third correlation value and a second correlation value for the third correlation value, wherein the first correlation value being a correlation value at the code phase, the second correlation value being a correlation value at a phase shifted from the code phase by X chip (0&lt;X&lt;1), and the third correlation value being a correlation value at a phase shifted from the code phase by Y chip (1&lt;Y); and a code phase error estimating unit which estimates the code phase error by using the ratio and an offset value of the ratio determined in advance for each of the spread codes when the reception signal is a multipath signal. 
     When the reception signal is direct wave signal, the ratio between the first correlation value for the third correlation value and the second correlation value for the third correlation value becomes a constant value corresponding to the spread codes. When the reception signal is a multipath signal, the ratio varies in the manner similar to sin waves. Also, the ratio of the two correlation values differs according to different spread codes. Thus, the code phase error can be estimated by using the ratio and the offset of the ratio determined for each spread codes when the reception signal is multipath signal. 
     A code phase calculating method according to a third aspect of the invention includes: performing correlation calculation for I and Q components of a reception signal as a positioning signal modulated in spread codes and a replica signal in the spread codes; judging code phase based on the result of the correlation calculation; calculating an angle formed by position vectors of the correlation values at advanced phase and delayed phase from the code phase in advanced and delayed directions by N chip (0&lt;N&lt;1) plotted on IQ coordinates; calculating a ratio between a first correlation value for a third correlation value and a second correlation value for the third correlation value, wherein the first correlation value being a correlation value at the code phase, the second correlation value being a correlation value at a phase shifted from the code phase by X chip (0&lt;X&lt;1), and the third correlation value being a correlation value at a phase shifted from the code phase by Y chip (1&lt;Y); judging whether the reception signal is a multipath signal by using the angle and the ratio; estimating a code phase error by using the ratio and an offset value of the ratio determined in advance for each of the spread codes when the reception signal is a multipath signal; and correcting the code phase by using the estimated code phase error when the reception signal is judged as a multipath signal. 
     A code phase calculating device according to a fourth aspect of the invention includes: a correlation calculating unit which performs correlation calculation for I and Q components of a reception signal as a positioning signal modulated in spread codes and a replica signal in the spread codes; a code phase judging unit which judges code phase based on the result of the correlation calculation; an angle calculating unit which calculates an angle formed by position vectors of the correlation values at advanced phase and delayed phase from the code phase in advanced and delayed directions by N chip (0&lt;N&lt;1) plotted on IQ coordinates; a ratio calculating unit which calculates ratio between a first correlation value for a third correlation value and a second correlation value for the third correlation value, wherein the first correlation value being a correlation value at the code phase, the second correlation value being a correlation value at a phase shifted from the code phase by X chip (0&lt;X&lt;1), and the third correlation value being a correlation value at a phase shifted from the code phase by Y chip (1&lt;Y) a ratio of the first correlation value for the third correlation value to the second correlation value for the third correlation value; and a multipath signal judging unit which judges whether the reception signal is multipath signal by using the angle and the ratio; a code phase error estimating unit which estimates a code phase error by using the ratio and an offset value of the ratio determined in advance for each of the spread codes when the reception signal is a multipath signal; and a code phase correcting unit which corrects the code phase by using the estimated code phase error when the reception signal is judged as a multipath signal. 
     When the reception signal is not a multipath signal (i.e., direct wave signal), the angle formed by the position vectors of the correlation values at the advanced phase and delayed phase plotted on the IQ coordinates becomes a constant value (theoretically zero) as will be described in detail in the following embodiment. When the reception signal is a multipath signal, the angle varies in the manner similar to sin waves. When the reception signal is direct wave signal, the ratio between the first correlation value for the third correlation value and the second correlation value for the third correlation value becomes a constant value corresponding to the spread codes. When the reception signal is a multipath signal, the ratio varies in the manner similar to sin waves. Thus, whether the reception signal is multipath signal can be determined by judging variations in the “angle formed by two position vectors” and the “ratio of the two correlation values” Also, the ratio of the two correlation values differs according to different spread codes. Thus, the code phase error can be estimated by using the ratio and the offset of the ratio determined for each spread codes when the reception signal is a multipath signal. When the reception signal is judged as a multipath signal, the code phase can be corrected by using the estimated code phase error. 
     It is preferable that the judgment whether the reception signal is a multipath signal or not is based on judgments includes: judgment whether the angle exceeds a predetermined angle variation width or not; judgment whether the ratio exceeds a predetermined ratio variation width or not; and judgment that the reception signal is a multipath signal when the angle exceeds the predetermined angle variation width and when the ratio exceeds the predetermined ratio variation width in the code phase calculating method. 
     According to this structure, the reception signal is judged as a multipath signal when the angle formed by the two position vectors exceeds the predetermined angle variation width and when the ratio of the correlation values exceeds the predetermined ratio variation width. 
     It is preferable that the positioning signal is transmitted from a positioning satellite and has the different spread codes for each of the positioning satellite, and that either or both of the predetermined angle variation width and the predetermined ratio variation width are changed for each of the positioning satellite in the code phase calculating method. 
     According to this structure, the positioning signal is transmitted from a positioning satellite and has different spread codes for each of the positioning satellite, and either or both of the predetermined angle variation width and the predetermined ratio variation width are changed for each of the positioning satellite. When the spread codes are different, the angle formed by the two position vectors and the ratio of the two correlation values differ in case of the reception signal which is not multipath signal. Thus, judgment of multipath signal can be made with higher accuracy by varying the angle variation width and the ratio variation width for each positioning satellite. As a result, a code phase calculation with higher accuracy can be performed. 
     It is preferable that the judgment whether the reception signal is a multipath signal or not is based on judgments includes: judgment whether the angle exceeds a predetermined angle variation width or not; judgment whether the ratio exceeds a first ratio variation width or not; judgment whether the ratio exceeds a second ratio variation width having threshold condition wider than the first ratio variation width or not; and judgment that the reception signal is a multipath signal 1) when the angle exceeds the predetermined angle variation width and when the ratio exceeds the first ratio variation width, or 2) when the ratio exceeds the second ratio variation width in the code phase calculating method. 
     According to this structure, the reception signal is judged as a multipath signal when 1) the angle formed by the two position vectors exceeds the predetermined angle variation width and when the ratio of the two correlation values exceeds the first ratio variation width, or 2) the ratio of the two correlation values exceeds the second ratio variation width. 
     It is preferable that the positioning signal is transmitted from a positioning satellite and has different spread codes for each of the positioning satellite, and that at least one of the predetermined angle variation width, the first ratio variation width, and the second ratio variation width is changed for each of the positioning satellite in the code phase calculating method. 
     According to this structure, the positioning signal is transmitted from a positioning satellite and has the different spread codes for each of the positioning satellite, and at least one of the predetermined angle variation width, the first ratio variation width, and the second ratio variation width is changed for each of the positioning satellite. When the spread codes are different, the ratio of the two correlation values differs in case of the reception signal which is not multipath signal. Thus, judgment of multipath signal can be made with higher accuracy by varying the first ratio variation width and the second ratio variation width for each positioning satellite. As a result, judgment of multipath signal with higher accuracy can be performed. 
     A program according to a fifth aspect of the invention is a program under which a processor contained in a positioning device performs the code phase error estimating method described above or the code phase calculating method described above. 
     According to this program, operation and advantage similar to those of the methods or devices described above can be offered by the program read and performed by the processor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIGS. 1A through 1C  show peak detection of correlation values. 
         FIG. 2  shows an example of a correlation result of a multipath signal. 
         FIG. 3  shows error ERR. 
         FIG. 4  shows definition of PE value. 
         FIG. 5  shows the relationship between the PE value and the error ERR. 
         FIGS. 6A and 6B  show an example of a correlation result of a direct wave signal. 
         FIGS. 7A and 7B  show an example of a correlation result of a multipath signal. 
         FIGS. 8A and 8B  show definition of vector angle. 
         FIG. 9  shows the relationship between the vector angle and the error ERR. 
         FIG. 10  shows judgment whether a reception signal is multipath signal or not. 
         FIG. 11  shows an internal structure of a cellular phone. 
         FIG. 12  shows a circuit structure of a baseband processing circuit unit. 
         FIG. 13  shows an example of data structure of flag judgment range table. 
         FIG. 14  shows an example of data structure of PE offset table. 
         FIG. 15  shows an example of ROM structure. 
         FIG. 16  shows an example of RAM structure. 
         FIG. 17  is a flowchart of baseband process. 
         FIG. 18  is a flowchart of multipath detection process performed during the baseband processing. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A preferred embodiment according to the invention is hereinafter described with reference to the drawings. In the following example, a cellular phone having GPS positioning function will be discussed, but the invention is not limited to this example. 
     Principle 
     (A) Judgment of Multipath Signal 
     A method for judging whether a reception signal is a multipath signal or not is described. According to a GPS receiving device, a GPS satellite signal as a positioning signal transmitted from a GPS satellite as positioning satellite is captured using a spread code called C/A codes. More specifically, the GPS satellite signal is modulated by a spread code, namely a C/A code. Then, correlation calculation between the reception signal of the GPS satellite signal and a replica code which replicates the C/A code of the GPS satellite signal (capture target satellite signal) from the target GPS satellite (capture target satellite) is performed. This correlation calculation is carried out while shifting the frequency and phase of the replica codes. The correlation value obtained by correlation calculation will have a maximum value when the frequency of the replica code coincides with the frequency of the reception signal and when the phase of the replica code coincides with the phase of the reception signal. The phase of the C/A code and carrier wave frequency (Doppler frequency) contained in the GPS satellite signal are obtained through detection of the phase and frequency at which the correlation value is at the maximum, and the GPS satellite signal is thus captured. A unique C/A code is given to each GPS satellite, which enables to distinguish between and capture desired GPS satellite signals. When the maximum correlation value does not reach a predetermined value, it is not judged as a peak value. In this case, the reception signal is not judged as a target satellite signal. Then, correlation calculation is performed for other target satellite signals. 
     The GPS satellite constantly changes its position, and the distance between the GPS satellite and the GPS receiving device (pseudo range) changes accordingly. Thus, for dealing with the variations in the pseudo range, the GPS receiving device executes tracking process for tracking the captured GPS satellite signal. 
       FIGS. 1A through 1C  show detection of phase (peak phase) at which the correlation value becomes the maximum (peak). These figures illustrate an example of self-correlation value of C/A codes, showing code phases on the horizontal axis and correlation values on the vertical axis. As shown in the figures, the self-correlation values of C/A codes are represented by a substantially symmetrical triangular shape having the peak value at the top. Thus, the correlation value at the phase delayed from the peak phase and that at the phase advanced from the peak phase by the same amount are equivalent to each other. 
     Accordingly, the correlation values at the phase (early phase) advanced by a predetermined amount (such as ⅓ of a chip) from the code phase (punctual phase) as the currently tracking target and the phase (late phase) delayed therefrom by the same amount are calculated. Then, the punctual phase is controlled such that a correlation value Pl at the late phase (late correlation value) and a correlation value Pe at the early phase (early correlation value) become equal to each other. More specifically, when the early phase value Pe coincides with the late correlation value Pl as shown in  FIG. 1A , it is considered that a punctual phase Pp coincides with the peak phase. When the early correlation value Pe is larger than the late correlation value Pl as shown in  FIG. 1B , the punctual phase is delayed from the peak phase. In this case, the punctual phase is advanced. When the early correlation value Pe is smaller than the late correlation value Pl as shown in  FIG. 1C , the punctual phase is advanced from the peak phase. In this case, the punctual phase is delayed. 
     In the multipath environment, the signal received by the GPS receiving device (reception signal) is a signal (multipath signal) produced by superimposing an indirect wave signal such as reflection waves reflected by buildings, ground or the like, transmissive waves transmitted through obstacles, and diffraction waves diffracted through obstacles on a GPS satellite signal (direct wave signal) transmitted from a GPS satellite. 
       FIG. 2  shows a correlation result of a multipath signal in a graph showing correlation values of a direct wave signal, an indirect wave signal, and a multipath signal produced by combining (superimposing) the direct wave signal and the indirect wave signal. As shown in the figure, the correlation values of the indirect wave signal form a substantially triangular shape similar to the correlation values of the direct wave signal. However, the peak value of the correlation values (correlation peak value) of the indirect wave signal is smaller than the correlation peak value of the direct wave signal. This is because the signal intensity of the GPS satellite signal transmitted from the GPS satellite is lowered while reflected by buildings or ground or transmitted through obstacles. Also, the peak phase of the indirect wave signal is delayed from the peak phase of the direct wave signal. This is because the transmission route from the GPS satellite to the GPS receiving device is extended by reflection of the GPS satellite signal transmitted from the GPS satellite by buildings or ground, diffraction of the GPS satellite signal through obstacles, or for other reasons. Since the correlation value of the multipath signal is the sum of the correlation value of the direct wave signal and the correlation value of the indirect wave signal, the triangular shape is distorted and becomes asymmetric with respect to the peak value as the center. As a result, the punctual phase of the multipath signal does not coincide with the peak phase as shown in  FIG. 3 . The phase difference between the peak phase and the punctual phase is hereinafter referred to as “error ERR”. The indirect wave signal has delay phase from that of the direct wave signal, and the correlation peak value of the indirect wave signal is smaller than the correlation peak value of the direct wave signal. Thus, the punctual phase becomes delay phase from the peak phase. 
     In this embodiment, two parameters “PE value” and “vector angle” are defined, and it is judged whether the reception signal is multipath signal or not based on these parameters. In addition, the error ERR of the code phase is calculated to correct the code phase obtained by signal capture and tracking. 
       FIG. 4  shows definition of the PE value in an example of the correlation result of the reception signal. As illustrated in this figure, the PE value is defined by equation (1) based on the correlation value Pp of the punctual phase (punctual correlation value), a correlation value Pn at a phase advanced from the punctual phase by 1 chip or larger, and a correlation value Pa at a code phase advanced from the punctual phase by N chip. The N value lies in the range of 0&lt;N&lt;1, and the example in the figure shows the case of N=⅔. That is, the PE value represents the ratio between the punctual correlation value Pp for the correlation value Pn and the correlation value Pa for the correlation value Pn. The correlation value Pn is a correlation value at a phase away from the punctual phase by 1 chip or larger and thus considered as a correlation value of a noise floor.
 
 PE =( Pp−Pn )/( Pa−Pn )   (1)
 
     The PE value and the error ERR have a relationship described herein.  FIG. 5  shows the relationship between the PE value of the reception signal and the error ERR when the effect of the multipath changes from “absence” to “presence”. In this figure having the horizontal axis as a common time axis, a solid line indicates PE values with elapse of time, and a broken line indicates error ERR with elapse of time. 
     As can be seen from the figure, the reception signals received by the GPS receiving device are only direct wave signals under the condition of “absence” of the effect of multipath. In this case, the error ERR becomes substantially zero, and the PE value is a constant value. This is because the curved shape of the correlation value of the direct wave signal does not change with elapse of time. However, the inclination of the triangle differs according to PRN codes, and the PE value varies according to GPS satellites. 
     On the other hand, under the condition of “presence” of the effect of multipath, the reception signals become multipath signals produced by superimposing indirect wave signals on direct wave signals. In this case, the error ERR and the PE value vary with elapse of time. This is caused by the change of indirect wave signals according to the change of the relative position relationship between the GPS satellite signal and the GPS receiving device with shift of the GPS satellite and the GPS receiving device. As a result, the curved shape of the correlation value of the multipath signal varies. That is, the error ERR and the PE value vary since the correlation values Pp and Pa of the reception signal shown in  FIG. 4  change. The fluctuations in the PE value can be approximated by sin waves, and the amplitude is determined by the signal intensity relationship between the direct wave signal and the indirect wave signal and the difference in carrier frequencies. The PE value and the error ERR vary with elapse of time substantially in the same manner. More specifically, the PE value increases as the error ERR increases, and the PE value decreases as the error ERR decreases. 
     The vector angle is defined in the following manner.  FIGS. 6A and 6B  show the correlation result of the direct wave signal.  FIG. 6A  is a graph showing the correlation value of the direct wave signal relative to code phase, and  FIG. 6B  is a graph on which correlation values P at the respective code phases shown in  FIG. 6A  are plotted on IQ coordinate plane which has the horizontal axis indicating Q components of the correlation values and the vertical axis indicating I components of the correlation values. The correlation value P is expressed as P=√(I 2 +Q 2 ). As can be seen from  FIG. 6B , the correlation value P of the direct wave signal is distributed substantially along a line passing the origin O on the IQ coordinate plane. More specifically, the I components and Q components of correlation values P 0  and P 4  at code phases CP 0  and CP 4  are both zero, and plotted at the origin O on the IQ coordinate plane. The I components and Q components of correlation values P 1  through P 3  at the code phases CP 1  through CP 3  are not zero, and thus plotted at positions away from the origin O. Particularly the correlation value P 2  at the code phase (peak phase) CP 2  at which the correlation value P becomes the maximum is plotted at the furthermost position from the origin O. More specifically, the correlation values P from the phase CP 0  advanced from the peak phase CP 2  by 1 chip or more to the phase CP 4  delayed from the peak phase CP 2  by 1 chip or more shift away from the origin O on the IQ coordinate plane in the negative to positive direction of the code phase, and reach the furthermost position at the peak phase. Then, the correlation values P again return to the origin O, leaving a substantially linear trace. In the example shown in the figure, the substantially linear trace has an angle of about 45 degree to the Q axis. However, this angle varies according to the phase of the carrier waves of the direct wave signal, the method of determining the IQ coordinates, and other conditions. 
       FIGS. 7A and 7B  show the correlation results of the multipath signal produced by combining direct wave signal and indirect wave signal shown in  FIGS. 2 and 3 .  FIG. 7A  is a graph showing correlation value of the multipath signal relative to code phase, and  FIG. 7B  shows correlation values at respective code phases shown in  FIG. 7A  plotted on IQ coordinate plane. As shown in  FIG. 7B , the correlation value P of the multipath signal is distributed such that a closed curved trace can be produced on the IQ coordinate plane. More specifically, the I component and Q component of correlation value P 0  at the code phase CP 0  are both zero, and plotted at the origin O on the IQ coordinate plane. The I components and Q components of correlation values P 1  through P 4  at the code phases CP 1  through CP 4  are not zero, and thus plotted at positions away from the origin O. Particularly the correlation value P 2  at the peak phase CP 2  is plotted at the farthermost position from the origin O. More specifically, the correlation values P of the multipath signal shift away from the origin O on the IQ coordinate plane, and reach the furthermost position at the peak phase. Then, the correlation values P again return to the origin  0 , leaving substantially closed curved trace. 
       FIGS. 8A and 8B  show the early and late correlation values P of the multipath signal correlation values P plotted on the IQ coordinate plane.  FIG. 8A  shows correlation values of the multipath signal, and  FIG. 8B  shows the correlation values at the respective phases shown in  FIG. 8A  plotted on the IQ coordinate plane. In  FIG. 8B , position vector directed from the origin O toward the position of the early correlation value Pe is referred to as “early correlation vector”, and position vector directed toward the position of the late correlation value Pl is referred to as “late correlation vector”. An angle θ formed by the early correlation vector and the late correlation vector is defined as “vector angle”. Since the correlation values Pl and Pe are equal to each other, the lengths of the early correlation vector and the late correlation vector on the IQ coordinate plane are similarly equal to each other. 
     The following relationship between the vector angle and the error ERR holds.  FIG. 9  shows the relationship between the vector angle and the error ERR when the condition of the multipath effect shifts from “presence” to “absence”. In this figure having the horizontal axis is a common time axis, a solid line indicates the vector angle with elapse of time, and a broken line indicates the error ERR with elapse of time. 
     Under the “absence” condition of the effect of the multipath, the reception signals are only direct wave signals. In this case, the error ERR is zero, and the vector angle is a constant value (theoretically zero). This is because the correlation values P are distributed along a substantially linear track on the IQ coordinate plane as shown in  FIGS. 6A and 6B . Theoretically, the early correlation value is equal to the late correlation value in the direct wave signal, and thus the vector angle becomes zero. In actual situation, however, correlation is calculated while shifting phase by predetermined phase width. Thus, the vector angle becomes a constant value determined according to the performance of the hardware. 
     On the other hand, under the “presence” condition of the effect of multipath, the reception signals are multipath signals, and the error ERR and the vector angle vary with elapse of time. The variations in the vector angle can be approximated by sin waves, and the amplitude is determined according to the relationship between signal intensities of direct wave signal and indirect wave signal and difference of carrier wave frequencies. Also, under the “presence” condition of the effect of multipath, the vector angle comes closer to the “absence” condition of the effect of multipath (constant value close to zero) as the error ERR becomes larger. On the contrary, the vector angle becomes larger as the error ERR becomes smaller. 
     It is judged whether the reception signal is multipath signal or not based on the relationships between the PE value and the error ERR and between the vector angle and the error ERR. When the error ERR is sufficiently large, the reception signal is judged as multipath signal. 
     More specifically, as shown in  FIG. 10 , judgment ranges are determined for the PE value and the vector angle. This figure shows the error ERR, the PE value, and the vector angle with elapse of time when the multipath effect shifts from “absence” condition to “presence” condition. In the figure having the horizontal axis as a common time axis, a broken line indicates the error ERR with elapse of time, a solid line indicates the PE value with elapse of time, and an alternate long and short dash line indicates the vector angle with elapse of time. 
     As shown in the figure, judgment ranges B and C are determined for the PE value. The judgment ranges B and C have a common center value, and this center value is equal to the PE value of the reception signal constituted by only direct wave signal (that is, a predetermined value according to C/A codes contained in direct wave signal). The width of the judgment range C is so determined as to be larger than the judgment range B. The PE value of the direct wave signal differs according to C/A codes of the GPS satellite signal contained in the corresponding direct wave signal. Thus, the center value of the judgment ranges B and C differs according to the GPS satellite as capture target. A judgment range A is determined for the vector angle. The center value of the judgment range A is equal to the value of vector angle when the reception signal is constituted by direct wave signal. 
     It is judged that the reception signal is multipath signal when at least either “Condition A: the PE value is out of the judgment range B and the vector angle is out of the judgment range A”, or “Condition B: the PE value is out of the judgment range C” is satisfied. It is thus judged that the reception signal is not multipath signal when neither of these conditions is satisfied. The reason for these judgments is as follows. 
     Under the “absence” condition of the effect of multipath, the PE value becomes a constant value according to the capture target satellite, and the vector angle becomes a constant value (theoretically zero). That is, neither “Condition A” and “Condition B” are satisfied, and the reception signal is not judged as multipath signal. On the other hand, under the “presence” condition of the effect of multipath, the PE value varies approximately in the same manner as the variation of the error ERR. Also, the absolute value of the vector angle decreases as the absolute value of the error ERR increases, and increases as the absolute value of the error ERR decreases. Thus, depending on the relationship between the error ERR and the vector angle, the case where “Condition A” is not satisfied may occur even when the reception signal is multipath signal. In the example shown in the figure, “Condition A” is not satisfied but the absolute value of the error ERR is large during the respective periods around the time t 1 , t 3 , and t 5 . In this case, the reception signal is judged as multipath signal under “Condition B” regardless of the value of the vector angle when the PE value is large to some extent. 
     (B) Correction of Error ERR 
     Correction of code phase when the reception signal is judged as multipath signal is now explained. As can be seen from  FIG. 5 , the variations in the PE value and the error ERR with elapse of time (variation direction) substantially coincide with each other. Thus, the error ERR can be calculated according to the following equation (2).
 
 ERR =( PE −Offset)/ k    (2)
 
     As discussed above, the PE value of the direction wave signal differs according to PRN codes of the GPS satellite signal. The “Offset” in the equation (2) shown above is an offset value for adjusting the PE value which differs according each PRN code. The value “k” is a coefficient for scale conversion. As shown in  FIG. 3 , the actual peak phase can be obtained by advancing the punctual phase as the code phase acquired by signal tracking by the amount of the error ERR. 
     Structure 
       FIG. 11  is a block diagram showing the internal structure of a cellular phone  1  as a GPS receiving device. As shown in the figure, the cellular phone  1  includes a GPS antenna  10 , a GPS receiving unit  20 , a host CPU (central processing unit)  51 , an operation unit  52 , a display unit  53 , a ROM (read only memory)  54 , a RAM (random access memory)  55 , a cellular-phone wireless communication circuit unit  60 , and a cellular-phone antenna  70 . 
     The GPS antenna  10  is an antenna which receives RF (radio frequency) signal containing GPS satellite signal transmitted from a GPS satellite. The GPS satellite signal is a signal spectrum-modulated by C/A codes as PRN codes, and superimposed on carrier waves in L 1  band having carrier wave frequency of 1.57542 [GHz]. 
     The GPS receiving unit  20  captures and extracts the GPS satellite signal from the RF signal received by the GPS antenna  10 , and performs position measurement calculation based on navigation message or the like extracted from the GPS satellite signal to calculate the current position. The GPS receiving unit  20  has an RF receiving circuit unit  30  and a baseband processing circuit unit  40 . 
     The RF receiving circuit unit  30  has a surface acoustic wave (SAW) filter  31 , a low noise amplifier (LNA)  32 , a local oscillation signal producing unit  33 , a multiplying unit  34 , an amplifying unit  35 , and an A/D converting unit  36 . The RF receiving circuit unit  30  receives signals by so-called super-heterodyne system. 
     The SAW filter  31  is a band-pass filter which transmits signals in a predetermined band range included in the RF signal inputted from the GPS antenna  10 , and blocks and outputs frequency components out of the band range. The LNA  32  is a low-noise amplifier which amplifies signals inputted from the SAW filter  31  and outputs the amplified signals. The local oscillation signal producing unit  33  is constituted by an oscillator such as a local oscillator (LO) and outputs local oscillation signals. The multiplying unit  34  has a multiplier which combines a plurality of signals, and down-converts the RF signal inputted from the LNA  32  into an intermediate frequency signal (IF signal) by integrating the RF signal by the local oscillation signal produced by the local oscillation signal producing unit  33 . Though not shown in the figure, the multiplying unit  34  down-converts the reception signal into the IF signal by multiplying the RF signal by local oscillation signals whose phases are shifted by 90 degrees from one another, and separates into the same phase component (I signal) and the orthogonal component (Q signal). The amplifying unit  35  amplifies the IF signal (I signal and Q signal) inputted from the multiplying unit  34  at a predetermined amplification rate. The A/D converting unit  36  converts the signal (analog signal) inputted from the amplifying unit  35  into digital signal. Thus, the RF receiving circuit unit  30  outputs the I signal and Q signal of the IF signal. 
     The baseband processing circuit unit  40  captures and tracks the GPS satellite signal from the IF signal inputted from the RF receiving circuit unit  30  to perform processes such as pseudo range calculation and position measurement calculation based on the navigation message, time information and the like extracted from the decoded data.  FIG. 12  illustrates the circuit structure of the baseband processing circuit unit  40 . As shown in this figure, the baseband processing unit  40  has a memory  41 , a replica code producing unit  42 , a correlation calculating unit  43 , a CPU  44 , a ROM  45 , and a RAM  46 . 
     The memory  41  samples the I signal and Q signal of the IF signal inputted from the RF receiving circuit unit  30  at predetermined time intervals, and stores the sampled signals. The replica code producing unit  42 , in accordance with the control signal from the CPU  44 , produces and outputs replica codes (replica signal) simulating PRN codes of the GPS satellite signal from the GPS satellite as the capture target. The correlation calculating unit  43  performs correlation calculation between the sampling data of the I signal and Q signal of the IF signal stored in the memory  41  and the replica codes inputted from the replica code producing unit  42  while shifting the phase of the replica codes. 
     The CPU  44  collectively controls the respective sections of the baseband processing circuit unit  40 , and performs various calculations containing baseband processing. In the baseband processing, the CPU  44  produces replica codes corresponding to C/A codes of the GPS satellite signal as the capture target. Then, the CPU  44  detects the C/A codes and the code phase contained in the GPS satellite signal based on the correlation calculation result (correlation value) obtained by the correlation calculating unit  43  to capture and track the GPS satellite signal. For tracking the GPS satellite signal, the correlation value of the phase currently tracked (punctual phase) is varied such that the late correlation value coincides with the early correlation value as discussed with reference to  FIGS. 1A to 1C . The data of the GPS satellite successfully captured (such as satellite number) is stored in capture satellite data  461 . 
     Measurement information for position measurement calculation is determined based on the correlation result of the reception signal. The measurement information is information about the reception signal used in position measurement calculation performed by the CPU  44 , and contains information on reception frequency and code phase. The measurement information is stored in measurement information  462 . 
     Then, the CPU  44  performs a multipath judging process for judging whether the reception signal from the captured GPS satellite is a multipath signal. In the multipath judging process, the PE value is initially calculated. More specifically, as discussed with reference to  FIG. 4 , the correlation value Pp of the code phase (punctual phase) currently tracked, the correlation value Pa at the phase advanced from the punctual phase by ⅔ chip, and the correlation value Pn advanced from the punctual phase by 1 chip or more are obtained. Then, the PE value is calculated according to equation (1). 
     The CPU  44  also calculates the vector angle. More specifically, as discussed with reference to  FIGS. 8A and 8B , the late correlation value and the early correlation value are plotted on the IQ coordinate plane, and the angle θ formed by the late correlation vector directed from the origin O toward the position of the late correlation value and the early correlation vector directed toward the position of the early correlation value is determined as “vector angle”. 
     Then, flags F 1  through F 3  are determined by comparing the calculated PE value and vector angle value with the predetermined judgment ranges A through C. More specifically, the PE value is compared with the judgment ranges B and C. Subsequently, the flag F 2  is set at “1” when the PE value is out of the judgment range B and the flat F 3  is set at “1” when the PE value is out of the judgment range C. The Vector angle is compared with the judgment range A, and the flag F 1  is set at “1” when the vector angle is out of the judgment range A. The flags F 1  through F 3  have the initial value “0”. The set values of the flags F 1  through F 3  are stored in flag data  463 . 
     The judgment ranges A through C are defined in flag judgment range table  452 .  FIG. 13  shows an example of data structure of the flag judgment range table  452 . The flag judgment range table  452  stores the respective center values of the judgment ranges A through C and the widths from each center value as reference in the positive direction and negative direction in correspondence with the respective center values. 
     The CPU  44  judges whether the reception signal is multipath signal or not based on the combinations of the set values of the flags F 1  through F 3 . More specifically, the CPU  44  judges that the reception signal is multipath signal when at least either Condition A: both the flags F 1  and F 2  are “1” or Condition B: the flag F 3  is “1” is satisfied, and judges that the reception signal is not multipath signal when neither of these conditions are satisfied. 
     When it is judged that the reception signal is multipath signal by this multipath judgment process, error correction process is performed. In the error correction process, the error ERR at the code phase is calculated according to equation (2) based on the PE value calculated by the multipath judgment process. In this case, the error ERR is calculated using PE Offset according to the capture target satellite with reference to PE offset table  463 . 
     The PE offset table is data table which defines the PE Offsets of the respective GPS satellites.  FIG. 14  shows an example of data structure of the PE offset table  453 . The PE offset table  453  stores the PE Offset for each GPS satellite in correspondence therewith. 
     After the error ERR is calculated, the CPU  44  advances the code phase by the amount of the calculated error ERR to correct measurement information about the captured GPS satellite. Then, data of the captured GPS satellite signal is decoded based on the corrected measurement information to extract navigation message and perform position measurement calculation for measuring the current position of the cellular phone  1  by calculating the pseudo range. The calculated measurement position is stored in measurement result data  464 . 
     The ROM  45  stores system programs for controlling the respective sections of the baseband processing circuit unit  40  and the RF receiving circuit unit  30  under the control of CPU, and various programs and data for performing various processes including baseband process and the like.  FIG. 15  shows the structure of ROM  45 . As can be seen from this figure, the ROM  45  stores baseband program  451  for performing baseband process as program, and the flag judgment range table  452  and the PE offset table  453  as data. 
     The RAM  46  is used as the operation area of the CPU  44 , and temporality stores programs and data read from the ROM  45 , calculation results calculated by the CPU  44  according to various programs and the like.  FIG. 16  shows the structure of the RAM  46 . As can be seen from this figure, the RAM  46  stores the capture satellite data  461 , the measurement information  462 , the flag data  463 , and the measurement result data  464 . 
     Returning to  FIG. 11 , the host CPU  51  collectively controls the respective sections of the cellular phone  1  according to various types of programs such as the system program stored in the ROM  54 . More specifically, the host CPU  51  chiefly performs processes for providing call function as telephone, and other various functions including navigation function for displaying navigation screen on the display unit  53  where the current position of the cellular phone  1  inputted from the baseband processing circuit unit  40  is plotted on a map. 
     The operation unit  52  is an input device constituted by operation keys and button switches to output operation signals to the host CPU  51  according to operation of the user. Various commands such as start or end command of position measurement are inputted by the operation of the operation unit  52 . The display unit  53  is a display device constituted by LCD (liquid crystal display) and the like to display a display screen (such as navigation screen and time information) based on a display signal inputted from the host CPU  51 . 
     The ROM  54  stores the system program for controlling the cellular phone  1  under the control of the host CPU  51 , and various programs and data for providing navigation function and others. The RAM  55  is used as the operation area of the host CPU  51 , and temporarily stores programs and data read from the ROM  54 , data inputted from the operation unit  52 , calculation results performed by the host CPU  51  according to various programs. 
     The cellular-phone wireless communication circuit unit  60  is a communication circuit unit for cellular phone including RF conversion circuit, baseband processing circuit, and the like to transmit and receive wireless signals under the control of the host CPU  51 . The cellular-phone antenna  70  is an antenna for transmitting and receiving cellular-phone wireless signals to and from a wireless base station provided by a communication service provider of the cellular phone  1 . 
     Flow of Process 
       FIG. 17  is a flowchart showing the flow of the baseband process performed by the CPU  44 . As shown in this figure, the CPU  44  initially selects capture target satellites based on track information on GPS satellite such as Almanac (step A 1 ). Then, the CPU  44  performs the process of Loop A for the selected capture target of the respective satellites. 
     In Loop A, the replica code producing unit  42  produces replica codes of C/A codes associated with the capture target satellites, and calculates measurement information for position measuring calculation based on correlation calculation results of reception signals from the capture target satellites inputted from the correlation calculating unit  43  (step A 3 ). More specifically, the receiving frequency and phase at which the correlation value becomes the peak correlation value are specified and defined as measurement information of the capture target satellites. After the process of Loop A is finished for all the capture target satellites, the process of Loop B is performed for the captured GPS satellites (captured satellites). In Loop B, multiplath detection process for judging whether the reception signals from the target captured satellites are multipath signals or not is performed (step A 5 ). 
       FIG. 18  is a flowchart showing the multipath detection process. As shown in this figure, the CPU  44  provides initial setting of “0” for all the flags F 1  through F 3  (step B 1 ). Then, the CPU  44  calculates PE values based on the correlation results of the reception signals from the target captured satellites inputted from the correlation calculating unit  43  (step B 3 ). Also, the CPU  44  calculates vector angles based on early correlation values and late correlation values inputted from the correlation calculating unit  43  (step B 5 ). 
     Subsequently, the CPU  44  compares the calculated PE values with the predetermined judgment ranges B and C with reference to the flag judgment range table  452 . When the PE value is out of the judgment range B (step B 7 : YES), the CPU  44  sets the flag F 2  at “1” (step B 9 ). When the PE value is out of the judgment range C (step B 11 : YES), the CPU  44  sets the flag F 3  at “1” (step B 13 ). Also, the CPU compares the vector angles with the predetermined judgment range A. When the vector angle is out of the judgment range A (step B 15 : YES), the CPU  44  sets the flag F 1  at “11” (step B 17 ). 
     Next, the CPU  44  judges whether the reception signals from the target captured satellites are multipath signals or not based on the set values of the flags F 1  through F 3 . More specifically, when at least either Condition A: both flags F 1  and F 2  are “1” or Condition B: flag F 3  is “1” is satisfied (step B 19 : YES), it is judged that the reception signal is multipath signal (step B 21 ). When neither of these conditions are satisfied (step B 19 : NO), it is judged that the reception signal is not multipath signal (step B 23 ). After these processes, the multipath detection process ends. 
     When it is judged that the reception signal is multipath signal as a result of the multipath detection process (step A 7 : YES), error correction process for correcting errors in the measurement information is performed. More specifically, the CPU  44  reads PE Offset corresponding to the PRN codes of the target captured satellite with reference to the PE offset table  453  (step A 9 ), and calculates error ERR according to equation (1) based on the PE Offset thus read and the PE value of the target captured satellite (step A 11 ). Then, the code phase is advanced by the amount of the calculated error ERR to correct the measurement information of the target captured satellite (step A 13 ). 
     When the process in Loop B for all the target captured satellites is finished, the CPU  44  performs position measuring calculation for calculating the current position of the cellular phone  1  using the measurement information on the respective captured satellites (step A 15 ) and outputs the calculated measurement positions to the host CPU  51  (step A 17 ). Then, it is judged whether the position measurement is finished or not. When the process is not finished (step A 19 : NO), the process returns to step A 1 . When the process is finished (step A 19 : YES), the baseband process ends. 
     Effect and Advantage 
     According to this embodiment, the “PE value” as the ratio between the correlation value at the punctual phase and the correlation value at the phase advanced from the punctual phase by N (0&lt;N&lt;1) chip is calculated based on the correlation calculation result of the reception signal. Also, the “vector angle” as the angle formed by the position vectors directed toward the early correlation value and the late correlation value from the origin O when the early correlation value and the late correlation value are plotted on the IQ coorinate plane. Then, it is determined by the PE value whether the reception signal is multipath signal based on the judgment results whether the PE value is inside or outside of the judgment ranges B and C and whether the vector angle is inside or outside of the judgment range A. When the reception signal is determined as multipath signal, the error ERR is calculated to correct the code phase appropriately. As a result, the position measurement accuracy can be enhanced even when the cellular phone  1  is positioned in the multipath environment. 
     MODIFIED EXAMPLE 
     The invention is not limited to the embodiment described and depicted herein, but may be practiced otherwise without departing from the scope and spirit of the invention. 
     (A) Judgment of Multlpath Signal 
     For example, the reception signal is judged as multipath signal when at least one of the two conditions (Conditions A and B) is satisfied in this embodiment. However, only one of these conditions may be used for this judgment. In this case, the ranges of the judgment ranges A, B and C may be widened or narrowed. 
     (B) Electronic Device 
     In this embodiment, the example of the cellular phone having the GPS function has been discussed. However, the invention is similarly applicable to other electronic device such as portable navigation device, car navigation system, PDA (personal digital assistants), and watch. 
     (C) Applicable System 
     While the system using GPS has been discussed in this embodiment, the invention is similarly applicable to other satellite positioning systems such as GALILEO using CDMA system similarly to the GPS system. Moreover, the invention is applicable to systems which transmit signals modulated by direct spectrum diffusion system such as system which uses wireless LAN wireless signals conforming with IEEE802.11b standard as positioning signals.