Abstract:
A Global Positioning System (GPS) forms a C/A code sequence by summing, beginning from a polarity inversion boundary determined by a correlation peak position detector, chips at corresponding positions in PN frames constituting each bit of navigation data; calculates pseudo ranges by computing correlation between the C/A code sequence and a reference C/A code sequence generated by the GPS terminal; and determines the position of the GPS terminal using the pseudo ranges and navigation data. The navigation data detected inside the GPS terminal is used when a received electric field detected by a received electric field intensity detector is greater than a threshold level, and the navigation data received from an external system is used when the received electric field is below the threshold level. Thus, the number of PN frames to be integrated is limited because the polarity inversion boundaries of the navigation data are detected, and hence the sensitivity (S/N ratio) is sufficient. Communication between a terminal and a base station is not always required for determining the position because the GPS terminal does not always obtain the Doppler information from the base station, reducing communication cost.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a Global Positioning System GPS and a Global Positioning method for precisely determining a location by receiving GPS signals from satellites. 
     2. Description of Related Art 
     Many satellites orbit the earth, and continuously transmit radio waves at a carrier frequency of 1575.42 GHz. The radio waves are phase modulated by pseudo-random sequences, and a unique pattern is assigned to each satellite so that the different radio waves can be easily identified. As a typical pseudo-random sequence, is known a regularly modulated code pattern called a C/A code (clear and acquisition code) available to the public. Furthermore, the radio waves carry navigation data necessary for users to perform positioning, such as satellite orbit information, satellite correction data, correction coefficients of the ionosphere, etc. The navigation data are transmitted by means of polarity inversions in the C/A code sequence. 
     FIG. 13 is a diagram showing the C/A code sequence. As shown in FIG. 13, the C/A code sequence is a regularly arranged code sequence with its data consisting of 20 PN frames, each which consists of 1023 bits of one millisecond long. Thus, the navigation data is a 50 bit per second signal consisting of 1000 PN frames per second. The polarity of the C/A code sequence is reversed in accordance with the polarity of the bits of the navigation data. 
     FIG. 14 is a block diagram showing a configuration of a conventional Global Positioning System disclosed in U.S. Pat. No. 5,663,734. In this figure, the reference numeral  101  designates a base station having a GPS receiving antenna  102  and a transmitting and receiving antenna  103 . The reference numeral  104  designates a remote unit. 
     The remote unit  104  comprises an RF (radio frequency) to IF (intermediate frequency) converter  106  with a GPS receiving antenna  105 ; an A/D converter  107  for converting the analog signal from the converter  106  to a digital signal; a memory (digital snapshot memory)  108  for recording the output of the A/D converter  107 ; and a general purpose programmable digital signal processor  109  (called DSP from now on) for processing the signal fed from the memory  108 . 
     The remote unit  104  further comprises a program EPROM  110  connected to the DSP  109 , a frequency synthesizer  111 , a power regulator  112 , a write address circuit  113 , a microprocessor  114 , a RAM (memory)  115 , an EEPROM  116 , and a modem  118  which has a transmitting and receiving antenna  117 , and is connected to the microprocessor  114 . 
     Next, the operation of the conventional GPS will be described. 
     The base station  101  commands the remote unit  104  to perform a measurement via a message transmitted over a data communication link  119 . The base station  101  also sends within this message Doppler information for the satellites in view, which is a form of satellite data information. This Doppler information typically is in the format of frequency information, and the message will specify an identification of the particular satellites in view. This message is received by the modem  118  in the remote unit  104 , and is stored in the memory  108  connected to the microprocessor  114 . 
     The microprocessor  114  handles data information transfer between the modem  118  and the DSP  109  and write address circuit  113 , and controls the power management functions in the remote unit  104 . 
     Once the remote unit  104  receives a command (e.g., from the base station  101 ) for GPS processing together with the Doppler information, the microprocessor  114  activates the RF to IF converter  106 , A/D converter  107  and memory  108  via the power regulator  112  and controlled power lines  120   a - 120   d,  thereby providing full power to these components. This causes the signal from the GPS satellite which is received by the antenna  105  to be down-converted to an IF frequency, followed by conversion to digital data. 
     A contiguous set of such data, typically corresponding to a duration of 100 milliseconds to one second (or even longer), is stored in the memory  108 . 
     Pseudo range calculation is executed by the DSP  109  that uses a fast Fourier transform (FFT) algorithm, which permits very rapid computation of the pseudo ranges by performing quickly a large number of correlation operations between a locally generated reference and the received signals. The fast Fourier transform algorithm permits a simultaneous and parallel search of all positions, thus speeding up the required computation process. 
     Once the DSP  109  completes its computation of the pseudo ranges for each of the in view satellites, it transmits this information to the microprocessor  114  through an interconnect bus  122 . 
     Then, the microprocessor  114  utilizes the modem  118  to transmit the pseudo range data over the data link  119  to the base station  101  for final position computation. 
     In addition to the pseudo data, a time lag may simultaneously be transmitted to the base station  101  that indicates the elapsed time from the initial data collection in the memory  108  to the time of transmission over the data link  119 . This time lag improves the capability of the base station  101  to perform position calculation, since it allows the computation of the GPS satellite positions at the time of data collection. 
     The modem  118  utilizes a separate transmitting and receiving antenna  117  to transmit and receive messages over the data link  119 . The modem  118  includes a communication receiver and a communication transmitter, which are alternately connected to the transmitting and receiving antenna  117 . Similarly, the base station  101  may use a separate antenna  103  to transmit and receive data link messages, thus allowing continuous reception of GPS signals via the GPS receiving antenna  102  at the base station  101 . 
     It is expected that the position calculations in the DSP  109  will require less than a few seconds of time, depending upon the amount of the data stored the memory  108  and the speed of the DSP  109  or several DSPs. 
     As described above, the memory  108  captures a record corresponding to a relatively long period of time. The efficient processing of this large block of data using fast convolution methods contributes to the ability to process signals at low received levels such as when reception is poor due to partial blockage from buildings, trees etc. 
     All pseudo ranges for visible GPS satellites are computed using the same buffered data. This provides improved performance relative to continuous tracking GPS receivers in situations such as urban blockage conditions in which the signal amplitude is rapidly changing. 
     The signal processing carried out by the DSP  109  will now be described with reference to FIG.  13 . The objective of the processing is to determine the timing of the received waveform with respect to a locally generated waveform. Furthermore, in order to achieve high sensitivity, a very long portion of such a waveform, typically 100 milliseconds to one second, is processed. 
     The received GPS signal (C/A mode) is constructed from a high rate (1 MHz) repetitive pseudo random (PN) pattern (PN frame) of 1023 symbols, and successive PN frames are added to one another. For example, there are 1000 PN frames over a period of one second. The first such frame is coherently added to the next frame, the result added to the third frame, followed by the additions as shown in FIGS.  15 (A)- 15 (E). The result is a signal having a duration of one PN frame (=1023 chips). The phase of this sequence is compared to a local reference sequence to determine the relative timing between the two, thus establishing the pseudo range. 
     With the foregoing configuration, the conventional Global Positioning System carries out preprocessing operation which precedes the correlation calculations, and which is called “preliminary integration of the received GPS signal” to implement high sensitivity. In this process, the preliminary integration is carried out for 5-10 PN frames to avoid reduction in the integrals due to polarity inversions in the navigation data. 
     The C/A code sequence in the GPS received signal can change its phases, that is, have polarity inversions at the transitions of the bits of the navigation. Therefore, the signal components (chips) may cancel out each other in the integration (cumulative summing) process because of the polarity inversions at the bits of the navigation data in the C/A code sequence, hindering sufficient improvement in the sensitivity (S/N ratio). 
     In other words, the conventional system does not detect the polarity inversions in the navigation data. 
     This limits the theoretical number of data to be integrated, and hence presents a problem of providing only insufficient improvement in the sensitivity (S/N ratio). 
     In addition, every time it determines its position (called “positioning” from now on), the remote unit functioning as a terminal collects Doppler information from the base station, calculates pseudo ranges to the visible satellites, and determines its position by transmitting the distance information to the server. Thus, the positioning always requires communication with the server, offering a problem of entailing communication cost. 
     SUMMARY OF THE INVENTION 
     The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to provide a highly sensitive Global Positioning System and Global Positioning method that can reduce the communication cost by limiting communications with a server only to a case where the receiving sensitivity is insufficient, and that can achieve stable reception inside buildings or in the blockage therefrom. 
     According to a first aspect of the present invention, there is provided a Global Positioning System including an external system that receives a GPS signal from a satellite and extracts navigation data and Doppler information from the GPS signal, and a GPS terminal that is connected to the external system through a communication medium and receives the GPS signal from the satellite, wherein each bit of the navigation data consists of a plurality of PN frames each of which consists of many chips arranged in a prescribed pattern, the GPS terminal comprising: a frequency converter for converting a frequency of the GPS signal received by the GPS terminal; an A/D converter for converting the GPS signal passing through the frequency conversion into corresponding GPS data; a memory for storing the GPS data for a predetermined time interval; a Doppler correction section for performing Doppler correction of the stored GPS data using Doppler information one of the GPS terminal and the external system obtains; means for sequentially dividing the GPS data subjected to the Doppler correction into a plurality of data blocks with a length of a navigation data bit in accordance with navigation data supplied from one of the GPS terminal and the external system; means for calculating in each of the data blocks a cumulative sum of chips at corresponding positions in individual PN frames in the data block; means for multiplying each cumulative sum by a corresponding navigation data bit, and for outputting a plurality of products; means for summing up the plurality of products at respective chip positions; and means for computing correlation between a set of sums of the products and a PN code sequence generated in the GPS terminal, and for calculating a pseudo range between the GPS terminal and the satellite from a correlation peak position at which the correlation becomes maximum. 
     Here, the Global Positioning System may further comprise means for shifting divisions of the data blocks if the correlation peak is less than a prescribed level, and for supplying new data blocks to the means for calculating a cumulative sum. 
     The means for shifting divisions of the data blocks may further shift the divisions to maximize the correlation, and the means for computing correlation may determine the pseudo range from the correlation peak position. 
     The Global Positioning System may further comprise a received electric field detector for detecting intensity of a received electric field, wherein the means for sequentially dividing the GPS data may divide, when the received electric field is greater than a predetermined level, the GPS data passing through the Doppler correction into the data blocks in accordance with the navigation data obtained from the GPS signal received by the GPS terminal, and may divide, when the received electric field is less than the predetermined level, the GPS data in accordance with the navigation data supplied from the external system. 
     The Global Positioning System may further comprise a position determining section for determining a position of the GPS terminal from the pseudo ranges and the navigation data which is extracted from the GPS signal received by the GPS terminal when the received electric field is greater than a prescribed level, and which is supplied from the external system when the received electric field is less than the prescribed level. 
     The GPS terminal may further comprise a position determining section for determining a position of the GPS terminal from the pseudo ranges obtained by the GPS terminal and the navigation data received by the external system. 
     The means for shifting divisions of the data blocks may shift the divisions of the data blocks by a predetermined amount if the correlation peak value is less than a prescribed level to enable detection of the correlation peak position. 
     The means for shifting divisions of the data blocks may shift the divisions of the data blocks roughly at a first step, and then slightly at a second step to converge to the correlation peak value. 
     The Doppler correction section may carry out the Doppler correction of the C/A code sequence in the GPS signal received by the GPS terminal using the Doppler information obtained from the navigation data supplied by one of the GPS terminal and the external system. 
     The Doppler correction section may carry out the Doppler correction of the C/A code sequence by performing the Doppler correction on the GPS signal received by the GPS terminal. 
     The received electric field detector may change a correlation calculation interval in response to the electric field level detected. 
     The received electric field detector may change a summation interval in response to the electric field level detected. 
     According to a second aspect of the present invention, there is provided a GPS terminal connected through a communication medium to an external system that receives a GPS signal from a satellite and extracts navigation data and Doppler information from the GPS signal, wherein the GPS terminal receives the GPS signal from the satellite, and each bit of the navigation data consists of a plurality of PN frames each of which consists of many chips arranged in a prescribed pattern, the GPS terminal comprising: a frequency converter for converting a frequency of the GPS signal received by the GPS terminal; an A/D converter for converting the GPS signal passing through the frequency conversion into corresponding GPS data; a memory for storing the GPS data for a predetermined time interval; a Doppler correction section for performing Doppler correction of the stored GPS data using Doppler information one of the GPS terminal and the external system obtains; means for sequentially dividing the GPS data subjected to the Doppler correction into a plurality of data blocks with a length of a navigation data bit in accordance with navigation data supplied from one of the GPS terminal and the external system; means for calculating in each of the data blocks a cumulative sum of chips at corresponding positions in individual PN frames in the data block; means for multiplying each cumulative sum by a corresponding navigation data bit, and for outputting a plurality of products; means for summing up the plurality of products at respective chip positions; and means for computing correlation between a set of sums of the products and a PN code sequence generated in the GPS terminal, and for calculating a pseudo range between the GPS terminal and the satellite from a correlation peak position at which the correlation becomes maximum. 
     According to a third aspect of the present invention, there is provided a Global Positioning method in a system including an external system that receives a GPS signal from a satellite and extracts navigation data and Doppler information from the GPS signal, and a GPS terminal that is connected to the external system through a communication medium and receives the GPS signal from the satellite, wherein each bit of the navigation data consists of a plurality of PN frames each of which consists of many chips arranged in a prescribed pattern, the Global Positioning method comprising the steps of: converting a frequency of the GPS signal received by the GPS terminal; carrying out A/D conversion of the GPS signal passing through the frequency conversion into corresponding GPS data; storing the GPS data for a predetermined time interval; performing Doppler correction of the stored GPS data using Doppler information one of the GPS terminal and the external system obtains; sequentially dividing the GPS data subjected to the Doppler correction into a plurality of data blocks with a length of a navigation data bit in accordance with navigation data supplied from one of the GPS terminal and the external system; calculating in each of the data blocks a cumulative sum of chips at corresponding positions in individual PN frames in the data block; multiplying each cumulative sum by a corresponding navigation data bit, and for outputting a plurality of products; summing up the plurality of products at respective chip positions; computing correlation between a set of sums of the products and a PN code sequence generated in the GPS terminal; and calculating a pseudo range between the GPS terminal and the satellite from a correlation peak position at which the correlation becomes maximum. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an outline of the operation of an embodiment 1 of the Global Positioning System and Global Positioning method in accordance with the present invention; 
     FIG. 2 is a block diagram showing a specific configuration of a terminal as shown in FIG. 1; 
     FIG. 3 is a block diagram showing a configuration of the terminal as shown in FIG. 2 in more detail; 
     FIG. 4 is a flowchart illustrating the operation of a central server; 
     FIG. 5 is a flowchart illustrating the operation of a CPU in the Global Positioning System and Global Positioning method; 
     FIG. 6 is a flowchart illustrating the operation of a CPU in the Global Positioning System and Global Positioning method; 
     FIG. 7 is a flowchart illustrating the operation of a CPU in the Global Positioning System and Global Positioning method; 
     FIG. 8 is a diagram showing a structure of a GPS signal (C/A code sequence); 
     FIG. 9 is a diagram showing a data structure of the GPS signal (C/A code sequence); 
     FIG. 10 is a diagram illustrating a method of detecting polarity inversions in navigation data; 
     FIG. 11 is a flowchart illustrating a method of detecting polarity inversions in navigation data; 
     FIGS. 12A-12C are diagrams illustrating processing of a correlation result (correlation peak data) between a secondary sum result and a C/A code sequence; 
     FIG. 13 is a diagram illustrating a C/A code sequence; 
     FIG. 14 is a block diagram showing a configuration of a conventional Global Positioning System; and 
     FIG. 15 is a diagram illustrating a method of detecting a C/A code sequence in the conventional Global Positioning System. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described with reference to the accompanying drawings. 
     Embodiment 1 
     FIG. 1 is a block diagram showing an outline of the operation of an embodiment 1 of the Global Positioning System and Global Positioning method in accordance with the present invention. In this figure, the reference numeral  1  designates a satellite in view;  2  designates a central server as an external system; and  4  designates a terminal connected to the central server  2  via a wire or wireless communication medium  5 . The central server  2  has a receiving antenna  3  installed in a vantage location for receiving a GPS signal from the satellite in view  1 , and extracts navigation data and Doppler data from the GPS signal. The terminal  4  comprises a receiving antenna  6  for receiving the GPS signal from the satellite  1 . 
     Next, an outline of the operation of the present embodiment 1 will be described. 
     First, receiving the GPS signal from the receiving antenna  3  installed in a vantage location, the central server  2  makes a decision as to whether the S/N ratio is good or not (step ST 1 ), calculates Doppler data (step ST 2 ) and calculates GPS navigation data (step ST 3 ). 
     On the other hand, the terminal  4  detects the intensity of the electric field received by the receiving antenna  6  (step ST 4 ), and makes a decision as to whether the received electric field is good or not (step ST 5 ). If the decision result is positive (YES), the terminal  4  extracts navigation data and Doppler data on the terminal side (step ST 6 ), whereas if the decision result is negative (NO), the terminal  4  collects required navigation data and Doppler data from the central server  2  (step ST 7 ). Subsequently, the terminal  4  determines a memory interval and a calculation interval in accordance with the received electric field intensity, and stores the received GPS signal in the memory (step ST 8 ). 
     After that, the terminal  4  makes the Doppler correction of the received GPS signal (step ST 9 ), divides the data by the navigation data bit length, detects points at which a correlation value takes maximum value by a correlation peak position detector (step ST 10 ), and obtains a pseudo range from the peak position. The terminal  4  carries out the position computation using the pseudo ranges and the previously extracted navigation data (step ST 14 ). 
     As described above, the present embodiment 1 makes a decision as to whether the received electric field is good or not, and communicates with the central server  2  only when the received electric field is bad. This makes it possible to sharply reduce the communication cost. 
     FIG. 2 is a block diagram showing a concrete configuration of the terminal  4  as shown in FIG.  1 . In FIG. 2, the reference numeral  7  designates a receiving section;  8  and  9  designates an analog-to-digital converter (called “A/D converter” from now on);  11  designates a received electric field intensity detector;  12  designates a memory (DRAM);  13  designates a processing section;  14  designates an antenna mounted on a radio unit  15  for exchanging data with the central server  2  by radio;  16  designates a modem (or terminal adapter) connected to the radio unit  15 ; and  17  designates an I/O (input/output) circuit interposed between the modem  16  and a bus  18 . 
     The receiving section  7  comprises a down converter  20  with bandpass filters  19  and  24  connected to its input and output terminals; a reference oscillator  21 ; a frequency synthesizer  22 ; a frequency divider  23  for dividing the output frequency of the frequency synthesizer  22  to generate a clock signal; and an I/Q converter  25 . The processing section  13  comprises CPUs  26 ,  27  and  28  connected to the bus  18 ; a memory (RAM)  29  connected to the bus  18 ; and a DSP  30  connected to the CPU  27 . The CPUs  26 - 28  are connected with memories (ROM)  26   a - 28   a,  respectively. 
     FIG. 3 is a block diagram showing the terminal  4  as shown in FIG. 2 in more detail in connection with the central server  2 , in which the same reference numerals designate the same or like portions to those of FIG. 2, and the description thereof is omitted here. The central server  2  comprises a GPS reference receiving section  31 , a navigation data extracting section  32  for extracting navigation data contained in the GPS data, a Doppler information computing section  33 , a signal combiner  36  and a signal transmitter and receiver  37 . 
     The terminal  4  comprises an interval determining section  41  for determining an interval of the memory space and that of the correlation calculation in response to the output of the received electric field intensity detector  11 ; a navigation data extracting section  43  connected to the memory area S; a data transmitting and receiving section  48  connected to the communication medium  5  via a switch  47 ; a Doppler information extracting section  45  connected to the output of the navigation data extracting section  43  and to the output of the data transmitting and receiving section  48 ; a Doppler correction section  46  connected to the memory area H; a navigation data extracting section  49  connected to the output of the data transmitting and receiving section  48 ; and a position determining section for determining the position from the pseudo ranges and the navigation data fed from the memory area V. The pseudo ranges are obtained by detecting correlation between the output of the memory area V and the output of a C/A code sequence generator  51  by a correlation peak position detector  52 . 
     The sections from the navigation data extracting section  43  to the position determining section  57  are not separately installed, but their functions are carried out by the processing section  13  comprising the CPUs  26 - 28  and the DSP  30 , for example. Besides, although the three CPUs are shown for the sake of simplifying the description, a single CPU can accomplish the same functions in practice. 
     Next, the operation of the present embodiment 1 will be described. 
     FIG. 4 is a flowchart illustrating the operation of the central server  2 . In the central server  2 , the GPS reference receiving section  31  receives the GPS signal (step ST 16 ), first. Subsequently, the Doppler information computing section  33  calculates the Doppler shift (step ST 17 ); the navigation data extracting section  32  extracts the GPS navigation data (step ST 18 ); and the signal combiner  36  combines them. Then, in response to a request from the terminal  4  for the data, the signal transmitter and receiver  37  transmits the Doppler shift and the navigation data to the terminal  4  (step ST 19 ). 
     FIGS. 5-7 are flowcharts illustrating operations of the CPUs  26 - 28  in the Global Positioning System. First, on the CPU  26  side in the terminal  4 , the antenna  6  receives the GPS signal (step ST 21 ), and supplies the received GPS signal to the received frequency converter  20  which converts it to a predetermined frequency using the local oscillator frequency fed from the reference oscillator  21  via the frequency synthesizer  22 . Subsequently, the I/Q converter  25  carries out the I/Q conversion of the output of the received frequency converter  20  (step ST 22 ), thereby extracting an I signal and a Q signal. The A/D converters  8  and  9  carry out the A/D conversion of these signals. 
     On the other hand, the received electric field intensity detector  11  detects and determines the received electric field intensity: an electric field level  1  (extremely faint); an electric field level  2  (weak); or an electric field level  3  (normal) (steps ST 23 -ST 25 ). A memory interval and correlation calculation interval determining section  41  determines in response to the electric field level a memory interval τ 1 , a memory interval τ 2  or a memory interval τ 3  (steps ST 26 -ST 28 ), and stores (updates) the GPS signal passing through the A/D converters  8  and  9  into the memory area S in the memory  12  in accordance with the time interval determined above (step ST 29 ). 
     The navigation data extracting section  43  reads the content of the memory area S to extract the navigation data. The Doppler information extracting section  45  extracts the Doppler information not only from the signal fed from the navigation data extracting section  43 , but also from the signal sent from the central server  2  fed via the data transmitting and receiving section  48 , and stores the Doppler information to a memory area D. The Doppler correction section  46  reads the data from the memory area D (step ST 30 ), carries out the Doppler correction of the data in the memory area S, and stores the corrected data in the memory area H (step ST 31 ). 
     Subsequently, on the CPU  27  side, it reads the electric field intensity from the memory area L (step ST 32 ), and when the electric field level is  3  (normal), the CPU  27  reads the navigation data from the memory area V that stores the navigation data fed from the navigation data extracting section  43  (step ST 32   b ). On the other hand, when the electric field level is  1  or  2  (very faint or faint), the CPU  27  reads the navigation data which is sent from the central server  2  and stored in the memory area V via the transmitting and receiving section  48  and the navigation data extracting section  49  (step ST 32   a ). Then, the CPU  27  collects the GPS signal and navigation data from the memory areas H and V, respectively, in the interval corresponding to the determined memory interval τ 1  , τ 2  or τ 3  (step ST 33 ). The correlation peak position detector  52  divides the GPS signal read out of the memory area H into a plurality of data blocks in accordance with respective bits of the navigation data read out of the memory area V; sums up the corresponding chips in the 20 PN frames constituting each data block (corresponding to each bit of the navigation data); multiplies the resultant sums by the corresponding bits of the navigation data to form a plurality of products; sums up the products over the memory interval τ 1 , τ 2  or τ 3 ; and computes the correlation between the summed up result (C/A code sequence) and the C/A code sequence generated by the C/A code sequence generator  51  (step ST 34 ). This process will be described in more detail later. 
     Afterward, the correlation peak position detector  52  shifts the navigation data along the time axis such that the correlation peak value becomes maximum, and iterates the same correlation calculations to determine the correlation peak position as the boundary of the polarity inversion of the navigation data, and as the pseudo ranges (ST 36  and ST 37 ). 
     Then, the correlation peak position detector  52  carries out the position computation from the navigation data and the pseudo ranges, and outputs the location (steps ST 38  and ST 40 ). 
     The CPU  28  reads the content in the memory area L (step ST 41 ), and decides as to whether the electric field level is equal to or greater than  3  (normal) (step ST 42 ). If the decision result is positive (YES), the CPU  28  turns off the switch  47  to disconnect the central server  2 , thereby halting data transferring (step ST 43 ). In contrast, if the decision result is negative (NO), that is, if the received electric field level is  1  or  2  (very weak or weak), it turns on the switch  47  to collect the data from the central server  2  (step ST 44 ), and stores through the Doppler information extracting section  45  the navigation data to the memory area VS and the Doppler information to the memory area D for an interval corresponding to the received electric field level (step ST 45 ). 
     As described above, the present embodiment 1 is configured such that it decides the level of the received electric field, and makes a communication with the central server  2  only when the received electric field is insufficient. This makes it possible to sharply reduce the communication cost. Furthermore, the present embodiment 1 is configured such that the correlation peak position detector  52  sums up the values at the same chip positions in the individual periods of the regularly arranged C/A code sequence consisting of multiple chips, and sums up the C/A code sequence using the changing boundary from an increase to decrease or vice versa in the summed up result as the start point of the data summation. This makes it possible to solve the problem in the conventional system in that only insufficient improvement in the sensitivity (S/N ratio) is achieved because the signal components are canceled out in the integral (accumulating). Thus, a high sensitivity Global Positioning System can be implemented by positively detecting the C/A code sequence buried in noise, and the pseudo ranges. 
     An example of the correlation peak position detector  52  will now be described in more detail. 
     This example carries out the following steps: it successively divides the regularly arranged C/A code sequence consisting of a lot of chips into data blocks with a length of one bit of the navigation data beginning from any desired position; takes a cumulative sum of the corresponding chips in individual PN frames in each data block of the C/A code sequence; sums up the resultant cumulative sums with matching their polarities in accordance with the navigation data detected in the GPS terminal or with the navigation data sent from the central server; carries out the correlation calculation between the summed up result and the C/A code sequence; and adopts the correlation peak position as the start position of the data summation. 
     FIG. 8 illustrates the relationship between the number of PN frames and the number of chips for M navigation data lengths. 
     As shown in this figure, each bit of the navigation data consists of 20 PN frames, and one PN frame consists of 1023 chips. Thus, one PN frame (C/A code sequence=PN code sequence) of the GPS signal passing through the Doppler correction and stored in the memory consists of 1023 chips. 
     Although each PN frame of the GPS signal consists of 1023 chips, the A/D converters must convert it at a sampling rate twice that or greater for accurate transmission of the information according to the sampling theorem. Thus, the number of the signal sampling of the memory S and memory V is twice that or greater. 
     Accordingly, the 1023 chips are stored in the number of samples of 1023×2i (i=1, 2, 3, . . . ), for example. 
     For the sake of simplicity, the following description is made in terms of the chips of the C/A code sequence. 
     In FIG. 5, the CPU  26  carries out the Doppler correction for each satellite on the basis of the Doppler information (Doppler frequency shift for each satellite) read from the memory D (step ST 30  and ST 31 ), and stores the corrected values to the memory H. As for the signal undergone the Doppler correction by the CPU  27  of FIG. 5, the regularly arranged C/A code sequence consisting of multiple chips is divided into multiple data blocks beginning from an arbitrary position, each data block having a length of one bit of the navigation data. Subsequently, sampled data of corresponding 1023 chips (=1023×2i sampled data, where i=1, 2 or 3, for example) in the 20 PN frames in each data block are subjected to the first summation over the interval of one bit of the navigation data beginning from the initial divided position, thereby resulting in 1023×2i sums for each data block. 
     The GPS data stored is data consisting of the regular sequence of the PN frames, and the phase of the C/A code sequence (PN code sequence) may be reversed in accordance of the polarity of each 20 millisecond long bit of the navigation data. A phase inversion position of the C/A code sequence agrees with that of the navigation data. 
     FIG. 9 shows the relationships between the navigation data, PN frames and chips for M bits of the navigation data. In this figure, d(i,j,k) designates the sampled data of an with chip in a jth PN frame in a kth navigation data bit. The data d(i,j,k) are stored in the memory over the prescribed time interval (M bits of the navigation data in the present embodiment). It is unknown where the data starts in the GPS signal. 
     FIG. 10 illustrates the data stored in the memory. The GPS signal (C/A code sequence) is divided at every internal or external navigation data bit length interval, and the data corresponding to M bits of the navigation data are stored in the memory. Thus, the total of 1023×20×M chips of the C/A code sequence corresponding to the M bits of the navigation data are stored in the memory. 
     The data stored in the memory are shown in a matrix with 20×M rows and 1023 columns, in which 20×M corresponds to the number of PN frames for M navigation data bits, and 1023 corresponds to the number of chips per PN frame. Thus, the data next to the 1023th column of the first row is the first column data of the second row. Likewise, the data next to the 1023th column of the second row is the first column data of the third row. Iterating such arrangement up to the 20×Mth row and 1023 th column. 
     When dividing the GPS signal (navigation data) at every external or internal navigation data bit length interval, the initial positions of the divisions usually disagree with the initial positions at which the phase inversion of the GPS signal takes place, that is, the true initial positions of the navigation data bit in the GPS signal. 
     The correlation peak position detector  52  matches them as much as possible by the following process. 
     This method will be described in detail with reference to FIG.  11 . 
     First, in FIG. 10, the data elements in the 20×1023 matrix obtained by dividing the GPS signal in accordance with the navigation data bit D 1  are defined by d(i,j,k). Then, the following sums are calculated. 
     
       
           S   1 ( D   1 )= d (1,1,1)+ d (2,1,1)+ d (3,1,1)+ . . . + d (20,1,1) 
       
     
     
       
           S   2 ( D   1 )= d (1,2,1)+ d (2,2,1)+ d (3,2,1)+ . . . + d (20,2,1) 
       
     
     
       
           S   3 ( D   1 )= d (1,3,1)+ d (2,3,1)+ d (3,3,1)+ . . . + d (20,3,1) 
       
     
     
       
           S   i ( D   1 )= d (1, i, 1)+ d (2, i, 1)+ d (3, i, 1)+ . . . + d (20, i, 1) 
       
     
     
       
           S   1023 ( D   1 )= d (1,1023,1)+ d (2,1023,1)+ . . . + d (20,1023,1) 
       
     
     Subsequently, the sums S 1 (D 1 ), S 2 (D 1 ), . . . S i (D 1 ), . . . S 1023 (D 1 ) are each multiplied by the navigation data bit D 1  (=−1 or +1), thereby obtaining the following products. 
     
       
           D   1   ×S   1 ( D   1 ),  D   1   ×S   2 ( D   1 ), . . . ,  D   1   ×S   i ( D   1 ) , . . . , D   1   ×S   1023 ( D   1 ). 
       
     
     Likewise, as for the data divided in accordance with the navigation data bit D 2 , the following sums are calculated. 
     
       
           S   1 ( D   2 )= d (1,1,2)+ d (2,1,2)+ d (3,1,2)+ . . . + d (20,1,2) 
       
     
     
       
           S   2 ( D   2 )= d (1,2,2)+ d (2,2,2)+ d (3,2,2)+ . . . + d (20,2,2) 
       
     
     
       
           S   3 ( D   2 )= d (1,3,2)+ d (2,3,2)+ d (3,3,2)+ . . . + d (20,3,2) 
       
     
     
       
           S   i ( D   2 )= d (1 ,i, 2)+ d (2, i, 2)+ d (3, i, 2)+ . . . + d (20, i, 2) 
       
     
     
       
           S   1023 ( D   2 )= d (1,1023,2)+ d (2,1023,2)+ . . . + d (20,1023,2) 
       
     
     Subsequently, the sums S 1 (D 2 ), S 2 (D 2 ), . . . S i (D 2 ), . . . S 1023 (D 2 ) are each multiplied by the navigation data bit D 2  (=−1 or +1), thereby obtaining the following products. 
     
       
           D   2   ×S   1 ( D   2 ),  D   2   ×S   2 ( D   2 ), . . . ,  D   2   ×S   i ( D   2 ), . . . ,  D   2   ×S   1023 ( D   2 ). 
       
     
     In just the same manner, as for the data divided in accordance with the navigation data bit D M , the following sums are calculated. 
     
       
           S   1 ( D   M )= d (1,1, M )+ d (2,1, M )+ d (3,1, M )+ . . . + d (20,1, M ) 
       
     
     
       
           S   2 ( D   M )= d (1,2, M )+ d (2,2, M )+ d (3,2, M )+ . . . + d (20,2, M ) 
       
     
     
       
           S   3 ( D   M )= d (1,3, M )+ d (2,3, M )+ d (3,3, M )+ . . . + d (20,3, M ) 
       
     
     
       
           S   i (D M )= d (1, i,M )+ d (2, i,M )+ d (3, i,M )+ . . . + d (20, i,M ) 
       
     
     
       
           S   1023 ( D   M )= d (1,1023, M )+ d (2,1023, M )+ . . . + d (20,1023, M ) 
       
     
     Subsequently, the sums S 1 (D M ), S 2 (D M ), . . . S i (D M ), . . . S 1023 (D M ) are each multiplied by the navigation data bit D M  (=−1 or +1), thereby obtaining the following products. 
     
       
           D   M   ×S   1 ( D   M ),  D   M   ×S   2 ( D   M ), . . . ,  D   M   ×S   i ( D   M ), . . . ,  D   M   ×S   1023 ( D   M ) 
       
     
     After that, the following data are calculated. 
     
       
           C   1   =D   1   ×S   1 ( D   1 )+ D   2   ×S   1 ( D   2 )+ . . . + D   M   ×S   1 ( D   M ) 
       
     
     
       
           C   2   =D   1   ×S   2 ( D   1 )+ D   2   ×S   2 ( D   2 )+ . . . + D   M   ×S   2 ( D   M ) 
       
     
     
       
           C   3   =D   1   ×S   3 ( D   1 )+ D   2   ×S   3 ( D   2 )+ . . . + D   M   ×S   3 ( D   M ) 
       
     
     
       
           C   4   =D   1   ×S   4 ( D   1 )+ D   2   ×S   4 ( D   2 )+ . . . + D   M   ×S   4 ( D   M ) 
       
     
     
       
           C   1023   =D   1   ×S   1023 ( D   1 )+ D   2   ×S   1023 ( D   2 )+ . . . + D   M   ×S   1023 ( D   M ) 
       
     
     Then, the correlation is calculated between the data sequence consisting of the elements C 1 , C 2 , C 3 , C 4 , . . . , C 1023  and the 1023 data elements in the C/A code sequence generated inside the GPS terminal  4  (step ST 34 ). 
     The accuracy of the correlation calculation values will increase with the number of the sampling points. FIG. 12 shows an example of providing two sampling points per chip to improve the accuracy of the correlation peak position: FIG.  12 ( a ) illustrates a case where a true peak comes at the center of the two sampling points;  12 ( b ) illustrates a case where the true peak slightly deviates from a sampling point; and  12 ( c ) illustrates a case where the true peak agrees with one of the sampling points. Here, the peak position Xp (local position) is obtained by Xp=P 2 /(P 1 +P 2 )·τ/2, where τ is a bit length. 
     Subsequently, the CPU  27  makes a decision as to whether the correlation peak value is equal to or greater than a predetermined value (step ST 51 ). If the decision result at step ST 51  is positive (YES), the CPU  27  enters into a fine adjusting mode (step ST 51 a). 
     Specifically, it searches for a convergent point at which the peak value becomes greatest with shifting the navigation data (that is, the division positions of the data blocks) by +Δn chip or −Δn chip (step ST 52 ). 
     If the CPU  27  finds the convergent point at step ST 53 , it obtains the correlation peak position Xp, and stores the maximum peak position (pseudo range) with the corresponding satellite number (step ST 54 ). 
     If the decision result at step ST 53  is negative (NO), the CPU  27  iterates the fine adjustment of the navigation data with slightly shifting it by +Δn chip or −Δn chip until it converges at step ST 53 , and if it converges within the predetermined value, the CPU obtains the correlation peak position Xp, and stores the maximum peak value (pseudo range) with the corresponding satellite number (step ST 54 ). 
     In contrast, if the decision result at step ST 51  is negative (NO), the CPU  27  sequentially shift each navigation data such that it can detect the correlation peak position, and iterates steps ST 34  and ST 57  with shifting the navigation data until the correlation peak exceeds the predetermined level at step ST 51 . If the shift amount exceeds a prescribed value at step ST 56  during the iteration loop, the CPU  27  decides at step ST 58  whether the Doppler correction value is normal or not. If the Doppler correction value is within a prescribed value, the CPU rereads and corrects the Doppler correction value again at step ST 59 , and iterates the steps ST 34 -ST 54 . In the course of this, if the Doppler correction value exceeds the prescribed value at step ST 58 , the CPU  27  makes a decision that it cannot detect the pseudo range with the present satellite, stores the result with the satellite number (step ST 60 ), and proceeds to obtaining pseudo ranges for other satellites. 
     The foregoing steps ST 34 , ST 51 -ST 59  are processings for distinguishing the correlation peak position. 
     As described above, the present embodiment 1 divides into the data blocks the regularly arranged C/A code sequence including a series of PN frames consisting of multiple chips at every navigation data bit interval beginning at any desired position; cumulatively sums up the chips at the same positions in the PN frames in the data block; further sums up the cumulative sums with matching their polarities in accordance with the internally detected navigation data or externally supplied navigation data from the central server; carries out the correlation calculation between the resultant sum and the reference C/A code sequence; and makes the correlation peak position as the start position of the data summation. This enables the C/A code sequence to be cumulatively summed up efficiently without the adverse effect due to the polarity inversions in the navigation data, and makes it possible to positively receive the C/A code sequence in such a bad environment as in a tunnel or building in which the receiving sensitivity is poor. 
     For example, applying the above described Global Positioning System to a mobile phone so that the position detected by Global Positioning System is marked on a map displayed on the basis of the data read from a memory will provide a mobile phone with a highly accurate navigation function that is effective in a poor receiving sensitivity environment such as in a tunnel or building. 
     Incidentally, in the foregoing embodiment, the correlation calculation can be achieved using FFT or IFFT. 
     The correlation calculation can be replaced by the number of the agreements between the internally generated C/A code sequence and the received C/A code sequences, or by the degree of the agreement.