Patent Publication Number: US-10317850-B2

Title: Positioning apparatus, electronic timepiece, positioning control method and recording medium

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a positioning apparatus, an electronic timepiece, a positioning control method, and a recording medium. 
     2. Description of the Related Art 
     There is a multi-functional electronic timepiece which is able to perform various functions in addition to displaying the time. In such electronic timepieces, operation is performed based on a clock signal generated by an oscillating circuit. When a function in which controlling timing is important is performed, accuracy of such clock signal becomes important. Specifically, the oscillating frequency easily changes according to the temperature conditions. In view of the above, there are conventionally known oscillators including a temperature compensating circuit and oscillators including a configuration to suppress change in temperature. 
     Functions among the various functions in the electronic timepiece include calculating the accurate date and time and the present position by receiving radio waves from a positioning satellite and transmitting and receiving various types of data using various communication standards such as Bluetooth (registered trademark) or WLAN (wireless LAN). When communication is performed, clock frequencies, operation frequencies of various units operating based on clock frequencies, and frequencies which are constant multiplications of the above overlap with the reception frequencies and noise occurs. As a result, the reception sensitivity decreases. In view of the above, there is a technique which changes the clock signal frequencies and the operation frequencies when the radio waves are received so that the constant multiplications of the above do not overlap with the reception frequencies. For example, Japanese Patent Application Laid-Open Publication No. 2012-182538 discloses setting a plurality of operation frequencies in advance, and when the reception sensitivity decreases in one operation frequency, the operation frequency is changed to a different operation frequency to avoid overlapping. Overlapping of frequencies due to influence of change in oscillating frequencies because of temperature change can also be avoided. 
     However, to constantly use an oscillator with high accuracy in a positioning apparatus and an electronic timepiece means to perform operation more than necessary to maintain accuracy. With this, electric power consumption increases and efficiency worsens. On the other hand, if the accuracy is simply reduced, the reception sensitivity reduces. Then, operation to acquire radio waves needs to be frequently performed and this consumes large amounts of electric power. Consequently, in this case also the efficiency worsens. 
     SUMMARY OF THE INVENTION 
     A positioning apparatus, an electronic timepiece, a positioning control method, and a recording medium which are able to efficiently perform positioning operations are disclosed. 
     According to one embodiment, there is provided a positioning apparatus including: a first oscillator which outputs a clock signal; a second oscillator which outputs a clock signal which is more accurate than the first oscillator; a receiver which receives a radio wave from a positioning satellite; and a positioning controller which calculates a present position based on positioning information calculated from the received radio wave, wherein, the positioning controller controls switching of a supply source of the clock signal supplied to the receiver and the positioning controller between the first oscillator and the second oscillator depending on a reception status of the radio wave from the positioning satellite by the receiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a functional configuration of an electronic timepiece according to an embodiment of the present invention. 
         FIG. 2  is a diagram describing a format of a navigation message of a signal transmitted by a radio wave transmitted from a GPS satellite. 
         FIG. 3  is a flowchart showing a control procedure of a positioning control process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment is described with reference to the drawings. 
       FIG. 1  is a block diagram showing a functional configuration of an electronic timepiece  1  which is an embodiment of an electronic timepiece of the present invention, the electronic timepiece  1  including a satellite radio wave receiving module  5  which is an embodiment of the positioning apparatus of the present invention. 
     The electronic timepiece  1  according to the present embodiment is a watch including a band, and the watch is attached to an arm of a user by using the band. 
     The electronic timepiece  1  includes, a microcomputer  4 , a satellite radio wave receiving module  5 , an antenna A 1 , an operation member  61 , a display  62 , an annunciator  63 , and an electric power supplying unit  70 . 
     The microcomputer  4  includes a host controller  40  (a timekeeping controller), an oscillating circuit  46 , a frequency dividing circuit  47 , and a timekeeping circuit  48  (timekeeping unit). 
     The host controller  40  includes a host CPU  41  (Central Processing Unit) and a RAM  42  (Random Access Memory). 
     The host CPU  41  is a processor which performs various calculating processes. The calculating process performed by the host CPU  41  is mainly a process regarding counting and displaying the date/time as the clock of the electronic timepiece  1 . This is performed continuously and repeatedly for a long period of time with a low load. 
     The RAM  42  provides a work memory space in the host CPU  41  to store temporary data and setting data. The setting data includes local time setting information  421  which is the setting of time-zones and summer time rules (that is, whether summer time exists, term, and hours shifted) of the date/time to be used and displayed in the electronic timepiece  1 . 
     The host controller  40  stores the time-zone and the summer time rules calculated based on present position information calculated by the satellite radio wave receiving module  5  in the RAM  42  as local time setting information  421  (performs local time setting). The host controller  40  converts the present date/time kept by the timekeeping circuit  48  to local time of the present position according to the local time setting information  421 . With this, the local time is utilized and displayed. 
     The oscillating circuit  46  generates a signal with a predetermined frequency and outputs the signal. The signal is generated using a crystal oscillator (oscillator). Such crystal oscillator can be externally attached to the microcomputer. 
     The frequency dividing circuit  47  outputs the frequency dividing signal dividing the frequency signal input from the oscillating circuit  46  at a set dividing ratio. The setting of the dividing ratio can be changed with the host CPU  41 . 
     The timekeeping circuit  48  counts and holds the present date/time (at least the present time) by counting the frequency dividing signal with the predetermined frequency input from the frequency dividing circuit  47 . The present date/time counted by the timekeeping circuit  48  can be corrected (controlled) with a control signal from the host CPU  41  based on the accurate present date/time calculated by a satellite radio wave receiving processor  50 . The timekeeping circuit  48  can be hardware such as a counter, or the host CPU  41  can use software to count using the RAM  42 . 
     The satellite radio wave receiving module  5  is a module which performs positioning operation by receiving the radio wave from the positioning satellite. The satellite radio wave receiving module  5  includes a satellite radio wave receiving processor  50 , a DCXO  55  (digital control crystal oscillator) (first oscillator), a TCXO  56  (temperature compensating crystal oscillator) (second oscillator), a switch  57 , and a ROM  58 . 
     The satellite radio wave receiving processor  50  receives radio waves (satellite radio waves) transmitted from a positioning satellite of a satellite positioning system such as a GPS (Global Positioning System) of the US through the antenna A 1 . With this, the satellite radio wave receiving processor  50  calculates information such as date/time information (present date/time information) and positioning satellite position information (information including orbit information such as ephemeris, position information, speed information and acceleration speed information). The satellite radio wave receiving processor  50  performs positioning calculation to calculate the accurate present date/time and present position based on the above information. The satellite radio wave receiving processor  50  includes a receiver  51 , a module controller  52  (positioning controller, computer), and a storage  53  (estimate information storage, position storage) and is formed as one as an LSI. The operation of the satellite radio wave receiving processor  50  is performed based on a clock signal with a predetermined frequency (for example, about 26 MHz) selectively input from the DCXO  55  or TCXO  56  through the switch  57 . 
     The receiver  51  includes an acquiring unit  511  which performs an acquiring operation such as receiving and detecting the radio waves from the positioning satellite which is the reception target and identifying the positioning satellite and identifying the phase of the transmitting signal of the positioning satellite. The receiver  51  also includes a tracking unit  512  which tracks the acquired radio wave from the positioning satellite and continuously demodulates and calculates the signal. Regarding the operation of the receiver  51 , the acquiring unit  511  and the tracking unit  512  are controlled separately. When operation is not necessary, the acquiring unit  511  and the tracking unit  512  can each be stopped. 
     The module controller  52  performs processes such as controlling reception of the satellite radio wave and identifying the present date/time and calculating the present position (that is, positioning calculation) based on the received signal (positioning information). The module controller  52  includes a module CPU  521  and a memory  522 . 
     The module CPU  521  performs various calculating processes and controls the operation of the satellite radio wave receiving processor  50 . The module CPU  521  (module controller  52 ) has higher computing processing than the host CPU  41  (host controller  40 ), and is able to perform high load processing such as the above-described positioning calculation. Due to the above, the power consumption of the module CPU  521  is larger than power consumption during standby and power consumption when processes similar to the above are performed in the host CPU  41 . The module CPU  521  and the host CPU  41  are connected by a connection with high power consumption efficiency such as a I2C bus. 
     The memory  522  includes a volatile memory such as DRAM and SRAM providing a work memory space (includes cache memory) in the module CPU  521  and a ROM which stores initial setting data, etc. As the ROM, a non-volatile memory which can be rewritten and updated can be used in addition to or instead of the mask ROM. 
     In its own operation, the module controller  52  counts the date/time based on the date/time information kept by the timekeeping circuit  48  calculated from the host CPU  41 . The satellite radio wave receiving module  5  includes an RTC (Real Time Clock) different from the microcomputer  4 . Therefore, the module controller  52  is able to calculate the date/time from the RTC and count the date/time. The RTC counts the date/time using the clock signal with a predetermined frequency (for example, about 16 kHz) input from the oscillating circuit  46  or other oscillating circuit. 
     The storage  53  is an auxiliary storage which stores various setting data, history data and programs regardless of the electric power supply state. Such data includes, ephemeris (information regarding the present position of the positioning satellite) and almanac (estimate orbit information) data calculated from positioning satellites, most previous receiving timing data, and calculated present position history data. The storage  53  stores a correction table  531  according to the measurement value of a later-described thermistor  551  of the DCXO  55  and an acquiring table  532  (acquiring time estimate information) showing the relation between the parameter regarding the radio wave reception strength (for example, CNR) and the estimate information of the amount of time necessary for acquiring. For example, a flash memory is used as the storage  53 . 
     Separate from the units regarding the operation of the timepiece such as the host CPU  41 , the satellite radio wave receiving processor  50  is able to switch the electric power supply from the electric power supplying unit  70 . 
     The DCXO  55  digitally controls and adjusts the oscillating frequency of the crystal oscillator and outputs a predetermined frequency signal (clock signal). The DCXO  55  includes a thermistor  551  and adjusts the oscillating frequency based on the temperature detected by the thermistor  551 . The relation between the temperature measured by the thermistor  551  and the adjusting range of the oscillating frequency is held as table data in advance. The oscillating frequency of the DCXO  55  initially tends to become comparatively larger (±10 to 30 ppm, etc.) compared to TCXO  56  depending on the variation of the crystal oscillator. As the table data is successively updated based on the data regarding the relation with the temperature measured by the thermistor  551  calculated in use, the accuracy can be enhanced (±1 to 3 ppm, etc.). The adjustment control can be performed by the module controller  52  or any other structure as long as the process regarding enhancement of accuracy is possible. Here, the table data showing the relation is stored in the storage  53  as the correction table  531  and the module controller  52  performs the control process. 
     The TCXO  56  is a crystal oscillator including a temperature compensation function and a structure configured as a module can be used. The TCXO  56  compensates the oscillating frequency of the crystal oscillator which changes according to temperature and outputs a highly accurate clock signal which has a smaller frequency variation from the set frequency than the DCXO  55 , that is, a high frequency stability. Within the normal reception handling temperature range, the TCXO  56  is able to stably output the signal with a sampling frequency suitable according to the inverse spectrum spread (normally, ±1 ppm or lower, 0.1 ppm, etc.). The TCXO  56  internally includes a temperature compensation circuit, and supplies electric power for operation when the temperature compensation circuit operates so that the power consumption becomes larger than the DCXO  55 . 
     The clock frequency which is the target of output by the DCXO  55  and the TCXO  56  are the same. 
     The switch  57  includes a switching unit which alternately switches the supply source of the clock signal between the DCXO  55  and the TCXO  56  under the control of the module controller  52 . The switch  57  is also able to stop the operation of the DCXO  55  or the TCXO  56  which is not the supply source of the clock signal. Alternatively, the switching between operating and not operating can be directly performed by the module controller  52 . 
     The ROM  58  is a storage which stores the program  581  and the setting data  582  to execute various operations performed by the host CPU  41  and the module CPU  521 . Other than the mask ROM, the ROM  58  may include a nonvolatile memory such as a flash memory in which data can be rewritten and updated. The setting data  582  stored in the ROM  58  includes map data to calculate the time-zone in which the present position belongs, and the summer time setting. The host CPU  41  can directly access to the data of the ROM  58 . 
     The operation member  61  receives input from external operation such as user operation. For example, the operation member  61  includes one or a plurality of press button switches and outputs the signal according to the pressed press button switch to the host CPU  41 . 
     The display  62  displays various types of information under control by the host CPU  41 . The display  62  includes a display screen and a driving circuit. For example, a Liquid Crystal Display (LCD) is used as the display screen, and the driving circuit performs the driving operation regarding the display on the liquid crystal display screen. The contents displayed on the display  62  include information regarding present date/time (at least time) and position information according to the positioning result. 
     The annunciator  63  performs various notifying operations to the user. Mechanisms to generate notification include a speaker, a piezoelectric element which generates a beeping sound, a vibrating motor or the like. 
     The electric power supplying unit  70  supplies to each unit electric power necessary to operate each unit of the electronic timepiece  1 . The electric power supplying unit  70  supplies the electric power output from a battery  71  at an operation voltage for each unit. When the operation voltage is different depending on the operating site, the electric power supplying unit  70  converts the voltage using a regulator and outputs the voltage. For example, the battery  71  includes a solar panel which generates power depending on the entering light and a secondary cell which accumulates generated electric power. Alternatively, a detachable dry cell or rechargeable cell is stored in a storing unit and is used as the battery  71 . 
     Next, the positioning operation is described. 
     According to the satellite positioning performed by receiving the radio wave from the positioning satellite, radio waves are received from 3 or more positioning satellites (three-dimensionally 4 satellites; predetermined number necessary for calculating present position). The reception position is calculated with the present position of the plurality of positioning satellites and the shift in receiving timing of the radio waves (pseudo range). Here, the measurement of the shift amount of the receiving timing greatly depends on the accuracy of the supplied clock signal. 
     For example, the positioning satellite (GPS satellite) according to GPS (Global Positioning System) of the US is mainly used as the positioning satellite, but positioning satellites of other positioning system such as GLONASS of Russia can be used. Alternatively, the positioning satellites of a plurality of positioning systems can be used in combination to perform positioning. The signal transmitted from the positioning satellite includes the present date/time information and the information regarding the satellite position. The information is transmitted by spectrum spread in a format determined in the positioning system. The acquiring unit  511  uses code string data of a spread code (C/A code) of the positioning satellites held in advance and detects the signal with inverse spectrum spread. The C/A code is a pseudo random code string in which 1023 codes (chips) are arranged and is repeated at 1023 kHz (1 msec cycle). 
       FIG. 2  is a diagram describing a format of a navigation message in a signal transmitted by the radio wave transmitted from the GPS satellite. 
     According to GPS, frame data in the unit of 30 seconds is transmitted from each GPS satellite to be a total of 25 pages, and with this, all data is output in a cycle of 12.5 minutes. Each frame data includes 5 sub-frames (6 seconds). Each sub-frame includes 10 words (0.6 seconds each, in order, WORD  1  to WORD  10 ). The data format of WORD  1  and WORD  2  is the same in all sub-frames. In WORD  1 , after the Preamble which is a fixed code string of 8 bits (predetermined code string), a telemetry (TLM Message) of 14 bits is transmitted, then an Integrity status flag of 1 bit, 1 spare bit, and parity data of 6 bits are provided. The WORD  2  is shown with TOW-Count (also called Z-count) of 17 bits and then an Alert flag and an Anti-spoof flag each with 1 bit. Then, a sub-frame ID showing a sub-frame number (cycle number) of 3 bits is shown and 2 bits for matching the parity data and parity data of 6 bits are provided. 
     The data of WORD  3  and after is different depending on the sub-frame. In sub-frame  1 , WN (week number) of 10 bits and satellite health information (SV-Health) are included in WORD  3 , and satellite clock correction information is included in WORD  8  to WORD  10 . The sub-frames  2  and  3  mainly include ephemeris (orbit information) and a part of the sub-frame  4 , and the sub-frame  5  include almanac (estimate orbit information). 
     According to the GPS satellite and a Quasi-Zenith Satellite System (QZSS) of Japan conforming to the GPS satellite, the preamble transmitted once every 6 seconds is to be the reference code string and identification of the code string (securing of synchronizing) and decoding is performed. The ephemeris transmitted at a 30 second cycle is normally valid for 4 hours, and a valid ephemeris updated before the valid term ends needs to be calculated again. 
     According to the GLONASS satellite, the information of the position, speed, and acceleration speed (ephemeris) is transmitted once every 30 seconds and updated every predetermined time period (30 minutes). Therefore, a new (valid) ephemeris is calculated at every update. 
     The almanac transmits information of all positioning satellites with 12.5 minute cycles in the GPS satellite and 2.5 minute cycles in the GLONASS satellite with signals from the positioning satellites. The almanac can be calculated when the radio wave is received from any of the positioning satellites. 
     When the ephemeris is calculated and held, the present position can be calculated based on the present date/time and the shift of the receiving timing of the radio wave from each acquired positioning satellite. In the electronic timepiece  1 , the present position is calculated and output at a predetermined time interval. The predetermined time interval may be constant, such as every 5 seconds or every minute. Alternatively, the electronic timepiece  1  can include a measuring unit including an acceleration speed sensor or terrestrial magnetism sensor (both not shown), and the moving state can be measured and determined to suitably set and change a time interval according to the moving speed. 
     Next, the positioning control operation in the electronic timepiece  1  is described. 
     The electronic timepiece  1  is able to perform positioning operation by the satellite radio wave receiving processor  50  at a predetermined time interval and is able to calculate position data (history data). Here, in the electronic timepiece  1 , either the TCXO  56  or the DCXO  55  is selected as the supply source of the clock signal during operation of the satellite radio wave receiving processor  50 , and the oscillator selected according to the switching operation by the switch  57  under the control (switching control) of the module controller  52  outputs the clock signal. As described above, the accuracy of the signal (positive value and stability) is higher in the TCXO  56 . Therefore, normally, according to the reception status of the radio wave, the TCXO  56  is used when the apparatus is started or when the reception status is bad, that is, when the radio wave reception strength is low and/or when the position of the acquired positioning satellite is not preferable. When it is possible to calculate position information at an accuracy equal to or more than the reference from a sufficient number of positioning satellites (suitable satellite position) the DCXO  55  is used. 
     When the necessary position accuracy is different depending on the state of use, etc., the accuracy which is to be the reference for switching between the TCXO  56  and the DCXO  55  can be changed. 
       FIG. 3  is a flowchart showing a control procedure by the module controller  52  of the positioning control process performed by the satellite radio wave receiving module  5  according to the present embodiment. 
     The positioning control process starts when started by the host controller  40  and calculated by the positioning instruction, and the process continues until the positioning end instruction is received by interruption through the host computer  40  due to input on the operation member  61  or the like. 
     When the positioning control process starts, the module controller  52  (module CPU  521 ) performs the start operation and the initial setting of the satellite radio wave receiving processor  50  (step S 101 ). The module controller  52  considers the setting of the switch  57  as the input from the TCXO  56  (step S 102 ). 
     The module controller  52  sets the UTC date/time based on the present date/time input from the host controller  40  (step S 103 ). The module controller  52  starts the reception operation of the satellite radio wave by the receiver  51  (step S 104 ). 
     The module controller  52  determines whether the ephemeris data (information regarding present position of the positioning satellite) within the valid term is stored in the storage  53  for the set UTC date/time (step S 105 ). When it is determined that it is stored (“YES” in step S 105 ), the module controller  52  controls the acquiring unit  511  to perform acquiring operation of the radio wave from the positioning satellite (step S 111 ). Here, the module controller  52  calculates the positioning satellite predicted to be a visible state at the present based on the ephemeris or the almanac. With this, it is possible to acquire the radio wave from the above positioning satellite with priority. The process of the module controller  52  advances to step S 131 . 
     When it is determined that the valid ephemeris data is not stored (step S 105 , “NO”), the module controller  52  controls the acquiring unit  511  to perform the acquiring operation of the radio wave from all positioning satellites (step S 121 ). When the almanac is held, the positioning satellite predicted to be in the visible state at present is calculated based on the almanac, and the radio wave from such positioning satellite can be acquired with priority. The module controller  52  starts the tracking operation of the radio wave from the acquired positioning apparatus performed sequentially by the tracking unit  512  (step S 122 ). The module controller  52  calculates the remaining time (estimated time) that the acquiring operation can be continued until the timing that it is assumed that the preamble is received based on the present date/time (step S 123 ). Here, the module controller  52  considers the maximum shift range of the date/time calculated with the date/time information or estimated based on the previous date/time calculating history, etc. and calculates the minimum remaining time (minimum time). That is, when the error of the date/time counted by the timekeeping circuit  48  is ±0.5 seconds or less for each day, the maximum shift range is the time (seconds) multiplying 0.5 to the passed number of days (passed time) from the previous date/time calculating timing. 
     The module controller  52  determines the remaining time that the acquiring can be performed again when the oscillating circuit is switched to DCXO  55  (step S 124 ). The module controller  52  refers to the acquiring table  532  stored in the storage  53 , calculates the time (re-acquiring time) estimated to be necessary for acquiring with the DCXO  55  corresponding to the reception strength of the radio wave from the acquired positioning satellite (including index of the radio wave strength relative to the noise of the CNR, etc.), and compares the time with the remaining time. For example, the table shows actual values calculated in advance by examinations, and when the reception strength becomes lower than a predetermined level, the time necessary for acquiring drastically increases. When it is determined that there is no remaining time necessary for re-acquiring (no re-acquiring time) (step S 124 , “NO”), the process of the module controller  52  advances to step S 127 . When it is determined that there is remaining time necessary for re-acquiring (there is re-acquiring time) (step S 124 , “YES”), the module controller  52  switches the oscillating circuit which supplies the clock signal to the switch  57  to the DCXO  55  (step S 125 ). Then, the module controller  52  performs the acquiring operation again (step S 126 ). Then, the process of the module controller  52  advances to step S 127 . 
     When the process advances to step S 127 , the module controller  52  controls the tracking unit  512  to track the radio wave from the acquired positioning satellite and demodulates the signal to identify the preamble. The module controller  52  decodes and deciphers the signal as the reference of the position of the identified preamble and calculates the date/time information and ephemeris data (step S 127 ). 
     The module controller  52  determines whether calculating the necessary date/time information and ephemeris data is finished (step S 128 ). When it is determined that the calculating is not finished (step S 128 , “NO”), the process of the module controller  52  returns to step S 127 . When it is determined that the calculating is finished, the process of the module controller  52  advances to step S 131 . 
     When the process advances to step S 131 , the module controller  52  calculates the present position at a predetermined time interval while performing the tracking operation of the acquired satellite (step S 131 ). The module controller  52  stores the calculated present position as the history data with the present date/time in the storage  53 . The module controller  52  outputs the control signal to the display  62  and displays the present position on the display  62 . The module controller  52  determines whether the radio waves from the positioning satellites in a reference number or more are acquired (step S 132 ). The reference number here is the number of satellites necessary for positioning, that is, usually more than 3 or 4 satellites, and a number of satellites so that sufficient accuracy is maintained and in which positioning can be continued even if some of the radio waves from the acquired positioning satellites fall out of the acquired state, for example, 6 to 8 satellites. When it is determined that this is not acquired (step S 132 , “NO”), the process of the module controller  52  returns to step S 111  and the acquiring operation by the acquiring unit  511  is continued or restarted. 
     When it is determined that the radio waves from the positioning satellites in a number of the reference number or more are acquired (step S 132 , “YES”), the module controller  52  ends the acquiring operation by the acquiring unit  511  and performs calculating of the present position in a predetermined time interval while continuing the tracking operation by the tracking unit  512  (step S 133 ). 
     The module controller  52  determines whether the tracking is continued in a state in which the radio waves from the positioning satellites in a number equal to or larger than the number necessary for positioning is acquired (step S 134 ). Here, the module controller  52  also determines whether the ephemeris of the positioning satellite regarding the acquired radio wave is within the valid term. Here, the valid term of the ephemeris can be set to a shorter time (for example, 2 hours) than the above-described valid term of the ephemeris (4 hours). When it is determined that the radio waves are not acquired from the positioning satellites in a number equal to or larger than the number necessary for positioning (even if acquired, the ephemeris is not within the valid term) (step S 134 , “NO”), the module controller  52  determines whether the supply source of the clock signal is the DCXO  55  (step S 151 ). When it is determined that it is the DCXO  55  (step S 151 , “YES”), the module controller  52  advances the process to step S 162  and controls the switching unit  57  to switch the supply source to TCXO  56  (step S 162 ). Then, the process of the module controller  52  advances to step S 163 . When it is determined that it is not the DCXO  55  (step S 151 , “NO”), the process of the module controller  52  advances to step S 163 . 
     When it is determined that the radio waves are acquired from the positioning satellites in a number equal to or more than the number necessary for positioning (step S 134 , “YES”), the module controller  52  determines whether the calculated position accuracy is a reference or more (step S 135 ). For example, DOP (Dilution of Precision) is used as the index of the position accuracy. When it is determined that it is not the reference or more (step S 135 , “NO”), the module controller  52  determines whether the supply source of the clock signal is the DCXO  55  (step S 161 ). When it is determined that it is the DCXO  55  (step S 161 , “YES”), the module controller  52  controls the switch  57  to switch the supply source to the TCXO  56  (step S 162 ). Then, the process advances to step S 163 . When it is determined that it is not the DCXO  55  (step  161 , “NO”), the process of the module controller  52  returns to step S 133 . 
     When it is determined that the calculated position accuracy is a reference or more (step S 135 , “YES”), the module controller  52  determines whether the supply source of the clock signal is the TCXO  56  (step S 141 ). When it is determined that it is the TCXO  56  (step S 141 , “YES”), the module controller  52  controls the switch  57  to switch the supply source to the DCXO  55  (step S 142 ). Then, the process of the module controller  52  advances to step S 163 . When it is determined that it is not the TCXO  56  (step S 141 , “NO”), the process of the module controller  52  returns to step S 133 . 
     When the process advances from the process in steps S 142 , S 151 , and S 162  to step S 163 , the module controller  52  controls the acquiring unit  511  to perform the acquiring operation of the radio waves from the positioning satellite again (step S 163 ). In this case, the module controller  52  can omit the radio waves of the presently acquired positioning satellite from the target of re-acquiring. The radio waves of the acquired positioning satellite include the phase shift of the clock signal being small and not falling outside the acquiring when the supply source of the clock signal is switched (mainly, switching to the more accurate TCXO  56 , but not limited to this). The module controller  52  determines whether the valid ephemeris is held for the positioning satellite transmitting the acquired radio waves (step S 164 ). When it is determined that it is not held (step S 164 , “NO”), the process of the module controller  52  advances to step S 127 . When it is determined that it is held (step S 164 , “YES”), the process of the module controller  52  returns to step S 133 . 
     Among the above processes, steps S 131  and S 133  are included in the position identifying step (position identifying unit). Steps S 125 , S 142 , and S 162  are included in the clock switching step (clock switching unit). 
     As described above, the satellite radio wave receiving module  5  according to the present embodiment includes, a DCXO  55  which outputs the clock signal, a TCXO  56  which outputs a clock signal more accurate than the DCXO  55 , a receiver  51  which receives a radio wave from a positioning satellite, and a module controller  52  which calculates a present position based on positioning information calculated from a received radio wave, wherein, the module controller  52  controls switching a supply source of the clock signal to the receiver  51  and the module controller  52  between the DCXO  55  or the TCXO  56  depending on a receiving state of a radio wave from a positioning satellite by a receiver  51 . 
     As described above, since power consumption of oscillators greatly differ depending on the accuracy, two types of oscillators with high and low accuracy (power consumption) are provided. With this, the power consumption can be reduced while maintaining the reception accuracy necessary for positioning according to receiving status. Therefore, positioning can be performed more efficiently. 
     The module controller  52  controls the switching according to accuracy of the calculated present position. That is, the module controller  52  switches the supply source of the clock signal between the DCXO  55  and the TCXO  56  according to whether the acquired positioning satellite used in calculating the present position is suitably positioned in a scattered state. With this, the accuracy of the clock signal is not forcibly reduced and the calculation of the present position is not made difficult. On the other hand, when it is determined that there will be no problems in calculating the present position even if the accuracy of the clock signal is decreased, the accuracy of the clock signal is suitably decreased so that the power consumption can be decreased. 
     The module controller  52  controls the receiver  51  to perform acquiring operation of the radio wave from the positioning satellite in a state receiving supply of the clock signal from the TCXO  56 . When the positioning satellite is acquired in a number which is a number of satellites necessary for calculating the present position, the module controller  52  determines whether the acquiring operation can be performed again within an estimated time until the preamble can be detected in the demodulated signal from the acquired radio wave. When it is determined that the acquiring operation can be performed again, the supply source of the clock signal is switched to the DCXO  55 , and the receiver  51  performs the acquiring operation again. As described above, at the start of receiving, when there is plenty of time for processing, the supply source of the clock signal is switched to the DCXO  55  when it is possible during such time period. Therefore, it is possible to efficiently select the suitable clock signal and start positioning without unnecessary extension of receiving time. 
     The module controller  52  considers the reception strength of the acquired radio wave and calculates the re-acquiring time necessary to perform acquiring operation again from the acquiring table  532 . Then, the module controller  52  determines whether acquiring can be performed again. As described above, the switching to the DCXO  55  is performed only when it is possible to perform switching without difficulty within an assumption based on the actual reception strength that the acquiring can be performed again. Therefore, the time for acquiring operation and the time for calculating the present position after acquiring is not extended more than necessary. 
     The storage  53  stores the acquiring table  532  showing the relation between the reception strength of the radio wave from the positioning satellite (including relative strength index of the noise such as CNR) and re-acquiring time. With this, the time necessary for acquiring can be easily estimated and suitable process timing can be controlled. The acquiring table  532  can be stored and updated in a flash memory, etc. With this, the accuracy can be enhanced according to the actual measurements. 
     The module controller  52  calculates the shortest estimated time based on the maximum shift range assumed in the estimated time. That is, error occurs in the estimated time according to the date/time counted in the timekeeping circuit  48 . Therefore, by considering such error and estimating the estimating time to be short, it is possible to prevent the receiving timing of the preamble passing while acquiring is performed. With this, it is possible to prevent unnecessary prolonging of the time necessary to finish calculating the ephemeris data and positioning can be started without delay. 
     The module controller  52  assumes the maximum shift range based on the passed time from the timing that the previous positioning is performed. The date/time which is counted by the timekeeping circuit  48  can be corrected with the accurate date/time calculated at positioning. The shift later on may increase according to the maximum error determined in the oscillating circuit  46 . Therefore, by considering the maximum shift range caused by the maximum error, it is possible to prevent wasting time on unsuccessful detecting of the preamble or calculating of the date/time due to the unexpected shift. 
     The storage  53  stores the information (ephemeris) regarding the present position of the positioning satellite. When a valid (within the valid term) ephemeris is held, the module controller  52  starts reception of the receiver  51  in a state receiving supply of the clock signal from the TCXO  56 , and performs positioning calculating. That is, when the positioning can be performed soon after acquiring the radio wave from the satellite, instead of forcibly decreasing the accuracy, priority is placed on performing the positioning. With this, the user-friendliness is not decreased. 
     Since the temperature compensation crystal oscillator (TCXO) is used as the highly accurate oscillator, temperature change which provides a large influence to the variation of the oscillating frequency is suitably compensated, and positioning can be performed securely even in a situation with bad reception. 
     The digital control crystal oscillator (DCXO) is used as the oscillator with an accuracy lower than the TCXO  56 . Therefore, with the lowest level accuracy of the crystal oscillator and the circuit configuration which can suitably control the measuring value of the thermistor  551 , the positioning can be maintained at a low power consumption within a suitable range of good reception status while suppressing increase of the cost and size. 
     The electronic timepiece  1  according to the present embodiment includes the above-described satellite radio wave receiving module  5 , a timekeeping circuit  48  which counts the present date/time, and a display  62  which displays the time according to the present date/time. 
     As described above, according to the electronic timepiece  1 , the present position can be calculated while suppressing the power consumption. 
     The electronic timepiece  1  includes a host controller  40  which performs local time setting according to the present position based on the information of the present position identified by the operation of the satellite radio wave receiving module  5 . With this, the user is able to understand the accurate local time according to the present position easily and suitably by utilizing the positioning process. That is, the user is free from the troublesome burden of setting the local time. 
     The DCXO  55  and the TCXO  56  are used to perform the positioning operation by the above-described positioning control method. With this, the operation can be performed efficiently and the power can be suitably consumed while maintaining quick calculating of the present position within a suitable accuracy. 
     The program  581  to control the above-described positioning control process is installed and the module CPU  521  (module controller  52 ) executes the process with software. With this, the process to easily and suitably calculate the present position can be performed efficiently. 
     The present invention is not limited to the above-described embodiment, and various changes can be made. 
     For example, the present embodiment includes the DCXO  55  and the TCXO  56  as the oscillating circuit. Alternatively, an OCXO (oven controlled crystal oscillator) can be used as an oscillator with higher accuracy. The DCXO  55  does not need to be digitally controlled, and a normal VCXO (voltage controller crystal oscillator) can be controlled by voltage based on the measured temperature of the thermistor  551 . If the minimum accuracy and stability necessary for receiving the radio wave from the positioning satellite can be secured, instead of using crystal oscillators, oscillators using ceramic oscillators or oscillators using Si (silicon) based MEMS (Micro Electro Mechanical Systems) oscillators can be used. 
     According to the above-described embodiment, when the number of acquired satellites decreases and is insufficient, re-acquiring is always performed by switching to transmission of clock signals from TCXO  56 , however, when the reception situation is good, it is not always necessary to switch to the TCXO  56 . Moreover, when re-acquiring can be performed by receiving supply of the clock signal from the DCXO  55 , re-acquiring can be performed when there is sufficient number of acquired satellites, that is, 4 or 5 satellites. 
     According to the present embodiment, the shift range of the detecting timing of the preamble is estimated by the passing time from the previous positioning timing. In addition to the above, temperature history within the passed time can also be considered. When the shift range is large (6 seconds or more) compared to the transmitting frequency (once every 6 seconds) of the preamble, a constant value (for example, 3 seconds) can be set, or the value can be set to 0 seconds and switching to the DCXO  55  is not performed at this time. 
     According to the above-described embodiment, the preamble is detected to be the reference position of the code string, but the code string showing the value of the TOW-Count according to the present date/time can be detected. 
     According to the present embodiment, the ephemeris is received and calculated, but when the ephemeris can be calculated through communication units, the decoding and calculating of the ephemeris based on the radio wave received from the positioning satellite does not have to be performed. In this case, the ephemeris can be stored in the ROM  58  (non-volatile memory). Other information such as the correction table  531  and the acquiring table  532  can be stored in the ROM  58  accessible from the host controller  40 . 
     According to the above-described embodiment, the electronic timepiece  1  is a watch, but the electronic timepiece  1  can be in any form. For example, the electronic timepiece  1  can be a portable clock, a small clock placed and attached inside a vehicle or the like. 
     The satellite radio wave receiving module  5  is not limited to those provided in the electronic timepiece  1 . The satellite radio wave receiving module  5  can be used as a dedicated positioning apparatus with a display or operation member. The satellite radio wave receiving module  5  can also be provided in other apparatuses such as an activity monitor which measures the number of steps of the user and calories consumed by the user, a pulse monitor, a sphygmomanometer, or the like. In these cases, the host CPU  41  and the timekeeping circuit  48  do not have to be provided separately, and the module CPU  521  can perform unitary calculation control. When the satellite radio wave receiving module  5  is started, the date-time information is calculated from the above-described RTC. When the satellite radio wave receiving module  5  is stopped or positioning ends, the date/time of the RTC is corrected to a correct date/time based on the positioning result. 
     According to the above description, the ROM  58  which may include a non-volatile memory is provided as an example as the computer readable medium including the program  581  for the operation process such as the positioning control process regarding the process by the module controller  52 . Alternatively, other computer readable mediums may be employed, such as a HDD (Hard Disk Drive) or a portable recording medium such as a CD-ROM or DVD disk. As a medium providing data of the program of the present invention through communication wires, a carrier wave can be applied to the present invention. 
     The detailed description of the configuration, control contents and process shown in the present embodiment can be suitably changed without leaving the scope of the present invention. 
     The embodiments of the present invention are described above, but the scope of the present invention is not limited to the above-described embodiment. The scope of the present invention is limited to the invention as claimed and its equivalents. 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2016-180148 filed on Sep. 15, 2016 the entire contents of which are incorporated herein by reference.