Abstract:
A time adjustment device has a satellite signal reception unit that receives satellite signals transmitted segmented into segments from a positioning information satellite; a time information acquisition unit that acquires time information from the satellite signal; a segment identifier acquisition unit that acquires a segment identifier for the segment containing the time information; a corresponding identifier calculation unit that calculates a corresponding segment identifier that corresponds to the segment identifier based on the time information; and a segment identifier evaluation unit that determines if the segment identifier is correct based on the segment identifier and the corresponding segment identifier.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Japanese Patent application No. 2007-199090 is hereby incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a time adjustment device that corrects the time based on signals from a positioning information satellite such as a GPS satellite, to a timekeeping device that has the time adjustment device, and to a time adjustment method. 
         [0004]    2. Description of Related Art 
         [0005]    The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on a known orbit, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time) with extremely high precision. 
         [0006]    A GPS receiver that receives signals from GPS satellites must receive the TOW (Time Of Week) signal contained in the signals from a GPS satellite in order to get the time information transmitted by the GPS satellite. The TOW signal is the GPS time, and more specifically is the number of seconds from the beginning of each week. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-2001-59864. 
         [0007]    The received signals are transmitted from GPS satellites orbiting at an altitude of approximately 20,000 km, and the received data may therefore contain errors. A parity check is therefore applied to detect if there is an error in the data. 
         [0008]    A parity check involves segmenting the transmitted data stream into a specified number of blocks, appending the result (parity bit) of an operation on the bits of each block to the end of each block, and transmitting this parity bit with the data. The receiver determines there are no errors in the data if the appended parity bit matches the result of the same operation applied to the bits of the data block. 
         [0009]    If a different data stream with the same parity is received, however, the parity check may determine that the data stream is correct, and the time may therefore be adjusted based on the wrong time information. 
       SUMMARY OF INVENTION 
       [0010]    A time adjustment device, a timekeeping device with a time adjustment device, and a time adjustment method according to the present invention enable quickly and reliably determining if there are any errors in the time information received from a positioning information satellite. 
         [0011]    A time adjustment device according to a preferred aspect of the invention has a satellite signal reception unit that receives satellite signals transmitted segmented into segments from a positioning information satellite; a time information acquisition unit that acquires time information from the satellite signal; a segment identifier acquisition unit that acquires a segment identifier for the segment containing the time information; a corresponding identifier calculation unit that calculates a corresponding segment identifier that corresponds to the segment identifier based on the time information; and a segment identifier evaluation unit that determines if the segment identifier is correct based on the segment identifier and the corresponding segment identifier. 
         [0012]    This aspect of the invention has a time information acquisition unit that acquires time information from the satellite signal, a segment identifier acquisition unit that acquires a segment identifier for the segment containing the time information, a corresponding identifier calculation unit that calculates a corresponding segment identifier that corresponds to the segment identifier based on the time information, and a segment identifier evaluation unit that determines if the segment identifier is correct based on the segment identifier and the corresponding segment identifier. 
         [0013]    The content of the time information differs according to the segment in which it is contained. For example, the Z count, which is an example of time information, in subframe  1 , which is an example of a segment identifier, is the time of the beginning of the next segment, which is subframe  2  in this example. 
         [0014]    The time information is thus intrinsically related to the segment containing the time information. As a result, the time information acquisition unit acquires the time information (such as the Z count) and the segment identifier acquisition unit gets the segment identifier (such as the subframe  1  identifier). 
         [0015]    The corresponding segment identifier calculation unit can thus compute a corresponding segment identifier (such as the value “1” after the operation is completed) based on the Z count. The result of this operation is a reliable value because there is a constant, fixed relationship between the Z count and the subframe number ( 1 ). 
         [0016]    The segment identifier evaluation unit then compares this corresponding segment identifier (such as  1 ) with the acquired segment identifier (such as subframe  1 ) to quickly and reliably determine if the time information (such as the Z count) is correct. 
         [0017]    Because the numeric values “1” match in this example, the information is determined to be correct. If these values differ, however, there is an error in the received satellite signal, and the satellite signal is received again. 
         [0018]    Even in cases in which the parity check that is conventionally applied can overlook data errors, the method of the invention can reliably and unfailingly determine if the data is correct, and can therefore shorten the time synchronization time. 
         [0019]    Preferably, the satellite signal contains page information for identifying a plurality of identical segment identifiers identifying segments containing different information. The time adjustment device also has a page information acquisition unit that acquires the page information, a corresponding page information calculation unit that calculates corresponding page information corresponding to the page information based on the time information, and a page information evaluation unit that determines if the page information is correct based on the page information and the corresponding page information. 
         [0020]    In this aspect of the invention the satellite signal contains page information for identifying a plurality of identical segment identifiers identifying segments containing different information, and the time adjustment device also has a page information acquisition unit that acquires the page information, a corresponding page information calculation unit that calculates corresponding page information corresponding to the page information based on the time information, and a page information evaluation unit that determines if the page information is correct based on the page information and the corresponding page information. 
         [0021]    More particularly, the satellite signal contains a plurality of, such as 25, identical segment identifiers, such as subframe  5 . While the segment identifier, subframe  5  in this example, is the same, the content of the corresponding segments is different, and the 25 segment identifiers are therefore discriminated from each other using page information such as a page number. 
         [0022]    This page information, such as the page number, is closely related to the time information, such as the Z count, by means of the segment identifier, such as subframe  5 . For example, the time information of the Z count in subframe  5  of page  25 , the page information, denotes the time of the beginning of the next subframe, subframe  1 . 
         [0023]    The page information, such as page  25 , in the segment identifier, such as subframe  5 , thus has a constant regular relationship to the time information, such as the Z count. 
         [0024]    The page information acquisition unit thus acquires page information (such as “25”) that discriminates the segment identifier (such as subframe  5 ). The corresponding page calculation unit then computes the corresponding page information (such as “25”) for the page information based on the acquired time information (such as the Z count). 
         [0025]    The resulting value is reliable because the page information (such as “25”) in the segment identifier (such as subframe  5 ) has a constant ordered relationship to the time information (such as the Z count). 
         [0026]    The page information evaluation unit then determines if the actual page information is correct based on the actual page information acquired by the page information acquisition unit and the computed corresponding page information (such as “25”). 
         [0027]    If these values (“25” in this example) match, the information is correct. If the values do not match, there is an error in the received satellite signal and the satellite signal is received again. 
         [0028]    Even in cases in which the parity check that is conventionally applied can overlook data errors, the method of the invention can reliably and unfailingly determine if the data is correct, and can therefore shorten the time synchronization time. 
         [0029]    In another aspect of the invention the segments contain a plurality of subdivisions, and of these subdivisions the time information subdivision containing the time information contains subdivision identification information enabling discriminating the time information subdivision from other subdivisions; and the time adjustment device also has a time information subdivision signal reception determination unit that determines if the satellite signal reception unit received the satellite signal for the time information subdivision; and a subdivision identification information evaluation unit that determines if the subdivision identification information is present in the satellite signal for the time information subdivision. 
         [0030]    In this aspect of the invention the segments contain a plurality of subdivisions, and of these subdivisions the time information subdivision containing the time information contains subdivision identification information enabling discriminating the time information subdivision from other subdivisions; and the time adjustment device also has a time information subdivision signal reception determination unit that determines if the satellite signal reception unit received the satellite signal for the time information subdivision; and a subdivision identification information evaluation unit that determines if the subdivision identification information is present in the satellite signal for the time information subdivision. 
         [0031]    The segments (such as subframes) of the satellite signal contain a handover word (HOW) (ten words constitute one subframe) as an example of the time information subdivision, and if this time information subdivision (such as the HOW) is received, the time information (such as the Z count) is also received simultaneously. 
         [0032]    The time information subdivision (such as the HOW) contains subdivision identification information (such as a prescribed bit being 0) that enables discriminating the time information subdivision from other subdivisions (such as other words). 
         [0033]    As a result, if the time information subdivision signal reception determination unit determines that the satellite signal corresponding to the time information subdivision (such as the HOW) was received, the subdivision identification information evaluation unit determines if the subdivision identification information (such as the prescribed bit being set to 0) is in the time information subdivision (such as the HOW). If the subdivision identification information is present, the time information subdivision (such as the HOW) is determined to be correct. If the subdivision identification information is not present, the subdivision is determined to not be the time information subdivision (such as the HOW), and the satellite signal is received again. 
         [0034]    Even in cases in which the parity check that is conventionally applied can overlook data errors, the method of the invention can reliably and unfailingly detect the subdivision containing the time information, and can therefore shorten the time synchronization time. 
         [0035]    Another aspect of the invention is a timekeeping device with a time adjustment device having a satellite signal reception unit that receives satellite signals transmitted segmented into segments from a positioning information satellite; a time information acquisition unit that acquires time information from the satellite signal; a segment identifier acquisition unit that acquires a segment identifier for the segment containing the time information; a corresponding identifier calculation unit that calculates a corresponding segment identifier that corresponds to the segment identifier based on the time information; and a segment identifier evaluation unit that determines if the segment identifier is correct based on the segment identifier and the corresponding segment identifier. 
         [0036]    Another aspect of the invention is a time adjustment method wherein a satellite signal reception unit has a satellite signal reception process that receives satellite signals transmitted segmented into segments from a positioning information satellite; a time information acquisition unit has a time information acquisition process that acquires time information from the satellite signal; a segment identifier acquisition unit has a segment identifier acquisition process that acquires a segment identifier for the segment containing the time information; a corresponding identifier calculation unit has a corresponding identifier calculation process that calculates a corresponding segment identifier that corresponds to the segment identifier based on the time information; and a segment identifier evaluation unit has a segment identifier evaluation process that determines if the segment identifier is correct based on the segment identifier and the corresponding segment identifier. 
         [0037]    Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1  is a schematic diagram of a wristwatch with a GPS time adjustment device as an example of a timekeeping device with a time adjustment device according to the present invention. 
           [0039]      FIG. 2  is a block diagram of the main internal hardware arrangement of the wristwatch with GPS receiver. 
           [0040]      FIG. 3  is a block diagram showing the main software configuration of the wristwatch with GPS receiver. 
           [0041]      FIG. 4  is a block diagram showing the data stored in the program storage unit in  FIG. 3 . 
           [0042]      FIG. 5  is a block diagram showing the data stored in the data storage unit in  FIG. 3 . 
           [0043]      FIG. 6  is a flow chart of the operation of the wristwatch with GPS receiver according to the first embodiment of the invention. 
           [0044]      FIG. 7  is a flow chart of the operation of the wristwatch with GPS receiver according to the first embodiment of the invention. 
           [0045]      FIG. 8  schematically shows the structure of a GPS signal. 
           [0046]      FIG. 9  schematically shows the HOW word shown in  FIG. 8B . 
           [0047]      FIG. 10  is a block diagram of the main internal software configuration of the wristwatch with a GPS time adjustment device as an example of a timekeeping device with a time adjustment device according to a second embodiment of the invention. 
           [0048]      FIG. 11  is another block diagram of the main internal software configuration of the wristwatch with a GPS time adjustment device as an example of a timekeeping device with a time adjustment device according to a second embodiment of the invention. 
           [0049]      FIG. 12  is a flow chart of the operation of the wristwatch with a GPS time adjustment device according to a second embodiment of the invention. 
           [0050]      FIG. 13  is a schematic diagram showing five of the ten words in one subframe shown in  FIG. 8A . 
           [0051]      FIG. 14  is a table of the page ID/subframe SVID values in  FIG. 11 . 
           [0052]      FIG. 15  is a block diagram of the main internal software configuration of the wristwatch with a GPS time adjustment device as an example of a timekeeping device with a time adjustment device according to a third embodiment of the invention. 
           [0053]      FIG. 16  is a flow chart of the operation of the wristwatch with GPS time adjustment device according to a third embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0054]    Preferred embodiments of the present invention are described below with reference to the accompanying figures. Note that the following embodiments are preferred specific implementations of the invention and therefore describe some technically preferred limitations, but the scope of the invention is not limited thereto unless specifically stated as required by the invention. 
       Embodiment 1 
       [0055]      FIG. 1  shows a wristwatch with a GPS time adjustment device  10  (referred to herein as a GPS wristwatch  10 ) as an example of a timekeeping device with a time adjustment device according to the present invention.  FIG. 2  is a block diagram of the main internal hardware arrangement of the GPS wristwatch  10  shown in  FIG. 1 . 
         [0056]    As shown in  FIG. 1 , the GPS wristwatch  10  has a dial  12  with hands  13  including a long hand and a short hand on the face, and a display  27  such as an LED display for presenting information and messages. The display  27  is not limited to an LED device and could be an LCD or an analog display. 
         [0057]    As also shown in  FIG. 1  the GPS wristwatch  10  also has an antenna  11 . This antenna  11  is used for receiving signals from a GPS satellite  15  circling the Earth on a fixed orbit in space. The GPS satellite  15  is an example of a positioning information satellite that orbits the Earth. 
         [0058]    As shown in  FIG. 2 , the GPS wristwatch  10  has an internal time display device  21  and GPS receiver  20 , and components for functioning as a computer. 
         [0059]    More particularly, the time display device  21  in this embodiment of the invention functions as an electronic timepiece. The components of the GPS wristwatch  10  shown in  FIG. 2  are described below. 
         [0060]    As shown in  FIG. 2  the GPS wristwatch  10  has a bus  16 . Connected to this bus  16  are a CPU (central processing unit)  17 , RAM (random access memory)  18 , and ROM (read-only memory)  19 . 
         [0061]    The GPS receiver  20  for receiving satellite signals transmitted from the GPS satellites  15  is also connected to the bus  16 . The time display device  21  is also connected to the bus  16 . 
         [0062]    More specifically, the GPS receiver  20  includes the antenna  11 , an RF unit that converts the signals received by the antenna  11  to an intermediate frequency, and a baseband unit that demodulates the received signals. 
         [0063]    The GPS receiver  20  is more specifically a device for extracting a GPS signal from the signals received from a GPS satellite  15  in  FIG. 1  by means of the antenna  11 , an RF unit, and a baseband unit. The GPS receiver  20  is thus an example of a satellite signal reception unit. 
         [0064]    The GPS signal (an example of a satellite signal) contains highly precise GPS time information (Z count) that is based on an atomic clock. The GPS signal is described further below. 
         [0065]    The GPS receiver  20  is thus an example of a time information acquisition unit. 
         [0066]    The time display device  21  connected to the bus  16  has a real-time clock (RTC)  22 , which is an IC device (integrated circuit device) in this aspect of the invention, and a display  27 . A solar battery  23  is also connected to the bus  16  as the power supply. 
         [0067]    The bus  16  thus is an internal bus with addresses and data paths that function to connect all other devices. Various operating programs and information are stored in ROM  19 , which is also connected to the bus  16 . The CPU  17  uses RAM  18  to execute the programs and access ROM  19 . 
         [0068]      FIG. 3  is a block diagram showing the general software configuration of the GPS wristwatch  10 . 
         [0069]    As shown in  FIG. 3  the GPS wristwatch  10  has a control unit  28 . The control unit  28  runs the programs stored in the program storage unit  30 , and processes the data stored in the data storage unit  40 . 
         [0070]    The program storage unit  30  and data storage unit  40  are shown as discrete units in  FIG. 3 , but the data and programs are not actually stored separately and are simply shown this way for convenience. 
         [0071]      FIG. 4  is a block diagram showing the data stored in the program storage unit  30  in  FIG. 3 .  FIG. 5  is a block diagram showing data stored in the data storage unit  40  in  FIG. 3 . 
         [0072]      FIG. 6  and  FIG. 7  are flow charts describing the main steps in the operation of the GPS wristwatch  10  according to this embodiment of the invention. 
         [0073]    The operation of the GPS wristwatch  10  according to this embodiment of the invention is described next with reference to the flow charts in  FIG. 6  and  FIG. 7 . The programs and data shown in  FIG. 4  and  FIG. 5  are also described below in conjunction with the operation of the GPS wristwatch  10 . 
         [0074]    In order to adjust the time of the RTC  22  in  FIG. 1 , the GPS wristwatch  10  first starts the operation for receiving the satellite signal from a GPS satellite  15  in step ST 1  in  FIG. 6 . More specifically, the satellite signal reception program  31  in  FIG. 4  runs. Searching for a GPS satellite  15  to capture then begins in step ST 2 . This is an operation for capturing the signal transmitted by a GPS satellite  15  orbiting the Earth, and is executed by the GPS receiver  20  in  FIG. 1 , or more particularly by executing the GPS satellite search program  32  in  FIG. 4 . 
         [0075]    Control then goes to step ST 3 . Whether a search for all satellites has been completed is determined in step ST 3 . More specifically, whether the search for all of the GPS satellites  15  orbiting the Earth has been completed is determined by executing the terminate all-satellite search decision program  33  in  FIG. 4 . If searching for all GPS satellites  15  is finished, the GPS satellite  15  capture operation ends in order to not waste power. 
         [0076]    If searching for all GPS satellites  15  has not ended in step ST 3 , control goes to step ST 4 . Whether a GPS satellite  15  has been captured is determined in step ST 4  by executing the satellite capture detection program  34  (an example of a satellite signal reception step). 
         [0077]    If it is decided in step ST 4  that a GPS satellite  15  has not been captured, control returns to step ST 2 . If it is decided in step ST 4  that a GPS satellite  15  was captured, control goes to step ST 5  to determine if the reception time has timed out. This is to prevent the GPS receiver  20  in  FIG. 1  from continuing to operate for a long time when signal reception is not possible and thus prevent excessive power consumption. 
         [0078]    If reception has timed out in step ST 5 , the procedure loops to step ST 2 . If reception has not timed out in step ST 5 , control goes to step ST 6  to determine if the Z count has been acquired. The structure of the satellite signal transmitted from the GPS satellite  15  is described first below before describing step ST 6 . 
         [0079]      FIG. 8  schematically describes a GPS signal. 
         [0080]    As shown in  FIG. 8A , the GPS satellite  15  transmits signals in data frame units and transmits one frame every 30 seconds. Each frame consists of five subframes, and one subframe is transmitted every 6 seconds. Each subframe contains 10 words (1 word is transmitted every 0.6 second). 
         [0081]    The first word in each subframe is a telemetry (TLM) word storing the TLM data, and each TLM word starts with a preamble as shown in  FIG. 8B . 
         [0082]    The TLM word is followed by a handover word HOW storing the HOW (handover word) data, and each HOW starts with the time of week (TOW) indicating the GPS time information (Z count) of the GPS satellite  15 . 
         [0083]    The Z count stores the time of the beginning of the TLM in the next subframe. 
         [0084]    The GPS time is the number of seconds since 00:00:00 Sunday night of each week, and is reset to zero at precisely 00:00:00 every Sunday night. 
         [0085]    Referring again to step ST 6  in  FIG. 6 , whether the TOW data in the HOW of the satellite signal shown in  FIG. 8B  received from the GPS satellite  15  was received is determined in step ST 6  (an example of a time information acquisition step). 
         [0086]    If the TOW data was received, the time data (Z count) acquired by processing the TOW data renders highly precise GPS time information. 
         [0087]    More specifically, whether the Z count was acquired is determined by running the Z count acquirability decision program  35  in  FIG. 4 . The acquired Z count, or more particularly the time at the beginning of the preamble of the TLM word in the next subframe, is stored as the Z count  41  in  FIG. 5  by the Z count storage program  36  in  FIG. 4 . This Z count is a value such as 00:10:00. 
         [0088]    If this time (Z count) is the time derived by operating on subframe  1 , this time (Z count) is the time at the rise of the preamble of the TLM word in the next subframe, subframe  2 . 
         [0089]    However, the satellite signal from the GPS satellite  15  received by the GPS receiver  20  in  FIG. 1  may contain errors as described above. If there are errors, the acquired data will be wrong and it will therefore not be possible to accurately correct the time of the RTC  22  in  FIG. 1 . 
         [0090]    This embodiment of the invention therefore executes the steps starting from step ST 7  when the Z count data is acquired in step ST 6 . However, if step ST 6  determines that the Z count was not acquired, control returns to step ST 4 . 
         [0091]    The subframe ID is acquired in step ST 7  (an example of a subframe identifier acquisition step).  FIG. 9  schematically describes the handover word HOW in  FIG. 8B . 
         [0092]    As shown in  FIG. 9  the HOW is 30 bits long, and the TOW data is carried in bits  1  to  17 . Bits  20  to  22  contain the subframe ID, which is the number of the subframe carrying the HOW. 
         [0093]    More specifically, if the HOW in  FIG. 9  is the HOW in subframe  1 , a value of 1 is sent as the subframe ID. 
         [0094]    Step ST 7  in  FIG. 6  therefore acquires the subframe ID from bit  20  to bit  22  following the TOW in  FIG. 9 , and stores the result as the subframe ID  42  in  FIG. 5 . 
         [0095]    More specifically, the subframe ID acquisition and storage program  37  in  FIG. 4  runs to save the subframe ID, such as 1 in this example, as the subframe ID  42 . 
         [0096]    This subframe ID acquisition and storage program  37  is an example of a segment identifier acquisition unit, the subframe is an example of a signal segment, and the subframe ID number is an example of a segment identifier. 
         [0097]    Control then goes to step ST 8  to calculate the subframe ID based on the Z count (an example of a corresponding identifier calculation step). That is, the time referred to as the Z count in the TOW data in  FIG. 9  is the time of the beginning of the preamble at the start of the next subframe, and this time information is therefore intrinsically related to the subframes. 
         [0098]    More particularly, the Z count is acquired by multiplying the TOW data by four. The subframe ID is synchronized to the Z count, and starts from subframe  1  at the beginning of each week. The following equation can therefore be applied to the TOW data to predict the ID of the subframe containing the TOW data, that is, the subframe number. 
         [0000]      subframe ID=(( TOW+ 4)mod5)+1 
         [0099]    where mod is the remainder of dividing X by Y if XmodY. 
         [0100]    More specifically, the subframe ID calculation program  38  runs in step ST 8  to compute the above equation. That is, the TOW data that is the basis of the Z count is obtained from the Z count  41  in  FIG. 5 , and the TOW data is substituted in the above equation to get the subframe ID. 
         [0101]    The subframe ID thus computed is then saved as the calculated subframe ID  43  in  FIG. 5 , and in this example is 1. 
         [0102]    The subframe ID calculation program  38  is an example of a corresponding identifier calculation unit, and the calculated subframe ID  43  is an example of a corresponding segment identifier. 
         [0103]    Control then goes to step ST 9 . In step ST 9  the computed subframe ID is compared with the acquired subframe ID to detect a match (this step is an example of a segment identifier evaluation step). 
         [0104]    That is, the subframe ID comparison program  39  in  FIG. 4  operates to compare the subframe ID  42  with the calculated subframe ID  43  in  FIG. 5  and determine if they match. 
         [0105]    If they are the same, the received satellite signal is not the wrong signal, the TOW data is from the corresponding subframe, and the Z count (time information) acquired from the TOW data is highly reliable. 
         [0106]    If the values do not match, there is an error in the data and the satellite signal must be received again. 
         [0107]    The subframe ID comparison program  39  is thus an example of a segment identifier evaluation unit. 
         [0108]    If step ST 9  does not detect a match, control returns to step ST 4 . Control goes to step ST 10  if a match is confirmed. 
         [0109]    Step ST 10  detects if the parity check was successful. This parity check is done using bit  29  and bit  30  in the previous word, and bit  25  and bit  28  in the current word as shown in  FIG. 9 . The result of the parity check determines if there is an error in the data. More specifically, the parity check is done by the parity check program  131  in  FIG. 4 . 
         [0110]    Whether there are any errors in the satellite signal data received from the GPS satellite  15  is confirmed by a parity check in this embodiment of the invention. However, because the parity check determines if an operation on certain bits in a specified data stream matches a certain value, there are cases in which the parity check could pass even though there are errors, and the Z count could be calculated and the time could be adjusted as a result of this parity check. The time cannot be adjusted with precision if this happens. 
         [0111]    To prevent this, this embodiment of the invention does not rely only on the parity check, but also predicts the subframe ID by an operation applied to the received Z count, and determines if the predicted calculated subframe ID  43  matches the actually received subframe ID  42 . Whether the received data is correct can thus be quickly and reliably determined. 
         [0112]    If the parity check passes in step ST 10 , satellite signal reception ends in step ST 11 , and the time of the RTC  22  in  FIG. 1  is adjusted in step ST 12 . More specifically, the time adjustment program  132  in  FIG. 4  is executed. 
         [0113]    If the parity check fails in step ST 10 , control returns to step ST 4 . 
       Embodiment 2 
       [0114]      FIG. 10  and  FIG. 11  are block diagrams showing the main internal software configuration of a wristwatch with a GPS time adjustment device  100  described below as an example of a timekeeping device with a time adjustment device according to this second embodiment of the invention.  FIG. 12  is a flow chart describing the main steps in the operation of the wristwatch with a GPS time adjustment device  100  according to this embodiment of the invention. 
         [0115]    More specifically,  FIG. 10  shows the data stored in the program storage unit  300  in this embodiment of the invention, and  FIG. 11  shows the data stored in the data storage unit  400  in this embodiment of the invention. 
         [0116]    The configuration of the wristwatch with GPS time adjustment device  100  according to this embodiment of the invention has many parts in common with the GPS wristwatch  10  described in the first embodiment, like parts are therefore identified by the same reference numerals, and the differences therebetween are described below. 
         [0117]    The operation of this embodiment of the invention is described next referring to the flow chart in  FIG. 12 . This embodiment executes steps ST 1  to ST 9  as shown in  FIG. 6  and  FIG. 7  and described in the first embodiment above. 
         [0118]    Steps ST 10  to ST 12  of the first embodiment are also executed in the same way. This embodiment thus differs from the first embodiment in inserting steps as shown in  FIG. 12  between step ST 9  and step ST 10  in the first embodiment. 
         [0119]    Therefore, before the parity check is executed in step ST 10  in  FIG. 12 , whether the number of the received subframe is 5 is first determined in step ST 21 . 
         [0120]    More specifically, the subframe ID validation program  331  in  FIG. 10  runs and references the subframe ID  42  in  FIG. 11 . 
         [0121]    Subframe  5  is described next. The satellite signal from a GPS satellite  15  contains five subframes as shown in  FIG. 8 . Of these subframes, subframe  5  contains almanac data containing orbital information for all of the GPS satellites. The almanac is large, cannot be contained in one subframe, and is therefore distributed between 25 subframes  5 . 
         [0122]    The satellite signal thus contains 25 subframes identified as subframe  5 , and each subframe  5  can be uniquely identified by a page number, such as subframe  5  of page  3 . 
         [0123]    Therefore, if the subframe ID  42  acquired in step ST 7  is 5, that is, subframe  5  is detected, which page this subframe  5  was acquired from must be determined. 
         [0124]    This embodiment of the invention therefore detects in step ST 21  in  FIG. 12  whether subframe  5  was received, and control goes to step ST 22  if subframe  5  is detected. If subframe  5  is not detected, control goes to step ST 10 . 
         [0125]    In step ST 22  the SVID of the received subframe  5  is acquired, and the page ID of this subframe  5  is acquired based on this subframe SVID. 
         [0126]    More specifically, the subframe SVID acquisition program  332  in  FIG. 10  operates as described further below. Note that the SVID is an example of page information. 
         [0127]    Each subframe contains ten words as shown in  FIG. 8A , and  FIG. 13  describes the five words in subframe  5 . 
         [0128]    As shown in  FIG. 13  the third word in the subframe contains data, called the SVID, corresponding to the page ID of the subframe. 
         [0129]      FIG. 14  is a table showing an example of the page ID/subframe SVID correlation data  441  in  FIG. 11 . 
         [0130]    More specifically, the page ID corresponding to the SVID shown in  FIG. 13 , that is, the page number data, is stored in the table shown in  FIG. 14 . 
         [0131]    After the subframe SVID acquisition program  332  in  FIG. 10  runs and gets the SVID value in word  3  in  FIG. 13 , the page ID acquisition program  333  in  FIG. 10  runs to get the page ID data  442  based on this SVID data and the page ID/subframe SVID correlation data  441  in  FIG. 14 . 
         [0132]    The acquired page ID is then saved as the page ID data  442  in  FIG. 11 . In this example the page ID is page  10 . 
         [0133]    The subframe SVID acquisition program  332  is an example of a page information acquisition unit. 
         [0134]    While it is good if the page ID data  442  is correct, the satellite signal transmitted from the GPS satellite  15  may contain errors. If these errors are not addressed, a correct Z count cannot be acquired and the time cannot be adjusted with good precision. 
         [0135]    The page ID is therefore calculated from the Z count data in  FIG. 11  in step ST 23 . 
         [0136]    The page ID (see  FIG. 14 ) and the Z count or other time information are closely related. For example, the time information of the Z count in subframe  5  of page  25  denotes the time of the beginning of the next subframe, subframe  1 , and the page information, such as page  25  for this subframe  5 , and time information, such as the Z count, have a fixed regular order. 
         [0137]    More specifically, the page IDs in  FIG. 14  are synchronized to the Z count of each subframe, and the page ID for the first page of each week is 1, that is, the signal restarts from page  1  every week. 
         [0138]    The page ID of subframe  5  from which the TOW data was detected, that is, the page number, can therefore be predicted based on the TOW data using the following equation. 
         [0139]    If the TOW equals 0, the page ID=1. 
         [0140]    If the TOW does not equal 0, [(page ID)=((TOW−1)mod25]+1 
         [0141]    where mod is the remainder of dividing X by Y for XmodY. 
         [0142]    The page ID calculation program  334  in  FIG. 10  applies the above equation to compute the page ID based on the acquired TOW, and stores the result as the calculated page ID data  443  in  FIG. 11 . 
         [0143]    The page ID calculation program  334  is an example of a corresponding page information operating unit. 
         [0144]    Whether the calculated page ID and the acquired ID are the same is then determined in step ST 24 . More specifically, the page ID comparison program  335  in  FIG. 10  runs to compare the page ID data  442  and the calculated page ID data  443  in  FIG. 11 . 
         [0145]    If the values match, the data of the actually received page ID is correct, and the time is therefore adjusted based on the data in step ST 12 . 
         [0146]    If the values do not match, there is a data error, control returns to step ST 4 , and the satellite signal is received again. 
         [0147]    The page ID comparison program  335  is thus an example of a page information evaluation unit. 
         [0148]    This embodiment of the invention can thus reliably determine the correctness of the page ID in subframes that differ according to the page number. 
         [0149]    This embodiment of the invention then also applies a parity check (step ST 10 ) to determine if there are any errors in the data. 
         [0150]    That is, this embodiment of the invention can quickly detect an error without performing a parity check. 
       Embodiment 3 
       [0151]      FIG. 15  is a block diagram showing the main internal software configuration of a wristwatch with a GPS time adjustment device  200  described below as an example of a timekeeping device with a time adjustment device according to this third embodiment of the invention.  FIG. 16  is a flow chart describing the main steps in the operation of the wristwatch with a GPS time adjustment device  200  according to this embodiment of the invention. 
         [0152]    More specifically,  FIG. 15  shows the data stored in the program storage unit  500  in this embodiment of the invention. 
         [0153]    The configuration of the wristwatch with GPS time adjustment device  200  according to this embodiment of the invention has many parts in common with the GPS wristwatch  10  described in the first embodiment, like parts are therefore identified by the same reference numerals, and the differences therebetween are described below. 
         [0154]    The operation of this embodiment of the invention is described next referring to the flow chart in  FIG. 16 . This embodiment executes steps ST 1  to ST 5  as shown in  FIG. 6  and  FIG. 7  and described in the first embodiment above. 
         [0155]    Steps ST 6  to ST 12  of the first embodiment are also executed in the same way. This embodiment thus differs from the first embodiment in inserting steps as shown in  FIG. 16  between step ST 5  and step ST 6  in the first embodiment. 
         [0156]    More specifically, if a satellite signal is captured in step ST 4  in  FIG. 16  and reception does not time out in step ST 5 , whether the HOW was detected is determined in step ST 31 . If the HOW was detected, whether bit  29  and bit  30  in the HOW are set to 0 is determined in step ST 32 . 
         [0157]    This word is an example of a subdivision, and the HOW is an example of a time information subdivision. That bit  29  and bit  30  equal 0 is an example of subdivision identification information. 
         [0158]    The meaning of adding these steps is described next. As shown in  FIG. 9  and described in the first embodiment, the time data (Z count) is acquired based on the TOW data contained in the HOW. Therefore, if a part of the received satellite signal other than the HOW is mistakenly recognized as the HOW word, and bits  1  to  17  are read as the TOW data to get the Z count (time data), the time cannot be correctly adjusted with precision. 
         [0159]    In this embodiment of the invention, therefore, the wristwatch with GPS time adjustment device  200  determines in step ST 32  if data believed to be the HOW data was acquired. If data believed to be the HOW data was acquired, step ST 32  determines if the HOW data is really a correct HOW word. 
         [0160]    If the HOW is correct or not is determined as follows. As shown in  FIG. 9  the HOW contains information that is not found in the other words, and this information is that bit  29  and bit  30  are always 0. 
         [0161]    This embodiment of the invention uses this property of the HOW word to determine if the received data believed to be a HOW is actually a correct handover word. 
         [0162]    This is further described with reference to  FIG. 16 . The HOW detection program  531  in  FIG. 15  runs first in step ST 31 , and the wristwatch with GPS time adjustment device  200  determines if data believed to be the HOW of the satellite signal from the GPS satellite  15  was detected. If the data was not detected, control returns to step ST 4 . If the data was detected, control goes to step ST 32 . 
         [0163]    The HOW bits  29  and  30  confirmation program  532  in  FIG. 15  then runs in step ST 32 . More specifically, this program  532  determines if bit  29  and bit  30  in the received data believed to be a HOW are both set to 0. If bit  29  and bit  30  are 0, control goes to step ST 6 . If either bit  29  or bit  30  is not 0, control returns to step ST 4 . 
         [0164]    The HOW detection program  531  is thus an example of a HOW detection program  531 , and the HOW bits  29  and  30  confirmation program  532  is an example of a subdivision identification information evaluation unit. 
         [0165]    Because this embodiment of the invention can verify if the received data is the handover word containing the TOW time data, erroneously recognizing other data as the HOW and acquiring erroneous time data based thereon can be quickly prevented. 
         [0166]    The invention is not limited to the embodiments described above. The foregoing embodiments are described using GPS satellites that orbit the Earth as an example of a positioning information satellite. However, the positioning information satellite of the invention is not so limited, and includes geostationary satellites and quasi-zenith satellites, for example. 
         [0167]    The invention being thus described, it will be obvious that it may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.