Abstract:
An apparatus, method and computer program storage device cooperate to provide a quality position estimation of a mobile communication device by determining a reference area. The reference area includes the position of the mobile communication device as well as a plurality of transmitters. Transmitters that are detected as being outside of the reference area are excluded from contributing to the position estimate. The reference area is centered at a standard position, which may be a previous position or a GPS based position, for example. The range of the reference area includes a component that considers the range of a transmitter, as well as a GPS error or an estimated move distance of the mobile communication device.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to a position estimating apparatus, a position estimating method, and a computer program product. 
       BACKGROUND ART 
       [0002]    Recently, receiver apparatuses capable of receiving wireless signals transmitted from satellites have been installed in moving bodies such as vehicles, mobile telephones, and the like. By using GPS (Global Positioning System) positioning, it is possible to estimate the position of the moving body in which the receiver apparatus is installed. Such position estimating technology that uses a receiver apparatus is an important base technology in a wide range of fields such as navigation, security and entertainment. However, position estimating technology based on GPS positioning requires a long time at startup for supplementary synchronization and is difficult to use inside buildings and underground that are out of range for the wireless signals from satellites. 
         [0003]    Also, as disclosed in Japanese Laid-Open Patent Publication No. 2008-104029 for example, a method of estimating the position of a wireless terminal based on the signal strength at the wireless terminal of wireless signals transmitted from base stations on a wireless LAN (Local Area Network) has been proposed. More specifically, it is possible to estimate the position of the wireless terminal based on position information of respective base stations registered in advance and the distances between the base stations and the wireless terminal calculated from the respective signal strengths of the wireless signals. Since base stations on a wireless LAN are also set up inside buildings and underground, by using this method of estimating, it is possible to carry out position estimation inside buildings and underground which has been difficult for position estimating technology based on GPS positioning. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL 1: JP 2008-104029A 
       
     
       SUMMARY 
     Technical Problem 
       [0005]    However, a case can be imagined where, due to movement of the base station or the like, the registered base station position information is erroneous. If position estimation is carried out based on such erroneous base station position information, the precision of the position estimation will fall. Such concern has significantly increased in recent years due to the spread of mobile base stations that are carried by the user. 
         [0006]    The present disclosure aims to provide a novel and improved position estimating apparatus, position estimating method, and computer program product that are capable of suppressing a fall in the precision of position estimation. 
       Solution to Problem 
       [0007]    According to one embodiment, a position estimation apparatus includes 
         [0008]    a reference area determining unit determines a reference area that includes a position of a mobile communications station, and 
         [0009]    a position estimation unit that estimates the position of the mobile communication station based on position information of a plurality of transmitters within the reference area. 
         [0010]    One aspect of the apparatus is that the reference area determining unit determines the reference area by combining an estimated move distance of the apparatus with a predetermined communication range of at least one of the plurality of transmitters. 
         [0011]    Another aspect is that the apparatus includes a movement detector that estimates the estimated move distance of the apparatus. 
         [0012]    Another aspect is that the movement detector being an accelerometer. 
         [0013]    According to a different aspect, the apparatus includes 
         [0014]    a GPS-based location device that detects location information based on wireless satellite transmissions, wherein 
         [0015]    the reference area determining unit determines the reference area by combining a GPS uncertainty amount with a predetermined communication range of at least one of the plurality of transmitters. 
         [0016]    According to another aspect, the device includes 
         [0017]    a computer readable storage device that stores a previous position estimation result for the apparatus in association with time, wherein 
         [0018]    and the reference area determining unit determines the reference area by using the previous position estimation result as a standard. 
         [0019]    According to another aspect, the reference area determining unit determines the reference area by calculating the estimated move distance. 
         [0020]    According to another aspect the position estimation unit estimates the move distance by measuring a propagation distance from the plurality of transmitters. 
         [0021]    According to another aspect the position estimation unit excludes from consideration transmitters outside of the reference area based on signal information from respective of the plurality of transmitters. 
         [0022]    According to another aspect the position estimation unit estimates the position of the mobile communication station by calculation of 
         [0000]    
       
         
           
             
               
                 
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         [0000]    wherein 
         [0023]    O being a position, A i  being position information of an ith transmitter, and W i  being a weighting coefficient. 
         [0024]    According to another aspect the apparatus also includes a computer readable storage unit that stores transmitter reliability information regarding whether a particular transmitter has been detected as having a low reliability with at least a predetermined frequency of occurrence, and 
         [0025]    the position estimation unit excludes transmitters having a stored reliability below a predetermined threshold when estimating the position of the mobile communication station. 
         [0026]    According to another aspect, the position estimation unit excludes transmitters having a received signal characteristic indicating the transmitter is outside of the reference area. 
         [0027]    The apparatus may also include a communications interface that receives transmitter signal information and reference area information from the mobile communication station, and sends an estimate of the position of the mobile communication station to the mobile communication station. 
         [0028]    Optionally, the apparatus includes the mobile communication station. 
         [0029]    In a process based embodiment, the method includes 
         [0030]    determining with a processing device a reference area including a position of a mobile communications station; and 
         [0031]    estimating the position of the mobile communication station based on position information of a plurality of transmitters within the reference area. 
         [0032]    According to a computer readable storage device embodiment, the device has computer readable instructions that when executed by a processing device perform a position estimation method comprising: 
         [0033]    determining with the processing device a reference area including a position of a mobile communications station; and 
         [0034]    estimating the position of the mobile communication station based on position information of a plurality of transmitters within the reference area. 
         [0035]    In a mobile communications device embodiment, the device includes 
         [0036]    a receiver that receives transmissions from a plurality of transmitters; 
         [0037]    a reference area determining unit that determines a reference area that includes therein a position of the mobile communications station; and 
         [0038]    a communication interface that sends 
         [0039]    signal information from the transmissions of the plurality of transmitters, and 
         [0040]    the reference area to a remote device, 
         [0041]    and receives from the remote device an estimated position of the mobile communication station after the remote device used the signal information and reference area to estimate the position of the mobile communication station by using transmissions from a subset of the plurality of transmitters located within the reference area and excluding transmissions from a subset of the plurality of transmitters located outside of the reference area. 
         [0042]    In an apparatus-centric embodiment, that communications with a remote device, optionally available as a cloud resource, the method includes 
         [0043]    receiving transmissions from a plurality of transmitters; 
         [0044]    determining with a processing device a reference area that includes therein a position of the mobile communications station; 
         [0045]    sending signal information from the transmissions of the plurality of transmitters to a remote device; 
         [0046]    sending the reference area to the remote device; and 
         [0047]    receiving from the remote device an estimated position of the mobile communication station after the remote device used the signal information and reference area to estimate the position of the mobile communication station by using transmissions from a subset of the plurality of transmitters located within the reference area and excluding transmissions from a subset of the plurality of transmitters located outside of the reference area. 
         [0048]    In an apparatus-centric embodiment, that communications with a remote device, optionally available as a cloud resource, instructions are stored on a computer readable medium that when executed by a processing device perform a position estimation method comprising: 
         [0000]    receiving transmissions from a plurality of transmitters;
 
determining with a processing device a reference area that includes therein a position of the mobile communications station;
 
sending signal information from the transmissions of the plurality of transmitters to a remote device;
 
sending the reference area to the remote device; and
 
receiving from the remote device an estimated position of the mobile communication station after the remote device used the signal information and reference area to estimate the position of the mobile communication station by using transmissions from a subset of the plurality of transmitters located within the reference area and excluding transmission from a subset of the plurality of transmitters located outside of the reference area.
 
       Advantageous Effects of Invention 
       [0049]    According to embodiments of the present disclosure described above, it is possible to suppress a fall in the precision of position estimation. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0050]      FIG. 1  is a diagram useful in explaining the configuration of a position estimating system according to embodiments of the present disclosure. 
           [0051]      FIG. 2  is a diagram useful in explaining specific examples of position information of base stations stored by a wireless terminal. 
           [0052]      FIG. 3  is a diagram useful in explaining specific examples of measurement information. 
           [0053]      FIG. 4  is a diagram useful in showing examples of estimation results for different estimation methods. 
           [0054]      FIG. 5  is a functional block diagram showing the configuration of a wireless terminal according to a first embodiment of the present disclosure. 
           [0055]      FIG. 6  is a diagram useful in explaining specific examples of position estimation results stored by an estimation result storage unit. 
           [0056]      FIG. 7  is a diagram useful in explaining a specific example of a reference area. 
           [0057]      FIG. 8  is a flowchart showing the operation of the wireless terminal according to the first embodiment. 
           [0058]      FIG. 9  is a functional block diagram showing the configuration of a wireless terminal according to a second embodiment of the present disclosure. 
           [0059]      FIG. 10  is a diagram useful in explaining a specific example of a reference area. 
           [0060]      FIG. 11  is a flowchart showing the operation of the wireless terminal according to the second embodiment. 
           [0061]      FIG. 12  is a functional block diagram showing the configuration of a wireless terminal according to a third embodiment of the present disclosure. 
           [0062]      FIG. 13  is a diagram useful in showing specific examples of low reliability base station information stored by a low reliability base station information storage unit. 
           [0063]      FIG. 14  is a diagram useful in showing specific examples of mobile base station information stored by a mobile base station information storage unit. 
           [0064]      FIG. 15  is a flowchart showing an example operation of the wireless terminal according to the third embodiment. 
           [0065]      FIG. 16  is a flowchart showing an example operation of the wireless terminal according to the third embodiment. 
           [0066]      FIG. 17  is a diagram useful in explaining the configuration of a position estimating system according to a fourth embodiment. 
           [0067]      FIG. 18  is a functional block diagram showing the configuration of a position estimating apparatus according to the fourth embodiment. 
           [0068]      FIG. 19  is a sequence chart showing the operation of the position estimating system according to the fourth embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0069]    Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the appended drawings. Note that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. 
         [0070]    Also, in the present specification and drawings, in some cases a plurality of structural elements that have effectively the same functional configuration are distinguished from one another by appending different letters to the same reference numeral. For example, a plurality of structural elements with effectively the same functional configuration are distinguished as necessary as the “base stations  30 A,  30 B, and  30 C”. However, when it is not especially necessary to distinguish between the plurality of structural elements with effectively the same functional configuration, the same reference numeral is used. For example, the base stations  30 A,  30 B, and  30 C are referred to simply as the “base station  30 ” when it is not especially necessary to distinguish between them. 
         [0071]    Embodiments of the present disclosure will now be described in the order indicated below. 
         [0072]    1. Fundamental Configuration of Position Estimating System 
         [0073]    2. First Embodiment 
         [0074]    2-1. Configuration of Wireless Terminal According to First Embodiment 
         [0075]    2-2. Operation of Wireless Terminal According to First Embodiment 
         [0076]    3. Second Embodiment 
         [0077]    3-1. Configuration of Wireless Terminal According to Second Embodiment 
         [0078]    3-2. Operation of Wireless Terminal According to Second Embodiment 
         [0079]    4. Third Embodiment 
         [0080]    4-1. Configuration of Wireless Terminal According to Third Embodiment 
         [0081]    4-2. Operation of Wireless Terminal According to Third Embodiment 
         [0082]    5. Fourth Embodiment 
         [0083]    5-1. Configuration of Position Estimating System According to Fourth Embodiment 
         [0084]    5-2. Configuration of Position Estimating Apparatus According to Fourth Embodiment 
         [0085]    5-3. Operation of Position Estimating System According to Fourth Embodiment 
         [0086]    6. Conclusion 
       1. FUNDAMENTAL CONFIGURATION OF POSITION ESTIMATING SYSTEM 
       [0087]    As described in detail below in the “2. First Embodiment” to “5. Fourth Embodiment” sections for example, the present disclosure has a variety of possible implementations. The “position estimating apparatus  20 ” or “wireless terminal  40 ” according to such embodiments includes: 
         [0088]    (1) a base station information storage unit ( 216 ,  416 ) that stores position information for base stations; and 
         [0089]    (2) a position estimating unit ( 236 ,  436 ) that estimates measurement information of wireless signals based on the position information of base stations whose position information stored in the base station information storage unit is in a limited range and measurement information for the signal strengths of wireless signals transmitted from such base stations. 
         [0090]    First, the fundamental configuration that is common to the respective embodiments will be described with reference to  FIGS. 1 to 3 . 
         [0091]      FIG. 1  is a diagram useful in explaining the configuration of a position estimating system  1  according to the embodiments of the present disclosure. As shown in  FIG. 1 , the position estimating system  1  according to the embodiments of the present disclosure includes a plurality of base stations  30  and a wireless terminal  40 . 
         [0092]    Each base station  30  controls communication between communication apparatuses that are spatially distributed. As examples, the base stations  30  are capable of controlling wireless communication between the wireless terminal  40  and another wireless terminal (not shown) that are both located in the respective signal ranges of the base stations  30  and/or controlling communication between the wireless terminal  40  and a communication apparatus that is connected by wires to a base station  30 . More specifically, the base stations  30  may be base stations on a wireless LAN (Local Area Network) based on WiFi (Wireless Fidelity) Standard, LTE (Long Term Evolution) base stations, GSM (Global System for Mobile Communications) base stations, or BLUETOOTH (registered trademark) base stations. 
         [0093]    In addition, the base stations  30  form a wireless network by regularly transmitting a beacon signal, for example. Here, the expression “beacon signal” includes a beacon signal including a base station ID identifying each base station  30 . This means that it is possible for the wireless terminal  40  to specify the base station  30  that transmitted a beacon signal that has been received from the base station ID included in the beacon signal. 
         [0094]    The wireless terminal  40  is capable of wirelessly transmitting and receiving various data in accordance with control by the base stations  30 . For example, the wireless terminal  40  is capable of receiving content data from a content distribution server (not shown) and/or transmitting and receiving electronic mail to or from another wireless terminal via the base stations  30 . Note that various data, for example, audio data (such as music, a performance, or a radio program), image data (such as a movie, a television program, a video program, photographs, artwork, or drawings), games, and software, can be given as examples of the content data. 
         [0095]    As examples, the wireless terminal  40  may be an information processing apparatus such as a PC (personal computer), a home video processing apparatus (a DVD recorder, video deck, or the like), a mobile telephone, a PHS (Personal Handyphone System), a mobile music player, a mobile video processing apparatus, a PDA (Personal Digital Assistant), a home game console, a mobile game console, or a home appliance. 
         [0096]    When a wireless signal (for example, a beacon signal) transmitted from a base station  30  is received, the wireless terminal  40  is also capable of measuring the signal strength of such wireless signal. The wireless terminal  40  also stores position information for the respective base stations  30  and is capable of estimating the position of the wireless terminal  40  based on the position information of the respective base stations  30  and the measurement information for the wireless signals. Specific examples of the position information of the respective base stations  30  stored by the wireless terminal  40  and the measurement information will now be described. 
         [0097]      FIG. 2  is a diagram useful in explaining specific examples of the position information of the base stations  30  stored by the wireless terminal  40 . As shown in  FIG. 2 , the wireless terminal  40  stores base station information of a plurality of base stations  30  made up of base station IDs and position information. Note that in the present specification, for ease of explanation, it is assumed that the code assigned to each base station and the base station ID are the same. 
         [0098]    Although the position information is shown in simplified form as “position information A” and “position information B” in  FIG. 2 , the position information may be expressed by a format using latitude and longitude, a format using x and y coordinates, a format using absolute coordinates, a format using vectors, or the like. 
         [0099]      FIG. 3  is a diagram useful in showing specific examples of the measurement information. As shown in  FIG. 3 , the measurement information includes signal strength information for each base station  30 . For example, in the example shown in  FIG. 3 , the signal strength at the wireless terminal  40  of a wireless signal transmitted from the base station  30 A is “−90 dBm”. 
         [0100]    The wireless terminal  40  according to the embodiments of the present disclosure estimates the position information of the wireless terminal  40  based on the measurement information described earlier and the position information for each base station  30 . For example, the wireless terminal  40  estimates a position O of the wireless terminal  40  in accordance with the mathematical formulas shown below. 
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         [0101]    Note that “Ai” in Formula 1 shows the position information of the ith base station registered in the wireless terminal  40 . Accordingly, when the position information of a base station is expressed by longitude and latitude, the wireless terminal  40  applies Formula 1 to both the longitude and the latitude. Also, as shown in Formula 2, “Wi” is a weighting coefficient obtained based on distS (O,Ai) showing the distance between the wireless terminal  40  and the ith base station estimated from the signal strength. As shown in Formula 3, W is the sum of the weighting coefficients Wi. 
         [0102]    Note that the method of estimating the position of the wireless terminal  40  is not limited to a method using Formula 1 given above and the wireless terminal  40  may for example use the position of the base station  30  that transmitted the signal with the highest signal strength received by the wireless terminal  40  as an estimate of the position of the wireless terminal  40 . Alternatively, the wireless terminal  40  may use a position at the center of the base stations  30  that have transmitted signals received with a signal strength of a specified value or higher by the wireless terminal  40  as an estimate of the position of the wireless terminal  40 . It is also possible to realize highly robust estimation of the position by applying a least-squares method. 
         [0103]    However, a case can be imagined where, due to movement of a base station  30  or the like, the position information registered for the base station  30  is erroneous. If position estimation is carried out based on such erroneous position information, the precision of the position estimation will fall. This will now be described in detail with reference to  FIG. 4 . 
         [0104]      FIG. 4  is a diagram useful in showing examples of estimation results for different estimation methods. In  FIG. 4 , a situation is imagined where “position information A” to “position information C” and “position information X” showing positions A to C and position X have been registered as the position information of the base station  30 A to the base station  30 C and the base station  30 X. In this situation, a case is considered where a wireless terminal  40  located at position U receives wireless signals from the base station  30 A to the base station  30 C and the base station  30 X and carries out position estimation of the wireless terminal  40  based on the signal strength information of the respective wireless signals. 
         [0105]    Note that the signal strength of the wireless signal transmitted by a base station  30  falls in accordance with a specified rule as the distance from the base station  30  increases. That is, the signal strength of the wireless signal at the wireless terminal  40  can be converted into the distance between the base station  30  that transmitted such wireless signal and the wireless terminal  40 . For this reason, in  FIG. 4 , converted distance values of the signal strengths are shown inside parentheses. Also, the base station  30 X is assumed to not be present at the position X in reality and to have moved to the position X′. 
         [0106]    Here, when the wireless terminal  40  provisionally uses the center of gravity of the registered positions of the base stations  30 A to  30 C and the base station  30 X as an estimate of the position of the wireless terminal  40 , the wireless terminal  40  will use the position S as the estimate of the position of the wireless terminal  40  in spite of the wireless terminal  40  being actually located at the position U. Also, by using a robust median method that is less affected by errors, the wireless terminal  40  is capable of obtaining a position T, for example that is closer to the actual position U of the wireless terminal  40  than the position S. 
         [0107]    However, if position estimation for the wireless terminal  40  is carried out using the position information of the base station  30 X, it is expected that the position estimation result will be distant from the actual position regardless of the estimation method used. Here, although it would be conceivably possible to carry out position estimation without using the position information of the base station  30 X that is distant from the base station  30 A to the base station  30 C, in reality, a case where the position information of the base station  30 X is correct and the position information of the base station  30 A to the base station  30 C is erroneous can also be imagined. For this reason, it would not be appropriate to carry out position estimation without using the position information of the base station  30 X based only on the fact that the base station  30 X is very distant from the other base stations  30 . 
         [0108]    By focusing on the situation described above, the embodiments of the present disclosure were conceived. By carrying out position estimation using position information of highly reliable base stations  30 , the wireless terminal  40  or the position estimating apparatus  20  according to the embodiments of the present disclosure are capable of suppressing a drop in the precision of position estimation. Such embodiments of the present disclosure will now be described in detail. 
       2. FIRST EMBODIMENT 
       [0109]    2-1. Configuration of Wireless Terminal According to First Embodiment 
         [0110]      FIG. 5  is a functional block diagram showing the configuration of a wireless terminal  40 - 1  according to a first embodiment of the present disclosure. As shown in  FIG. 5 , the wireless terminal  40 - 1  according to the first embodiment includes a communication unit  412 , a measurement unit  414 , a base station information storage unit  416 , an accelerometer  420 , a moved distance calculating unit  424 , an estimation result storage unit  428 , a reference area determining unit  432 , and a position estimating unit  436 . Although described in detail later, the wireless terminal  40 - 1  according to the first embodiment encompasses a function as a position estimating apparatus that estimates the position of the wireless terminal  40 - 1 . 
         [0111]    The communication unit  412  is an interface for base stations  30  in the periphery of the wireless terminal  40 - 1  and functions as a reception unit that receives wireless signals transmitted from the base stations  30  and as a transmission unit that transmits wireless signals to the base stations  30 . 
         [0112]    The measurement unit  414  measures the signal strength of wireless signals received by the communication unit  412  from the respective base stations  30  separately for each transmitting base station  30 . By measuring signal strength using the measurement unit  414 , as one example measurement information such as that described with reference to  FIG. 3  is obtained. 
         [0113]    As described with reference to  FIG. 2  for example, the base station information storage unit  416  stores base station information made up of base station IDs and position information of the respective base stations  30 . Note that as examples, the base station information storage unit  416  may be a storage medium such as a nonvolatile memory, a magnetic disk, an optical disc, or an MO (Magneto Optical) disc. Here, EEPROM (Electrically Erasable Programmable Read-Only Memory) and EPROM (Erasable Programmable ROM) can be given as examples of nonvolatile memory. A hard disk, a disk-shaped magnetic disk, and the like can be given as examples of a magnetic disk. A CD (Compact Disc), a DVD-R (Digital Versatile Disc-Recordable), and a BD (Blu-Ray Disc (Registered Trademark)) and the like can be given as examples of an optical disc. Also, although the base station information storage unit  416  and the estimation result storage unit  428  are shown as different functional blocks in  FIG. 5 , the functions of both storage units may be provided by the same storage medium or by different storage media. 
         [0114]    The accelerometer  420  functions as a motion detection unit that detects acceleration (movement) of the wireless terminal  40 - 1 . The moved distance calculating unit  424  calculates the movement speed of the wireless terminal  40 - 1  in a given period from the history of the acceleration of the wireless terminal  40 - 1  obtained by the accelerometer  420 . The moved distance calculating unit  424  is also capable of calculating the moved distance by integrating the movement speed during a given period. Note that the accelerometer  420  is described above merely as one example of a motion detection unit and a “motion detection unit” for the present disclosure is not limited to the accelerometer  420 . For example, it is possible to use a compass, a gyrosensor or other type of sensor as the motion detection unit. 
         [0115]    The estimation result storage unit  428  stores previous position estimation results for the wireless terminal  40 - 1  produced by the position estimating unit  436 . Specific examples of the position estimation results stored by the estimation result storage unit  428  will now be described with reference to  FIG. 6 . 
         [0116]      FIG. 6  is a diagram useful in explaining specific examples of position estimation results stored by the estimation result storage unit  428 . As shown in  FIG. 6 , the estimation result storage unit  428  stores position information of the wireless terminal  40 - 1  estimated in the past by the position estimating unit  436  in association with the time at which such estimates were made. 
         [0117]    The reference area determining unit (limited range determining unit)  432  reads the previous estimated position information from the estimation result storage unit  428  and determines a reference area (limited range) that uses a position shown by the estimated position information as a standard. The position estimating unit  436  estimates the position of the wireless terminal  40 - 1  based on the position information of the base stations  30  whose position information stored in the base station information storage unit  416  is inside the reference area and the signal strength information of wireless signals transmitted from such base stations  30 . The reference area will now be described in detail with reference to  FIG. 7 . 
         [0118]      FIG. 7  is a diagram useful in explaining a specific example of the reference area. The position N shown in  FIG. 7  is the previous position estimation result. The wireless terminal  40 - 1  is thought to be located in a range (the range shown by the broken line in  FIG. 7 ) that is within the estimated distance moved by the wireless terminal  40  from the time of the previous position estimation calculated by the moved distance calculating unit  424  from the position N. 
         [0119]    Accordingly, base stations that transmit wireless signals that can be received by the wireless terminal  40 - 1  at the present time are thought to be located in a region that is centered on the position N and has a sum of the estimated moved distance and the signal range of the base stations  30  as a radius. For this reason, if the typical signal range (area range) of a base station  30  is 300 m, as shown in  FIG. 7 , the reference area determining unit  432  determines, as the reference area, a region that is centered on the position N and has a value produced by adding the estimated moved distance and 300 m as a radius. 
         [0120]    In this case, the position A to the position C are inside the reference area and the position X is outside the reference area. For this reason, the position estimating unit  436  estimates the position U of the wireless terminal  40 - 1  according to Formula 1 described above, for example, based on the position information A to position information C of the base stations  30 A to  30 C showing the position A to the position C stored in the base station information storage unit  416  and the signal strength information of the wireless signals transmitted from the base stations  30 A to  30 C. 
         [0121]    Meanwhile, the position estimating unit  436  does not use the position information X of the base station  30 X showing the position X stored in the base station information storage unit  416  and the signal strength information of the wireless signal transmitted from the base station  30 X in position estimation for the wireless terminal  40 - 1 . In this way, according to the first embodiment of the present disclosure, by not using the position information of a base station  30  that has a high probability of being erroneous and the signal strength information of the wireless signal from such base station  30  in position estimation, it is possible to suppress a drop in the precision of the position estimation. 
         [0122]    2-2: Operation of Wireless Terminal According to First Embodiment 
         [0123]    The configuration of the wireless terminal  40 - 1  according to the first embodiment of the present disclosure has been described above. Next, the operation of the wireless terminal  40 - 1  according to the first embodiment will be described with reference to  FIG. 8 . 
         [0124]      FIG. 8  is a flowchart showing the operation of the wireless terminal  40 - 1  according to the first embodiment. As shown in  FIG. 8 , first, the measurement unit  414  of the wireless terminal  40 - 1  measures the signal strengths of the wireless signals received by the communication unit  412  from the base stations  30  in the periphery (S 304 ). 
         [0125]    Also, the reference area determining unit  432  searches the estimation result storage unit  428  for the most recent position estimation result of the wireless terminal  40 - 1  (S 308 ) and the moved distance calculating unit  424  calculates the moved distance of the wireless terminal  40 - 1  from the most recent time at which position estimation was carried out (S 312 ). 
         [0126]    Next, the reference area determining unit  432  determines the reference area based on the most recent position estimation result for the wireless terminal  40 - 1  and the moved distance calculated by the moved distance calculating unit  424  (S 316 ). 
         [0127]    After this, the position estimating unit  436  searches the base station information storage unit  416  for position information of the base stations that transmitted the wireless signals whose signal strengths have been measured by the measurement unit  414  (S 320 ). The position estimating unit  436  then determines outliers that are position information outside the reference area out of the pieces of position information that have been found (S 324 ). Next, the position estimating unit  436  estimates the position information of the wireless terminal  40 - 1  based on the position information of the base stations  30  inside the reference area (S 328 ). 
       3. SECOND EMBODIMENT 
       [0128]    The first embodiment of the present disclosure has been described above. Next, a second embodiment of the present disclosure will be described. As described in detail below, the second embodiment of the present disclosure differs to the first embodiment in that the reference area is determined in accordance with a position determination result produced by a different method to the position estimating unit  436 . 
         [0129]    3-1: Configuration of Wireless Terminal According to Second Embodiment 
         [0130]      FIG. 9  is a functional block diagram showing the configuration of a wireless terminal  40 - 2  according to the second embodiment of the present disclosure. As shown in  FIG. 9 , the wireless terminal  40 - 2  according to the second embodiment includes the communication unit  412 , the measurement unit  414 , the base station information storage unit  416 , a GPS positioning unit  440 , and a reference area determining unit  444 . Note that since functional blocks such as the communication unit  412 , the measurement unit  414 , and the base station information storage unit  416  are the same as those described in the first embodiment, detailed description thereof is omitted here. 
         [0131]    The GPS positioning unit  440  receives navigation messages from satellites and calculates the present position of the wireless terminal  40 - 2  from ephemeris information included in the navigation messages. Note that the GPS positioning unit  440  is merely one example of a position determining unit that determines a position according to a different method to the position estimating unit  436  and such “position determining unit” can be realized by a variety of implementations. 
         [0132]    The reference area determining unit  444  (limited range determining unit) determines the reference area with the present position of the wireless terminal  40 - 2  calculated by the GPS positioning unit  440  as a standard. The position estimating unit  436  estimates the position of the wireless terminal  40 - 2  based on the position information of the base stations  30  whose position information stored in the base station information storage unit  416  is inside the reference area and signal strength information for the wireless signals transmitted from such base stations  30 . The reference area will now be described in more detail with reference to  FIG. 10 . 
         [0133]      FIG. 10  is a diagram showing a specific example of the reference area. The position O shown in  FIG. 10  is the positioning result produced by the GPS positioning unit  440 . The wireless terminal  40 - 2  is believed to be located in a region that is within the range of the GPS positioning error from the position O (i.e., in the region shown by the broken line shown in  FIG. 10 ). 
         [0134]    Accordingly, the base stations that transmitted the wireless signals that can be received at the present time by the wireless terminal  40 - 2  are believed to be located in a region that is centered on the position O and has the sum of the GPS positioning error and the signal range of a base station  30  as a radius. For this reason, if the typical signal range (area range) of a base station  30  is 300 m, as shown in  FIG. 10 , the reference area determining unit  444  determines, as the reference area, a region that is centered on the position O and has a value produced by adding the GPS positioning error and 300 m as a radius. 
         [0135]    In this case, the position A to the position C are inside the reference area and the position X is outside the reference area. For this reason, the position estimating unit  436  estimates the position U of the wireless terminal  40 - 2  according to Formula 1 described above, for example, based on the position information A to position information C of the base stations  30 A to  30 C showing the position A to the position C stored in the base station information storage unit  416  and the signal strength information of the wireless signals transmitted from the base stations  30 A to  30 C. 
         [0136]    Meanwhile, the position estimating unit  436  does not use the position information X of the base station  30 X showing the position X stored in the base station information storage unit  416  and the signal strength information of the wireless signal transmitted from the base station  30 X in position estimation of the wireless terminal  40 - 2 . In this way, according to the second embodiment of the present disclosure, in the same way as the first embodiment, by not using the position information of a base station  30  that has a high probability of being erroneous and the signal strength information of the wireless signal from such base station  30  in position estimation, it is possible to suppress a drop in the precision of the position estimation. 
         [0137]    3-2: Operation of Wireless Terminal According to the Second Embodiment 
         [0138]    The configuration of the wireless terminal  40 - 2  according to the second embodiment of the present disclosure has been described above. Next, the operation of the wireless terminal  40 - 2  according to the second embodiment will be described with reference to  FIG. 11 . 
         [0139]      FIG. 11  is a flowchart showing the operation of the wireless terminal  40 - 2  according to the second embodiment. As shown in  FIG. 11 , first, the measurement unit  414  of the wireless terminal  40 - 2  measures the signal strengths of the wireless signals received by the communication unit  412  from the base stations  30  in the periphery (S 344 ). Also, the GPS positioning unit  440  calculates the present position of the wireless terminal  40 - 2  (S 348 ). Note that the processing in S 344  and S 348  may be implemented in parallel. 
         [0140]    The reference area determining unit  444  then determines the reference area based on the positioning result produced by the GPS positioning unit  440  (S 352 ). After this, the position estimating unit  436  searches the base station information storage unit  416  for position information of the base stations that transmitted the wireless signals whose signal strengths have been measured by the measurement unit  414  (S 356 ). 
         [0141]    Next, the position estimating unit  436  determines outliers that are position information outside the reference area out of the pieces of position information that have been found (S 360 ). The position estimating unit  436  then estimates the position information of the wireless terminal  40 - 2  based on the position information of the base stations  30  inside the reference area (S 364 ). 
       4. THIRD EMBODIMENT 
       [0142]    The second embodiment of the present disclosure has been described above. Next, a wireless terminal  40 - 3  according to a third embodiment of the present disclosure will be described in detail with reference to  FIGS. 12 to 16 . 
         [0143]    4-1: Configuration of Wireless Terminal According to Third Embodiment 
         [0144]      FIG. 12  is a functional block diagram showing the configuration of the wireless terminal  40 - 3  according to the third embodiment of the present disclosure. As shown in  FIG. 12 , the wireless terminal  40 - 3  according to the third embodiment includes the communication unit  412 , the measurement unit  414 , the base station information storage unit  416 , the GPS positioning unit  440 , the reference area determining unit  444 , a low reliability base station information storage unit  448 , a mobile base station determining unit  452 , and a mobile base station information storage unit  456 . 
         [0145]    Note that since functional blocks such as the communication unit  412 , the measurement unit  414 , the base station information storage unit  416 , the GPS positioning unit  440 , and the reference area determining unit  444  are the same as those described in the second embodiment, detailed description thereof is omitted here. 
         [0146]    As described above in the second embodiment and the like, the position estimating unit  436  searches the base station information storage unit  416  for position information of the base stations that transmitted the wireless signals whose signal strengths have been measured by the measurement unit  414  and determines outliers that are position information outside the reference area out of the pieces of position information that have been found. 
         [0147]    Here, the position information determined by the position estimating unit  436  to be an outlier is believed to be position information of a base station  30  that has moved after registration in the base station information storage unit  416 . This means that there is the possibility that a base station  30  with position information determined to be an outlier by the position estimating unit  436  is a mobile base station carried by the user. However, there are also cases where even a home base station will move due to movement of the user. For this reason, in the present embodiment, it is specified whether the base station  30  is a mobile base station based on the frequency with which, or the number of times that, the position information has been determined to be an outlier. This is described in detail below. 
         [0148]    The low reliability base station information storage unit  448  stores the base station IDs of base stations with position information that has been determined by the position estimating unit  436  to be an outlier. Specific examples of the low reliability base station information stored by the low reliability base station information storage unit  448  will now be described with reference to  FIG. 13 . 
         [0149]      FIG. 13  is a diagram useful in showing specific examples of the low reliability base station information stored by the low reliability base station information storage unit  448 . As shown in  FIG. 13 , the low reliability base station information storage unit  448  stores base station IDs showing low reliability base stations in association with report times showing when such base stations were reported (added) as low reliability base stations. For example, in  FIG. 13 , it is shown that the base station  30 X was reported as a low reliability base station at “2010/7/14 10:34:56”. In this way, the low reliability base station information storage unit  448  functions as a history information storage unit that stores history information for each piece of low reliability base station information that has been reported. 
         [0150]    The mobile base station determining unit  452  refers to the low reliability base station information storage unit  448  and determines that a base station  30  that has been reported as a low reliability base station with high frequency is a mobile base station. For example, the mobile base station determining unit  452  may determine that a base station  30  that has been reported as a low reliability base station more than a set number of times, such as twice, five times, or ten times within a specified period, such as one week, one month, or three months, is a mobile base station. 
         [0151]    In more detail, for the example shown in  FIG. 13 , since the base station  30 X has been reported as a low reliability base station four times in the period from “2010/7/14” to “2010/7/18”, the mobile base station determining unit  452  may determine that the base station  30 X is a mobile base station. 
         [0152]    Information on the base station  30  determined by the mobile base station determining unit  452  to be a mobile base station is added to the mobile base station information storage unit  456 . For example, when the base station  30 X has been determined to be a mobile base station by the mobile base station determining unit  452 , as shown in  FIG. 14 , the base station ID of the base station  30 X is added to the mobile base station information storage unit  456 . Note that the base station ID may be added to the mobile base station information storage unit  456  in an offline manner based on a base station ID reported by the user and/or information reported from a vendor or carrier. 
         [0153]    The information on the base station  30  determined by the mobile base station determining unit  452  to be a mobile base station may be deleted from the base station information storage unit  416 . For example, when the base station  30 X has been determined by the mobile base station determining unit  452  to be a mobile base station, the base station information, such as the position information and the like, of the base station  30 X may be deleted from the base station information storage unit  416 . 
         [0154]    Since a mobile base station is not present at a fixed location as described above, if information on such a mobile base station were used when estimating the position of the wireless terminal  40 - 3 , there would be the risk of a fall in the precision of the position estimation. 
         [0155]    For this reason, the position estimating unit  436  of the wireless terminal  40 - 3  may estimate the position of the wireless terminal  40 - 3  by selectively using signal strength information of base stations  30  whose position information show positions in the reference area and that are not stored in the mobile base station information storage unit  456  as mobile base stations, and also the position information of such base stations  30 . According to this configuration, it is possible to improve the precision of position estimation for a wireless terminal  40 . 
         [0156]    4-2: Operation of Wireless Terminal According to Third Embodiment 
         [0157]    The configuration of the wireless terminal  40 - 3  according to the third embodiment of the present disclosure has been described above. Next, the operation of the wireless terminal  40 - 3  according to the third embodiment will be described with reference to  FIGS. 15 and 16 . 
         [0158]      FIGS. 15 and 16  are flowcharts showing an example operation of the wireless terminal  40 - 3  according to the third embodiment. As shown in  FIG. 15 , first, when measurement information for wireless signals has been acquired by the measurement unit  414  of the wireless terminal  40 - 3 , the position estimating unit  436  acquires, from the base station information storage unit  416 , the base station information of the base stations that transmitted such wireless signals (S 504 ). 
         [0159]    After this, the wireless terminal  40 - 3  carries out the processing in S 508  to S 520  for each piece of base station information S acquired in S 504 . More specifically, the position estimating unit  436  refers to the mobile base station information storage unit  456  and determines whether the base station information S is base station information of a base station  30  registered as a mobile base station (S 508 ). 
         [0160]    Also, the position estimating unit  436  determines whether the position information included in the base station information S is inside the reference area (S 512 ). After this, if it has been determined that the position information included in the base station information S is outside the reference area, the wireless terminal  40 - 3  adds the base station information S to the low reliability base station information storage unit  448  (S 516 ) and excludes such base station information S from use in position estimation (S 520 ). Also, if it has been determined in S 508  that the base station information S is base station information of a base station  30  registered as a mobile base station, the wireless terminal  40 - 3  excludes such base station information S from use in position estimation (S 520 ). 
         [0161]    The position estimating unit  436  then uses the remaining base station information and measurement information to estimate the position information of the wireless terminal  40 - 3  (S 524 ). 
         [0162]    Meanwhile, the mobile base station determining unit  452  executes the processing in S 528  to S 536  shown in  FIG. 16  at arbitrary timing, such as when a specified interval has passed or when new information has been added to the low reliability base station information storage unit  448 . 
         [0163]    More specifically, the mobile base station determining unit  452  refers to the low reliability base station information storage unit  448  (S 528 ) and determines whether a base station  30  has been reported as a low reliability base station with high frequency (S 532 ). After this, the mobile base station determining unit  452  adds information relating to a base station  30  that has been reported as a low reliability base station with high frequency to the mobile base station information storage unit  456  (S 536 ). Note that the mobile base station determining unit  452  may delete information relating to the base station  30  that has been reported as a low reliability base station with high frequency from the base station information storage unit  416 . 
       5. FOURTH EMBODIMENT 
       [0164]    The first to third embodiments of the present disclosure have been described above. 
         [0165]    In the first to third embodiments of the present disclosure, examples have been described where a function for determining outliers among the base stations  30 , a position estimating function, a function for determining mobile base stations, and the like have been implemented in a wireless terminal  40 . However, as described below as the fourth embodiment, it is also possible to implement such functions in a position estimating apparatus  20 . 
         [0166]    5-1. Configuration of Position Estimating System According to Fourth Embodiment 
         [0167]      FIG. 17  is a diagram useful in showing the configuration of a position estimating system  2  according to the fourth embodiment. As shown in  FIG. 17 , the position estimating system  2  according to the fourth embodiment includes a position estimating apparatus  20 , a plurality of base stations  30 , and a wireless terminal  40 . 
         [0168]    On receiving a wireless signal transmitted from a base station  30  (for example, a beacon signal), the wireless terminal  40  is capable of measuring the signal strength of the wireless signal. After this, the wireless terminal  40  transmits measurement information including the measured signal strength information and the base station ID of the base station  30  to the position estimating apparatus  20 . The wireless terminal  40  also transmits reference area information for enabling the position estimating apparatus  20  to specify a reference area. Note that the previous estimated position and estimated moved distance described in the first embodiment and the positioning result produced by GPS described in the second embodiment can be given as examples of the reference area information. 
         [0169]    Note that there are no particular limitations on the format for expressing the signal strength information. As examples, the signal strength information may be expressed in “Dbm” units or by a value, such as “40%” or “80%”, that is the measured value expressed as a proportion of a set value (for example, a saturation value for the signal strength), or may be a converted distance value for the signal strength. 
         [0170]    The position estimating apparatus  20  stores position information for each base station  30  and is capable of estimating the position information of a wireless terminal  40  based on the position information of the respective base stations  30  and measurement information received from the wireless terminal  40 . The position estimating apparatus  20  then transmits the estimated position information to the wireless terminal  40 . 
         [0171]    Note that the position estimating apparatus  20  may communicate with the wireless terminal  40  via a communication network that includes wired or wireless transfer paths. More specifically, the communication network may include a public network such as the Internet, a telephone network, or a satellite communication network, various types of LAN (Local Area Network) including Ethernet (registered trademark), an LTE core network, a WAN (Wide Area Network), and the like. The communication network may also include a dedicated network such as an IP-VPN (Internet Protocol-Virtual Private Network). 
         [0172]    As described above, in the fourth embodiment, the position estimation for the wireless terminal  40  is carried out by the position estimating apparatus  20 . In the same way, in the fourth embodiment, the determination of the reference area, the determination of outliers, the determination of mobile base stations, and the like may also be carried out by the position estimating apparatus  20 . The configuration and operation of the position estimating apparatus  20  according to the fourth embodiment will now be described. 
         [0173]    5-2: Configuration of Position Estimating Apparatus According to Fourth Embodiment 
         [0174]      FIG. 18  is a functional block diagram showing the configuration of the position estimating apparatus  20  according to the fourth embodiment of the present disclosure. As shown in  FIG. 18 , the position estimating apparatus  20  according to the fourth embodiment includes a communication unit  212 , a base station information storage unit  216 , a position estimating unit  236 , a reference area determining unit  244 , a low reliability base station information storage unit  248 , a mobile base station determining unit  252 , and a mobile base station information storage unit  256 . 
         [0175]    The communication unit  212  is an interface for transmitting and receiving information to and from the wireless terminal  40 . As one example, the communication unit  212  has functions as a reception unit that receives measurement information and reference area information from the wireless terminal  40  and as a transmission unit that transmits a position estimation result to the wireless terminal  40 . 
         [0176]    In the same way as the base station information storage unit  416  described above in the first to third embodiments, the base station information storage unit  216  stores base station information made up of base station IDs and position information of the respective base stations  30 . 
         [0177]    The reference area determining unit  244  determines the reference area based on the reference area information received by the communication unit  212  from the wireless terminal  40 . Since methods of determining the reference area based on the reference area information were described in the first and second embodiments, detailed description thereof is omitted here. 
         [0178]    However, when the position estimating apparatus  20  stores a history of the position estimation results for each wireless terminal  40 , it is possible for the reference area determining unit  244  to specify the reference area based on the history of position estimation results stored in the position estimating apparatus  20  and an estimated moved distance received from the wireless terminal  40  as the reference area information. 
         [0179]    Since the position estimating unit  236 , the low reliability base station information storage unit  248 , the mobile base station determining unit  252 , and the mobile base station information storage unit  256  can be effectively the same configurations as the position estimating unit  436 , the low reliability base station information storage unit  448 , the mobile base station determining unit  452 , and the mobile base station information storage unit  456  described in the third embodiment, detailed description thereof is omitted here. 
         [0180]    In short, base station IDs showing base stations  30  that have low reliability as fixed base stations are recorded in the low reliability base station information storage unit  248  and the mobile base station determining unit  252  determines mobile base stations by referring to the low reliability base station information storage unit  248 . Information relating to a base station  30  determined by the mobile base station determining unit  252  to be a mobile base station is added to the mobile base station information storage unit  256  and is deleted from the base station information storage unit  216 . 
         [0181]    The position estimating unit  236  estimates the position of the wireless terminal  40  by selectively using the measurement information of base stations  30  which have position information showing positions inside the reference area and are not stored in the mobile base station information storage unit  256  as mobile base stations, and also position information of such base stations  30 . 
         [0182]    5-3: Operation of the Position Estimating System According to the Fourth Embodiment 
         [0183]    The configuration of the position estimating apparatus  20  according to the fourth embodiment of the present disclosure has been described above. Next, the operation of the position estimating system  2  according to the fourth embodiment will be described with reference to  FIG. 19 . 
         [0184]      FIG. 19  is a sequence chart showing the operation of the position estimating system  2  according to the fourth embodiment. As shown in  FIG. 19 , the wireless terminal  40  first measures the signal strengths of the wireless signals received from the base stations  30  in the periphery (S 604 ). The wireless terminal  40  acquires the reference area information (S 508 ) and transmits the reference area information and the measurement information acquired in S 604  to the position estimating apparatus  20  (S 612 ). 
         [0185]    After this, the position estimating unit  236  of the position estimating apparatus  20  searches the base station information storage unit  216  for base station information relating to the measurement information received from the wireless terminal  40  (S 616 ). The reference area determining unit  244  determines the reference area based on reference area information received from the wireless terminal  40  (S 620 ). 
         [0186]    In addition, the position estimating unit  236  determines mobile base stations and determines outliers as described above and estimates the position of the wireless terminal  40  using the measurement information of base stations  30  that have position information showing positions inside the reference area and have not been stored in the mobile base station information storage unit  256 , and position information of such base stations  30  (S 628 ). After this, the communication unit  212  transmits the position information of the wireless terminal  40  estimated by the position estimating unit  236  to the wireless terminal  40  (S 632 ). 
       6. CONCLUSION 
       [0187]    As described above, according to the embodiments of the present disclosure, by estimating the position of the wireless terminal  40  based on the position information of the base stations  30  showing positions in the reference area, it is possible to improve the precision of position estimation for the wireless terminal  40 . 
         [0188]    In addition, according to the third and fourth embodiments of the present disclosure, by estimating the position of the wireless terminal  40  without using the position information of base stations  30  that have a high probability of being mobile base stations, it is possible to significantly improve the precision of the position estimation. 
         [0189]    Although preferred embodiments of the present disclosure have been described in detail with reference to the attached drawings, the present disclosure is not limited to the above examples. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof. 
         [0190]    As one example, the respective steps in the processing of the position estimating system  2  or the wireless terminal  40  in this specification do not need to be carried out in a time series in the order in which such steps are given in the sequence charts or flowcharts. For example, the respective steps in the processing of the wireless terminal  40  may be carried out in a different order to the order written in the flowcharts and/or may be carried out in parallel. 
         [0191]    It is also possible to produce a computer program for causing hardware such as a CPU, a ROM, and a RAM incorporated in the position estimating apparatus  20  or the wireless terminal  40  to achieve the same functions as the respective structural elements of the position estimating apparatus  20  or the wireless terminal  40  described earlier. A storage medium on which such computer program is recorded may also be provided. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               20  Position estimating apparatus 
               30  Base station 
               40 ,  40 - 1 ,  40 - 2 ,  40 - 3  Wireless terminal 
               212 ,  412  Communication unit 
               216 ,  416  Base station information storage unit 
               236 ,  436  Position estimating unit 
               244 ,  432 ,  444  Reference area determining unit 
               248 ,  448  Low reliability base station information storage unit 
               252 ,  452  Mobile base station determining unit 
               256 ,  456  Mobile base station information storage unit 
               420  Accelerometer 
               424  Moved distance calculating unit 
               428  Estimation result storage unit 
               440  GPS positioning unit