Patent Publication Number: US-11644556-B2

Title: Position measurement device, position measurement method, and program recording medium

Description:
This application is a National Stage Entry of PCT/JP2018/023050 filed on Jun. 18, 2018, which claims priority from Japanese Patent Application 2017-120117 filed on Jun. 20, 2017, the contents of all of which are incorporated herein by reference, in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a position measurement device, a position measurement method, and a program which measure a position of a target. 
     BACKGROUND ART 
     There is a technique of receiving a reflected wave of an electromagnetic wave transmitted toward a range including a specific area, and performing, by using the received reflected wave, position measurement of a target located in the specific area. For example, presence or absence of a target can be determined by scanning a reflected wave for each section uniquely determined by a distance and an angle from an antenna which transmits and receives an electromagnetic wave, and measuring spectral intensity of the section. Generally, in position measurement of a target in a specific area, all sections covering the whole specific area are scanned. 
     NPL 1 discloses a tracking technique of scanning all sections of a specific area, and, when a target is discovered, performing position measurement by restricting a scan range to a region including the target. The technique of NPL 1 performs position measurement for all sections in a specific area with low frequency, and, when a target is discovered at a certain scan opportunity, performs position measurement with high frequency at a next scan opportunity, mainly for a region to which the target seems to move. 
     PTL 1 discloses a radar device which uses beat signals acquired from a plurality of antennas, and detects a target at a plurality of detection angles. The device of PTL 1 selects a target for calculation from detected targets, and changes, based on position information of the target for calculation, an interval between some detection angles among a plurality of detection angles at which a target is detected next, in such a way as to be different from an interval between other detection angles. The device of PTL 1 enables to set a small interval between inspection angles of an angle range requiring detailed sensing, and set a large interval between other inspection angles, and therefore, enables to improve detection accuracy of a target, and shorten a detection time of the target. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] Japanese Unexamined Patent Application Publication No. 2009-128016 
       
    
     Non Patent Literature 
     
         
         [NPL 1] Kohei Sato, Hiroyoshi Yamada, Hiroyuki Tsuji, Yoshio Yamaguchi, “Experimental Study on Tracking of Multiple-Moving Persons in Indoor Environment Using MIMO Doppler Radar”, Proceedings of the IEICE general conference (CD-ROM), vol. 2015, p.ROMBUNNO BS-1-5, February 2015. 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     A general position measurement technique has a problem that detection of spectral intensity is performed in all sections in a specific area, and therefore, a scan time becomes long. 
     In contrast, the techniques of NPL 1 and PTL 1 enable to shorten a scan time by restricting a scan range to a target in a specific area. However, the techniques of NPL 1 and PTL 1 scan all sections of the specific area until discovering a target, and therefore, it takes time to discover a target newly entering the specific area. In other words, the techniques of NPL 1 and PTL 1 have a problem of, when a target newly enters a specific area, being unable to measure a position of the target in the specific area in real time. 
     In order to solve the problem described above, an object of the present invention is to provide a position measurement device which enables to shorten a time required for detecting a target entering a specific area, and perform position measurement of the target located in the specific area in real time. 
     Solution to Problem 
     A position measurement device according to one aspect of the present invention includes: a storage means for storing area information for setting a first region along a boundary of a specific area; an area setting means for acquiring the area information from the storage means, and setting the first region, based on the acquired area information; and a position measurement means for acquiring the first region from the area setting means, setting the acquired first region to a verification region, performing position measurement of a target located in the verification region, and updating the area information stored in the storage means. 
     A position measurement method according to one aspect of the present invention includes: setting a first region, based on area information acquired from a storage means for storing the area information for setting the first region along a boundary of a specific area; setting the first region to a verification region; performing position measurement of a target located in the verification region; and updating the area information stored in the storage means. 
     A program according to one aspect of the present invention causes a computer to execute: processing of setting a first region, based on area information acquired from a storage means for storing the area information for setting the first region along a boundary of a specific area; processing of setting the first region to a verification region; processing of performing position measurement of a target located in the verification region; and processing of updating the area information stored in the storage means. 
     Advantageous Effects of Invention 
     According to the present invention, it becomes possible to provide a position measurement device which enables to shorten a time required for detecting a target entering a specific area, and perform position measurement of the target located in the specific area in real time. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram illustrating a configuration of a position measurement device according to a first example embodiment of the present invention. 
         FIG.  2    is a block diagram illustrating a configuration of an area setting unit of the position measurement device according to the first example embodiment of the present invention. 
         FIG.  3    is a conceptual diagram for describing a measurement target area of the position measurement device according to the first example embodiment of the present invention. 
         FIG.  4    is a table that compiles information stored in an information storage unit of the position measurement device according to the first example embodiment of the present invention. 
         FIG.  5    is a diagram illustrating one example of a range spectrum calculated by a position measurement unit of the position measurement device according to the first example embodiment of the present invention. 
         FIG.  6    is a diagram for describing a mechanism in which the position measurement device according to the first example embodiment of the present invention performs distance measurement by using a frequency modulated continuous wave. 
         FIG.  7    is a diagram illustrating one example of a result of performing direction-of-arrival estimation by using a range spectrum by the position measurement unit according to the first example embodiment of the present invention. 
         FIG.  8    is a conceptual diagram for describing a mechanism in which a position of a target is determined by the position measurement device according to the first example embodiment of the present invention. 
         FIG.  9    is a diagram for describing a parameter when the position measurement device according to the first example embodiment of the present invention uses a two-dimension polar coordinate. 
         FIG.  10    is a diagram for describing a parameter when the position measurement device according to the first example embodiment of the present invention uses a three-dimension polar coordinate. 
         FIG.  11    is a flowchart for describing an operation of the position measurement device according to the first example embodiment of the present invention. 
         FIG.  12    is a conceptual diagram for describing a position measurement method in a general position measurement scheme. 
         FIG.  13    is a conceptual diagram for describing a tracking method in a general position measurement scheme. 
         FIG.  14    is a block diagram illustrating a configuration of a position measurement device according to a second example embodiment of the present invention. 
         FIG.  15    is a block diagram illustrating a configuration of a pre-scan unit of the position measurement device according to the second example embodiment of the present invention. 
         FIG.  16    is a conceptual diagram for describing a measurement target area when the position measurement device according to the second example embodiment of the present invention performs distance measurement. 
         FIG.  17    is a conceptual diagram for describing a measurement target area when the position measurement device according to the second example embodiment of the present invention performs angle measurement. 
         FIG.  18    is a flowchart for describing an operation of the position measurement device according to the second example embodiment of the present invention. 
         FIG.  19    is a block diagram illustrating a configuration of a position measurement device according to a third example embodiment of the present invention. 
         FIG.  20    is a block diagram illustrating a configuration of a pre-scan unit of the position measurement device according to the third example embodiment of the present invention. 
         FIG.  21    is a conceptual diagram for describing a measurement target area of the position measurement device according to the third example embodiment of the present invention. 
         FIG.  22    is a flowchart for describing an operation of the position measurement device according to the third example embodiment of the present invention. 
         FIG.  23    is a block diagram illustrating one example of a hardware configuration which executes processing of the position measurement device according to each example embodiment of the present invention. 
     
    
    
     EXAMPLE EMBODIMENT 
     Example embodiments of the present invention will be described below by using the drawings. However, limitation being technically preferable in order to implement the present invention is placed on example embodiments described below, but does not limit the scope of the invention to the description below. Note that, in all the drawings used for description of the example embodiments below, the same reference sign is given to a similar part, unless there is a particular reason. Moreover, in the example embodiments below, a repeated description may be omitted in relation to a similar configuration and operation. 
     First Example Embodiment 
     First, a position measurement device according to a first example embodiment of the present invention is described with reference to the drawings. The position measurement device according to the present example embodiment measures a position of a target in a specific area by transmitting an electromagnetic wave to a range including the specific area where position measurement of a target is performed, and receiving and then analyzing a reflected wave of the transmitted electromagnetic wave. 
     (Configuration) 
       FIG.  1    is a block diagram illustrating a configuration of a position measurement device  1  according to the present example embodiment. As in  FIG.  1   , the position measurement device  1  includes a signal transmission unit  11 , a signal reception unit  12 , a beat signal generation unit  13 , an information storage unit  14 , an area setting unit  15 , and a position measurement unit  17 . 
     The signal transmission unit  11  generates a transmission signal transmitted toward a measurement target area via a transmission antenna. The signal transmission unit  11  outputs the generated transmission signal to the transmission antenna and the beat signal generation unit  13 . A generation signal generated by the signal transmission unit  11  is transmitted toward the measurement target area via at least one transmission antenna. For example, the signal transmission unit  11  generates a pulse wave, a continuous wave, and a frequency modulated continuous wave (hereinafter, FMCW), as a transmission signal. An example using the FMCW is described below. 
     The signal reception unit  12  receives, as a reception signal, a reflected wave of a transmission signal via at least one reception antenna. The signal reception unit  12  outputs the received reception signal to the beat signal generation unit  13 . When there are a plurality of reception antennas, the signal reception unit  12  separately outputs, to the beat signal generation unit  13 , reception signals each acquired from each reception antenna. 
     The beat signal generation unit  13  acquires a transmission signal from the signal transmission unit  11 , and acquires a reception signal from the signal reception unit  12 . The beat signal generation unit  13  generates an intermediate frequency signal (hereinafter, an IF signal) by synthesizing the acquired transmission signal and the reception signal. The beat signal generation unit  13  outputs the generated IF signal to the position measurement unit  17 . 
     When there are a plurality of transmission antennas or reception antennas, the beat signal generation unit  13  synthesizes a transmission signal and a reception signal for each pair of a transmission antenna and a reception antenna. For example, when transmission antennas are configured by Tx 1  and Tx 2  and reception antennas are configured by Rx 1  and Rx 2 , four pairs of (Tx 1 , Rx 1 ), (Tx 1 , Rx 2 ), (Tx 2 , Rx 1 ), and (Tx 2 , Rx 2 ) are formed. In this case, the beat signal generation unit  13  generates an IF signal for each of the four pairs. 
     The information storage unit  14  (also referred to as a storage unit) stores information (hereinafter, area information) for calculating a step-in area (also referred to as a first region). Moreover, the information storage unit  14  stores information (hereinafter, target information) for calculating a tracking area (also referred to as a second region). Note that, when no tracking area is set, the information storage unit  14  may store area information. 
     The step-in area is a region set along a boundary of a specific area. Note that, although an example of setting a step-in area inside a boundary of a specific area is described in the present example embodiment, a step-in area may be set outside a boundary of a specific area. Moreover, a step-in area may be a fixed region, or may be a region which dynamically changes in shape and area each time position measurement is performed. 
     A tracking area is a range in which a target is estimated, based on a past position measurement result, to be located in current position measurement. A tracking area may be set in a range of a fixed distance from a past target position, or may be set in a range in which a distance from a target position dynamically changes. 
     The area setting unit  15  acquires area information with reference to the information storage unit  14 , and sets a step-in area along a boundary of a specific area using the acquired area information. The area setting unit  15  outputs the set step-in area to the position measurement unit  17 . Moreover, the area setting unit  15  sets a tracking area for performing position measurement of a target being tracked. The area setting unit  15  outputs the set tracking area to the position measurement unit  17 . Note that, when only a target located in a step-in area is targeted for detection, the area setting unit  15  may not set a tracking area. 
     As in  FIG.  2   , the area setting unit  15  includes a step-in area setting unit  151  (also referred to as a first region setting unit), and a tracking area setting unit  153  (also referred to as a second region setting unit). The step-in area setting unit  151  refers to the information storage unit  14 , and sets a step-in area having any width from a boundary of a specific area. The tracking area setting unit  153  refers to the information storage unit  14 , and sets a tracking area for performing position measurement of a target being tracked. Note that, when only a target located in a step-in area is targeted for detection, the tracking area setting unit  153  may be omitted. 
     The position measurement unit  17  has a first function of setting a scan area. The position measurement unit  17  gives the step-in area setting unit  151  an instruction to acquire a step-in area, and acquires the step-in area from the step-in area setting unit  151 . Moreover, the position measurement unit  17  gives the tracking area setting unit  153  an instruction to acquire a tracking area, and acquires the tracking area from the tracking area setting unit  153 . The position measurement unit  17  sets, to a scan area (also referred to as a verification region), a region combining a tracking area and a step-in area. Note that, when only a target located in a step-in area is targeted for detection, the position measurement unit  17  gives the area setting unit  15  which does not include the tracking area setting unit  153 , an instruction to acquire a tracking area. When only a target located in a step-in area is targeted for detection, the position measurement unit  17  sets the step-in area to a scan area. 
     Moreover, the position measurement unit  17  has a second function of performing position measurement in a scan area. The position measurement unit  17  acquires an IF signal from the beat signal generation unit  13 , performs position measurement of a scan area, based on the acquired IF signal, and calculates a position spectrum of a section in the scan area. 
     Furthermore, the position measurement unit  17  has a third function of updating information in the information storage unit  14 . The position measurement unit  17  determines, based on the calculated position spectrum, whether a target is located in an area, and updates, according to a determination result, information held in the information storage unit  14 . 
     The above is the description of an overview of the components of the position measurement device  1  according to the present example embodiment. Note that a configuration (referred to as an intermediate signal generation unit) combining the signal transmission unit  11 , the signal reception unit  12 , and the beat signal generation unit  13  inside a broken line in  FIG.  1    may be configured outside without being included in the position measurement device  1 . In this case, the position measurement device  1  may receive an IF signal generated by an intermediate signal generation unit configured outside. 
     [Measurement Target Area] 
     Here, a measurement target area to which a transmission signal is transmitted from the position measurement device  1  is described.  FIG.  3    is a conceptual diagram for describing a measurement target area A 11  of the position measurement device  1 . A specific area A 12 , a step-in area A 13 , an internal area A 14 , and a tracking area A 15  are set inside the measurement target area A 11 . 
     In the example of  FIG.  3   , a target  111  and a target  112  are located inside the specific area A 12 . The target  111  is a target whose position is already measured in previous position measurement. On the other hand, the target  112  is a target which has been located outside the specific area A 12  in previous position measurement, but newly entered the specific area A 12  in current position measurement. 
     The measurement target area A 11  (inside a fan shape of a full line) is a region in which the position measurement device  1  is able to perform position measurement of a target by a reflected wave of an electromagnetic wave received via an antenna. 
     The specific area A 12  (inside a rectangular shape of a broken line) is a region set within a range of the measurement target area A 11  in order for the position measurement device  1  to perform position measurement of a target. 
     The step-in area A 13  (a region between the rectangular shape of the broken line and a rectangular shape of a full line) is a region set along a boundary of the specific area A 12 . In the example of  FIG.  3   , the step-in area A 13  is set inside the boundary of the specific area A 12 . Note that, as long as a target entering the specific area A 12  can be detected, the step-in area A 13  may be set in a part including outside of the boundary of the specific area A 12 . 
     The internal area A 14  (inside the rectangular shape of the full line) is a region included in the specific area A 12  and set inside the step-in area A 13 . 
     The tracking area A 15  (inside a circular shape of a full line) is a region in which a target is estimated, based on a previous scan result, to be located at current scan timing. 
     In the present example embodiment, position measurement of all targets located inside the specific area A 12  is enabled by performing position measurement of a scan area including the step-in area A 13  and the tracking area A 15 . In the present example embodiment, a region being a scan target is limited, and therefore, position measurement of a target can be performed in a short time as compared with a scheme which scans all regions of the specific area A 12 . 
     Now, characteristic components of the position measurement device  1  according to the present example embodiment are described in detail. In relation to the area setting unit  15 , description is given below separately for the step-in area setting unit  151  and the tracking area setting unit  153 . 
     [Information Storage Unit] 
     First, the information storage unit  14  is described.  FIG.  4    is a table (an information table  140 ) indicating one example of information held by the information storage unit  14 . The information storage unit  14  holds information including area information and target information. 
     Area information is information used in order to set the step-in area A 13 . The information table  140  in  FIG.  4    includes, as one example of the area information, a specific area coordinate, a step-in coordinate, and a step-in width. Note that, when the step-in area A 13  is set at width from the boundary of the specific area A 12 , only a step-in range may be included as area information. Moreover, when a width of a step-in area dynamically changes, it is preferable that the information storage unit  14  holds a target speed and a previous scan time (a previous position measurement time) as area information. 
     A specific area coordinate is a coordinate indicating a range of the specific area A 12 . In the example of  FIG.  3   , coordinates (x1, y1), (x2, y2), (x3, y3), and (x4, y4) of four vertexes of the specific area A 12  represented by the rectangular shape of the broken line are set as specific area coordinates. Note that, when the specific area A 12  is not rectangular, a coordinate indicating a shape of the specific area A 12  may be designated as a specific area coordinate. Note that information representing a shape of the specific area A 12  may be not a coordinate but a mathematical expression. 
     When width of the step-in area A 13  is a fixed value, a step-in coordinate is a coordinate for calculating the width of the step-in area A 13 . In the example of  FIG.  3   , coordinates (x5, y5), (x6, y6), (x7, y7), and (x8, y8) of four vertexes of the internal area A 14  represented by the rectangular shape of the full line are step-in coordinates. On the other hand, when width of the step-in area A 13  dynamically changes, the width is set using a target speed and a previous scan time. 
     Target information is information used in order to calculate a tracking area. In  FIG.  4   , a target identifier and a target position are included in target information. 
     A target identifier is an identifier uniquely allocated to an individual target. A target identifier is used in order to determine which target has exited when information relating to a target that has exited to outside of the specific area A 12  is deleted. Note that, when information about a target determined to be located inside the specific area A 12  in the past is preserved, a target identifier may be omitted. From now on, it is assumed that information relating to a target that has exited to the outside of the specific area A 12  is deleted, and an example of using a target identifier is described. 
     A target position is a coordinate indicating a position of a target. A target position is linked with a target identifier uniquely allocated to an individual target. 
     For example, when the target  111  given a target identifier “ 111 ” is located at (x9, y9), and the target  112  given a target identifier “ 112 ” is located at (x10, y10), a target position and a target identifier are held as in  FIG.  4   . 
     When the tracking area A 15  is in a fixed range from a target position, the information storage unit  14  stores a tracking range in addition to a target position and a target identifier. Moreover, when the tracking area A 15  dynamically changes, the information storage unit  14  stores a target speed and a previous scan time. Note that, when a plurality of targets are located inside the specific area A 12 , and a target speed of each target is different, the information storage unit  14  stores a target speed of each target. 
     Additionally, a tracking area may be set based on a movement prediction model of a target. In this case, the information storage unit  14  stores information relating to movement prediction model generation. For example, information relating to movement prediction model generation includes position information, a measurement time, and the like of a target in the past. 
     The above is the detailed description of the information storage unit  14 . Note that a step-in coordinate, a step-in range, a target speed, and a tracking range may be predetermined fixed values, or may be values input via a non-illustrated user interface (UI). 
     [Step-in Area Setting Unit] 
     Next, the step-in area setting unit  151  is described. The step-in area setting unit  151  acquires area information from the information storage unit  14 . The step-in area setting unit  151  sets a step-in area, based on the acquired area information, and outputs the set step-in area to the position measurement unit  17 . 
     When a step-in area is fixed, the step-in area setting unit  151  acquires, as area information, a specific area coordinate and at least either a step-in coordinate or a step-in range from the information storage unit  14 . The step-in area setting unit  151  sets a step-in area, based on the acquired area information. 
     When a step-in coordinate is used, the step-in area setting unit  151  sets, to a step-in area, a region between a range set by the step-in coordinate and a range set by the specific area coordinate. On the other hand, when a step-in range is used, the step-in area setting unit  151  sets, to a step-in area, a region having a width of the step-in range from a boundary of the specific area coordinate. 
     When a step-in area dynamically changes, the step-in area setting unit  151  acquires a specific area coordinate, a target speed, and a previous scan time from the information storage unit  14 . In this case, the step-in area setting unit  151  updates the previous scan time held in the information storage unit  14  to a current time. The step-in area setting unit  151  calculates a step-in area range R step-in  by applying, to Equation 1, a target speed V and a difference time T diff  from the previous scan time to the current time. Note that, in Equation 1, α is a constant.
 
 R   step-in   =α×V×T   diff   (1)
 
     As in Equation 1, the step-in area range R step-in  is a value being proportional to the target speed V and the difference time T diff . A part from the boundary of the specific area A 12  to the step-in area range R step-in  is a step-in area A step-in . 
     The above is the detailed description of the step-in area setting unit  151 . 
     [Tracking Area Setting Unit] 
     Next, the tracking area setting unit  153  is described. The tracking area setting unit  153  acquires target information from the information storage unit  14 . The tracking area setting unit  153  sets a tracking area, based on the target information acquired from the information storage unit  14 , and outputs the set tracking area to the position measurement unit  17 . 
     When a tracking area is fixed, the tracking area setting unit  153  acquires a target position and a tracking range from the information storage unit  14 , and sets a tracking area. In relation to the target  111  in  FIG.  4   , the tracking area setting unit  153  acquires a target position (x9, y9) and a tracking range (5 m), and sets a range with a radius of 5 m around the target position (x9, y9) to a tracking area. 
     When a tracking area dynamically changes, the tracking area setting unit  153  acquires, as target information, a target position, the target speed V, and a previous scan time. The tracking area setting unit  153  sets a tracking area, based on the acquired target information. In this case, in relation to the target  111  in  FIG.  4   , the tracking area setting unit  153  updates the previous scan time to a current time. The tracking area setting unit  153  calculates a tracking range R tracking  by applying, to Equation 2, the target speed V and a difference time T diff  from the previous scan time to the current time. Note that, in Equation 2, β is a constant.
 
 R   tracking   =β×V×T   diff   (2)
 
     A tracking area set for the target  111  is a region with the radius R tracking  around the target position (x9, y9). The tracking range R tracking  is a value being proportional to the target speed V and the difference time T diff . Note that a tracking area is not limited to a circular shape. For example, the tracking range R tracking  may be set by using a Kalman filter or the like, based on movement prediction of a target. When the Kalman filter is used, the tracking area setting unit  153  performs movement prediction by acquiring, from the information storage unit  14 , past position information of a target and information of a time in which the target is present at the position, and sets a tracking area. 
     For example, when the number of targets being tracked is N, the tracking area setting unit  153  performs calculation of Equation 2 in relation to each target (N is a natural number). Then, the tracking area setting unit  153  sets, to a tracking area A tracking , a union of tracking areas A tracking_n  each set for each target by Equation 3 (n is a natural number).
 
 A   tracking   =A   tracking_1   ∪A   tracking_2   ∪ . . . ∪A   tracking_N   (3)
 
     The above is the detailed description of the tracking area setting unit  153 . 
     [Position Measurement Unit] 
     Next, the position measurement unit  17  is described. The position measurement unit  17  includes the following three functions. 
     The first function of the position measurement unit  17  is a function of setting a scan area. The position measurement unit  17  sets a scan area using a step-in area acquired from the step-in area setting unit  151 , and a tracking area acquired from the tracking area setting unit  153 . 
     The position measurement unit  17  calculates a scan area A scan  by the following Equation 4 or 5 using an acquired step-in area A step-in  and a tracking area A tracking . Note that, in Equation 5, A interest  indicates the whole specific area A 12 .
 
 A   scan   =A   step-in   ∪A   tracking   (4)
 
 A   scan   =A   step-in ∪( A   tracking   ∩A   interest )  (5)
 
     Equation 4 adds a condition that a region outside the specific area A 12  is also designated as the scan area A scan  when the tracking area A tracking  includes outside of the specific area A 12 . On the other hand, Equation 5 adds a condition that a region outside the specific area A 12  is not designated as the scan area A scan  when the tracking area A tracking  includes the outside of the specific area A 12 . 
     The second function of the position measurement unit  17  is a function of performing position measurement in a scan area. The position measurement unit  17  performs position measurement in a scan area using an IF signal acquired from the beat signal generation unit  13 . A section in the scan area A scan  is a range included in a set in Expression 6, among sections U (r scan , θ scan ) represented by a distance r scan  and an angle θ scan .
 
{ U ( r   scan ,θ scan )|( r   scan ×cos θ scan   ,r   scan ×sin θ scan )∈ A   scan }  (6)
 
     In the example of  FIG.  3   , a section U 1  is a section inside the step-in area A 13  indicated by a distance r and an angle θ. 
     From now on, a method of calculating spectral intensity of a section U (r scan , θ scan ) as a position spectrum is illustrated. 
     First, in order to calculate spectral intensity of a section, the position measurement unit  17  calculates a spectrum (hereinafter, a range spectrum) from the position measurement device  1  to a target. When the FMCW is applied, the position measurement unit  17  calculates a range spectrum P range  by Fourier-transforming an IF signal IF signal  (Equation 7).  FIG.  5    is one example of a graph illustrating a relation between a difference frequency ΔF calculated by use of Equation 7, and spectral intensity.
 
 P   range   =FFT ( IF   signal )  (7)
 
       FIG.  6    is a graph for describing a mechanism of performing position measurement by applying the FMCW. The position measurement unit  17  calculates a distance r to a target by applying, to Equation 8, the difference frequency ΔF, a sweep time T sweep , a frequency bandwidth BW, and a parameter of light speed c. 
     
       
         
           
             
               
                 
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     Next, the position measurement unit  17  calculates a position spectrum of a section by calculating a spectrum of an angle of a target at any distance from the position measurement unit  17 . The position measurement unit  17  detects a direction of the target located at the distance r, by implementing a direction-of-arrival estimation method for a range spectrum P range (r) of a distance from a reception antenna to the target. Moreover, the position measurement unit  17  may implement, by using Equation 8, a direction-of-arrival estimation method for a range spectrum of the difference frequency ΔF associated with the distance r. 
     An example of using a beamformer method as the direction-of-arrival estimation method is described below. Note that Capon or a linear prediction method may be used as the direction-of-arrival estimation method. Moreover, multiple signal classification (MUSIC) may be used as the direction-of-arrival estimation method. Further, estimation of signal parameters via rotational invariance techniques (ESPRIT) may be used as the direction-of-arrival estimation method. 
     The position measurement unit  17  uses a weight vector a(θ) expressed by Equation 9, in order to direct a main lobe of an array antenna to the angle θ. Note that, in Equation 9, T indicates a transposed matrix, j indicates an imaginary unit, λ indicates a wavelength of a central frequency, K indicates the number of elements of a reception antenna, and d 1  indicates a distance from a reference point of a device to a position of an i-th element. 
     
       
         
           
             
               
                 
                   
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     The position measurement unit  17  calculates, as a position spectrum, spectral intensity P position  (r, θ) at the distance r and the angle θ, by using Equation 10. Note that the position measurement unit  17  may calculate 0 in a range satisfying the condition of Expression 6. In Equation 10, H represents a conjugate transposed matrix. Moreover, in Equation 10, a correlation matrix R xx (r) is calculated from the range spectrum P range (r) of any difference frequency component ΔF received from a plurality of reception antennas. Any difference frequency component ΔF is equivalent to any distance. 
     
       
         
           
             
               
                 
                   
                     
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       FIG.  7    is one example of a result of performing direction-of-arrival estimation using a range spectrum at a distance r 1  calculated by applying a difference frequency component ΔF 1  exceeding a threshold value in  FIG.  5    to Equation 8. The position measurement unit  17  performs position measurement of a target by calculating a position spectrum P position  (r, θ) using Equation 10, for the distance r scan  and the angle θ scan  satisfying the condition of Expression 6. As above, the position measurement unit  17  calculates the spectral intensity P position  (r, θ) in the section U (r scan , θ scan ) represented by a distance and an angle. 
     A scan area being a union of a tracking area and a step-in area does not include an internal area, and therefore, has a small area as compared with a specific area. Thus, the number of sections calculated by using Equation 10 is reduced, and a time required for position measurement is shortened by calculating within a range of a scan area, rather than calculating over the whole specific area including the internal area. 
     The third function of the position measurement unit  17  is a function of updating information in the information storage unit  14 . The position measurement unit  17  determines, by using calculated spectral intensity, whether a target is located inside an area, and updates information in the information storage unit  14  according to a determination result. For example, when spectral intensity in a certain section exceeds a preset threshold value, the position measurement unit  17  determines that a target is located in the section. 
     In  FIG.  5   , spectral intensity of a spectrum having a peak at difference frequency ΔF 1  exceeds a threshold value. Moreover, in  FIG.  7   , spectral intensity exceeds a threshold value at angles θ 1  and θ 2 , in relation to the distance r 1  calculated by using the difference frequency ΔF 1 . In other words, the position measurement unit  17  determines that targets are located at positions of coordinates indicated by (r 1  cos θ 1 , r 1  sin θ 1 ) and (r 1  cos θ 2 , r 1  sin θ 2 ). 
     When a target is detected in a scan area, the position measurement unit  17  updates or newly registers position information of the target held in the information storage unit  14 . 
     When newly registering position information (target position) of the target in the information storage unit  14 , the position measurement unit  17  registers the position information in association with an identifier (target identifier). An identifier may be attached in any way as long as a target being tracked can be identified. For example, the position measurement unit  17  uses a monotonously increasing numeral as an identifier. 
     On the other hand, in relation to a target which is not detected in a scan area, the position measurement unit  17  deletes information about the target from the information storage unit  14 . In the present function, the position measurement unit  17  deletes information about all targets from the information storage unit  14 , when no target is detected in a scan area. Moreover, when at least one target is detected in a scan area, the position measurement unit  17  deletes or updates information held in the information storage unit  14 , after determining whether or not a target currently being tracked is located inside a specific area. Further, when determining that a target for which a tracking area is set is located outside the area, the position measurement unit  17  deletes information about the target from the information storage unit  14 . 
     Herein, a method of determining that a target is located outside the specific area A 12  is described by use of  FIG.  8   . In  FIG.  8   , a target  113  is located in the step-in area A 13 , and a part of the tracking area A 15  set in line with the target  113  includes outside of the specific area A 12 . An area being the tracking area A 15  and being outside the specific area A 12  is a region which may not be set to a scan area, as described by using Equations 4 and 5. 
     In a case of performing position measurement for the whole tracking area A 15  (Equation 4), the position measurement unit  17  determines that the target  113  is outside the specific area A 12 , when a position of the target  113  is outside the specific area A 12 . 
     On the other hand, in a case of not performing position measurement for a region being the tracking area A 15  and being outside the specific area A 12  (Equation 5), the position measurement unit  17  determines that a position of a target is outside the specific area A 12 , when no target is detected in the tracking area A 15 . 
     The above is the description of details of the components of the position measurement device  1  according to the present example embodiment. Note that, although a two-dimensional coordinate system is used in the above description in order to simplify the description, the scheme according to the present example embodiment may replace the two-dimensional coordinate system with a two-dimensional polar coordinate illustrated in  FIG.  9    or a three-dimensional coordinate system illustrated in  FIG.  10   . In the example of  FIG.  9   , a position indicated by (X, Y) in the two-dimensional coordinate system is indicated by (r cos θ, r sin θ) in the two-dimensional polar coordinate. Moreover, in the example of  FIG.  10   , a position indicated by (X, Y, −Z) in the three-dimensional coordinate system is indicated by (R sin θ 1  cos θ 2 , R sin θ 1  sin θ 2 , −R cos θ 1 ) in a three-dimensional polar coordinate. Note that, although an example in which an antenna is located at an origin of each coordinate system is illustrated in each of  FIGS.  9  and  10   , an origin of each coordinate system may be set at a position other than a position of the antenna. 
     (Operation) 
     Next, an operation of the position measurement device  1  according to the present example embodiment is described with reference to the drawings.  FIG.  11    is a flowchart for describing the operation of the position measurement device  1 . Note that, although a component of the position measurement device  1  is designated as an operation agent below, the position measurement device  1  itself may be designated as an operation agent. 
     The flowchart in  FIG.  11    illustrates processing from a point of receiving a reflected wave of an electromagnetic wave sent toward a measurement target area to a point of performing position measurement of a target using a signal of the reflected wave. The processing in the flowchart in  FIG.  11    may be regularly executed, or may be irregularly executed. For example, the position measurement device  1  repeatedly executes the processing in the flowchart in  FIG.  11    every second. 
     In  FIG.  11   , first, the beat signal generation unit  13  generates an IF signal by mixing, for each pair of transmission/reception antennas, a transmission signal generated by the signal transmission unit  11  with a reception signal received by the signal reception unit  12  (step S 11 ). The beat signal generation unit  13  outputs the generated IF signal to the position measurement unit  17 . 
     Next, the step-in area setting unit  151  acquires area information from the information storage unit  14  in response to a request from the position measurement unit  17 , and sets a step-in area, based on the acquired area information (step S 12 ). The step-in area setting unit  151  outputs the set step-in area to the position measurement unit  17 . 
     When a set step-in area is fixed, the step-in area setting unit  151  is able to set a step-in area as follows. For example, the step-in area setting unit  151  acquires specific area coordinates and a step-in coordinates in  FIG.  4   , and sets, to a step-in area, a region between a line connecting the specific area coordinates and a line connecting the step-in coordinates. Moreover, for example, the step-in area setting unit  151  acquires specific area coordinates and a step-in range in  FIG.  4   , and sets, to a step-in area, a region within the step-in range from a line connecting the specific area coordinates. 
     When a step-in area dynamically changes, the step-in area setting unit  151  is able to set a step-in area as follows. For example, the step-in area setting unit  151  acquires specific area coordinates, a target speed, and a previous scan time in  FIG.  4   , determines width from a line connecting the specific area coordinates using an elapsed time from previous scan, and the target speed, and sets a step-in area. The step-in area setting unit  151  calculates a step-in area range by applying, to Equation 1, the target speed, and a difference time between the previous scan time and a current time. Then, the step-in area setting unit  151  updates all scan times in the information storage unit  14  to the current time. 
     Next, the tracking area setting unit  153  acquires target information from the information storage unit  14  in response to a request from the position measurement unit  17 , and sets a tracking area (step S 13 ). The tracking area setting unit  153  outputs the set tracking area to the position measurement unit  17 . Note that a target identifier associated with a target position may be acquired from the information storage unit  14  and then output to the position measurement unit  17  by the tracking area setting unit  153 , or may be acquired from the information storage unit  14  by the position measurement unit  17 . 
     When a tracking area is fixed, for example, the tracking area setting unit  153  acquires a target position and a tracking range in  FIG.  4   . Then, the tracking area setting unit  153  sets, to a tracking area, a region in a circle with a radius of the tracking range around the target position. 
     When a tracking area dynamically changes, the tracking area setting unit  153  is able to set a tracking area as follows. For example, the tracking area setting unit  153  acquires a target position, a target speed, and a previous scan time in  FIG.  4   , and determines a tracking range from an elapsed time from previous scan, and the target speed. Then, the tracking area setting unit  153  sets, to a tracking area, a region in a circle with a radius of the tracking range around the target position. 
     The tracking area setting unit  153  calculates a tracking range by applying, to Equation 2, the target speed, and a difference time between the previous scan time and the current time. Moreover, for example, the tracking area setting unit  153  is able to set a tracking area also by acquiring, from the information storage unit  14 , past position information of a target and information of a time at which the target has been present at the position, and performing movement prediction based on a movement prediction model. When the number of targets being tracked is N, the tracking area setting unit  153  sets a tracking area in relation to each target, and sets, to a tracking area, a union of tracking areas each set in relation to each target. 
     Next, the position measurement unit  17  sets a scan area to be a scan target, by use of the step-in area acquired from the step-in area setting unit  151 , and the tracking area acquired from the tracking area setting unit  153  (step S 14 ). The position measurement unit  17  calculates a scan area by applying the step-in area and the tracking area to Equation 4 or 5. 
     Next, the position measurement unit  17  performs position measurement for all sections inside the scan area, and determines whether a target is located inside the specific area A 12  (step S 15 ). The position measurement unit  17  calculates a position spectrum by use of Equation 10, for all the sections inside the scan area, by using the condition of Expression 6. The position measurement unit  17  determines that a target is located in a section in which intensity of a position spectrum exceeds a threshold value. 
     When determining that a target is located inside the specific area A 12  (Yes in step S 15 ), the position measurement unit  17  registers position information (target position) of the target in the information storage unit  14  (step S 16 ). Note that, in relation to a newly detected target, the position measurement unit  17  registers an identifier (target identifier) of the target in the information storage unit  14  in association with the position information. 
     On the other hand, when determining that a target is located outside the specific area A 12  (No in step S 15 ), the position measurement unit  17  deletes information about the target from the information storage unit  14  (step S 17 ). Specifically, the position measurement unit  17  deletes, from the information storage unit  14 , position information of a target coinciding with an identifier (target identifier) of the target determined to be located outside the specific area A 12 , together with the identifier. 
     The above is the description of the operation of the position measurement device  1  according to the present example embodiment. 
     (Advantageous Effect) 
     As above, the position measurement device according to the present example embodiment executes position measurement in relation to a scan area including a tracking area and a step-in area of a target already detected in previous position measurement. Thus, the position measurement device according to the present example embodiment is able to thoroughly detect, in a step-in area, a target entering a specific area, while tracking a target located inside the specific area. 
     With a general scheme, position measurement is performed for all sections of a specific area. On the other hand, the scheme according to the present example embodiment reduces a region to be scanned, by restricting to a tracking area and a step-in area and thus performing position measurement, and therefore, is able to shorten a required time of a scan. 
     In other words, according to the present example embodiment, a first advantageous effect that a scan time can be shortened can be acquired. A reason for this is that position measurement is performed in a step-in area set along an inner periphery of a boundary of a specific area, and therefore, processing is completed in a short time as compared with a case where position measurement is performed for all sections of the specific area. 
     Furthermore, according to the present example embodiment, a second advantageous effect that a target entering a specific area can be detected in real time can be acquired. A reason for this is that a target entering a specific area can be thoroughly detected in a step-in area. 
     (Related Art) 
     Herein, a related art of the scheme according to the present example embodiment is described with reference to the drawings. 
       FIG.  12    is an example of receiving a reflected wave of an electromagnetic wave transmitted from a transmission/reception antenna  10 , and scanning all sections of the specific area A 12 . A scheme in  FIG.  12    is able to determine presence or absence of the target  111  by scanning each section U uniquely determined by a distance R and an angle θ from the transmission/reception antenna  10 , and measuring spectral intensity of the section U. The example of  FIG.  12    has a time to scan all sections of the specific area A 12 . For example, assuming that the number of sections included in the specific area A 12  is N section , and a scan time required for position measurement for one section is T scan , a total scan time T total  is calculated by Equation 11.
 
 T   total   =N   section   ×T   scan   (11)
 
     In this way, the scheme in  FIG.  12    scans all sections of the specific area A 12 , and therefore, requires time to detect the target  112  entering the specific area A 12  within one position measurement time (total scan time T total ). 
       FIG.  13    is an example of, when performing position measurement for all sections of a specific area with the scheme in  FIG.  12    and then detecting the target  111 , tracking the detected target  111 . In the example of  FIG.  13   , all sections inside the specific area A 12  are scanned by low-frequency position measurement. Then, when the target  111  is detected inside the specific area, a range (the tracking area A 15 ) in which the target  111  is estimated to move is tracked by high-frequency position measurement in next position measurement. 
     A scheme in  FIG.  13    enables to shorten a scan time by tracking the target  111  with concentration. However, the scheme in  FIG.  13    does not enable to measure, in real time, a position of a target entering the specific area A 12  in a period of tracking the target  111  with concentration. 
     Moreover, the scheme in  FIG.  13    scans all sections inside the specific area A 12  by low-frequency position measurement, and therefore, when a wait time of position measurement is prolonged too much, detection of a target entering the specific area A 12  during the wait time is delayed. 
     Further, the schemes in  FIGS.  12  and  13    enables to increase measurement accuracy of position measurement by setting a fine interval between distance measurement and angle measurement. However, scanning too finely increases the number of sections to be scanned, and prolongs a required time for scanning. 
     The scheme according to the present example embodiment sets a step-in area in line with a boundary of a specific area, and performs position measurement of a target in the step-in area. Since a target always passes through the step-in area when entering the specific area, a target entering the specific area can be thoroughly detected. Moreover, in the scheme according to the present example embodiment, a target can be certainly detected in a step-in area by providing the step-in area in line with a distance in which the target moves in a scan time interval. Moreover, the scheme according to the present example embodiment enables to shorten a required time for position measurement by scanning a section inside a step-in area with concentration. In other words, the present example embodiment enables to shorten a scan time by performing position measurement of a limited scan area while tracking a detected target. 
     Second Example Embodiment 
     Next, a position measurement device according to a second example embodiment of the present invention is described with reference to the drawings. The present example embodiment is different from the first example embodiment in including a pre-scan function of measuring at least either a distance or a direction of a target. 
     (Configuration) 
       FIG.  14    is a block diagram illustrating a configuration of a position measurement device  2  according to the present example embodiment. As in  FIG.  14   , the position measurement device  2  includes a signal transmission unit  21 , a signal reception unit  22 , a beat signal generation unit  23 , an information storage unit  24 , an area setting unit  25 , a pre-scan unit  26 , and a position measurement unit  27 . 
     The signal transmission unit  21 , the signal reception unit  22 , the information storage unit  24 , and the area setting unit  25  are similar to the corresponding components of the position measurement device  1  according to the first example embodiment, and therefore, description thereof is omitted. 
     The beat signal generation unit  23  is similar to the component of the position measurement device  1 , but is different from the position measurement device  1  in outputting a generated IF signal to the pre-scan unit  26 . 
     The pre-scan unit  26  performs at least either distance measurement or direction measurement before position measurement by the position measurement unit  27 , and determines whether a target is located inside the specific area. 
     When a target is detected in a specific area, the pre-scan unit  26  outputs a range spectrum or an IF signal to the position measurement unit  27 . Note that, when a target is detected in a specific area, the pre-scan unit  26  may output distance information and direction information of the target to the position measurement unit  27 , in addition to a range spectrum or an IF signal. 
     On the other hand, when no target is detected in a specific area, the pre-scan unit  26  deletes target information held in the information storage unit  24 . 
     As in  FIG.  15   , the pre-scan unit  26  includes a distance measurement unit  261  which measures a distance to a target, and a direction measurement unit  263  which measures a direction of a target. 
     The distance measurement unit  261  measures a distance to a target, and determines whether the target is located inside a specific area. 
     When a target is detected in a specific area, the distance measurement unit  261  outputs an IF signal or a range spectrum to the position measurement unit  27 . In this instance, the distance measurement unit  261  may output acquired distance information of the target to the position measurement unit  27 . Note that the distance information of the target may have a plurality of candidates. On the other hand, when no target is detected in a specific area, the distance measurement unit  261  deletes information about the target held in the information storage unit  24 . 
     For example, the distance measurement unit  261  calculates a range spectrum by use of Equation 7. The distance measurement unit  261  determines presence or absence of a target by determining whether or not the acquired spectrum has a peak exceeding a set threshold value. When a distance r calculated using Equation 8 has a value indicating inside of a specific area A interest  with regard to a difference frequency ΔF having spectral intensity exceeding a threshold value, the distance measurement unit  261  outputs a range spectrum P range (r) of the difference frequency ΔF to the position measurement unit  27 . Note that the distance measurement unit  261  determines whether the distance r is inside the specific area, by whether the distance r is included in a set in Expression 12.
 
{ U ( r   interest ,θ interest )|( r   interest ×cos θ interest   ,r   interest ×sin θ interest )∈ A   interest }  (12)
 
     From now on, it is assumed that the distance r associated with difference frequencies ΔF 1  to ΔF M  of a range spectrum exceeding a threshold value is a set represented by Expression 13 (M is a natural number).
 
{ r   peak1   ,r   peak2   , . . . ,r   peakM }  (13)
 
     The distance measurement unit  261  outputs the set of Expression 13 and the range spectrum P range (r) to the position measurement unit  27 . 
     The direction measurement unit  263  performs direction-of-arrival estimation for an IF signal, measures a direction of a target from an acquired angle spectrum, and determines whether a target is located inside a specific area. 
     When a target detects a target inside a specific area, the direction measurement unit  263  outputs an IF signal to the position measurement unit  27 . On the other hand, when no target is detected in a specific area, the direction measurement unit  263  deletes target information held in the information storage unit  24 , in a way similar to the distance measurement unit  261 . In this instance, the direction measurement unit  263  may output acquired direction information of the target to the position measurement unit  27 . Note that the direction information of the target may have a plurality of candidates. 
     In the first example embodiment, the correlation matrix R xx (r) is acquired by using the range spectrum P range (r) having any distance r. In the present example embodiment, a correlation matrix R xx  is calculated by use of an IF signal. Specifically, the direction measurement unit  263  calculates an angle spectrum P angle (θ) by applying the correlation matrix R xx  to Equation 14, by using a beamformer method being a direction-of-arrival estimation method. Note that the direction measurement unit  263  may use a direction-of-arrival estimation method other than the beamformer method, as in the first example embodiment. Note that θ may be in a range satisfying the condition of Expression 12. 
     
       
         
           
             
               
                 
                   
                     
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     The direction measurement unit  263  determines presence or absence of a target in a specific area by determining whether a spectrum acquired by Equation 14 exceeds a threshold value. 
     From now on, it is assumed that an angle θ of an angle spectrum exceeding a threshold value is included in a set represented by Expression 15 (N is a natural number).
 
{θ peak1 ,θ peak2 , . . . ,θ peakN }  (15)
 
     The direction measurement unit  263  outputs the set of Expression 15 and the IF signal to the position measurement unit  27 . 
     Next, the position measurement unit  27  is described. The position measurement unit  27  includes three functions. Note that a first and third functions of the position measurement unit  27  are similar to those in the first example embodiment, and therefore, description thereof is omitted. 
     A second function of the position measurement unit  27  is a function of inputting an IF signal or a range spectrum from the pre-scan unit  26 , and performing position measurement of a scan area by use of the input IF signal or range spectrum. Note that, when inputting an IF signal, the second function of the position measurement unit  27  is the same as the second function of the position measurement unit  27  in the first example embodiment. 
     When acquiring distance information or direction information of a target from the pre-scan unit  26 , the position measurement unit  27  may calculate only a position spectrum regarding a distance included in the set of Expression 13 or a direction included in the set of Expression 15, at a time of calculating a position spectrum by using Equation 10. 
     As one example, a case where the distance measurement unit  261  detects a target in a region at a distance r from the position measurement device  2  is described. In this instance, the position measurement unit  27  acquires a range spectrum P range (r) having a distance r, from the distance measurement unit  261 . The position measurement unit  27  calculates a correlation matrix R xx (r) by using the range spectrum P range (r), and calculates a position spectrum P position  (r, θ) by using Equation 10. On the other hand, the position measurement unit  27  does not calculate a position spectrum in relation to a region which is not at the distance r in the scan area. 
     [Measurement Target Area] 
     Herein, a measurement target area to which a transmission signal is transmitted from the position measurement device  2  is described.  FIG.  16    is an example of performing distance measurement as a pre-scan, and  FIG.  17    is an example of performing direction measurement as a pre-scan. 
       FIG.  16    is a conceptual diagram for describing a measurement target area A 21  of the position measurement device  2  when distance measurement is performed as a pre-scan. Inside the measurement target area A 21 , a specific area A 22 , a step-in area A 23 , an internal area A 24 , a tracking area A 25 , a pre-scan area A 26 , and a position measurement area A 27  are set. Note that the specific area A 22 , the step-in area A 23 , the internal area A 24 , and the tracking area A 25  are similar to those in the first example embodiment, and therefore, description thereof is omitted. 
     In the example of  FIG.  16   , a target  211  and a target  212  are located inside the specific area A 22 . The target  211  is a target whose position is already measured in previous position measurement. On the other hand, the target  212  is a target which has been located outside the specific area A 22  in the previous position measurement, but newly entered the specific area A 22  in current position measurement. 
     In  FIG.  16   , a region in a range at a distance including the specific area A 22  in the measurement target area A 21  is designated as the pre-scan area A 26 . The distance measurement unit  261  detects a target located in the pre-scan area A 26 , and calculates a distance between the target and the position measurement device  2 . 
     Furthermore, in  FIG.  16   , the target  212  is located in a place at a distance r 1  from the position measurement device  2 . The position measurement unit  27  performs position measurement with regard to a region (the position measurement area A 27 ) at the distance r 1  from the position measurement device  2 , in the scan area combining the step-in area A 23  and the tracking area A 25 . 
       FIG.  17    is a conceptual diagram for describing the measurement target area A 21  of the position measurement device  2  when direction measurement is performed as a pre-scan. Inside the measurement target area A 21 , a specific area A 22 , a step-in area A 23 , an internal area A 24 , a tracking area A 25 , a pre-scan area A 28 , and a pre-scan area A 29  are set. Note that the specific area A 22 , the step-in area A 23 , the internal area A 24 , and the tracking area A 25  are similar to those in the example of  FIG.  16   , and therefore, description thereof is omitted. Additionally, in the example of  FIG.  17   , a target  211  and a target  212  are located inside the specific area A 22 , as in the example of  FIG.  16   . 
     In the example of  FIG.  17   , a target located in a region (being the same as the measurement target area A 21 ) in a range at a distance including the specific area A 22  in the measurement target area A 21  is detected, and a direction of the target from the position measurement device  2  can be detected. In  FIG.  17   , the target  212  is located in a direction at an angle θ 1  from the position measurement device  2 . The position measurement unit  27  performs position measurement with regard to a direction region (the pre-scan area A 28  and the pre-scan area A 29 ) at the angle θ 1 , in the scan area combining the step-in area A 23  and the tracking area A 25 , and therefore, a required time for position measurement is shortened. 
     (Operation) 
     Next, an operation of the position measurement device  2  according to the present example embodiment is described with reference to the drawings.  FIG.  18    is a flowchart for describing the operation of the position measurement device  2 . Note that, although a component of the position measurement device  2  is designated as an operation agent below, the position measurement device  2  itself may be designated as an operation agent. 
     The flowchart in  FIG.  18    illustrates processing from a point of receiving a reflected wave of an electromagnetic wave sent toward a measurement target area to a point of performing position measurement of a target by using a signal of the reflected wave, as in the first example embodiment. The processing illustrated by the flowchart in  FIG.  18    is repeated appropriately. 
     In  FIG.  18   , first, the beat signal generation unit  23  generates an IF signal by mixing, for each pair of transmission/reception antennas, a transmission signal generated by the signal transmission unit  21  with a reception signal received by the signal reception unit  22  (step S 21 ). The beat signal generation unit  23  outputs the generated IF signal to the pre-scan unit  26 . 
     Next, the pre-scan unit  26  determines whether a target is located inside a specific area (step S 22 ). When a target is detected inside the specific area (Yes in step S 22 ), the operation proceeds to step S 23 . On the other hand, when no target is detected inside the specific area (No in step S 22 ), the operation proceeds to step S 28 , and target information is deleted from the information storage unit  24  (step S 28 ). 
     When direction measurement is performed as a pre-scan in step S 22 , the pre-scan unit  26  calculates a range spectrum using Equation 7. The pre-scan unit  26  checks presence or absence of a target inside the specific area by whether a peak of the calculated spectrum exceeds a threshold value. 
     When following two conditions are satisfied (equivalent to Yes in step S 22 ), the pre-scan unit  26  outputs the IF signal or the range spectrum P range (r) to the position measurement unit  27 . A first condition is a condition that spectral intensity exceeds a threshold value at a difference frequency ΔF of the range spectrum. The second condition is a condition that a distance r associated with the ΔF acquired by Equation 8 is a value indicating inside of the specific area A interest . In this instance, the pre-scan unit  26  may output distance information of a target to the position measurement unit  27 . 
     Furthermore, when direction measurement is performed as a pre-scan in step S 22 , the pre-scan unit  26  performs direction-of-arrival estimation for the IF signal, and measures a direction from an acquired angle spectrum to a target. The pre-scan unit  26  calculates a correlation matrix R xx  by using the IF signal, and calculates an angle spectrum P angle (θ) by applying the calculated correlation matrix R xx  to Equation 14. 
     The pre-scan unit  26  checks presence or absence of a target inside the specific area A 22  by whether a peak of the calculated spectrum exceeds a threshold value. 
     When determining, by the direction measurement, that a target is present inside the specific area A 22  (equivalent to Yes in step S 22 ), the pre-scan unit  26  outputs the IF signal to the position measurement unit  27 . 
     In this instance, the pre-scan unit  26  may output direction information of the target to the position measurement unit  27 . 
     As in the first example embodiment, the area setting unit  25  acquires area information from the information storage unit  24  in response to a request from the position measurement unit  27 , and sets a step-in area, based on the acquired area information (step S 23 ). The area setting unit  25  outputs the set step-in area to the position measurement unit  27 . 
     Furthermore, as in the first example embodiment, the area setting unit  25  acquires target information from the information storage unit  24  in response to a request from the position measurement unit  27 , and sets a tracking area (step S 24 ). The area setting unit  25  outputs the set tracking area to the position measurement unit  27 . 
     As in the first example embodiment, the position measurement unit  27  acquires the step-in area and the tracking area from the area setting unit  25 . As in the first example embodiment, the position measurement unit  27  sets a scan area to be a scan target, by using the acquired step-in area and tracking area (step S 25 ). 
     The position measurement unit  27  calculates position spectrums for all sections of the scan area by using the IF signal or range spectrum acquired from the pre-scan unit  26 , and determines whether a target is located inside the specific area A 22  (step S 26 ). In this instance, the position measurement unit  27  may perform position measurement exclusively for a section coinciding with the distance information or the direction information of the target being an input from the pre-scan unit  26 . 
     When determining that a target is located inside the specific area A 22  (Yes in step S 26 ), the position measurement unit  27  registers position information (target position) of the target in the information storage unit  24 , as in the first example embodiment (step S 27 ). 
     On the other hand, when determining that a target is located outside the specific area A 22  (No in step S 26 ), the position measurement unit  27  deletes information about the target from the information storage unit  24  (step S 28 ). 
     The above is the description of the operation of the position measurement device  2  according to the present example embodiment. 
     (Advantageous Effect) 
     As above, in the present example embodiment, position measurement is not performed when no target is detected in a pre-scan. In other words, in the present example embodiment, when a target is located inside a specific area, a distance or a direction of the target is calculated by a pre-scan, and position measurement is performed in the position measurement unit in relation to a limited distance or direction in a scan area. Thus, according to the present example embodiment, a scan time can be shortened by reducing unnecessary position measurement, as compared with the first example embodiment. 
     Third Example Embodiment 
     Next, a position measurement device according to a third example embodiment of the present invention is described with reference to the drawings. The position measurement device according to the present example embodiment is different from that according to the first example embodiment in including a pre-scan function of measuring either a distance or a direction of a target located in a step-in area, and in skipping position measurement itself when no target is detected in a pre-scan. 
     (Configuration) 
       FIG.  19    is a block diagram illustrating a configuration of a position measurement device  3  according to the present example embodiment. As in  FIG.  19   , the position measurement device  3  includes a pre-scan unit  36 , in addition to a signal transmission unit  31 , a signal reception unit  32 , a beat signal generation unit  33 , an information storage unit  34 , an area setting unit  35 , and a position measurement unit  37 . 
     The signal transmission unit  31 , the signal reception unit  32 , and the information storage unit  34  are similar to components of the position measurement device  1  according to the first example embodiment, and therefore, description thereof is omitted. 
     The beat signal generation unit  33  is similar to the component of the position measurement device  1 , but is different from the position measurement device  1  in outputting a generated IF signal to the pre-scan unit  36 . 
     The area setting unit  35  is similar to the component of the position measurement device  1 , but is different from the position measurement device  1  in outputting a step-in area to the pre-scan unit  36 . 
     The pre-scan unit  36  acquires a step-in area from the area setting unit  35 . The pre-scan unit  36  separates the acquired step-in area into a distance measurement area for pre-scanning by distance measurement, and a direction measurement area for pre-scanning by direction measurement. Note that a step-in area is equivalent to a sum of a distance measurement area and a direction measurement area. 
     Then, the pre-scan unit  36  performs, before position measurement by the position measurement unit  37 , distance measurement in relation to a distance measurement area and direction measurement in relation to a direction measurement area, and determines whether a target is located inside the step-in area. 
     When a target is detected in the step-in area, the pre-scan unit  36  outputs a range spectrum or an IF signal to the position measurement unit  37 . Note that, when a target is detected in the step-in area, the pre-scan unit  36  may output distance information or direction information of the target to the position measurement unit  37 , in addition to a range spectrum or an IF signal. 
     On the other hand, when no target is detected inside the step-in area, the pre-scan unit  36  deletes target information held in the information storage unit  34 . 
     Herein, a detailed configuration of the pre-scan unit  36  is described. As in  FIG.  20   , the pre-scan unit  36  includes a distance measurement unit  361 , a direction measurement unit  363 , and an area assignment unit  365 . 
     The distance measurement unit  361  acquires an IF signal from the beat signal generation unit  33 . The distance measurement unit  361  performs distance measurement in relation to a distance measurement area assigned by the area assignment unit  365 , and determines whether a target is located inside the distance measurement area. 
     The distance measurement unit  361  outputs a range spectrum to the position measurement unit  37  regardless of whether a target is located inside the distance measurement area. In this instance, the distance measurement unit  361  may output information relating to a distance of a target to the position measurement unit  37 . Assuming that a distance measurement area assigned by the area assignment unit  365  is A h_step-in , whether a distance r is in a scan range can be determined by whether the distance r is included in a set of Expression 16.
 
{ U ( r   h_step-in )|( r   h_step-in ×cos θ, r   h_step-in ×sin θ)∈ A   h_step-in }  (16)
 
     The direction measurement unit  363  acquires an IF signal from the beat signal generation unit  33 . The direction measurement unit  363  performs direction measurement in relation to a direction measurement area assigned by the area assignment unit  365 , and determines whether a target is located inside the direction measurement area. 
     The direction measurement unit  363  outputs the IF signal to the position measurement unit  37  regardless of whether a target is located inside the direction measurement area. In this instance, the direction measurement unit  363  may output information relating to a direction of a target to the position measurement unit  37 . Assuming that a direction measurement area assigned by the area assignment unit  365  is A v_step-in , whether a direction θ is in a scan range can be determined by whether the direction θ is included in a set of Expression 17.
 
{ U (θ v_step-in )|( r ×cos θ v_step-in   ,r ×sin θ v_step-in )∈ A   v_step-in }  (17)
 
     The area assignment unit  365  (also referred to as an assignment unit) acquires a step-in area from the area setting unit  35 , and separates the step-in area into a distance measurement area and a direction measurement area. The area assignment unit  365  outputs the distance measurement area to the distance measurement unit  361 , and outputs the direction measurement area to the direction measurement unit  363 . 
     Next, the position measurement unit  37  is described. The position measurement unit  37  includes following three functions. Note that a first and third functions of the position measurement unit  37  are similar to those in the first example embodiment, and therefore, description thereof is omitted. 
     A second function of the position measurement unit  37  is a function of inputting an IF signal or a range spectrum from the pre-scan unit  26 , and performing position measurement of a scan area by using the input IF signal or range spectrum, as in the second example embodiment. However, in the present example embodiment, when no target is detected by the pre-scan unit  36 , position measurement may be performed in a new scan area A new_scan  in which a step-in area A step-in  is removed from a scan area A scan , as in Equation 18. Moreover, an area obtained by removing the distance measurement area and the direction measurement area from the scan area A scan  may be designated as the new scan area A new_scan .
 
 A   new_scan   =A   scan ∩   A   step-in     (18)
 
     [Measurement Target Area] 
     Herein, a measurement target area to which a transmission signal is transmitted from the position measurement device  3  is described.  FIG.  21    is a conceptual diagram for describing a measurement target area A 31  targeted for measurement by the position measurement device  3 . Inside the measurement target area A 31 , a specific area A 32 , a distance measurement area A 33 , a direction measurement area A 34 , a tracking area A 35 , a scan range A 36 , and a scan range A 37  are set. The distance measurement area A 33  and the direction measurement area A 34  configure a step-in area. Note that the specific area A 32  and the tracking area A 35  are similar to those in the first example embodiment, and therefore, description thereof is omitted. 
     In the example of  FIG.  21   , a target  311  located inside the specific area A 32 . The target  311  is a target whose position is already measured in previous position measurement. 
     The area assignment unit  365  outputs the distance measurement area A 33  in the step-in area to the distance measurement unit  361 . The distance measurement unit  361  pre-scans a scan range including the distance measurement area A 33  by distance measurement. In  FIG.  21   , the scan range A 36  is a range scanned in order to cover the distance measurement area A 33 . The distance measurement unit  361  outputs a range spectrum of the pre-scanned scan range A 36  to the position measurement unit  37 . 
     Furthermore, the area assignment unit  365  outputs the direction measurement area A 34  in the step-in area to the direction measurement unit  363 . The direction measurement unit  363  pre-scans, by direction measurement, a range including the direction measurement area A 34  in the step-in area. In  FIG.  21   , the scan range A 37  is a range scanned in order to cover the direction measurement area A 34 . The direction measurement unit  363  outputs an IF signal of the pre-scanned scan range A 37  to the position measurement unit  37 . 
     Note that, in the example of  FIG.  21   , a target is located in neither the distance measurement area A 33  nor the direction measurement area A 34 , and therefore, position measurement may be performed for the tracking area A 35  of the target  311 . 
     (Operation) 
     Next, an operation of the position measurement device  3  according to the present example embodiment is described with reference to the drawings.  FIG.  22    is a flowchart for describing the operation of the position measurement device  3 . Note that, although a component of the position measurement device  3  is designated as an operation agent below, the position measurement device  3  itself may be designated as an operation agent. 
     The flowchart in  FIG.  22    illustrates processing from a point of receiving a reflected wave of an electromagnetic wave sent toward a measurement target area to a point of performing position measurement of a target by use of a signal of the reflected wave, as in the first example embodiment. The processing in the flowchart in  FIG.  22    is repeated appropriately. 
     In  FIG.  22   , first, the beat signal generation unit  33  generates an IF signal by mixing, for each pair of transmission/reception antennas, a transmission signal generated by the signal transmission unit  31  with a reception signal received by the signal reception unit  32  (step S 301 ). The beat signal generation unit  33  outputs the generated IF signal to the pre-scan unit  36 . 
     Next, the area setting unit  35  acquires area information from the information storage unit  34  in response to a request from the pre-scan unit  36 , and sets a step-in area, based on the acquired area information (step S 302 ). The area setting unit  35  outputs the set step-in area to the pre-scan unit  36 . 
     Next, the area assignment unit  365  of the pre-scan unit  36  separates, into a distance measurement area and a direction measurement area, the step-in area acquired from the area setting unit  35  (step S 303 ). The area assignment unit  365  outputs the distance measurement area to the distance measurement unit  361 , and outputs the direction measurement area to the direction measurement unit  363 . 
     Next, the pre-scan unit  36  determines whether or not a target is located inside a step-in area configured by the distance measurement area and the direction measurement area (step S 304 ). The pre-scan unit  36  outputs a range spectrum and an IF signal to the position measurement unit  37 . 
     When detecting a target inside the step-in area (Yes in step S 304 ), the pre-scan unit  36  notifies the position measurement unit  37  that the target is detected (the operation proceeds to step S 307 ). When receiving the notification from the pre-scan unit  36 , the position measurement unit  37  requests the area setting unit  35  to acquire the step-in area and a tracking area. 
     On the other hand, when no target is detected inside the step-in area (No in step S 304 ), target information is deleted from the information storage unit  34  (step S 305 ). 
     When the target information is deleted from the information storage unit  34  in step S 305 , the pre-scan unit  36  refers to target information in the information storage unit  34 , and checks presence or absence of a target being tracked (step S 306 ). When a target being tracked is present (Yes in step S 306 ), the pre-scan unit  36  outputs an IF signal to the position measurement unit  37  (the operation proceeds to step S 307 ). On the other hand, when no target being tracked is present (No in step S 306 ), the processing along the flowchart in  FIG.  22    is finished. 
     The area setting unit  35  acquires area information from the information storage unit  34  in response to a request from the position measurement unit  37 , and sets a step-in area, based on the acquired area information (step S 307 ). The area setting unit  35  outputs the set step-in area to the position measurement unit  37 . Note that, when information about a step-in area is output from the pre-scan unit  36  to the position measurement unit  37 , step S 307  may be omitted. 
     The area setting unit  35  acquires target information from the information storage unit  34  in response to a request from the position measurement unit  37 , and sets a tracking area (step S 308 ). The area setting unit  35  outputs the set tracking area to the position measurement unit  37 . 
     The position measurement unit  37  acquires the step-in area and the tracking area from the area setting unit  35 . The position measurement unit  37  sets a scan area to be a scan target, by using the acquired step-in area and tracking area (step  309 ). 
     The position measurement unit  37  calculates position spectrums for all sections of the scan area by using the IF signal or range spectrum acquired from the pre-scan unit  36 , and determines whether a target is located inside the specific area (step S 310 ). However, when a tracking target is detected in step S 306  (Yes in step S 306 ), the position measurement unit  37  may perform position measurement only in a new scan area. 
     When determining that a target is located inside a specific area (Yes in step S 310 ), the position measurement unit  37  registers position information (target position) of the target in the information storage unit  34  (step S 311 ). 
     On the other hand, when determining that a target is located outside the specific area (No in step S 310 ), the position measurement unit  37  deletes information about the target from the information storage unit  34  (step S 312 ). 
     The above is the description of the operation of the position measurement device  3  according to the present example embodiment. 
     (Advantageous Effect) 
     As above, in the present example embodiment, position measurement is not performed when no target is detected in a step-in area by the pre-scan unit, in contrast to the second example embodiment. In other words, in the second example embodiment, position measurement is performed when a target is present inside a specific area, whereas, in the present example embodiment, position measurement is not performed unless the distance measurement unit and the position measurement unit sense a target even when a target is present inside a particular area. Thus, according to the present example embodiment, a scan time can be shortened by further reducing unnecessary position measurement, as compared with the second example embodiment. 
     (Hardware) 
     Herein, a hardware configuration which executes processing by the position measurement device according to each example embodiment of the present invention is described by citing an information processing device  90  in  FIG.  23    as one example. Note that the information processing device  90  in  FIG.  23    is a configuration example for executing processing by the position measurement device according to each example embodiment, and does not limit the scope of the present invention. 
     As in  FIG.  23   , the information processing device  90  includes a processor  91 , a main storage device  92 , an auxiliary storage device  93 , an input/output interface  95 , and a communication interface  96 . In  FIG.  23   , an interface is abbreviated as an I/F. The processor  91 , the main storage device  92 , the auxiliary storage device  93 , the input/output interface  95 , and the communication interface  96  are data-communicably connected to one another via a bus  99 . Moreover, the processor  91 , the main storage device  92 , the auxiliary storage device  93 , and the input/output interface  95  are connected to a network such as the Internet or an intranet via the communication interface  96 . 
     The processor  91  extracts a program stored in the auxiliary storage device  93  or the like into the main storage device  92 , and executes the extracted program. In the present example embodiment, a configuration using a software program installed in the information processing device  90  may be provided. The processor  91  executes processing by the position measurement device according to the present example embodiment. 
     The main storage device  92  has a region where a program is deployed. The main storage device  92  may be a volatile memory such as a dynamic random access memory (DRAM). Moreover, a non-volatile memory such as a magnetoresistive random access memory (MRAM) may be configured and added as the main storage device  92 . 
     The auxiliary storage device  93  stores various data. The auxiliary storage device  93  is configured by a local disk such as a hard disk or a flash memory. Note that the main storage device  92  may be configured in such a way as to store various data, and the auxiliary storage device  93  may be omitted. 
     The input/output interface  95  is an interface for connecting the information processing device  90  and peripheral equipment. The communication interface  96  is an interface for connecting to an external system or device through a network such as the Internet or an intranet, based on a standard or a specification. The input/output interface  95  and the communication interface  96  may be formed into a common interface as an interface for connecting to external device. 
     The information processing device  90  may be configured in such a way that input equipment such as a keyboard, a mouse, and a touch panel is connected to the information processing device  90  as required. The input equipment is used for input of information and setting. Note that, when a touch panel is used as input equipment, a display screen of display equipment may be configured in such a way as to double as an interface of the input equipment. Data communication between the processor  91  and input equipment may be mediated by the input/output interface  95 . 
     Furthermore, the information processing device  90  may be equipped with display equipment for displaying information. When being equipped with display equipment, the information processing device  90  preferably include a display control device (not illustrated) for controlling display of the display equipment. The display equipment may be connected to the information processing device  90  via the input/output interface  95 . 
     Still further, the information processing device  90  may be equipped with a disk drive as required. The disk drive is connected to the bus  99 . Between the processor  91  and a non-illustrated recording medium (program recording medium), the disk drive mediates reading of data and a program from the recording medium, writing of a processing result of the information processing device  90  into the recording medium, and the like. The recording medium can be implemented by an optical recording medium such as a compact disc (CD) or a digital versatile disc (DVD). Additionally, the recording medium may be implemented by a semiconductor recording medium such as a universal serial bus (USB) memory or a secure digital (SD) card, a magnetic recording medium such as a flexible disk, or another recording medium. 
     The above is one example of a hardware configuration for enabling the position measurement device according to each example embodiment of the present invention. Note that the hardware configuration in  FIG.  23    is one example of a hardware configuration for executing arithmetic processing by the position measurement device according to each example embodiment, and does not limit the scope of the present invention. Moreover, a program which causes a computer to execute processing relating to the position measurement device according to each example embodiment also falls within the scope of the present invention. Further, a program recording medium recording a program according to each example embodiment also falls within the scope of the present invention. 
     The components of the position measurement device according to each example embodiment can be combined in any way. Moreover, each component of the position measurement device according to each example embodiment may be implemented by software, or may be implemented by a circuit. 
     While the present invention has been described above with reference to the example embodiments, the present invention is not limited to the above-described example embodiments. Various changes which can be understood by a person skilled in the art can be made to a configuration and details of the present invention within the scope of the present invention. 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2017-120117, filed on Jun. 20, 2017, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  2 ,  3  Position measurement device 
           10  Transmission/reception antenna 
           11 ,  21 ,  31  Signal transmission unit 
           12 ,  22 ,  32  Signal reception unit 
           13 ,  23 ,  33  Beat signal generation unit 
           14 ,  24 ,  34  Information storage unit 
           15 ,  25 ,  35  Area setting unit 
           17 ,  27 ,  37  Position measurement unit 
           26 ,  36  Pre-scan unit 
           151  Step-in area setting unit 
           153  Tracking area setting unit 
           261 ,  361  Distance measurement unit 
           263 ,  363  Direction measurement unit 
           365  Area assignment unit