Abstract:
This invention provides communication devices that can increase the possibility of finding a person and further can shorten the time in which the person is found. A sought device ( 2 ) supplies a power to each of a plurality of components thereof during a first time period, thereby performing a reception process. A seeking device ( 1 ) repetitively transmits a call signal during a third time period that is longer than the first interval. When having received the call signal, the sought device ( 2 ) transmits a response signal which includes identification information of the sought device during a fourth time period. The seeking device ( 1 ) uses the response signal to estimate the distance and direction of the sought device ( 2 ), and causes the identification information of the sought device ( 2 ) and information related to the distance and direction of the sought device ( 2 ) to be displayed.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a search apparatus for searching another communication apparatus (search-target apparatus), and also to a communication system composed of a search apparatus and a search-target apparatus. 
       BACKGROUND ART 
       [0002]    Radio-wave beacon systems have been known, each configured to inform, when a mountain climber is lost due to an avalanche or the like, a rescuer carrying a reception apparatus, of the location where the mountain climber is lost, by transmission of a radio wave from a transmission apparatus (small transmitter) carried by the mountain climber (e.g., PTLs 1 and 2 and NPL 1). 
         [0003]    In this radio-wave beacon system, the transmission apparatus transmits a radio wave of 457 kHz as a rescue signal, and the reception apparatus thus can find out the direction and an approximate distance to the transmission apparatus based on the received radio wave. 
       CITATION LIST 
     Patent Literature 
       [0004]    PTL 1 
         [0005]    Japanese Patent Application Laid-Open No. 2003-198389 
         [0006]    PTL 2 
         [0007]    Japanese Patent Application Laid-Open No. 2005-229449 
         [0008]    NPL 1: Report of Conference on Improving Beacon Systems for Searching Lost Mountain Climbers (Conference on Improving Beacon Systems for Searching Lost Mountain Climbers by Ministry of International Affairs and Communications, Western Region (Hokuriku) General Communication Bureau, March 2005) 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    However, the radio-wave beacon system described above uses a medium-wave band frequency (457 kHz), and the transmission output of the radio-wave beacon system is limited within a micro-power range due to the restrictions under the Radio Law, so that the radio-wave receivable distance of the radio wave beacon system is up to approximately 100 m, and the radio wave beacon system cannot conduct a search when the distance in between increases to greater than 100 m. 
         [0010]    As disclosed in NPL 1, for this kind of communication systems, a communication system with a long communication distance is in demand for enabling a wide-range search (e.g., at least several hundred meters in radius). 
         [0011]    Moreover, for this kind of communication systems, apparatuses to be carried by mountain climbers, provided with the following features are in demand: being small and light; being capable of securing surely transmitting a radio wave without any operation of a mountain climber when the mountain climber is lost; and being capable of continuously operating for a long period (e.g., seven days or longer) via power saving, for example. 
         [0012]    In general, in rescuing a lost climber, for example, a plurality of searchers forms a rescue team to rescue a lost climber or the like. In rescue activities, it is important for each member of the rescue team to be aware of the actions of the other rescue members, such as an action that, for example, one of the searchers has found a lost climber. 
         [0013]    Development of the communication system that satisfies these demands further increases the possibility of finding a lost mountain climber or the like and expectedly reduces the time until the lost mountain climber or the like is found. 
         [0014]    An object of the present invention to provide, in consideration of the points mentioned above, a search apparatus and a communication system each capable of increasing the possibility of finding a lost mountain climber or the like and also capable of reducing the time until the lost mountain climber or the like is found, as compared with the related art. 
         [0015]    Another object of the present invention is to provide a search apparatus capable of allowing, when a search-target apparatus of a lost climber or the like is searched for by a plurality of search apparatuses, another search apparatus to easily confirm that one of the plurality of search apparatuses has been able to start communication with the search-target apparatus. 
       Solution to Problem 
       [0016]    A search apparatus according to the present invention is portable and performs radio communication with a portable search-target apparatus, the search apparatus including: a transmission section that transmits a call signal to the search-target apparatus; a reception section that receives a response signal from the search-target apparatus; and a control section that acquires, from the response signal, when the search-target apparatus is in communication with another search apparatus, identification information of the other search apparatus in communication with the search-target apparatus, and that causes the acquired identification information to be output. 
         [0017]    A search apparatus according to the present invention is portable and performs radio communication with a portable search-target apparatus, the search apparatus including: a transmission section that transmits a call signal to the search-target apparatus; a reception section that receives a response signal from the search-target apparatus or a response confirmation signal transmitted from another search apparatus in communication with the search-target apparatus, the response confirmation signal corresponding to the response signal; and a control section that acquires identification information of the search-target apparatus and identification information of the other search apparatus from the response signal or the response confirmation signal, and that causes the acquired identification information to be output. 
       Advantageous Effects of Invention 
       [0018]    According to the present invention, a search-target apparatus is no longer required to always transmit a periodical signal such as a beacon and is only required to transmit a response signal upon reception of a call signal from a search apparatus. Thus, the power consumption of the search-target apparatus is reduced as compared with the apparatus of the related art. For this reason, the search-target apparatus is capable of continuously operating for a long period (e.g., three months or more), thus making it possible to increase the possibility of finding a lost mountain climber, for example. 
         [0019]    According to the present invention, search apparatuses are capable of conducting a wide range search (e.g., 100 m to 5 km), thus increasing the possibility of finding a lost mountain climber or the like and making it possible to reduce the time until the lost mountain climber or the like is found. 
         [0020]    According to the present invention, in an attempt to search for one search-target apparatus, using a plurality of search apparatuses, in a case where one of the search apparatuses has been able to start communication with the search-target apparatus, another search apparatus is allowed to easily find out that the search-target apparatus has been found, and to easily confirm the search apparatus in communication with the search-target apparatus. For this reason, when a search is conducted by a team composed of a plurality of searchers, efficient and fast rescue activities can be performed for a lost climber or the like. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]      FIGS. 1A to 1D  are each an external view of a search apparatus according to Embodiment 1 of the present invention; 
           [0022]      FIG. 2  is an external view of a search-target apparatus according to Embodiment 1 of the present invention; 
           [0023]      FIG. 3  is a block diagram illustrating a configuration of the search apparatus according to Embodiment 1 of the present invention; 
           [0024]      FIG. 4  is a block diagram illustrating a configuration of the search-target apparatus according to Embodiment 1 of the present invention; 
           [0025]      FIG. 5  is a sequence diagram illustrating how communication is performed between the search apparatus and search-target apparatus according to Embodiment 1 of the present invention; 
           [0026]      FIG. 6  is another sequence diagram illustrating how communication is performed between the search apparatus and search-target apparatus according to Embodiment 1 of the present invention; 
           [0027]      FIG. 7  is a diagram illustrating a relationship between a distance and received signal strength; 
           [0028]      FIG. 8  is a diagram illustrating a directivity formed by the search apparatus according to Embodiment 1 of the present invention; 
           [0029]      FIG. 9  is a diagram illustrating an information table stored in a memory section of the search apparatus according to Embodiment 1 of the present invention; 
           [0030]      FIGS. 10A to 10F  are each a diagram illustrating a display screen of the search apparatus according to Embodiment 1 of the present invention; 
           [0031]      FIGS. 11A and 11B  illustrate a flowchart indicating an operation flow of the search apparatus according to Embodiment 1 of the present invention; 
           [0032]      FIG. 12  is a flowchart indicating an operation flow of the search-target apparatus according to Embodiment 1 of the present invention; 
           [0033]      FIG. 13  is a diagram illustrating an example of a use state of a search apparatus according to Embodiment 2 of the present invention; 
           [0034]      FIG. 14  is a sequence diagram illustrating how communication is performed between the search apparatus and a search-target apparatus according to Embodiment 2 of the present invention; 
           [0035]      FIGS. 15A and 15B  illustrate a flowchart indicating an operation flow of the search apparatus according to Embodiment 2 of the present invention; and 
           [0036]      FIG. 16  is a diagram illustrating a display screen of the search apparatus according to Embodiment 2 of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
       [0037]    Hereinafter, a description will be given of Embodiment 1 of the present invention with reference to the accompanying drawings. A communication system according to Embodiment 1 is composed of search apparatus  1  (see  FIGS. 1A to 1D , and  FIG. 3 ) and one or more search-target apparatuses  2  (see  FIGS. 2 and 4 ). Search apparatus  1  performs radio communication with each search-target apparatus  2 . In addition, search apparatus  1  includes all the functions which are provided to search-target apparatus  2  and is capable of performing radio communication with another search apparatus, as a search-target apparatus. 
       Communication Modes 
       [0038]    Search apparatus  1  according to the present invention executes one of the following communication modes based on a user indication. 
         [0039]    (1) Individual Search Mode 
         [0040]    (2) All Search Mode 
         [0041]    (3) Group Search Mode 
         [0042]    (4) Search-Target Mode 
         [0043]    (1) The individual search mode is a communication mode in which search apparatus  1  performs radio communication with single search-target apparatus  2  indicated by a user and estimates a relative position (distance and direction) of this single search-target apparatus  2 . Note that, search apparatus  1  is capable of previously registering search-target apparatuses  2  in association with registration numbers (e.g., 1 to 10). 
         [0044]    (2) The all search mode is a communication mode in which search apparatus  1  makes a request for transmission of a response signal to all search-target apparatuses  2  and then receives a response signal and identifies communicable search-target apparatus  2 . 
         [0045]    (3) The group search mode is a communication mode in which search apparatus  1  makes a request for transmission of a response signal to all the previously registered search-target apparatuses  2  and then receives a response signal and identifies communicable search-target apparatus  2 . 
         [0046]    (4) The search-target mode is a communication mode in which search apparatus  1  performs, as a search-target apparatus, radio communication with another search apparatus. 
       Structure of Search Apparatus  1   
       [0047]    First, a description will be given of a structure of search apparatus  1  using  FIGS. 1A to 1D .  FIGS. 1A to 1D  are a front view, a right side view, a rear view, and an A-A cross-sectional view, respectively. 
         [0048]    Search apparatus  1  is in size and weight that allow a user (standard individual) to carry around (for example, width W: 64 mm, height H: 107 mm, and thickness T: 13 mm and weight may be 70 g). 
         [0049]    Case  11  of search apparatus  1  has a rectangular cube and is formed using a non-conductive member. Front surface  11   a  of case  11  has a flat panel shape. Front surface  11   a  of case  11  is provided with display section  12  and operation section  13 . 
         [0050]    Display section  12  is provided on front surface  11   a  of case  11  and includes a screen composed of a liquid crystal display (LCD), for example. Note that, a description of a display screen in each communication mode will be given hereinafter. 
         [0051]    Operation section  13  is provided near the side of bottom surface  11   b  (lower end side) on front surface  11   a  of case  11  and includes a plurality of buttons. Operation section  13  converts the content of a button operation based on the user&#39;s intention into an electrical signal and transmits the electrical signal to central processing unit (CPU)  13  (see  FIG. 3 ). 
         [0052]    Side surfaces  11   c  and  11   d  of case  11  are thinner on the side of flat surface  11   e  (upper end side) than on the side of bottom surface  11   b  (lower end side) with respect to a center portion of case  11  as the boundary. Right side surface  11   c  is provided with power supply switch  14 . 
         [0053]    Case  11  houses substrate  15  therein. Antenna  101  is patterned on the side of flat surface  11   e  (upper end side) with respect to the center of substrate  15 . In addition, various circuits (see  FIG. 3 ) are mounted on substrate  15 . 
         [0054]    Antenna  101  is composed of: first antenna element  111  serving as a radiator at a center portion of antenna  101 ; and second and third antenna elements  112  and  113  serving as a wave conductor or a reflector at two sides of first antenna element  111 , respectively. 
         [0055]    The length of first antenna element  111  is ¼ of wavelength λ (λ/4). For example, when search apparatus  1  performs radio communication using 920 MHz, the length (λ/4) of first antenna element  111  is approximately 81.5 mm Second and third antenna elements  112  and  113  are identical in length and are shorter than first antenna element  111  in length. 
         [0056]    Antenna elements  111 ,  112 , and  113  are each composed of a folding pattern to be housed in a space of length H 1  (e.g., 40 mm) and are each patterned on two sides of substrate  15  and connected within a through hole. 
         [0057]    Recess portion  16  is provided at a center portion of rear surface  11   f  of case  11 . When the user holds search apparatus  1  by hand so as to be able to press the buttons on operation section  13  with the first digit (thumb), the user can put the second digit (index finger) into recess portion  16  and thus can stably hold search apparatus  1 . 
       Structure of Search-Target Apparatus  2   
       [0058]    Next, a description will be given of a structure of search-target apparatus  2  using 
         [0059]      FIG. 2 .  FIG. 2  is an external view (front view) of search-target apparatus  2  according to Embodiment 1. 
         [0060]    Search-target apparatus  2  is in size and weight that allow a user (standard individual) to carry around (for example, width W: 40 mm, height H: 63 mm, and thickness T: 13 mm and weight may be 20 g). 
         [0061]    Case  21  of search-target apparatus  2  has a rectangular cube and is formed using a non-conductive member. Front surface  21   a  of case  21  is provided with light emitting diode (LED)  22  and power supply switch  23   
         [0062]    LED  22  lights up (blinks) at a predetermined timing such as reception of a radio wave from search apparatus  1  in the individual search mode. 
         [0063]    Case  21  houses an antenna (see  FIG. 4 ) and a substrate (not illustrated) therein. In addition, various circuits (see  FIG. 4 ) are mounted on the substrate. 
         [0064]    Protruding portion  24  having hole  24   a  for inserting a string or the like therethrough is provided at a flat surface (upper end) of case  21 . Connecting the string inserted through hole  24   a  to a cloth (e.g., belt) of the user prevents search-target apparatus  2  from separating from the user even when the user encounters an avalanche, for example. 
       Circuit Configuration of Search Apparatus  1  (Block Diagram) 
       [0065]    Next, a description will be given of a circuit configuration of search apparatus  1  using  FIG. 3 .  FIG. 3  is a block diagram illustrating a configuration of search apparatus  1  according to Embodiment 1. Search apparatus  1  is mainly composed of case  11  (not illustrated in  FIG. 3 ), display section  12 , operation section  13 , power supply switch  14  (not illustrated in  FIG. 3 ), substrate  15  (not illustrated in  FIG. 3 ), antenna  101 , radio section  102 , control section  103 , sound section  104 , and battery  105 . Radio section  102  and control section  103  are mounted on substrate  15 . 
         [0066]    Radio section  102  performs radio signal processing. Radio section  102  includes transmission section  121 , reception section  122 , radio control section  123 , first clock  124 , first switch  125 , second switch  126 , and third switch  127 . 
         [0067]    Transmission section  121  applies radio transmission processing such as modulation, amplification, and/or up-conversion to a baseband digital signal output from CPU  131  and transmits a radio signal via first antenna element  111 . The frequency of the radio wave (e.g., call signal) transmitted from transmission section  121  is equal to or greater than 710 MHz but not greater than 960 MHz. 
         [0068]    Reception section  122  applies radio reception processing such as amplification, down-conversion, and/or demodulation to the radio signal received via first antenna element  111  and outputs a baseband digital signal to CPU  131 . In addition, reception section  122  measures a received signal strength (RSSI: received signal strength indicator) of the radio wave received via first antenna element  111  and outputs a measurement value (analog value) to radio control section  123 . 
         [0069]    Radio control section  123  controls each section in radio section  102  using a clock signal of first clock  124 . Radio control section  123  converts the measurement value of the received signal strength output from reception section  122  to a digital value and outputs the digital value to CPU  131 . Note that, control for switches  125 ,  126 , and  127  performed by radio control section  123  will be described hereinafter in detail. 
         [0070]    First clock  124  is a high-speed and highly-accurate clock and generates a reference clock signal of a predetermined frequency (e.g., 36 MHz) to be used within radio section  102 . 
         [0071]    First switch  125  connects transmission section  121  or reception section  122  to first antenna element  111  in accordance with an indication of radio control section  123 . Second switch  126  connects/disconnects between substrate  15  and second antenna element  112  in accordance with an indication of radio control section  123 . Third switch  127  connects/disconnects between substrate  15  and third antenna element  113  in accordance with an indication of radio control section  123 . 
         [0072]    Control section  103  performs baseband signal processing. Control section  103  includes CPU  131 , memory section  132 , second clock  133 , third clock  134 , and logical operation unit  135 . 
         [0073]    CPU  131  is a central processing unit of control section  103  and executes various programs using memory section  132  as a working memory. In particular, CPU  131  generates a signal to be transmitted with respect to search-target apparatus  2 , causes, upon reception of a radio wave from search-target apparatus  2 , predetermined information thus acquired to be displayed on display section  12  and an alarm sound to be output from sound section  104 . 
         [0074]    Memory section  132  includes a read only memory (ROM) and a random access memory (RAM) and stores various programs to be executed by CPU  131 , and various types of data. 
         [0075]    Second clock  133  is a low speed clock and generates a reference clock signal of a predetermined frequency (e.g., 32 kHz) to be used within control section  103  during a standby state, for example. Third clock  134  is a high-speed and highly-accurate clock and generates a reference clock signal of a predetermined frequency (e.g., 40 MHz) to be used within control section  103  during a communication state with search-target apparatus  2 , for example. 
         [0076]    Logical operation section  135  calculates, in cooperation with CPU  131 , a propagation delay time of a transmission path to search-target apparatus  2  based on a difference between a transmission timing of a transmission frame and a reference clock of third clock  134 , a difference between a reception timing of a reception frame and a reference clock of third clock  134 , as well as a difference between a transmission timing of a transmission frame and a reference clock of third clock  234  (see  FIG. 4 ), and a difference between a reception timing of a reception frame and a reference clock of third clock  234  in search-target apparatus  2 . 
         [0077]    Sound section  104  outputs an alarm sound from a speaker at a predetermined timing such as a case where an indication from the user is received. 
         [0078]    Battery  105  is housed in case  11 , and when the user make an indication to turn ON the power supply via power supply switch  14 , battery  105  supplies each section of search apparatus  1  with power. 
       Functions of CPU  131   
       [0079]    Next, a description will be given of functions of CPU  131  of search apparatus  1  using  FIG. 3 . CPU  131  includes signal generation section  131   a,  signal acquisition section  131   b,  distance estimation section  131   c,  and direction estimation section  131   d  as functions according to the present invention. 
         [0080]    Signal generation section  131   a  generates a digital signal sequence (transmission frame) containing various types of information based on an indication of the user (electric signal input from operation section  13 ) and outputs the generated sequence to transmission section  121 . In the individual search mode, signal generation section  131   a  includes, in the digital signal, identification information of single search-target apparatus  2  indicated by the user and identification information of search apparatus  1  of signal generation section  131   a.  In the all search mode, signal generation section  131   a  includes, in the digital signal, information indicating a request for transmission of a response signal to all search-target apparatuses  2 . In the group search mode, signal generation section  131   a  includes, in the digital signal, identification information of all search-target apparatuses  2  belonging to a group indicated by the user. In the search-target mode, upon reception of information indicating a request for transmission of identification information of the search apparatus of signal generation section  131   a  from signal acquisition section  131   b,  signal generation section  131   a  includes, in the digital signal, identification information of the search apparatus of signal generation section  131   a  and identification information of search apparatus  1  which is the communication counterpart. Note that, in the individual search mode, signal generation section  131   a  may include, in the digital signal (response confirmation signal), information indicating lighting or outputting of an alarm sound with respect to search-target apparatus  2 , based on an indication or the like from the user. 
         [0081]    In a communication mode other than the search-target mode, signal acquisition section  131   b  acquires the identification information of search-target apparatus  2  from the digital signal sequence (reception frame) of the received response signal and outputs the identification information of search-target apparatus  2  to display section  12 . In the search-target mode, signal acquisition section  131   b  acquires information from the digital signal sequence (reception frame) of the received call signal, and when the information indicates a request for transmission of the identification information of the search apparatus of signal acquisition section  131   b,  signal acquisition section  131   b  outputs the information to notify signal generation section  131   a,  accordingly. Note that, in the all search mode, signal acquisition section  131   b  may output to display section  12 , from among the acquired pieces of identification information of search-target apparatuses  2 , identification information of search-target apparatus  2  which corresponds to the largest received signal strength (1) or whose identification information matches the registered identification information (2), or all the pieces of identification information of search-target apparatuses  2  (3). 
         [0082]    In the individual search mode, distance estimation section  131   c  estimates the distance to search-target apparatus  2  based on the received signal strength of the radio wave measured by reception section  122  or the propagation delay time calculated by logical operation section  135  and outputs the estimation value to display section  12 . Note that, a detailed description of the distance estimation in Embodiment 1 will be given, hereinafter. 
         [0083]    In the individual search mode, direction estimation section  131   d  estimates the direction of search-target apparatus  2  based on the received signal strength when second switch  126  is ON but third switch  127  is OFF, and the received signal strength when second switch  126  is OFF but third switch  127  is ON, and outputs the estimation value to display section  12 . Note that, a detailed description of the direction estimation in Embodiment 1 will be given, hereinafter. 
       Circuit Configuration of Search-Target Apparatus  2  (Block Diagram) 
       [0084]    Next, a description will be given of a circuit configuration of search-target apparatus  2  using  FIG. 4 .  FIG. 4  is a block diagram illustrating a configuration of search-target apparatus  2  according to Embodiment 1. Search-target apparatus  2  is mainly composed of case  21  (not illustrated in  FIG. 4 ), LED  22 , power supply switch  23  (not illustrated in  FIG. 4 ), a substrate (not illustrated), antenna  201 , radio section  202 , control section  203 , sound section  204 , and battery  205 . 
         [0085]    Radio section  202  performs radio signal processing. Radio section  202  includes transmission section  221 , reception section  222 , radio control section  223 , first clock  224 , and first switch  225 . 
         [0086]    Transmission section  221  applies radio transmission processing such as modulation, amplification, and/or up-conversion to a baseband digital signal output from CPU  231  and transmits a radio signal via antenna element  201 . The frequency of the radio wave (e.g., response signal) transmitted from transmission section  221  is equal to or greater than 770 MHz but not greater than 960 MHz. 
         [0087]    Reception section  222  applies radio reception processing such as amplification, down-conversion, and/or demodulation to the radio signal received via antenna element  201  and outputs a baseband digital signal to CPU  231 . 
         [0088]    Radio control section  223  controls each section in radio section  202  using a clock signal of first clock  224 . 
         [0089]    First clock  224  is a high-speed and highly-accurate clock and generates a reference clock signal of a predetermined frequency (e.g., 36 MHz) to be used within radio section  202 . 
         [0090]    First switch  225  connects transmission section  221  or reception section  222  to antenna  201  in accordance with an indication of radio control section  223 . 
         [0091]    Control section  203  performs baseband signal processing. Control section  203  includes CPU  231 , memory section  232 , second clock  233 , and third clock  234 . 
         [0092]    CPU  231  is a central processing unit of control section  203  and executes various programs using memory section  232  as a working memory. In particular, CPU  231  generates a signal to be transmitted with respect to search apparatus  1 , and upon reception of a radio wave from search apparatus  1  or an indication from search apparatus  1 , CPU  231  causes LED  22  to light and an alarm sound to be output from sound section  204 . 
         [0093]    Memory section  232  includes a read only memory (ROM) and a random access memory (RAM) and stores various programs to be executed by CPU  231 , and various types of data. 
         [0094]    Second clock  233  is a low speed clock and generates a reference clock signal of a predetermined frequency (e.g., 32 kHz) to be used within control section  203  during a standby state, for example. Third clock  234  is a high-speed and highly-accurate clock and generates a reference clock signal of a predetermined frequency (e.g., 40 MHz) to be used within control section  203  during a communication state with search apparatus  1 , for example. 
         [0095]    Sound section  204  outputs an alarm sound from a speaker at a predetermined timing upon reception of a radio wave from search apparatus  1 . 
         [0096]    Battery  205  is housed in case  21 , and when the user makes an indication to turn ON the power supply via power supply switch  23 , battery  205  supplies each section of search-target apparatus  2  with power. 
       Functions of CPU  231   
       [0097]    Next, a description will be given of functions of CPU  231  of search-target apparatus  2  using  FIG. 4 . CPU  231  includes signal generation section  231   a  and signal acquisition section  231   b  as functions according to the present invention. 
         [0098]    Upon reception of information indicating a request for transmission of the identification information of search-target apparatus  2  of signal generation section  231   a  from signal acquisition section  231   b , signal generation section  231   a  generates a digital signal sequence (transmission frame) containing the identification information of search-target apparatus  2  and identification information of search apparatus  1  which is the communication counterpart and outputs the generated sequence to transmission section  221 . 
         [0099]    Signal acquisition section  231   b  acquires information from a digital signal sequence (reception frame) of the received call signal, and when the information indicates a request for transmission of identification of search-target apparatus  2  of signal acquisition section  231   b,  signal acquisition section  231   b  outputs the information to notify signal generation section  231   a,  accordingly. In addition, upon reception of a radio wave from search apparatus  1  or an indication from search apparatus  1 , signal acquisition section  231   b  causes LED  22  to light and an alarm to be output from sound section  204 . 
       Communication Sequence of Individual Search Mode 
       [0100]    Next, a description will be given of how search apparatus  1  and search-target apparatus  2  according to Embodiment 1 communicate with each other in the individual search mode, using a sequence diagram of  FIG. 5 . 
         [0101]    Search apparatus  1  enters the standby state after the power supply is turned ON until an indication is given from the user (button operation of operation section  13 ). During the standby state, no power is supplied to each section of search apparatus  1  in order to reduce power consumption (sleep state). However, operation section  13  and second clock  133  are supplied with power from battery  105 . Second clock  133  runs a low speed clock circuit to perform a count operation all the time. 
         [0102]    Search-target apparatus  2  enters the standby state after the power supply is turned ON. During the standby state, no power is supplied to each section of search-target apparatus  2  in order to reduce power consumption (sleep state). However, second clock  233  is supplied with power from battery  205 . Second clock  233  runs a low speed clock circuit to perform a count operation all the time. 
         [0103]    Search-target apparatus  2  supplies power to each section (active state) every first interval (e.g., 3 s) until the count value of second clock  233  expires. During the active state, search-target apparatus  2  performs reception processing in first period  321  (e.g., 3 ms). At this time, first clock  224  runs a clock circuit to perform a count operation. 
         [0104]    Search-target apparatus  2  returns to the sleep state in a case where search-target apparatus  2  has not been able to acquire information indicating a request for transmission of identification information of search-target apparatus  2  during first period  321 . 
         [0105]    During the standby state, when identification information of search-target apparatus  2 , which is the search-target, is indicated by the user, search apparatus  1  supplies power to each section and starts a search for search-target apparatus  2  (search state). At this time, first clock  124  and third clock  134  run a high-speed clock circuit to perform a count operation. 
         [0106]    Search apparatus  1  which has entered the search state first performs reception processing in second period  311  (e.g., 5 ms) in order to confirm that another search apparatus  1  transmits no radio wave. When receiving no radio wave from another search apparatus  1  during second period  311 , search apparatus  1  repeatedly transmits a call signal containing identification information of search apparatus  1 , identification information of search-target apparatus  2 , which is the search-target, and timing information indicating transmission timing of a response signal with respect to search-target apparatus  2 , in third period  312  (e.g.,  3 . 5  s), which is longer than the first interval. 
         [0107]    Upon reception of a call signal during any one of first periods  321 - 3 , search-target apparatus  2  starts communication with search apparatus  1  (communication state). At this time, third clock  234  runs a high-speed clock circuit to perform a count operation. In fourth period  322 - 1  (e.g., 2 ms) indicated by the timing information contained in the call signal, search-target apparatus  2  transmits a response signal containing the identification information of search target apparatus  2 , the identification information of search apparatus  1 , which is the communication counterpart, and distance information (a difference between the transmission timing of the transmission frame and the reference clock of third clock  234 , and a difference between the reception timing of the reception frame and the reference clock of third clock  234 ) (note that, the response signal to be transmitted for the first time after reception of a call signal is referred to as “call response signal” in the description to be given hereinafter). 
         [0108]    Search apparatus  1  performs reception processing in fifth period  313  (e.g., 3 ms). Upon reception of a call response signal, search apparatus  1  right away transmits a response confirmation signal containing the identification information of search apparatus  1  and the identification information of search-target apparatus  2 , which is the search target, in sixth period  314 - 1  (e.g., 2 ms). Note that, the baseband frequency of the response confirmation signal is different from that of the call signal. For this reason, even when receiving the response confirmation signal and the call signal from another search apparatus  1  at the same time, search-target apparatus  2  causes no interference. 
         [0109]    Search-target apparatus  2  performs reception processing in seventh period  323 - 1  (e.g.,  3  ms) right after fourth period  322 - 1 . Upon reception of a response confirmation signal, search-target apparatus  2  transmits again a response signal in fourth period  322 - 2  after a second interval (e.g., 100 ms), which is shorter than the first interval, elapses. 
         [0110]    Subsequently, until a disconnection indication is made by the user, the communication system repeats transmission/reception of a response signal and transmission/reception of a response confirmation signal. Search apparatus  1  estimates, every time receiving a response signal, the distance and direction of search-target apparatus  2  using the response signal, and causes the identification information of search-target apparatus  2  and information on the distance and direction of search-target apparatus  2  to be displayed on a screen of display section  12 . 
         [0111]    When a disconnection indication is made by the user, search apparatus  1  transmits a disconnection signal containing the identification information of search apparatus  1 , the identification information of search-target apparatus  2 , which is the search target, and information indicating transmission disconnection with respect to this search-target apparatus  2 , in subsequent sixth period  314 - 7 . 
         [0112]    Upon reception of the disconnection signal in corresponding seventh period  323 - 7 , search-target apparatus  2  transmits a disconnection response signal containing the identification information of search-target apparatus  2 , the identification information of search apparatus  1 , which is the communication counterpart, and information indicating reception of the disconnection signal, in subsequent fourth period  322 - 8 . 
         [0113]    In subsequent sixth period  314 - 8 , search apparatus  1  transmits a disconnection confirmation signal containing the identification information of search apparatus  1 , the identification information of search-target apparatus  2 , which is the search target, and information indicating reception of the disconnection response signal, and returns to the standby state. Search-target apparatus  2  returns to the standby state upon reception of the disconnection confirmation signal in subsequent seventh period  323 - 8 . 
       Communication Sequence of All Search Mode (Group Search) Mode 
       [0114]    Next, a description will be given of how search apparatus  1  and search-target apparatus  2  according to Embodiment 1 communicate with each other in the all search mode, using a sequence diagram of  FIG. 6 . Note that, the sequence of the group search mode is the same as that of the all search mode. In addition, how search apparatus  1  and search-target apparatus  2  operate is the same in all the communication modes, and has been already described using  FIG. 5 , so that the description thereof is omitted herein. 
         [0115]    In  FIG. 6 , an assumption is made that four search-target apparatuses  2  ( 2 - 1 ,  2 - 2 ,  2 - 3 , and  2 - 4 ) exist within a coverage area of a radio wave of search apparatus  1 . 
         [0116]    In the all search mode, search apparatus  1  in a communication state first performs reception processing in second period  311  (e.g., 5 ms) in order to confirm that another search apparatus  1  transmits no radio wave. When no radio wave has been received from another search apparatus  1  in second period  311 , search apparatus  1  repeatedly transmits a call signal containing information indicating a request for transmission of identification information with respect to all search-target apparatuses  2 , in third period  312  (e.g., 3.5 s), which is longer than the first interval. 
         [0117]    When each search-target apparatus  2  receives a call signal in any one of first periods  321 - 12 ,  321 - 22 ,  321 - 31 , and  321 - 41 , search-target apparatus  2  transmits a call response signal containing the identification information of search-target apparatus  2  in fourth period  322  after the first interval time elapses from the start of this first period  321 . 
         [0118]    After the expiration of third period  312 , search apparatus  1  receives a call response signal in fifth period  313  (e.g., 3.5 s), which is longer than the first interval. 
         [0119]    Subsequently, the communication system repeats transmission/reception of a call signal and transmission/reception of a call response signal a predetermined number of times N. Search apparatus  1  causes the identification information of search-target apparatus  2  which is contained in the received call response signal to be displayed on a screen of display section  12 . Search apparatus  1  and search-target apparatuses  2  return to the standby state thereafter. 
       Distance Estimation 
       [0120]    Next, a description will be given of a method of estimating a distance to search-target apparatus  2  by search apparatus  1  according to Embodiment 1. As a distance estimation method in the field of radio communications, a first distance estimation method based on a received signal strength, and a second distance estimation method based on a propagation delay time have been known. 
         [0121]      FIG. 7  is a diagram illustrating a relationship between a distance and a received signal strength. In  FIG. 7 , the horizontal axis represents distance (m) and the vertical axis represents received signal strength (dBm). As illustrated in  FIG. 7 , there is a correlation between the distance and received signal strength, and the longer the distance is, the lower the received signal strength will be. The first distance estimation method is a method of estimating a distance using the correlation between this distance and the received signal strength. 
         [0122]    As illustrated in  FIG. 7 , the longer the distance is, the smaller the amount of change in received signal strength in distance units will be. Accordingly, search apparatus  1  can accurately estimate a distance using the first distance estimation method when the distance to search-target apparatus  2  is short. Meanwhile, when the distance to search-target apparatus  2  is long, the variation in radio wave becomes moderate, so that the distance cannot be accurately estimated when the first distance estimation method is used. 
         [0123]    The second distance estimation method is a method of estimating a distance by multiplying the calculated propagation delay time by the speed of radio wave. The estimation accuracy of the second distance estimation method is substantially constant regardless of the distance. Moreover, when the distance to search-target apparatus  2  is long, the high level of estimation accuracy is not required as compared with the case where the distance is short. 
         [0124]    In view of the points mentioned above, distance estimation section  131   c  of search apparatus  1  according to Embodiment 1 estimates the distance to search-target apparatus  2  using the first distance estimation method based on the received signal strength when the measured received signal strength is greater than a predetermined threshold (e.g., − 50  dBm) (near distance mode), and when the measured received signal strength is not greater than the predetermined threshold (wide area mode), distance estimation section  131   c  of search apparatus  1  according to Embodiment 1 estimates the distance to search-target apparatus  2  using the second distance estimation method based on the propagation delay time. 
         [0125]    Note that, in Embodiment 1, it is possible to use hysteresis control to switch between the estimation methods, and a first threshold for switching from the first distance estimation method to the second distance estimation method and a second threshold (&gt;first threshold) for switching from the second distance estimation method to the first distance estimation method may be set. Thus, it is made possible to prevent the estimated distance goes up and down significantly in a short time because switching between the estimation methods occurs frequently. 
         [0126]    In addition, in Embodiment 1, it is possible to switch from the first distance estimation method to the second distance estimation method based on a magnitude relationship between the received signal strength and a third threshold, and switching from the second distance estimation method to the first distance estimation method may be performed based on a magnitude relationship between the propagation delay time and a fourth threshold. 
       Antenna Control and Direction Estimation 
       [0127]    Next, a description will be given of control of antenna  101  and an estimation method for a direction of search-target apparatus  2  by search apparatus  1  according to Embodiment 1. 
         [0128]    Radio control section  123  controls first switch  125  to connect first antenna element  111  and reception section  122 , when receiving a radio wave (response signal). 
         [0129]    In the individual search mode, radio control section  123  turns ON second switch  126  to connect between substrate  15  and second antenna element  112  and turns OFF third switch  127  to disconnect between substrate  15  and third antenna element  113 , in a first part period of a radio wave (response signal) receiving period. 
         [0130]    Second antenna element  112  and substrate  15  connected to each other become longer as a whole than first antenna element  111  (radiator) by this control and thus acts as a reflector. In addition, third antenna element  113  becomes shorter than first antenna element  111  (radiator) and thus acts as a director. 
         [0131]    As a result, first reception directivity  801  of  FIG. 8  is formed in the first part period. During this period, when the user holds search apparatus  1  substantially horizontally with the side of flat surface  11   e  (upper end side) facing forward, search apparatus  1  can receive a radio wave intensively from a right oblique forward direction. 
         [0132]    In addition, radio control signal  123  turns OFF second switch  126  to disconnect between substrate  15  and second antenna element  112  and turns ON third switch  127  to connect between substrate  15  and third antenna element  113  in a second part period of the radio wave (response signal) reception period. 
         [0133]    Third antenna element  113  and substrate  15  connected to each other become longer as a whole than first antenna element  111  (radiator) by this control and thus acts as a reflector. In addition, second antenna element  112  becomes shorter than first antenna element  111  (radiator) and thus acts as a director. 
         [0134]    As a result, second reception directivity  802  of  FIG. 8  is formed in the second part period. During this period, when the user holds search apparatus  1  substantially horizontally with the side of flat surface  11   e  (upper end side) facing forward, search apparatus  1  can receive a radio wave intensively from a left oblique forward direction. 
         [0135]    When search-target apparatus  2 , which is the search-target, exists on the right side of the user, the received signal strength in the first part period becomes higher than the received signal strength in the second part period. Reversely, when search-target apparatus  2 , which is the search-target, exists on the left side of the user, the received signal strength in the first part period becomes lower than the received signal strength in the second part period. 
         [0136]    Thus, according to this embodiment, the direction of search-target apparatus  2  can be estimated based on the degree of difference in received signal strength between the first part period and the second part period and a magnitude relationship. 
         [0137]    Note that, during the radio wave transmission of the individual search mode, the all search mode, the group search mode, and the search-target mode, radio control section  123  turns OFF second switch  126  and third switch  127 , and search apparatus  1  may perform non-directivity transmission and reception. 
         [0138]    In Embodiment 1, memory section  132  stores an information table indicating the estimated directions based on the degrees of differences in received signal strength and their signs (magnitude relationship) as illustrated in  FIG. 9 . Direction estimation section  131   d  estimates the direction of search-target apparatus  2  based on the received signal strengths measured in the first and the second part periods, respectively, and with reference to the information table in  FIG. 9 , and outputs the estimation result to display section  12 . 
         [0139]    Note that, direction estimation section  131   d  may average the received signal strengths measured a plurality of times and estimate the direction of search-target apparatus  2  using the average value. Thus, the influence of the propagation path variations can be absorbed. 
       Display Screen 
       [0140]    Next, a description will be given of information to be displayed on a screen of the search apparatus according to Embodiment 1 using  FIGS. 10A to 10F . 
         [0141]      FIG. 10A  illustrates a screen at start of the individual search mode when a search for already registered search-target apparatus  2  is started. On this screen, identification number (identification information)  1001  of search-target apparatus  2 , which is the search-target, is displayed together with the registration number. When a search execution is indicated by the user in this display state, search apparatus  1  executes the individual search mode for search-target apparatus  2  whose identification number is displayed on the screen. 
         [0142]      FIG. 10B  illustrates a screen at start of the individual search mode when a search for unregistered search-target apparatus  2  is started. On this screen, part  1002  where input of the identification number (identification information) of search-target apparatus  2 , which is the search target, is displayed. When a search execution is indicated by the user in this display state after completion of the input of identification number, search apparatus  1  executes the individual search mode (see  FIG. 5 ) for search-target apparatus  2  whose identification number is displayed on the screen. 
         [0143]      FIG. 10C  illustrates a screen at start of the all search mode. On this screen, letter  1003  of “ALL” is displayed. When a search execution is indicated by the user in this display state, search apparatus  1  executes the all search mode (see  FIG. 6 ). 
         [0144]      FIG. 10D  illustrates a screen at start of the group search mode. On this screen, letter  1004  of “ALL Registered” is displayed. When a search execution is indicated by the user in this display state, search apparatus  1  executes the group search mode for previously registered all search-target apparatuses  2  (see  FIG. 6 ). 
         [0145]      FIG. 10E  illustrates a screen when the individual search mode is executed, and a response signal is received from search-target apparatus  2 . On this screen, the following information is displayed: identification information  1005 - 1  of search-target apparatus  2  which has responded; information  1005 - 2  on the distance to search-target apparatus  2 ; information  1005 - 3  on the direction of search-target apparatus  2 ; and information  1005 - 4  indicating a received signal strength, for example. 
         [0146]    Note that, in this embodiment, updating of information  1005 - 3  on the direction of search-target apparatus  2  is limited to one step each. For example, when the result of the last direction estimation is left 75 degrees and the result of the subsequent direction estimation is 30 degrees, information indicating left 60 degrees, which corresponds to the estimation that has been made one step closer to right 30 degrees from left 75 degrees by one step (15 degrees) is displayed on the screen. Thereafter, when the results of the direction estimation are right 30 degrees in a row, pieces of information respectively indicating left 45 degrees, left 30 degrees, left 15 degrees, 0 degrees, right 15 degrees, and right 30 degrees are sequentially displayed on the screen. Thus, the influence of temporary and drastic variations in the estimated direction due to interception of radio waves, for example, can be absorbed. In addition, the directivity in  FIG. 8  is to vary depending how the user holds search apparatus  1 , for example, so that the direction estimated from a received signal strength does not necessarily match the actual direction of search-target apparatus  2 , but the direction estimation of Embodiment 1 makes it possible to roughly estimate the direction of search-target apparatus  2  and is effective enough. 
         [0147]      FIG. 10F  illustrates a screen when the all search mode or group search mode is executed, and a call response signal is received from search-target apparatus  2 . On this screen, pieces of identification information  1006  of all found search-target apparatuses  2  are displayed. At this time, pieces of identification information  1006  of search-target apparatuses  2  are displayed in descending order of received signal strengths from above. 
       Description of Flow 
       [0148]    Next, a description will be given of an operation flow of search apparatus  1  according to Embodiment 1 using  FIGS. 11A and 11B . 
         [0149]    Search apparatus  1  enters the sleep state when the power supply is turned ON (ST  1101 ) and wait for an indication from the user in this state (ST  1102 ). 
         [0150]    When the indication from the user is the individual search mode (ST  1102 : individual search mode), search apparatus  1  transmits a call signal to search-target apparatus  2 , which is the search-target (ST  1103  and ST  1104 ). Search apparatus  1  performs reception processing at a timing indicated to search-target apparatus  2  in the call signal (ST  1105 ). 
         [0151]    When no call response signal is successfully received in ST  1105  (ST  1106 : NO), search apparatus  1  repeats the steps from ST  1104  to ST  1106  (ST  1107 : NO, ST  1108 ). When no call response signal is successfully received even after transmission of the call signal M times (ST  1107 : YES), search apparatus  1  displays a message indicating that no response has been received on the screen of display section  12  (ST  1109 ). The flow then proceeds to ST  1132 . 
         [0152]    In a case where a call response signal is successfully received in ST  1105  (ST  1106 : YES), search apparatus  1  right away transmits a response confirmation signal to search-target apparatus  2  (ST  1110 ). In addition, search apparatus  1  measures the received signal strength of the response signal (ST  1111 ), estimates the distance to search-target apparatus  2  (ST  1112 ), and estimates the direction of search-target apparatus  2  (ST  1113 ). Search apparatus  1  displays the identification information of search-target apparatus  2  and information on the distance and direction of search-target apparatus  2  on the screen of display section  12  (ST  1114 ). 
         [0153]    When no search end indication is given from the user (ST  1115 : NO), and the timer that counts a predetermined time has not expired (ST  1116 : NO), search apparatus  1  performs again reception processing at predetermined intervals (ST  1117 ). 
         [0154]    When no response signal is successfully received in ST  1117  (ST  1118 : NO), search apparatus  1  repeats the steps from ST  1115  to ST  1117 . Meanwhile, when a response signal is successfully received in ST  1117  (ST  1118 : YES), the flow returns to ST  1110 , and search apparatus  1  repeats the steps from ST  1100  to ST  1117  (communication state). 
         [0155]    In this communication state, when a search end indication is given from the user (ST  1115 : YES), or when the timer that counts a predetermined time has expired (ST  1116 : YES), search apparatus  1  performs disconnection processing of disconnection signal transmission, disconnection response signal reception and disconnection confirmation signal transmission (ST  1119 ). The flow then advances to ST  1132 . 
         [0156]    In ST  1102 , when the indication from the user is the all search mode or group search mode (ST  1102 : all search mode, group search mode), search apparatus  1  transmits a call signal to all search-target apparatuses  2  or all previously-registered search-target apparatuses  2  at once (ST  1121 , ST  1122 ) and performs reception processing over a predetermined period (ST  1123 ). Search apparatus  1  repeats the steps of ST  1122  and ST  1123  N times (ST  1124 , ST  1125 ). 
         [0157]    When not even one response signal is successfully received in N times of ST  1123  (ST  1126 : NO), search apparatus  1  displays a message indicating that no response has been received on the screen of display section  12  (ST  1127 ). The flow then proceeds to ST  1132 . 
         [0158]    When at least one response signal is successfully received in N times of ST  1123  (ST  1126 : YES), search apparatus  1  displays the identification information of search-target apparatus  2  on the screen of display section  12  (ST  1128 ). 
         [0159]    When no indication to shift to the individual search mode or no search end indication is given from the user (ST  1129 : NO, ST  1130 : NO), and the timer that counts a predetermined time has not expired either (ST  1131 : NO), search apparatus  1  repeats the step of ST  1128 . 
         [0160]    After ST  1128 , when an indication to shift to the individual search mode is given (ST  1129 : YES), the flow moves to ST  1103 . In addition, when a search end indication is given from the user (ST  1130 : YES), or the timer that counts a predetermined time expires (ST  1131 : YES), the flow moves to ST  1132 . 
         [0161]    In ST  1132 , when the power supply is not turned OFF (ST  1132 : NO), the flow returns to ST  1101 . Meanwhile, when the power supply is turned OFF in ST  1132  (ST  1132 : YES), the flow ends. 
         [0162]    Note that, when the indication from the user is the search-target mode in ST  1102  (ST  1102 : search-target mode), the flow proceeds to ST  1201  and search apparatus  1  operates as search-target apparatus  2 . 
         [0163]    Next, a description will be given of an operation flow of search-target apparatus  2  according to Embodiment 1 using  FIG. 12 . 
         [0164]    Search-target apparatus  2  enters the sleep state when the power supply is turned ON (ST  1201 ) and periodically (at first intervals) performs reception processing (ST  1202 ). 
         [0165]    In a case where no call signal is successfully received in ST  1202  (ST  1203 : NO), search-target apparatus  2  returns to the sleep state in ST  1201 . 
         [0166]    In a case where a call signal is successfully received in ST  1202  (ST  1203 : YES), search-target apparatus  2  transmits a response signal to search apparatus  1  at the indicated timing (ST  1204 ) and right away performs reception processing (ST  1205 ). 
         [0167]    In a case where a response confirmation signal is successfully received in ST  1205  (ST  1206 : YES), search-target apparatus  2  transmits again a response signal to search apparatus  1  after a predetermined time (second interval) elapses (ST  1204 ). Subsequently, in a case where a response confirmation signal is successfully received (ST  1206 : YES), the steps of ST  1204  and ST  1205  are repeated. 
         [0168]    In a case where no response confirmation signal is received in ST  1205  (ST  1206 : NO), but a disconnection signal is received (ST  1207 : YES), search-target apparatus  2  performs disconnection processing of disconnection response signal transmission and disconnection confirmation signal reception (ST  1208 ). The flow then proceeds to ST  1209 . 
         [0169]    In a case where neither response confirmation signal nor disconnection signal is received in ST  1205  (ST  1206 : NO, ST  1207 : NO), the flow proceeds to ST  1209 . 
         [0170]    In ST  1209 , when the power supply is not turned OFF (ST  1209 : NO), the flow returns to ST  1201 . Meanwhile, when the power supply is turned OFF in ST  1209  (ST  1209 : YES), the flow ends. 
       Effects 
       [0171]    As has been described above, according to Embodiment 1, search-target apparatus  2  does not have to always transmit a periodical signal such as a beacon and only needs to transmit a response signal upon reception of a call signal from search apparatus  1 . Thus, the power consumption of search-target apparatus  2  is reduced as compared with the apparatus of the related art. For this reason, search-target apparatus  2  is capable of continuously operating for a long period (e.g., three months or more) and thus involves no concern for running out of a battery until the user descends from the mountain even when the user previously turns ON the power supply such as when the user enters the mountain area. Moreover, when the user previously turns ON the power supply, search-target apparatus  2  can perform radio communication with search apparatus  1  without the need for any additional user operation. For this reason, even when the user loses consciousness because of an avalanche or the like, search apparatus  1  can identify the position of search-target apparatus  2 . 
         [0172]    Search apparatus  1  and search-target apparatus  2  perform radio communication using a frequency of 710 MHz or higher but not higher than 960 MHz, so that the radio wave receivable distance of search-target apparatus  2  for a radio wave from search apparatus  1  is long, and search apparatus  1  is thus capable of conducting a wide area search (e.g., 100 m to 5 km). Note that, the use of an even higher frequency such as 2.4 GHz in WLAN reduces the antenna in length and reduces the receivable distance. Moreover, the use of an even higher frequency causes an increase in straightness of radio waves and reduction in wrap-around characteristics, so that the possibility of finding a lost climber or the like in an environment with obstacles is reduced. Meanwhile, the use of a lower frequency causes an increase in length of the antenna, thus causing an increase in size of the apparatus. 
         [0173]    Moreover, the use of the distance measurement method of Embodiment 1 enables, when search-target apparatus  2  is in a near distance area, search apparatus  1  to estimate the relative distance to search-target apparatus  2  with high accuracy, and when search-target apparatus  2  is not in a near distance area, the use of the method enables search apparatus  1  to estimate the relative distance to search-target apparatus  2  with predetermined accuracy. 
         [0174]    Moreover, the use of the antenna structure and switching control of Embodiment 1 makes it possible to reduce the circuit scale of an antenna portion and thus to reduce the size and weight of the entirety of the apparatus, as compared with a case where an array antenna is used. In first antenna element  111  (radiator) which receives radio waves, no switching of directivity is performed, so that the switching loss becomes the minimum. In addition, the switching timings of second switch  126  and third switch  127  are the same, so that only one control system is used for switching the switches. 
         [0175]    In addition, providing recess portion  16  at the center portion of rear surface  11   f  of case  11  of search apparatus  1  of Embodiment 1 allows, when the user holds search apparatus  1  by hand so as to be able to press the buttons of operation section  13  with the first digit (thumb), the user to put the second digit (index finger) into recess portion  16  to stably hold search apparatus  1 . In addition, this way of holding the apparatus does not involve fingers or hand around antenna element  111 ,  112 , or  113 , so that the influence on the antenna directivity is prevented, and the direction estimation accuracy can be stable. 
         [0176]    In addition, search-target apparatus  2  of Embodiment 1 causes LED  22  to light and an alarm sound to be output from sound section  204  upon reception of a radio wave from search apparatus  1  or an indication from search apparatus  1 . Thus, the user of search apparatus  1  can search for search-target apparatus  2  using his or her own visual or auditory perception. 
         [0177]    According to Embodiment 1, the possibility of finding a lost mountain climber or the like further increases, and a further reduction in the time required for finding the lost mountain climber can be expected. 
         [0178]    Note that, in Embodiment 1, a description has been given of the case where search-target apparatus  2  always transmits a call response signal upon reception of a call signal from search apparatus  1 , but the present invention is not limited to this case, and search-target apparatus  2  may previously store identification information of predetermined search apparatus  1 , and upon reception of a call signal from search apparatus  1 , search-target apparatus  2  may transmit a call response signal only when the stored pieces of identification information of search apparatuses  1  include one that matches the piece of identification information of search apparatus  1  included in this call signal. 
         [0179]    Thus, search apparatus  1  can cause only identification information of search-target apparatus  2  in which the identification information of search apparatus  1  has been registered. Moreover, search-target apparatus  2  can save power consumption because the number of times search-target apparatus  2  transmits a response signal is reduced. 
         [0180]    Furthermore, in Embodiment 1, a description has been given of the case where information on the estimated distance and direction of search-target apparatus  2  on the screen of an LCD or the like of search apparatus  1 , but the present invention is not limited to this case, and another display method may be used. For example, a plurality of LEDs may be provided at the front surface of case  11  of search apparatus  1 , and the position of the LED to light may be changed according to the distance and direction of search-target apparatus  2 , for example. 
         [0181]    In addition, in Embodiment 1, a description has been given of the case where identification information of search-target apparatus  2  is displayed on the screen of search apparatus  1 , but the present invention is not limited to this case, and search apparatus  1  may include a speaker, for example, and the identification information of search-target apparatus  2  may be output as audio from this speaker. 
         [0182]    In Embodiment 1, a description has been given of the case where recess portion  16  is provided at the center portion of rear surface llf of case  11  of search apparatus  1  to prevent any influence on the antenna directivity, but the present invention is not limited to this case, and it is also possible to prevent hands or fingers from being around antenna element  111 ,  112 , or  113  by another configuration such as providing a protruding portion at a position relatively closer to the side of fat panel  11   e  (upper end side) with respect to the center of rear surface  11   f.    
       Embodiment 2 
       [0183]    In a snow mountain or the like, a search for a lost climber is usually conducted in units of teams each composed of a plurality of members. In this case, it is important that rescue activities be conducted while each member of the team promptly notifies other members of information such as one indicating that the member has found the lost climber, to share such information within the team. 
         [0184]    In Embodiment 2, a description will be given of an application example of a search apparatus in case where a search is conducted by a team. Note that, a search apparatus and a search-target apparatus in Embodiment 2 are configured in a manner similar to those described in Embodiment 1, so that the description of the configurations will be omitted. 
       Description of Use State 
       [0185]      FIG. 13  is a diagram illustrating an exemplary use state of a search apparatus according to Embodiment 2.  FIG. 13  illustrates an example in which four searchers # 1 A, # 1 B, # 1 C, and # 1 D belonging to a single team use search apparatuses  1 A,  1 B,  1 C, and  1 D, respectively, and conduct a search for search-target # 2  provided with search-target apparatus  2 . In addition, an assumption is made that search-target apparatus  2  transmits a response signal to search apparatus  1 A carried by searcher # 1 A, and search apparatus  1 A is in communication with search-target apparatus  2 . 
         [0186]    In this case, in Embodiment 2, searchers # 1 B, # 1 C, and # 1 D other than searcher # 1 A can know that search apparatus  1 A of searcher # 1 A has received a response signal from search-target apparatus  2  of search-target # 2 , via search apparatuses  1 B,  1 C, and  1 D carried by searchers # 1 B, # 1 C, and # 1 D, respectively. More specifically, in Embodiment 2, each member can easily find out that one member of the team has successfully communicated with the search-target apparatus of the search-target without using another communication means. Hereinafter, a description will be given of the details of this case. 
       Communication Sequence 
       [0187]      FIG. 14  is a sequence diagram illustrating how the search apparatus and search-target apparatus according to Embodiment 2 communicate with each other. In  FIG. 14 , search apparatus  1 A carried by searcher # 1 A and search-target apparatus  2  carried by search-target # 2  have already been in communication with each other. An assumption is made that search apparatus  1 B carried by searcher # 1 B has started a search. 
         [0188]    In third period  312 B, search apparatus  1 B repeatedly transmits a call signal containing identification information of search apparatus  1 B, identification information of a search-target apparatus, which is the search-target, and timing information indicating transmission timing of a response signal with respect to this search-target apparatus  2  as has been described in Embodiment 1 ( FIG. 5 ). 
         [0189]    Subsequently, search apparatus  1 B receives a response signal transmitted from search-target apparatus  2  to search apparatus  1 A in fifth period  313 B. This response signal contains the identification information of search-target apparatus  2  and the identification information of search apparatus  1 A in communication, for example, so that search apparatus  1 B can recognize that search-target apparatus  2  is in communication with search apparatus  1 A. Upon reception of the response signal containing the identification information of another search apparatus, search apparatus  1 B performs, by control of signal acquisition section  131   b,  display processing for predetermined information to be described hereinafter. 
         [0190]    In the above description, the case has been described where search apparatus  1 B shifts to a display operation based on the information contained in the response signal transmitted from search-target apparatus  2 . However, in Embodiment 2, there may be a case where search apparatus  1 B shifts to a display operation based on the information contained in the response confirmation signal transmitted from search apparatus  1 A. 
         [0191]    Whether search apparatus  1 B receives the response signal from search-target apparatus  2  or the response confirmation signal transmitted from search apparatus  1 A in fifth period  313 B, depends on the position relationship between search apparatus  1 B and search-target apparatus  2  or search apparatus  1 A. Although it depends on the landform, in general, when the distance between search apparatus  1 B and search apparatus  1 A is shorter than the distance between search apparatus  1 B and search-target apparatus  2 , it is more likely that search apparatus  1 B receives a response confirmation signal from search apparatus  1 A in fifth period  313 B. 
       Description of Flow 
       [0192]      FIGS. 15A and 15B  illustrate a flowchart indicating the operation flow of the search apparatus according to Embodiment 2. Note that, in  FIGS. 15A and 15B , the steps common to  FIGS. 11A and 11B  are assigned the same reference numerals and the description of the steps will be omitted. In  FIGS. 15A and 15B , STS  1301  to  1304  are added as compared with  FIGS. 11A and 11B . 
         [0193]    In a case where a call response signal is successfully received in ST  1105  (ST  1106 : YES), search apparatus  1  determines the identification information contained in the call response signal or call signal (ST  1301 ). 
         [0194]    In a case where the identification information is one that is directed to search apparatus  1  (ST  1301 : YES), the flow moves to ST  1110 , and search apparatus  1  right away transmits a response confirmation signal to search-target apparatus  2  (ST  1110 ). 
         [0195]    Meanwhile, in a case where the identification information is one that is directed to another search apparatus (ST  1301 : NO), search apparatus  1  causes, by control of signal acquisition section  131   b,  the identification information of the other search apparatus to be displayed on display section  12  (ST  1302 ). 
         [0196]    Subsequently, when no search end indication is given from the user (ST  1303 : NO), and the timer that counts a predetermined time has not expired (ST  1304 : NO), search apparatus  1  repeats the step of ST  1302 . Meanwhile, when a search end indication is given from the user (ST  1303 : YES), or the timer that counts a predetermined time has expired (ST  1304 : YES), the flow proceeds to ST  1132 . 
       Display Screen 
       [0197]      FIG. 16  illustrates a screen of search apparatus  1  in ST  1302 . This screen is displayed after the screen illustrated in  FIG. 10A , in a case where search-target apparatus  2  is in communication with another search apparatus  1 . On this screen, identification information  1401  of search-target apparatus  2  and identification information  1402  of another search apparatus  1  in communication with this search-target apparatus  2  are displayed. 
       Effects 
       [0198]    As has been described above, according to Embodiment 2, a search apparatus can cause the identification information of another search apparatus in communication with a search-target apparatus to be displayed. Thus, each member can easily and promptly find out, without using another communication means, that one member of the team has successfully communicated with the search-target apparatus of the search-target. Accordingly, the all members of the team can promptly and efficiently perform rescue activities for a lost climber or the like in cooperation with each other. 
         [0199]    In addition, a search apparatus can acquire, from a response confirmation signal from another search apparatus in communication with a search-target apparatus, the identification information of the other search apparatus. Thus, there is a possibility for each member to find out, even when the member is out of the coverage area of a signal from the search-target apparatus, that one member of the team has successfully communicated with the search-target apparatus of the search-target. Thus, according to this embodiment, it is possible to increase the search area. 
         [0200]    In this embodiment, although the case has been described where, on the screen of search apparatus  1 , the identification information of search-target apparatus  2  and the identification information of another search apparatus are displayed, the present invention is not limited to this case. For example, search apparatus  1  may include a speaker and cause the identification information of search-target apparatus  2  and the identification information of another search apparatus to be output from the speaker as audio. 
         [0201]    This application is based upon and claims the benefit of priority from Japanese Patent Applications No. 2013-245284, filed on Nov. 27, 2013, and No. 2014-094510, filed on May 1, 2014, the contents of which including the specifications and drawings are incorporated herein by reference in their entirety. 
       INDUSTRIAL APPLICABILITY 
       [0202]    The present invention is suitable for use in a search apparatus for searching a position of another communication apparatus and also in a communication system composed of a search apparatus and a search-target apparatus. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  Search apparatus 
           2  Search-target apparatus 
           11 ,  21  Case 
           12  Display section 
           13  Operation section 
           14 ,  23  Power supply switch 
           15  Substrate 
           16  Recess portion 
           22  LED 
           101 ,  201  Antenna 
           102 ,  202  Radio section 
           103 ,  203  Control section 
           104 ,  204  Sound section 
           105 ,  205  Battery (Power supply section) 
           111  First antenna element 
           112  Second antenna element 
           113  Third antenna element 
           121 ,  221  Transmission section 
           122 ,  222  Reception section 
           123 ,  223  Radio control section 
           124 ,  224  First clock 
           125 ,  225  First switch 
           126  Second switch 
           127  Third switch 
           131 ,  231  CPU 
           131   a,    231   a  Signal generation section 
           131   b,    231   b  Signal acquisition section 
           131   c  Distance estimation section 
           131   d  Direction estimation section 
           132 ,  232  Memory section 
           133 ,  233  Second clock 
           134 ,  234  Third clock 
           135  Logical operation section