Patent Publication Number: US-2021165071-A1

Title: Radar apparatus, radar control apparatus, and radar system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a radar apparatus that transmits radio waves and detects radio waves reflected by a detection target, a radar control apparatus to be used in such a radar apparatus, and a radar system including such a radar apparatus. 
     BACKGROUND ART 
     In a radar system, a radio wave transmitted from a transmitter is reflected by a detection target and the reflected radio wave is received by a receiver. For example, PTL 1 discloses a radar apparatus that transmits a radio wave with horizontal polarization (horizontal polarization) and receives a radio wave with vertical polarization (vertical polarization). 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Unexamined Patent Application Publication No 2006-133108 
       
    
     SUMMARY OF THE INVENTION 
     It is desired in a radar system to detect various detection targets with high accuracy, and further improvement in accuracy is expected. 
     It is desirable to provide a radar apparatus, a radar control apparatus, and a radar system that make it possible to detect various detection targets with high accuracy. 
     A first radar apparatus according to one embodiment of the present disclosure includes a transmission unit, a reception unit, and a control unit. The transmission unit is configured to transmit radio waves. The reception unit includes a first receiver configured to receive a first reflected radio wave and a second receiver configured to receive a second reflected radio wave. The first reflected radio wave and the second reflected radio wave have different polarization characteristics from each other and are included in reflected radio waves that are the radio waves reflected by a detection target. The control unit is configured to control operations of the transmission unit and the reception unit, and configured to identify the detection target on the basis of the operation of the transmission unit, a first reception level at the first receiver, and a second reception level at the second receiver. 
     A second radar apparatus according to one embodiment of the present disclosure includes a transmission unit, a reception unit, and a control unit. The transmission unit includes a first transmitter configured to transmit a first radio wave and a second transmitter configured to transmit a second radio wave. The first radio wave and the second radio wave have different polarization characteristics from each other. The reception unit is configured to receive reflected radio waves that are radio waves reflected by a detection target. The radio waves include the first radio wave and the second radio wave. The control unit is configured to control operations of the transmission unit and the reception unit, and configured to identify the detection target on the basis of information as to which of the first transmitter and the second transmitter is operating, and a reception level at the reception unit. 
     A radar control apparatus according to one embodiment of the present disclosure is configured to perform: a first control operation of controlling an operation of a transmission unit configured to transmit radio waves a second control operation of controlling an operation of a reception unit including a first receiver configured to receive a first reflected radio wave and a second receiver configured to receive a second reflected radio wave, the first reflected radio wave and the second reflected radio wave having different polarization characteristics from each other and being included in reflected radio waves that are the radio waves reflected by a detection target; and an identification operation of identifying the detection target on the basis of the operation of the transmission unit, a first reception level at the first receiver, and a second reception level at the second receiver. 
     A radar system according to one embodiment of the present disclosure includes a radar apparatus, and a reflector provided for a detection target of the radar apparatus. The radar apparatus includes a transmission unit, a reception unit, and a control unit. The transmission unit is configured to transmit radio waves. The reception unit includes a first receiver configured to receive a first reflected radio wave and a second receiver configured to receive a second reflected radio wave. The first reflected radio wave and the second reflected radio wave have different polarization characteristics from each other and are included in reflected radio waves that are the radio waves reflected by the reflector. The control unit is configured to control operations of the transmission unit and the reception unit, and configured to identify the detection target on the basis of the operation of the transmission unit, a first reception level at the first receiver, and a second reception level at the second receiver. 
     In the first radar apparatus, the radar control apparatus, and the radar system according to one embodiment of the present disclosure, the transmission unit transmits the radio waves. Then, the radio waves are reflected by a reflector. The first receiver receives the first reflected radio wave included in the reflected radio waves, and the second receiver receives the second reflected radio wave included in the reflected radio waves. The first reflected radio wave and the second reflected radio wave have different polarization characteristics from each other. Then, the detection target is identified on the basis of the operation of the transmission unit, the first reception level at the first receiver, and the second reception level at the second receiver. 
     In the second radar apparatus according to one embodiment of the present disclosure, the first transmitter transmits the first radio wave and the second transmitter transmits the second radio wave. The first radio wave and the second radio wave have different polarization characteristics from each other. The first radio wave and the second radio wave have different polarization characteristics from each other. Then, the radio waves including the first radio wave and the second radio wave are reflected by the detection target. The reception unit receives the reflected radio waves. Then, the detection target is identified on the basis of the information as to which of the first transmitter and the second transmitter is operating, and the reception level at the reception unit. 
     In the first radar apparatus, the radar control apparatus, and the radar system according to one embodiment of the present disclosure, the first receiver configured to receive the first reflected radio wave and the second receiver configured to receive the second reflected radio wave are provided. The detection target is identified on the basis of the operation of the transmission unit, the first reception level at the first receiver, and the second reception level at the second receiver. This makes it possible to detect various detection targets with high accuracy. 
     In the second radar apparatus according to one embodiment of the present disclosure, the first transmitter configured to transmit the first radio wave and the second transmitter configured to transmit the second radio wave are provided. The detection target is identified on the basis of the information as to which of the first transmitter and the second transmitter is operating, and the reception level at the reception unit. This makes it possible to detect various detection targets with high accuracy. 
     It is to be noted that the effects described here are not necessarily limitative, and may include any of the effects described in the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram illustrating a configuration example of a radar system according to a first embodiment of the present disclosure. 
         FIG. 2  is a timing diagram illustrating operation of a transmission unit and a reception unit illustrated in  FIG. 1 . 
         FIG. 3  is a table illustrating examples of characteristics of various reflectors. 
         FIG. 4  is a configuration diagram according to a configuration example of a reflector. 
         FIG. 5  is a configuration diagram according to another configuration example of the reflector. 
         FIG. 6  is a configuration diagram according to another configuration example of the reflector. 
         FIG. 7  is a configuration diagram according to a configuration example of another reflector. 
         FIG. 8  is an explanatory diagram illustrating an operation example of the radar system illustrated in  FIG. 1 . 
         FIG. 9  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 1 . 
         FIG. 10  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 1 . 
         FIG. 11  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 1 . 
         FIG. 12  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 1 . 
         FIG. 13  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 1 . 
         FIG. 14  is a block diagram illustrating a configuration example of a radar apparatus according to a modification example. 
         FIG. 15  is a table illustrating examples of characteristics of various reflectors according to the modification example. 
         FIG. 16  is an explanatory diagram illustrating an operation example of a radar system according to the modification example. 
         FIG. 17  is an explanatory diagram illustrating another operation example of the radar system according to the modification example. 
         FIG. 18  is an explanatory diagram illustrating another operation example of the radar system according to the modification example. 
         FIG. 19  is an explanatory diagram illustrating another operation example of the radar system according to the modification example. 
         FIG. 20  is an explanatory diagram illustrating another operation example of the radar system according to the modification example. 
         FIG. 21  is an explanatory diagram illustrating another operation example of the radar system according to the modification example. 
         FIG. 22  is a block diagram illustrating a configuration example of a radar apparatus according to another modification example. 
         FIG. 23  is a configuration diagram illustrating a configuration example of a radar system according to a second embodiment. 
         FIG. 24  is a table illustrating examples of characteristics of various reflectors according to the second embodiment. 
         FIG. 25  is an explanatory diagram illustrating an operation example of the radar system illustrated in  FIG. 23 . 
         FIG. 26  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 23 . 
         FIG. 27  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 23 . 
         FIG. 28  is a configuration diagram illustrating a configuration example of a radar system according to a third embodiment. 
         FIG. 29  is an explanatory diagram illustrating an operation example of the radar system illustrated in  FIG. 28 . 
         FIG. 30  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 28 . 
         FIG. 31  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 28 . 
         FIG. 32  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 28 . 
         FIG. 33  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 28 . 
         FIG. 34  is an explanatory diagram illustrating another operation example of the radar system illustrated in  FIG. 28 . 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     In the following, some embodiments of the present disclosure are described in detail with reference to the drawings. It is to be noted that description is given in the following order. 
     1. First Embodiment 
     2. Second Embodiment 
     3. Third Embodiment 
     1. First Embodiment 
     [Configuration Example] 
       FIG. 1  illustrates a configuration example of a radar system (radar system  1 ) according to a first embodiment. The radar system  1  detects, for example, a vehicle (detection target) such as an automobile. The radar system  1  includes a radar apparatus  10  and a vehicle  90 . 
     The radar apparatus  10  uses radio waves to detect the detection target. In this example, the radar apparatus  10  is mounted on a vehicle and is configured to detect a position (distance and direction) of another vehicle (in this example, the vehicle  90 ) as seen from its own vehicle (own vehicle). The radar apparatus  10  includes a transmission unit  20 , a reception unit  30 , and a control unit  11 . 
     The transmission unit  20  transmits radio waves on the basis of an instruction from the control unit  11 , The transmission unit  20  includes a vertically polarized transmitter  20 V and a horizontally polarized transmitter  20 H. The vertically polarized transmitter  20 V transmits a radio wave WV with vertical polarization. The horizontally polarized transmitter  20 H transmits a radio wave WH with horizontal polarization. The vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H are configured to alternately transmit the radio wave WV and the radio wave WH on the basis of the instruction from the control unit  11  by, for example, operating alternately in time division. 
     The reception unit  30  receives, on the basis of an instruction from the control unit  11 , radio waves transmitted from the transmission unit  20  and reflected by the vehicle  90 . The reception unit  30  includes a vertically polarized receiver  30 V and a horizontally polarized receiver  30 H. The vertically polarized receiver  30 V receives a radio wave with vertical polarization. The horizontally polarized receiver  30 H receives a radio wave with horizontal polarization. The vertically polarized receiver  30 V and the horizontally polarized receiver  30 H are configured to continuously operate at all times on the basis of the instruction from the control unit  11 . 
       FIG. 2  illustrates an operation example of the transmission unit  20  and the reception unit  30 , where (A) illustrates the operation of the vertically polarized transmitter  20 V, (B) illustrates the operation of the horizontally polarized transmitter  20 H, (C) illustrates the operation of the vertically polarized receiver  30 V, and (D) illustrates the operation of the horizontally polarized receiver  30 H. In (A) and (B) of  FIG. 2 , “ON” indicates that radio waves are transmitted, and “OFF” indicates that transmission of radio waves is stopped. In (C) and (D) of  FIG. 2 , “ON” indicates a state of being ready to receive radio waves. 
     As illustrated in (A) and (B) of  FIG. 2 , in a period P 1 , the vertically polarized transmitter  20 V transmits the radio wave WV, and the horizontally polarized transmitter  20 H does not transmit the radio wave WH. Further, in a period P 2 , the horizontally polarized transmitter  20 H transmits the radio wave WH, and the vertically polarized transmitter  20 V does not transmit the radio wave WV Thus, the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H transmit radio waves by operating alternately in time division. Meanwhile, as illustrated in (C) and (D) of  FIG. 2 , the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H are configured to continuously operate at all times. 
     The control unit  11  ( FIG. 1 ) controls operation of the transmission unit  20  and the reception unit  30 . Specifically, the control unit  11  performs control to cause the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H to operate alternately in time division, and performs control to cause the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H to continuously operate at all times. 
     The control unit  11  includes a processor  12 . The processor  12  detects the position of the vehicle  90 , which is the detection target, on the basis of a result of the reception by the reception unit  30 . Further, the processor  12  also has a function of identifying, for example, the vehicle  90  on the basis of information as to which of the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H in the transmission unit  20  is operating, and respective reception levels at the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H. 
     The vehicle  90  is the detection target of the radar apparatus  10 . In this example, the own Vehicle equipped with the radar apparatus  10  is running on a lane  101 , and the vehicle  90  is running in front of the own vehicle on this lane  101 . Next to the lane  101  is provided a lane  102 , which is an oncoming lane. A lane marking  104  is provided between the lane  101  and the lane  102 . A guard rail  103  is disposed on one side of the lane  101 , and a guard rail  105  is disposed on one side of the lane  102 . 
     The vehicle  90  is installed with a reflector  91 . The reflector  91  reflects radio waves transmitted from the transmission unit  20  of the radar apparatus  10 . As the reflector  91 , it is possible to use one of a plurality of types of reflectors (five types of reflectors RA to RE in this example). 
       FIG. 3  illustrates examples of the five types of reflectors RA to RE. 
     The reflector RA emits a radio wave with horizontal polarization toward the radar apparatus  10  in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→horizontal polarization), and emits a radio wave with horizontal polarization toward the radar apparatus  10  in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→horizontal polarization). 
       FIG. 4  illustrates a configuration example of the reflector RA. In this example, the reflector RA includes a liquid crystal device  111  and reflective members  112  and  113 . The liquid crystal device  111  changes an orientation of a polarization plane of a radio wave, as with a liquid crystal display. Specifically, in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters, the liquid crystal device  111  reflects the radio wave WH toward the radar apparatus  10  while maintaining the orientation of the polarization plane. Further, in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters, the liquid crystal device  111  rotates the orientation of the polarization plane of the radio wave WV by 90 degrees while transmitting the radio wave, thereby emitting a radio wave with horizontal polarization. A rotational angle of the polarization plane in the liquid crystal device  111  is adjustable by, for example, changing a voltage applied to the liquid crystal device  111 . The reflective member  112  reflects the radio wave emitted from the liquid crystal device  111  toward the reflective member  113  while maintaining the orientation of the polarization plane. The reflective member  113  reflects the radio wave reflected at the reflective member  112  toward the radar apparatus  10  while maintaining the orientation of the polarization plane. 
       FIG. 5  illustrates another configuration example of the reflector RA. In this example, the reflector RA includes a vertically polarized antenna  121 , an amplifier  122 , and a horizontally polarized antenna  123 . The vertically polarized antenna  121  receives the radio wave WV transmitted from the vertically polarized transmitter  20 V and generates an electric signal corresponding to the radio wave WV The amplifier  122  amplifies the electric signal supplied from the vertically polarized antenna  121 . On the basis of the electric signal amplified in the amplifier  122 , the horizontally polarized antenna  123  emits a radio wave with horizontal polarization toward the radar apparatus  10 . Further, this reflector RA is configured to reflect the radio wave WH transmitted from the horizontally polarized transmitter  20 H toward the radar apparatus  10  while maintaining the orientation of the polarization plane. 
       FIG. 6  illustrates another configuration example of the reflector RA. In this example, the reflector RA includes a vertically polarized antenna  131 , a phase shifter  132 , an amplifier  133 , and an antenna  134 . The vertically polarized antenna  131  receives the radio wave WV transmitted from the vertically polarized transmitter  20 V and generates an electric signal corresponding to the radio wave WV The phase shifter  132  shifts a phase of the electric signal supplied from the vertically polarized antenna  131 . The amplifier  133  amplifies the electric signal whose phase has been shifted by the phase shifter  132 . On the basis of the electric signal amplified in the amplifier  122 , the antenna  134  emits a radio wave toward the radar apparatus  10 . Adjusting a phase quantity in the phase shifter  132  enables the antenna  134  to emit a radio wave with horizontal polarization. Further, this reflector RA is configured to reflect the radio wave WH transmitted from the horizontally polarized transmitter  20 H toward the radar apparatus  10  while maintaining the orientation of the polarization plane. 
     As illustrated in  FIG. 3 , the reflector RB emits, toward the radar apparatus  10 , a radio wave whose polarization plane is rotated by 45 degrees in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→45-degree polarization), and emits a radio wave with horizontal polarization toward the radar apparatus  10  in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→horizontal polarization). The reflector RB is implementable by, for example, adjusting the voltage applied to the liquid crystal device  111  in the configuration illustrated in  FIG. 4 . Further, the reflector RB is implementable by, for example, adjusting the phase quantity in the phase shifter  132  in the configuration illustrated in  FIG. 6 . 
     As illustrated in  FIG. 3 , the reflector RC emits a radio wave with vertical polarization toward the radar apparatus  10  in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→vertical polarization), and emits a radio wave with vertical polarization toward the radar apparatus  10  in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→vertical polarization). The reflector RC is implementable by using a configuration similar to that of the reflector RA. 
     As illustrated in  FIG. 3 , the reflector RD emits a radio wave with vertical polarization toward the radar apparatus  10  in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→vertical polarization), and emits a radio wave whose polarization plane is rotated by 45 degrees toward the radar apparatus  10  in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→45-degree polarization). The reflector RD is implementable by using a configuration similar to that of the reflector RB. 
     As illustrated in  FIG. 3 , the reflector RE emits a radio wave with horizontal polarization toward the radar apparatus  10  in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→horizontal polarization), and emits a radio wave with vertical polarization toward the radar apparatus  10  in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→vertical polarization). 
       FIG. 7  illustrates a configuration example of the reflector RE. In this example, the reflector RE includes a Faraday rotator  141  and a reflective member  142 . The Faraday rotator  141  rotates the orientation of the polarization plane of a radio wave to be transmitted by a predetermined angle (45 degrees in this example). The reflective member  142  reflects the radio wave emitted from the Faraday rotator  141  toward the Faraday rotator  141  while maintaining the orientation of the polarization plane. Further, in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters, the Faraday rotator  141  rotates the orientation of the polarization plane of the radio wave by 45 degrees while transmitting the radio wave, thereby emitting a radio wave with 45-degree polarization toward the reflective member  142 , and further rotates the orientation of the polarization plane of the radio wave reflected by the reflective member  142  by 45 degrees while transmitting the radio wave, thereby emitting a radio wave with horizontal polarization toward the radar apparatus  10 . In a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters, the Faraday rotator  141  rotates the orientation of the polarization plane of the radio wave by 45 degrees while transmitting the radio wave, thereby emitting a radio wave with 45-degree polarization toward the reflective member  142 , and further rotates the orientation of the polarization plane of the radio wave reflected by the reflective member  142  by 45 degrees while transmitting the radio wave, thereby emitting a radio wave with vertical polarization toward the radar apparatus  10 . A rotational angle of the polarization plane in the Faraday rotator  141  is adjustable by, for example, changing a magnetic field applied to the Faraday rotator  141 . 
     It is to be noted that a body of a vehicle not installed with the reflector  91  reflects a radio wave transmitted from the transmission unit  20  while maintaining the orientation of the polarization plane. Therefore, for example, this body emits a radio wave with vertical polarization in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→vertical polarization), and emits a radio wave with horizontal polarization in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→horizontal polarization). 
     As the reflector  91 , for example, it is possible to use one of such a plurality of types of reflectors (in this example, the five types of reflectors RA to RE). It is to be noted that these reflectors may also be installed in the guard rails  103  and  105 , for example. This makes it possible, in the radar system  1 , to identify the vehicle  90  and the guard rails  103  and  105  depending on the reflector  91 . 
     Here, the transmission unit  20  corresponds to a specific example of a “transmission unit” in the present disclosure. The reception unit  30  corresponds to a specific example of a “reception unit” in the present disclosure. The control unit  11  corresponds to a specific example of a “control unit” in the present disclosure. The vehicle  90  corresponds to a specific example of a “detection target” in the present disclosure. 
     [Operation and Workings] 
     Next, operation and workings of the radar system  1  of the present embodiment will be described. 
     (Outline of Overall Operation) 
     First, an outline of overall operation of the radar system  1  will be described with reference to  FIG. 1 . The transmission unit  20  transmits radio waves on the basis of an instruction from the control unit  11 , Specifically, the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization, and the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization. The radio waves transmitted from the transmission unit  20  are reflected by the reflector  91  of the vehicle  90 , for example. The reception unit  30  receives the radio waves reflected by the reflector  91  of the vehicle  90  on the basis of an instruction from the control unit  11 . Specifically, the vertically polarized receiver  30 V receives a radio wave with vertical polarization, and the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization. The processor  12  detects the position of the vehicle  90 , which is the detection target, on the basis of the result of the reception by the reception unit  30 . Further, the processor  12  identifies the vehicle  90  on the basis of the information as to which of the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H in the transmission unit  20  is operating and the respective reception levels at the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H. 
     (Detailed Operation) 
     Next, detailed description will be given on operations of identifying the detection target in the radar system  1  in a case where any of the five types of reflectors RA to RE is installed in the vehicle  90  and in a case where no reflector is installed in a vehicle. 
       FIG. 8  illustrates an operation example of the radar system  1  in a case where the reflector RA is used as the reflector  91 . In  FIG. 8 , solid lines indicate available radio waves in a case where the vertically polarized transmitter  20 V is transmitting the radio wave WV, and broken lines indicate available radio waves in a case where the horizontally polarized transmitter  20 H is transmitting the radio wave WH. 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RA, the reflector RA emits a radio wave with horizontal polarization toward the radar apparatus  10  (vertical polarization→horizontal polarization). Thus, the vertically polarized receiver  30 V hardly receives the radio wave reflected by the reflector RA, and the horizontally polarized receiver  30 H receives the radio wave reflected by the reflector RA. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the reflector RA, the reflector RA emits a radio wave with horizontal polarization toward the radar apparatus  10  (horizontal polarization→horizontal polarization). Thus, the vertically polarized receiver  30 V hardly receives the radio wave reflected by the reflector RA, and the horizontally polarized receiver  30 H receives the radio wave reflected by the reflector RA. 
       FIG. 9  illustrates an operation example of the radar system  1  in a case where the reflector RB is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RB, the reflector RB emits a radio wave with 45-degree polarization toward the radar apparatus  10  (vertical polarization→45-degree polarization). Thus, the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H receive the radio wave reflected by the reflector RB. The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave 45-degree polarization. Similarly, the reception level at the horizontally polarized receiver  30 H is lower than that in a case where the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization, because the radio wave received by the horizontally polarized receiver  30 H is a radio wave with 45-degree polarization. The reception level at the vertically polarized receiver  30 V and the reception level at the horizontally polarized receiver  30 H are substantially equal to each other. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the reflector RB, the reflector RB emits a radio wave with horizontal polarization toward the radar apparatus  10  (horizontal polarization→horizontal polarization). Thus, the vertically polarized receiver  30 V hardly receives the radio wave reflected by the reflector RB, and the horizontally polarized receiver  30 H receives the radio wave reflected by the reflector RB. 
       FIG. 10  illustrates an operation example of the radar system  1  in a case where the reflector RC is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RC, the reflector RC emits a radio wave with vertical polarization toward the radar apparatus  10  (vertical polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the reflector RC, and the horizontally polarized receiver  30 H hardly receives the radio wave reflected by the reflector RC. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the reflector RC, the reflector RC emits a radio wave with vertical polarization toward the radar apparatus  10  (horizontal polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the reflector RC, and the horizontally polarized receiver  30 H hardly receives the radio wave reflected by the reflector RC. 
       FIG. 11  illustrates an operation example of the radar system  1  in a case where the reflector RD is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RD, the reflector RD emits a radio wave with vertical polarization toward the radar apparatus  10  (vertical polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the reflector RD, and the horizontally polarized receiver  30 H hardly receives the radio wave reflected by the reflector RD. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the reflector RD, the reflector RD emits a radio wave with 45-degree polarization toward the radar apparatus  10  (horizontal polarization→45-degree polarization). Thus, the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H receive the radio wave reflected by the reflector RD. The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with 45-degree polarization. Similarly, the reception level at the horizontally polarized receiver  30 H is lower than that in a case where the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization, because the radio wave received by the horizontally polarized receiver  30 H is a radio wave with 45-degree polarization. The reception level at the vertically polarized receiver  30 V and the reception level at the horizontally polarized receiver  30 H are substantially equal to each other. 
       FIG. 12  illustrates an operation example of the radar system  1  in a case where the reflector RE is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RE, the reflector RE emits a radio wave with horizontal polarization toward the radar apparatus  10  (vertical polarization→horizontal polarization). Thus, the vertically polarized receiver  30 V hardly receives the radio wave reflected by the reflector RE, and the horizontally polarized receiver  30 H receives the radio wave reflected by the reflector RE. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the reflector RE, the reflector RE emits a radio wave with vertical polarization toward the radar apparatus  10  (horizontal polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the reflector RE, and the horizontally polarized receiver  30 H hardly receives the radio wave reflected by the reflector RE. 
       FIG. 13  illustrates an operation example of the radar system  1  in a case where the reflector  91  is not installed in a vehicle (vehicle  92 ). 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters a body of the vehicle  92 , the body of the vehicle  92  emits a radio wave with vertical polarization (vertical polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the body of the vehicle  92 , and the horizontally polarized receiver  30 H hardly receives the radio wave reflected by the body of the vehicle  92 . 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the body of the vehicle  92 , the body of the vehicle  92  emits a radio wave with horizontal polarization (horizontal polarization→horizontal polarization). Thus, the vertically polarized receiver  30 V hardly receives the radio wave reflected by the body of the vehicle  92 , and the horizontally polarized receiver  30 H receives the radio wave reflected by the body of the vehicle  92 . 
     As described above with reference to  FIGS. 8 to 12 , in the radar system  1 , the reception levels at the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H differ depending on whether the reflector  91  is installed in the other vehicle and which type of the reflector  91  is installed in a case where the reflector  91  is installed. This enables the radar system  1  to detect the reflector  91  on the basis of these reception levels. 
     As described above, in the radar system  1 , the transmission unit  20  transmits the radio wave WV with vertical polarization and the radio wave WH with horizontal polarization, and the reception unit  30  receives a radio wave with vertical polarization and a radio wave with horizontal polarization. Then, the processor  12  detects the reflector  91  on the basis of the information as to which of the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H in the transmission unit  20  is operating and the respective reception levels at the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H. This makes it possible, in the radar system  1 , to identify the vehicle  90  by identifying the reflector  91 . 
     Further, in the radar system  1 , the transmission unit  20  transmits the radio wave WV with vertical polarization and the radio wave WH with horizontal polarization, the reception unit  30  receives a radio wave with vertical polarization and a radio wave with horizontal polarization, and the reflector  91  that changes polarization characteristics of radio waves is used. This makes it possible to suppress influence of clutter, for example, or radio waves transmitted from an oncoming vehicle, making it possible to enhance detection accuracy. That is, for example, in a case where a transmission unit includes only the vertically polarized transmitter  20 V, a reception unit includes only the vertically polarized receiver  30 V, and a vehicle not installed with a reflector is to be detected, the radio wave WV transmitted from the transmission unit (the vertically polarized transmitter  20 V) is reflected by a body of the vehicle, which is the detection target, and is also reflected by, for example, unevenness of a road surface, etc. Therefore, the reception unit (the vertically polarized receiver  30 V) receives the radio wave reflected by the body of the vehicle, which is the detection target, and the unnecessary radio wave (clutter) reflected by the unevenness of the road surface, etc. In this case, detection accuracy can decrease, for example. In contrast, in the radar system  1 , the transmission unit  20  includes the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H, the reception unit  30  includes the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H, and the reflector  91  that changes polarization characteristics of radio waves is used. Thus, for example, in a case where the transmission unit  20  transmits the radio wave WV and the reflector RA (vertical polarization→horizontal polarization) is installed as the reflector  91  in the vehicle  90 , the reflector  91  emits a radio wave with horizontal polarization on the basis of the radio wave WV transmitted from the transmission unit  20 . Then, the horizontally polarized receiver  30 H of the reception unit  30  receives the radio wave reflected by the reflector  91 , and the vertically polarized receiver  30 V receives clutter. Thus, in this example, the reception level at the horizontally polarized receiver  30 H is less likely to be influenced by the clutter. It is consequently possible to enhance detection accuracy in the radar system  1 . 
     [Effects] 
     As described above, in the present embodiment, the transmission unit transmits a radio wave with vertical polarization and a radio wave with horizontal polarization, and the reception unit receives a radio wave with vertical polarization and a radio wave with horizontal polarization. This makes it possible to identify the detection target and enhance detection accuracy. 
     Modification Example 1-1 
     In the above embodiment, the five types of reflectors RA to RE are used, but this is not limitative. For example, the reflector RB rotates the polarization plane of the radio wave WV transmitted from the vertically polarized transmitter  20 V by 45 degrees, but a reflector that rotates the polarization plane of the radio wave WV by 20 degrees or a reflector that rotates the polarization plane of the radio wave WV by 70 degrees may be further used. Similarly, for example, the reflector RD rotates the polarization plane of the radio wave WH transmitted from the horizontally polarized transmitter  20 H by 45 degrees, but a reflector that rotates the polarization plane of the radio wave WH by 20 degrees or a reflector that rotates the polarization plane of the radio wave WH by 70 degrees may be further used. Further, for example, the reflector RE rotates the polarization plane of the radio wave WV transmitted from the vertically polarized transmitter  20 V by 90 degrees and rotates the polarization plane of the radio wave WH transmitted from the horizontally polarized transmitter  20 H by 90 degrees, but a reflector that rotates the polarization plane of the radio wave WV by 45 degrees and rotates the polarization plane of the radio wave WH by 45 degrees may be further used. Even in this case, for example, the processor  12  is able to identify these reflectors on the basis of the reception level at the vertically polarized receiver  30 V and the reception level at the horizontally polarized receiver  30 H. 
     Modification Example 1-2 
     In the above embodiment, the radar apparatus  10  detects the vehicle  90 , which is the detection target, by using vertical polarization and horizontal polarization, but this is not limitative. Hereinafter, a radar system  1 A according to the present modification example will be described in detail. 
       FIG. 14  illustrates a configuration example of a radar apparatus  10 A in the radar system  1 A. The radar apparatus  10 A includes the transmission unit  20 , a reception unit  30 A, and a control unit  11 A. 
     The reception unit  30 A receives radio waves transmitted from the transmission unit  20  and reflected by the vehicle  90  on the basis of an instruction from the control unit  11 A. The reception unit  30 A includes the vertically polarized receiver  30 V, the horizontally polarized receiver  30 H, a right-hand circularly polarized receiver  30 R, and a left-hand circularly polarized receiver  30 L, The right-hand circularly polarized receiver  30 R receives a radio wave with right-hand circular polarization. The left-hand circularly polarized receiver  30 L receives a radio wave with left-hand circular polarization. The vertically polarized receiver  30 V, the horizontally polarized receiver  30 H, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L are configured to continuously operate at all times on the basis of the instruction from the control unit  11 A. 
     The control unit  11 A controls operation of the transmission unit  20  and the reception unit  30 A. Specifically, the control unit  11 A performs control to cause the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H to operate alternately in time division, and performs control to cause the vertically polarized receiver  30 V, the horizontally polarized receiver  30 H, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L to continuously operate at all times. The control unit  11 A includes a processor  12 A. The processor  12 A detects the position of the vehicle  90 , which is the detection target, on the basis of a result of the reception by the reception unit  30 A. Further, the processor  12 A also has a function of identifying, for example, the vehicle  90  on the basis of information as to which of the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H in the transmission unit  20  is operating and respective reception levels at the vertically polarized receiver  30 V, the horizontally polarized receiver  30 H, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L of the reception unit  30 A. 
     As the reflector  91  installed in the vehicle  90 , it is possible to use one of a plurality of types of reflectors (in this example, 11 types of reflectors RA to RK), 
       FIG. 15  illustrates examples of the reflectors RF to RK. 
     The reflector RF emits a radio wave with right-hand circular polarization toward the radar apparatus  10 A in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→right-hand circular polarization), and emits a radio wave with left-hand circular polarization toward the radar apparatus  10 A in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→left-hand circular polarization). It is possible to configure the reflector RF by using a quarter-wave plate, for example. 
     The reflector RG emits a radio wave with left-hand circular polarization toward the radar apparatus  10 A in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→left-hand circular polarization), and emits a radio wave with right-hand circular polarization toward the radar apparatus  10 A in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→right-hand circular polarization). 
     The reflector RH emits a radio wave with right-hand circular polarization toward the radar apparatus  10 A in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→right-hand circular polarization), and emits a radio wave with horizontal polarization toward the radar apparatus  10 A in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→horizontal polarization). 
     The reflector RI emits a radio wave with left-hand circular polarization toward the radar apparatus  10 A in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→left-hand circular polarization nd emits a radio wave with horizontal polarization toward the radar apparatus  10 A in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→horizontal polarization). 
     The reflector RJ emits a radio wave with vertical polarization toward the radar apparatus  10 A in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→vertical polarization), and emits a radio wave with right-hand circular polarization toward the radar apparatus  10 A in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→right-hand circular polarization). 
     The reflector RK emits a radio wave with vertical polarization toward the radar apparatus  10 A in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→vertical polarization), and emits a radio wave with left-hand circular polarization toward the radar apparatus  10 A in a case where the radio wave WH transmitted from the horizontally polarized transmitter  20 H enters (horizontal polarization→left-hand circular polarization). 
       FIG. 16  illustrates an operation example of the radar system  1 A in a case where the reflector RF is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RF, the reflector RF emits a radio wave with right-hand circular polarization toward the radar apparatus  10 A (vertical polarization→right-hand circular polarization). Thus, the right-hand circularly polarized receiver  30 R, the vertically polarized receiver  30 V, and the horizontally polarized receiver  30 H receive the radio wave reflected by the reflector RF, and the left-hand circularly polarized receiver  30 L hardly receives the radio wave reflected by the reflector RE. The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with right-hand circular polarization. Similarly, the reception level at the horizontally polarized receiver  30 H is lower than that in a case where the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization, because the radio wave received by the horizontally polarized receiver  30 H is a radio wave right-hand circular polarization. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the reflector RF, the reflector RF emits a radio wave with left-hand circular polarization toward the radar apparatus  10 A (horizontal polarization→left-hand circular polarization). Thus, the left-hand circularly polarized receiver  30 L, the vertically polarized receiver  30 V, and the horizontally polarized receiver  30 H receive the radio wave reflected by the reflector RF, and the right-hand circularly polarized receiver  30 R hardly receives the radio wave reflected by the reflector RF. The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with left-hand circular polarization. Similarly, the reception level at the horizontally polarized receiver  30 H is lower than that in a case where the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization, because the radio wave received by the horizontally polarized receiver  30 H is a radio wave with left-hand circular polarization. 
       FIG. 17  illustrates an operation example of the radar system  1 A in a case where the reflector RG is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RG, the reflector RG emits a radio wave with left-hand circular polarization toward the radar apparatus  10 A (vertical polarization→left-hand circular polarization). Thus, the left-hand circularly polarized receiver  30 L, the vertically polarized receiver  30 V, and the horizontally polarized receiver  30 H receive the radio wave reflected by the reflector RG, and the right-hand circularly polarized receiver  30 R hardly receives the radio wave reflected by the reflector RG. The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with left-hand circular polarization. Similarly, the reception level at the horizontally polarized receiver  30 H is lower than that in a case where the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization, because the radio wave received by the horizontally polarized receiver  30 H is a radio wave with left-hand circular polarization. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the reflector RG, the reflector RG emits a radio wave with right-hand circular polarization toward the radar apparatus  10 A (horizontal polarization→right-hand circular polarization). Thus, the right-hand circularly polarized receiver  30 R, the vertically polarized receiver  30 V, and the horizontally polarized receiver  30 H receive the radio wave reflected by the reflector RG, and the left-hand circularly polarized receiver  30 L hardly receives the radio wave reflected by the reflector RG. The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with right-hand circular polarization. Similarly, the reception level at the horizontally polarized receiver  30 H is lower than that in a case where the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization, because the radio wave received by the horizontally polarized receiver  30 H is a radio wave with right-hand circular polarization. 
       FIG. 18  illustrates an operation example of the radar system  1 A in a case where the reflector RH is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RH, the reflector RH emits a radio wave with right-hand circular polarization toward the radar apparatus  10 A (vertical polarization→right-hand circular polarization). Thus, the right-hand circularly polarized receiver  30 R, the vertically polarized receiver  30 V, and the horizontally polarized receiver  30 H receive the radio wave reflected by the reflector RH, and the left-hand circularly polarized receiver  30 L hardly receives the radio wave reflected by the reflector RH. The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with right-hand circular polarization. Similarly, the reception level at the horizontally polarized receiver  30 H is lower than that in a case where the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization, because the radio wave received by the horizontally polarized receiver  30 H is a radio wave right-hand circular polarization. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the reflector RH, the reflector RH emits a radio wave with horizontal polarization toward the radar apparatus  10 A (horizontal polarization→horizontal polarization). Thus, the horizontally polarized receiver  30 H receives the radio wave reflected by the reflector RH, and the vertically polarized receiver  30 V, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L hardly receive the radio wave reflected by the reflector RH. 
       FIG. 19  illustrates an operation example of the radar system  1 A in a case where the reflector RI is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RI, the reflector RI emits a radio wave with left-hand circular polarization toward the radar apparatus  10 A (vertical polarization→left-hand circular polarization). Thus, the left-hand circularly polarized receiver  30 L, the vertically polarized receiver  30 V, and the horizontally polarized receiver  30 H receive the radio wave reflected by the reflector RI, and the right-hand circularly polarized receiver  30 R hardly receives the radio wave reflected by the reflector RI. The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with left-hand circular polarization. Similarly, the reception level at the horizontally polarized receiver  30 H is lower than that in a case where the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization, because the radio wave received by the horizontally polarized receiver  30 H is a radio wave with left-hand circular polarization. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the reflector RI, the reflector RI emits a radio wave with horizontal polarization toward the radar apparatus  10 A (horizontal polarization→horizontal polarization). Thus, the horizontally polarized receiver  30 H receives the radio wave reflected by the reflector RI, and the vertically polarized receiver  30 V, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L hardly receive the radio wave reflected by the reflector RI. 
       FIG. 20  illustrates an operation example of the radar system  1 A in a case where the reflector RJ is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RJ, the reflector RJ emits a radio wave with vertical polarization toward the radar apparatus  10 A (vertical polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the reflector RJ, and the horizontally polarized receiver  30 H, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L hardly receive the radio wave reflected by the reflector RJ. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the reflector RJ, the reflector RJ emits a radio wave with right-hand circular polarization toward the radar apparatus  10 A (horizontal polarization→right-hand circular polarization). Thus, the right-hand circularly polarized receiver  30 R, the vertically polarized receiver  30 V, and the horizontally polarized receiver  30 H receive the radio wave reflected by the reflector RJ, and the left-hand circularly polarized receiver  30 L hardly receives the radio wave reflected by the reflector RJ, The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with right-hand circular polarization. Similarly, the reception level at the horizontally polarized receiver  30 H is lower than that in a case where the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization, because the radio wave received by the horizontally polarized receiver  30 H is a radio wave with right-hand circular polarization. 
       FIG. 21  illustrates an operation example of the radar system  1 A in a case where the reflector RK is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RK, the reflector RK emits a radio wave with vertical polarization toward the radar apparatus  10 A (vertical polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the reflector RK, and the horizontally polarized receiver  30 H, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L hardly receive the radio wave reflected by the reflector RK. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization and this radio wave WH enters the reflector RK, the reflector RK emits a radio wave with left-hand circular polarization toward the radar apparatus  10 A (horizontal polarization left-hand circular polarization). Thus, the left-hand circularly polarized receiver  30 L, the vertically polarized receiver  30 V, and the horizontally polarized receiver  30 H receive the radio wave reflected by the reflector RK, and the right-hand circularly polarized receiver  30 R hardly receives the radio wave reflected by the reflector RK. The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with left-hand circular polarization. Similarly, the reception level at the horizontally polarized receiver  30 H is lower than that in a case where the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization, because the radio wave received by the horizontally polarized receiver  30 H is a radio wave with left-hand circular polarization. 
     As described above with reference to  FIGS. 8 to 12 and 16 to 21 , in the radar system  1 A, the reception levels at the vertically polarized receiver  30 V, the horizontally polarized receiver  30 H, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L differ depending on whether the reflector  91  is installed in the other vehicle and which type of the reflector  91  is installed in a case where the reflector  91  is installed. This enables the processor  12 A to detect the reflector  91  on the basis of information as to which of the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H is operating, and the respective reception levels at the vertically polarized receiver  30 V, the horizontally polarized receiver  30 H, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L. Thus, it is possible in the radar system  1 A to identify the vehicle  90 , for example. 
     It is to be noted that, in this example, the transmission unit  20  includes two transmitters (the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H) and the reception unit  30 A includes four receivers (the vertically polarized receiver  30 V, the horizontally polarized receiver  30 H, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L), but this is not limitative. For example, as in a radar apparatus  10 B illustrated in  FIG. 22 , a transmission unit may include four transmitters. This radar apparatus  10 B includes a transmission unit  20 B, the reception unit  30 A, and a control unit  11 B. 
     The transmission unit  20 B transmits radio waves on the basis of an instruction from the control unit  11 B. The transmission unit  20 B includes the vertically polarized transmitter  20 V, the horizontally polarized transmitter  20 H, a right-hand circularly polarized transmitter  20 R, and a left-hand circularly polarized transmitter  20 L. The right-hand circularly polarized transmitter  20 R transmits a radio wave WR with right-hand circular polarization. The left-hand circularly polarized transmitter  20 L transmits a radio wave WL with left-hand circular polarization. The vertically polarized transmitter  20 V, the horizontally polarized transmitter  20 H, the right-hand circularly polarized transmitter  20 R, and the left-hand circularly polarized transmitter  20 L are configured to cyclically transmit the radio wave WV, the radio wave WH, the radio wave WR, and the radio wave WL on the basis of the instruction from the control unit  11 B by, for example, operating cyclically in time division. 
     The control unit  11 B controls operation of the transmission unit  20 B and the reception unit  30 A. Specifically, the control unit  11 B performs control to cause the vertically polarized transmitter  20 V, the horizontally polarized transmitter  20 H, the right-hand circularly polarized transmitter  20 R, and the left-hand circularly polarized transmitter  20 L to operate cyclically in time division, and performs control to cause the vertically polarized receiver  30 V, the horizontally polarized receiver  30 H, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L to continuously operate at all times. The control unit  11 B includes a processor  12 B. The processor  12 B detects the position of the vehicle  90 , which is the detection target, on the basis of a result of the reception by the reception unit  30 A, Further, the processor  12 B also has a function of identifying, for example, the vehicle  90  on the basis of information as to which transmitter of the vertically polarized transmitter  20 V, the horizontally polarized transmitter  20 H, the right-hand circularly polarized transmitter  20 R, and the left-hand circularly polarized transmitter  20 L in the transmission unit  20  is operating and respective reception levels at the vertically polarized receiver  30 V, the horizontally polarized receiver  30 H, the right-hand circularly polarized receiver  30 R, and the left-hand circularly polarized receiver  30 L of the reception unit  30 A. 
     2. Second Embodiment 
     Next, description will be given on a radar system  2  according to a second embodiment. In the present embodiment, one transmitter is used to configure a transmission unit. It is to be noted that substantially the same components as those in the radar system  1  according to the first embodiment are denoted by the same reference numerals, and description thereof is omitted as appropriate. 
       FIG. 23  illustrates a configuration example of the radar system  2 . The radar system  2  includes a radar apparatus  40 . The radar apparatus  40  includes a transmission unit  50 , the reception unit  30 , and a control unit  41 . The transmission unit  50  transmits a radio wave on the basis of an instruction from the control unit  41 . The transmission unit  50  includes the vertically polarized transmitter  20 V The control unit  41  controls operation of the transmission unit  50  and the reception unit  30 . The control unit  41  includes a processor  42 . The processor  42  detects the position of the vehicle  90 , which is the detection target, on the basis of the result of the reception by the reception unit  30 . Further, the processor  42  is configured to identify, for example, the vehicle  90  on the basis of the respective reception levels at the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H. 
     As the reflector  91  installed in the vehicle  90 , it is possible to use one of a plurality of types of reflectors (two types of reflectors RA 2  and RB 2  in this example). 
       FIG. 24  illustrates examples of the two types of reflectors RA 2  and RB 2 . The reflector RA 2  emits a radio wave with horizontal polarization toward the radar apparatus  10  in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→horizontal polarization). The reflector RB 2  emits a radio wave whose polarization plane is rotated by 45 degrees toward the radar apparatus  10  in a case where the radio wave WV transmitted from the vertically polarized transmitter  20 V enters (vertical polarization→45-degree polarization). 
       FIG. 25  illustrates an operation example of the radar system  2  in a case where the reflector RA 2  is used as the reflector  91 . If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RA 2 , the reflector RA 2  emits a radio wave with horizontal polarization toward the radar apparatus  40  (vertical polarization→horizontal polarization). Thus, the vertically polarized receiver  30 V hardly receives the radio wave reflected by the reflector RA, and the horizontally polarized receiver  30 H receives the radio wave reflected by the reflector RA. 
       FIG. 26  illustrates an operation example of the radar system  2  in a case where the reflector RB 2  is used as the reflector  91 . If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RB 2 , the reflector RB 2  emits a radio wave with 45-degree polarization toward the radar apparatus  40  (vertical polarization→45-degree polarization). Thus, the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H receive the radio wave reflected by the reflector RB 2 . The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with 45-degree polarization. Similarly, the reception level at the horizontally polarized receiver  30 H is lower than that in a case where the horizontally polarized receiver  30 H receives a radio wave with horizontal polarization, because the radio wave received by the horizontally polarized receiver  30 H is a radio wave with 45-degree polarization. The reception level at the vertically polarized receiver  30 V and the reception level at the horizontally polarized receiver  30 H are substantially equal to each other. 
       FIG. 27  illustrates an operation example of the radar system  2  in a case where no reflector is installed in a vehicle (vehicle  92 ). If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters a body of the vehicle  92 , the body of the vehicle  92  emits a radio wave with vertical polarization (vertical polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the body of the vehicle  92 , and the horizontally polarized receiver  30 H hardly receives the radio wave reflected by the body of the vehicle  92 . 
     As described above, in the radar system  2 , the reception unit  30  receives a radio wave with vertical polarization and a radio wave with horizontal polarization. Then, the control unit  41  detects the reflector  91  on the basis of the respective reception levels at the vertically polarized receiver  30 V and the horizontally polarized receiver  30 H. Thus, it is possible in the radar system  2  to identify the vehicle  90 , for example. 
     Further, in the radar system  2 , the reception unit  30  receives a radio wave with vertical polarization and a radio wave with horizontal polarization, and the reflector  91  that changes polarization characteristics of radio waves is used. This makes it possible to suppress influence of clutter, for example, or radio waves transmitted from an oncoming vehicle, making it possible to enhance detection accuracy. 
     As described above, in the present embodiment, the reception unit receives a radio wave with vertical polarization and a radio wave with horizontal polarization. This makes it possible to identify the detection target and enhance detection accuracy. 
     Modification Example 2-1 
     In the above embodiment, the transmission unit  50  is configured by using the vertically polarized transmitter  20 V, but this is not limitative. Alternatively, for example, the horizontally polarized transmitter  20 H may be used to configure a transmission unit. 
     Modification Example 2-2 
     The modification examples of the first embodiment may be applied to the radar system  2  according to the above embodiment. 
     3. Third Embodiment 
     Next, description will be given on a radar system  3  according to a third embodiment. In the present embodiment, one receiver is used to configure a reception unit. It is to be noted that substantially the same components as those in the radar system  1  according to the first embodiment are denoted by the same reference numerals, and description thereof is omitted as appropriate. 
       FIG. 28  illustrates a configuration example of the radar system  3 . The radar system  3  includes a radar apparatus  60 . The radar apparatus  60  includes the transmission unit  20 , a reception unit  70 , and a control unit  61 . The reception unit  70  receives a radio wave on the basis of an instruction from the control unit  61 . The reception unit  70  includes the vertically polarized receiver  30 V. The control unit  61  controls operation of the transmission unit  20  and the reception unit  70 . The control unit  61  includes a processor  62 . The processor  62  detects the position of the vehicle  90 , which is the detection target, on the basis of the result of the reception by the reception unit  70 . Further, the processor  62  also has a function of identifying, for example, the vehicle  90  on the basis of information as to which of the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H in the transmission unit  20  is operating, and a reception level at the vertically polarized receiver  30 V. 
     As the reflector  91  installed in the vehicle  90 , it is possible to use one of a plurality of types of reflectors (in this example, the five types of reflectors RA to RE illustrated in  FIG. 3 ). 
       FIG. 29  illustrates an operation example of the radar system  3  in a case where the reflector RA is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RA, the reflector RA emits a radio wave with horizontal polarization toward the radar apparatus  60  (vertical polarization→horizontal polarization). Thus, the vertically polarized receiver  30 V hardly receives the radio wave reflected by the reflector RA. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization, and this radio wave WH enters the reflector RA, the reflector RA emits a radio wave with horizontal polarization toward the radar apparatus  60  (horizontal polarization→horizontal polarization). Thus, the vertically polarized receiver  30 V hardly receives the radio wave reflected by the reflector RA. 
       FIG. 30  illustrates an operation example of the radar system  3  in a case where the reflector RB is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits radio wave WV with vertical polarization and this radio wave WV enters the reflector RB, the reflector RB emits a radio wave with 45-degree polarization toward the radar apparatus  60  (vertical polarization→45-degree polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the reflector RB. The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with 45-degree polarization. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization, and this radio wave WH enters the reflector RB, the reflector RB emits a radio wave with horizontal polarization toward the radar apparatus  60  (horizontal polarization→horizontal polarization). Thus, the vertically polarized receiver  30 V hardly receives the radio wave reflected by the reflector RB. 
       FIG. 31  illustrates an operation example of the radar system  3  in a case where the reflector RC is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RC, the reflector RC emits a radio wave with vertical polarization toward the radar apparatus  60  (vertical polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the reflector RC. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization, and this radio wave WH enters the reflector RC, the reflector RC emits a radio wave with vertical polarization toward the radar apparatus  60  (horizontal polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by reflector RC. 
       FIG. 32  illustrates an operation example of the radar system  3  in a case where the reflector RD is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RD, the reflector RD emits a radio wave with vertical polarization toward the radar apparatus  60  (vertical polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the reflector RD. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization, and this radio wave WH enters the reflector RD, the reflector RD emits a radio wave with 45-degree polarization toward the radar apparatus  60  (horizontal polarization→45-degree polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the reflector RD. The reception level at the vertically polarized receiver  30 V is lower than that in a case where the vertically polarized receiver  30 V receives a radio wave with vertical polarization, because the radio wave received by the vertically polarized receiver  30 V is a radio wave with 45-degree polarization. In other words, the reception level at the vertically polarized receiver  30 V is lower than the reception level in a case where the radio wave is reflected by the reflector RC ( FIG. 31 ). 
       FIG. 33  illustrates an operation example of the radar system  3  in a case where the reflector RE is used as the reflector  91 . 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the reflector RE, the reflector RE emits a radio wave with horizontal polarization toward the radar apparatus  60  (vertical polarization→horizontal polarization). Thus, the vertically polarized receiver  30 V hardly receives the radio wave reflected by the reflector RE. 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization, and this radio wave WH enters the reflector RE, the reflector RE emits a radio wave with vertical polarization toward the radar apparatus  60  (horizontal polarization→vertical polarization). Thus, the vertically polarized receiver  30 V receives the radio wave reflected by the reflector RE. 
       FIG. 34  illustrates an operation example of the radar system  1  in a case where no reflector is installed in a vehicle (vehicle  92 ). 
     If the vertically polarized transmitter  20 V transmits the radio wave WV with vertical polarization and this radio wave WV enters the body of the vehicle  92 , the body of the vehicle  92  emits a radio wave with vertical polarization (vertical polarization→vertical polarization). Therefore, the vertically polarized receiver  30 V receives the radio wave reflected by the body of the vehicle  92 . The reception level at the vertically polarized receiver  30 V differs from the reception level in a case where the radio wave is reflected by the reflector RB ( FIG. 30 ), because the vertically polarized receiver  30 V receives the radio wave reflected by the body of the vehicle  92 . 
     Further, if the horizontally polarized transmitter  20 H transmits the radio wave WH with horizontal polarization, and this radio wave WH enters the body of the vehicle  92 , the body of the vehicle  92  emits a radio wave with horizontal polarization (horizontal polarization→horizontal polarization). Thus, the vertically polarized receiver  30 V hardly receives the radio wave reflected by the body of the vehicle  92 . 
     As described above, in the radar system  3 , the transmission unit  20  transmits the radio wave WV with vertical polarization and the radio wave WH with horizontal polarization. Then, the processor  62  detects the reflector  91  on the basis of information as to which of the vertically polarized transmitter  20 V and the horizontally polarized transmitter  20 H in the transmission unit  20  is operating, and the reception level at the vertically polarized receiver  30 V. Thus, it is possible in the radar system  3  to identify the vehicle  90 , for example. 
     Further, in the radar system  3 , the transmission unit  20  transmits a radio wave with vertical polarization and a radio wave with horizontal polarization, and the reflector  91  that changes polarization characteristics of radio waves is used. This makes it possible to suppress influence of clutter, for example, or radio waves transmitted from an oncoming vehicle, making it possible to enhance detection accuracy. 
     As described above, in the present embodiment, the transmission unit transmits a radio wave with vertical polarization and a radio wave with horizontal polarization. This makes it possible to identify the detection target and enhance detection accuracy. 
     Modification Example 3-1 
     In the above embodiment, the reception unit  70  is configured by using the vertically polarized receiver  30 V, but this is not limitative. Alternatively, for example, the horizontally polarized receiver  30 H may be used to configure a reception unit. 
     Modification Example 3-2 
     The modification examples of the first embodiment may be applied to the radar system  3  according to the above embodiment. 
     Although the present technology has been described above with reference to some embodiments and modification examples, the present technology is not limited to these embodiments, etc., and may be modified in a variety of ways. 
     For example, in the above embodiments, the present technology is applied to an automotive radar, but this is not limitative. The present technology is applicable to a variety of uses. Specifically, for example, the present technology may be used in a factory or the like. 
     It is to be noted that the effects described herein are merely illustrative and are not limitative, and may include other effects. 
     It is to be noted that the present technology may have the following configurations. 
     (1) 
     A radar apparatus including: 
     a transmission unit configured to transmit radio waves; 
     a reception unit including a first receiver configured to receive a first reflected radio wave and a second receiver configured to receive a second reflected radio wave, the first reflected radio wave and the second reflected radio wave having different polarization characteristics from each other and being included in reflected radio waves that are the radio waves reflected by a detection target; and 
     a control unit configured to control operations of the transmission unit and the reception unit, and configured to identify the detection target on the basis of the operation of the transmission unit, a first reception level at the first receiver, and a second reception level at the second receiver. 
     (2) 
     The radar apparatus according to (1), in which 
     the detection target includes a reflector configured to change polarization characteristics of the radio waves, and 
     the reflected radio waves are the radio waves reflected by the reflector. 
     (3) 
     The radar apparatus according to (1) or (2), in which 
     polarization of the first reflected radio wave and the second reflected radio wave is linear polarization, and 
     an orientation of a polarization plane of the first reflected radio wave differs from an orientation of a polarization plane of the second reflected radio wave. 
     (4) 
     The radar apparatus according to (1) or (2), in which 
     polarization of the first reflected radio wave and the second reflected radio wave is circular polarization, and 
     a rotational direction of a polarization plane of the first reflected radio wave differs from a rotational direction of a polarization plane of the second reflected radio wave. 
     (5) 
     The radar apparatus according to any one of (1) to (4), in which 
     the reflected radio waves include a third reflected radio wave whose polarization characteristic differs from the polarization characteristic of the first reflected radio wave and the polarization characteristic of the second reflected radio wave, and 
     the reception unit further includes a third receiver configured to receive the third reflected radio wave. 
     (6) 
     The radar apparatus according to any one of (1) to (5), in which 
     the radio waves include a first radio wave and a second radio wave having different polarization characteristics from each other, and 
     the transmission unit includes a first transmitter configured to transmit the first radio wave and a second transmitter configured to transmit the second radio wave. 
     (7) 
     The radar apparatus according to (6), in which the control unit is configured to control the operation of the transmission unit to cause the first transmitter to operate in a first period and the second transmitter to operate in a second period, and is configured to control the operation of the reception unit to cause both the first receiver and the second receiver to operate in the first period and the second period. 
     (8) 
     The radar apparatus according to (6) or (7), in which the control unit is configured to identify the detection target on the basis of information as to which of the first transmitter and the second transmitter is operating, the first reception level, and the second reception level. 
     (9) 
     A radar apparatus including: 
     a transmission unit including a first transmitter configured to transmit a first radio wave and a second transmitter configured to transmit a second radio wave, the first radio wave and the second radio wave having different polarization characteristics from each other; 
     a reception unit configured to receive reflected radio waves that are radio waves reflected by a detection target, the radio waves including the first radio wave and the second radio wave; and 
     a control unit configured to control operations of the transmission unit and the reception unit, and configured to identify the detection target on the basis of information as to which of the first transmitter and the second transmitter is operating, and a reception level at the reception unit. 
     (10) 
     A radar control apparatus configured to perform: 
     a first control operation of controlling an operation of a transmission unit configured to transmit radio waves; 
     a second control operation of controlling an operation of a reception unit including a first receiver configured to receive a first reflected radio wave and a second receiver configured to receive a second reflected radio wave, the first reflected radio wave and the second reflected radio wave having different polarization characteristics from each other and being included in reflected radio waves that are the radio waves reflected by a detection target; and 
     an identification operation of identifying the detection target on the basis of the operation of the transmission unit, a first reception level at the first receiver, and a second reception level at the second receiver. 
     (11) 
     A radar system including: 
     a radar apparatus; and 
     a reflector provided for a detection target of the radar apparatus, 
     the radar apparatus including
         a transmission unit configured to transmit radio waves;   a reception unit including a first receiver configured to receive a first reflected radio wave and a second receiver configured to receive a second reflected radio wave, the first reflected radio wave and the second reflected radio wave having different polarization characteristics from each other and being included in reflected radio waves that are the radio waves reflected by the reflector; and   a control unit configured to control operations of the transmission unit and the reception unit, and configured to identify the detection target on the basis of the operation of the transmission unit, a first reception level at the first receiver, and a second reception level at the second receiver.       

     This application claims the benefit of Japanese Priority Patent Application No. 2018-075865 filed with the Japan Patent Office on Apr. 11, 2018, the entire contents of which are incorporated herein by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.