Patent Publication Number: US-11041953-B2

Title: Object detecting device and sensor device

Description:
TECHNICAL FIELD 
     The present invention relates to an object detecting device for detecting an object existing in a space, and also relates to a sensor device mounting the object detecting device thereon. 
     BACKGROUND ART 
     The following method is known as a method for enhancing a detection probability by an object detecting device for detecting an object existing in a space. 
     A method is known as enhancing a detection probability of an object by receiving a signal reflecting off the object existing in the space with a plurality of object detecting devices, combining reception signals of the plurality of object detecting devices, and detecting the object from a combined signal, as compared with the case of detecting the object from a reception signal of a single object detecting device. 
     In the following non-patent literature 1, it is disclosed that a detection probability of an object to be detected is enhanced by acquiring position information of the object prior to combining reception signals of a plurality of object detecting devices, and performing coherent combining using the position information. 
     CITATION LIST 
     
         
         Non-Patent Literature 1: “Wideband Distributed Coherent Aperture Radar,” 2014 IEEE Radar Conference, pp. 1114-1117, May 2014. 
       
    
     SUMMARY OF INVENTION 
     When combining the reception signals of the plurality of object detecting devices, the detection probability of the object may be enhanced by acquiring in advance the position information of the object to be detected and performing the coherent combining using the position information. However, there has been a problem that, in a case where the position information of the object to be detected cannot be acquired in advance, the detection probability of the object cannot be enhanced. 
     The present invention has been made to solve the above problem, and an object thereof is to provide an object detecting device that is capable of enhancing a detection probability of an object to be detected without acquiring position information of the object in advance. 
     In addition, another object of the present invention is to provide a sensor device on which the foregoing object detecting device is mounted. 
     An object detecting device according to the present invention is provided with: a beat signal extractor to receive a signal reflecting off an object to be detected and extract a beat signal from the received signal; a spectral analyzer to analyze a spectrum of the beat signal extracted by the beat signal extractor and a spectrum of a beat signal extracted by another object detecting device; a search range width setter to set a search range width for frequency; a combination target selector to determine, for each spectrum analyzed by the spectral analyzer, a frequency search range having the search range width set by the search range width setter, and select, for each of the analyzed spectra, a frequency of a combination target from among the frequencies in the determined search range by comparing spectral components of the frequencies in the determined search range; a frequency corrector to calculate individual frequency correcting amounts from differences between each of the frequencies of the combination targets selected by the combination target selector, and correct a frequency of the beat signal extracted by the beat signal extractor and a frequency of the beat signal extracted by said another object detecting device in accordance with each of the calculated frequency correcting amounts; a combiner to combine the beat signals, each of whose frequencies has been corrected by the frequency corrector; and an object detector to detect the object from a combined beat signal obtained by the combiner. 
     According to the present invention, there is provided: a beat signal extractor to receive a signal reflecting off an object to be detected and extract a beat signal from the received signal; a spectral analyzer to analyze a spectrum of the beat signal extracted by the beat signal extractor and a spectrum of a beat signal extracted by another object detecting device; a search range width setter to set a search range width for frequency; a combination target selector to determine, for each spectrum analyzed by the spectral analyzer, a frequency search range having the search range width set by the search range width setter, and select, for each of the analyzed spectra, a frequency of a combination target from among the frequencies in the determined search range by comparing spectral components of the frequencies in the determined search range; a frequency corrector to calculate individual frequency correcting amounts from differences between each of the frequencies of the combination targets selected by the combination target selector, and correct a frequency of the beat signal extracted by the beat signal extractor and a frequency of the beat signal extracted by said another object detecting device in accordance with each of the calculated frequency correcting amounts; and a combiner to combine the beat signals, each of whose frequencies has been corrected by the frequency corrector. Therefore, there is an effect that the detection probability of an object to be detected can be enhanced without acquiring the position information of the object in advance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an explanatory diagram illustrating a relation between a plurality of object detecting devices and an object to be detected according to Embodiment 1 of the present invention. 
         FIG. 2  is a structural diagram illustrating a sensor device according to Embodiment 1 of the present invention, on which an object detecting device is mounted. 
         FIG. 3  is a structural diagram illustrating an object detecting device  1  according to Embodiment 1 of the present invention. 
         FIG. 4A  is an explanatory diagram illustrating a transmission wave and a reflected wave whose frequencies change over time, and  FIG. 4B  is an explanatory diagram illustrating a beat signal whose beat frequency changes over time. 
         FIG. 5  is a structural diagram illustrating a signal processing circuit  20  of the object detecting device  1  according to Embodiment 1 of the present invention. 
         FIG. 6  is a hardware structural diagram of the signal processing circuit  20  of the object detecting device  1  according to Embodiment 1 of the present invention. 
         FIG. 7  is a hardware structural diagram when the signal processing circuit  20  is realized by a computer. 
         FIG. 8  is a flowchart illustrating processing details of the signal processing circuit  20 . 
         FIG. 9  is an explanatory diagram for explaining a difference in beat frequency between the object detecting device  1   a  and the object detecting device  1   b  or  1   c.    
         FIG. 10A  is an explanatory diagram illustrating a transmission signal including a plurality of pulses,  FIG. 10B  is an explanatory diagram illustrating a reception signal including a plurality of pulses,  FIG. 10C  is an explanatory diagram illustrating a transmission wave and a reflected wave whose frequencies change over time, and  FIG. 10D  is an explanatory diagram illustrating a beat signal whose beat frequency changes over time. 
         FIG. 11A  is an explanatory diagram illustrating an output signal of a beat signal extracting circuit  17 ,  FIG. 11B  is an explanatory diagram illustrating a spectrum when the Doppler shift does not occur in the reflected wave,  FIG. 11C  is an explanatory diagram illustrating a spectrum when the Doppler shift occurs in the reflected wave, and  FIG. 11D  is an explanatory diagram enlarging a vicinity of a frequency of 0 Hz in  FIG. 11C . 
         FIG. 12  is a structural diagram illustrating another type of a signal processing circuit  20  of the object detecting device  1  according to Embodiment 1 of the present invention. 
         FIG. 13  is a structural diagram illustrating a signal processing circuit  20  of an object detecting device  1  according to Embodiment 3 of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, in order to explain the present invention in more detail, embodiments for carrying out the present invention will be described with reference to the accompanying drawings. 
     Embodiment 1 
       FIG. 1  is an explanatory diagram illustrating a relation between a plurality of object detecting devices and an object to be detected according to Embodiment 1 of the present invention. 
     Although  FIG. 1  illustrates an example using three object detecting devices  1   a ,  1   b , and  1   c , it is not limited to three as far as two or more object detecting devices are used. 
     In the example of  FIG. 1 , the object detecting device  1   a  radiates toward a space a transmission wave such as a radio frequency (RF) signal, and a reflected wave reflecting off and returning from an object  2  to be detected reaches the object detecting device  1   a , the object detecting device  1   b , and the object detecting device  1   c.    
     In Embodiment 1, the object detecting devices  1   a ,  1   b , and  1   c  may be expressed as an object detecting device  1  when it is not needed to distinguish those devices from each other. 
       FIG. 2  is a structural diagram illustrating a sensor device according to Embodiment 1 of the present invention, on which the object detecting device  1  is mounted. 
     In  FIG. 2 , an object monitor  3  is a processor for performing a process of monitoring the object  2  detected by the object detecting device  1 . 
     A monitoring process for the object  2  performed by the object monitor  3  includes: a process of monitoring change in the position and velocity of the object  2  by recording a relative velocity v r  and a distance R 1  to the object  2  detected by the object detecting device  1 , and; a prediction process of predicting a future position and a future velocity of the object  2 . Since the prediction process for predicting the future position and velocity of the object is a known technique, a detailed explanation will be omitted here. 
     The sensor device of  FIG. 2  includes an object monitoring device such as a target tracking device or a radar device. 
     In Embodiment 1, an example will be described, in which the object detecting device  1  detects the object  2  by using a distance measurement system called a frequency modulation continuous wave (FMCW) system. 
       FIG. 3  is a structural diagram illustrating the object detecting device  1  according to Embodiment 1 of the present invention. 
     In  FIG. 3 , a transmission signal generating circuit  11  is implemented by, for example, a combiner, an oscillator, a processor, a digital to analog converter (DAC), and the like. The transmission signal generating circuit  11  generates a transmission signal subjected to frequency modulation in which the frequency changes over time, and outputs the transmission signal to a transmission high-frequency circuit  12  and a beat signal extracting circuit  17 . 
     The processor may be, for example, a field-programmable gate array (FPGA), a Digital Signal Processor (DSP), a central processor (CPU), or the like. 
     Note that, when the transmission signal generating circuit  11  uses the DAC, a filter for removing harmonics may be provided at the output side of the DAC. 
     The transmission high-frequency circuit  12  performs: a frequency conversion process of converting a frequency of the transmission signal output from the transmission signal generating circuit  11  into a carrier frequency; a filter process of removing spurious emissions and out-of-band frequencies of the transmission signal; a power amplification process of amplifying power of the transmission signal; and the like. The transmission high-frequency circuit  12  outputs, as a transmission wave to a transmission antenna  13 , the transmission signal obtained through the foregoing processes. 
     The transmission antenna  13  radiates to the space the transmission wave output from the transmission high-frequency circuit  12 . 
     In the example of  FIG. 1 , the transmission wave is radiated to the space from the object detecting device  1   a  among the three object detecting devices  1   a ,  1   b , and  1   c , and transmission waves from the object detecting devices  1   b  and  1   c  are not radiated to the space. However, it is assumed that each of the object detecting devices  1   b  and  1   c  has the transmission signal generating circuit  11 , the transmission high-frequency circuit  12 , and the transmission antenna  13 , similarly to the object detecting device  1   a . It is also assumed that, in the object detecting devices  1   a ,  1   b , and  1   c , an output timing of the transmission signal from the transmission signal generating circuit  11  to the beat signal extracting circuit  17  is synchronized. 
     Note that, when the object detecting device  1  radiating the transmission wave to the space is always the object detecting device  1   a , and the object detecting devices  1   b  and  1   c  do not radiate the transmission wave to the space, the object detecting device  1   b  and  1   c  do not have to include the transmission signal generating circuit  11 , the transmission high-frequency circuit  12 , and the transmission antenna  13 . In this case, in order to extract a beat signal from a reception signal, the object detecting devices  1   b  and  1   c  need to acquire, through communication or the like, the transmission signal generated by the object detecting device  1   a.    
     A beat signal extractor  14  includes a reception antenna  15 , a reception high-frequency circuit  16 , and the beat signal extracting circuit  17 . The beat signal extractor  14  receives a signal reflecting off the object  2  to be detected, and extracts a beat signal from the received signal. 
     After the transmission wave is radiated from the transmission antenna  13  to the space, the reception antenna  15  receives a reflected wave for the transmission wave, which reflects off the object  2  to be detected. 
     The reception high-frequency circuit  16  performs: a frequency conversion process of converting the frequency of the reception signal of the reflected wave received by the reception antenna  15  into, for example, an intermediate (IF) frequency; a filter process of removing an image frequency during the frequency conversion and an unnecessary frequency included in the reception signal; a power amplification process of amplifying power of the reception signal by an amplifier such as a low noise amplifier (LNA); and the like. The reception high-frequency circuit  16  outputs to the beat signal extracting circuit  17  the reception signal obtained through the foregoing processes. 
     The beat signal extracting circuit  17  is implemented by, for example, a mixer or the like. The beat signal extracting circuit  17  extracts a beat signal from the reception signal by multiplying the reception signal output from the reception high-frequency circuit  16  and the transmission signal output from the transmission signal generating circuit  11  together. 
     The beat signal indicates a difference between a component of frequency modulation in the transmission signal and the frequency of the reception signal. 
       FIG. 4  is an explanatory diagram illustrating the transmission wave and reflected wave and the beat signal. 
       FIG. 4A  illustrates the transmission wave and reflected wave whose frequencies change over time.  FIG. 4B  illustrates the beat signal whose beat frequency changes over time. 
     The transmission wave is delayed due to a space propagation time from the object detecting device  1   a  to the object  2  to be detected, and is subjected to the Doppler shift. As a result, the reflected wave reaches the reception antenna  15  with a frequency which is different from that of the transmission wave. 
     Note that, at the object detecting device  1   a , a frequency shift occurs, which is twice as much as a Doppler shift caused by a relative moving velocity in a direction on a straight line connecting the object detecting device  1   a  and the object  2  to be detected. 
     At the object detecting device  1   b , a frequency shift occurs, which is a sum of the foregoing Doppler shift regarding the object detecting device  1   a  and a Doppler shift caused by a relative moving velocity in a direction on a straight line connecting the object detecting device  1   b  and the object  2 . 
     At the object detecting device  1   c , a frequency shift occurs, which is a sum of the foregoing Doppler shift regarding the object detecting device  1   a  and a Doppler shift caused by a relative moving velocity in a direction on a straight line connecting the object detecting device  1   c  and the object  2 . 
     A reception signal processor  18  includes an analog-to-digital converter (ADC)  19  and a signal processing circuit  20 . The reception signal processor  18  performs a process of detecting the object  2  to be detected by using a beat signal extracted by the beat signal extractor  14  and a beat signal extracted from each of the other object detecting devices  1 . 
     The ADC  19  converts the beat signal extracted by the beat signal extractor  14  into a digital signal and outputs a digital beat signal to the signal processing circuit  20 . 
     The signal processing circuit  20  performs a process of detecting the object  2  by using the digital beat signal output from the ADC  19 , the digital beat signal transmitted from each of the other object detecting devices  1 , and the like. 
     A position and velocity information outputting device  21  is implemented by, for example, a global positioning system (GPS) receiver, a velocimeter, and the like. The position and velocity information outputting device  21  detects the position and velocity of the object detecting device  1  and outputs position and velocity information indicating the position and velocity of the object detecting device  1 . Although it is desirable that the position of the object detecting device  1  is detected with higher accuracy, the position accuracy of the GPS signal received by the GPS receiver may be sufficient for that of the object detecting device  1 . 
     A multiplexer  22  multiplexes the digital beat signal having passed through a filter  31  of the signal processing circuit  20  illustrated in  FIG. 5  and the position and velocity information output from the position and velocity information outputting device  21 , and outputs the multiplexed signal to a communication device  23 . 
     The communication device  23  transmits the multiplexed signal output from the multiplexer  22  to the other object detecting devices  1 , and receives a multiplexed signal transmitted from each of the other object detecting devices  1 . For example, assuming that the communication device  23  is a communication device in the object detecting device  1   a , the communication device  23  transmits the multiplexed signal to the object detecting devices  1   b  and  1   c , and receives multiplexed signals transmitted from the object detecting devices  1   b  and  1   c . The transmission/reception of the multiplexed signal can be performed by wired or wireless communication. 
     A demultiplexer  24  de-multiplexes the multiplexed signal received by the communication device  23 , and outputs a digital beat signal and the position and velocity information to the signal processing circuit  20 . 
     A detected-object information displaying device  25  is implemented by, for example, a display, a graphics processing circuit, and the like. The detected-object information displaying device  25  displays information indicating a distance to the object  2  detected by the signal processing circuit  20  and a relative velocity. 
       FIG. 5  is a structural diagram illustrating the signal processing circuit  20  of the object detecting device  1  according to Embodiment 1 of the present invention.  FIG. 6  is a hardware structural diagram of the signal processing circuit  20  of the object detecting device  1  according to Embodiment 1 of the present invention. 
     In  FIGS. 5 and 6 , the filter  31  is implemented by, for example, a filter circuit  51  including a high-pass filter. The filter  31  removes a clutter component from the digital beat signal output by the ADC  19 . 
     The spectral analyzers  32   a ,  32   b , and  32   c  are implemented by, for example, a semiconductor integrated circuit mounting a CPU, or a spectral analysis circuit  52  using a one-chip microprocessor and the like. 
     The spectral analyzer  32   a  performs a process of analyzing a spectrum of the digital beat signal from which the clutter has been removed by the filter  31 . 
     Each of the spectral analyzers  32   b  and  32   c  performs a process of analyzing a spectrum of the digital beat signal output from the demultiplexer  24 , that is, a spectrum of the digital beat signal from which the clutter has been removed by a filter  31  provided in each of the other object detecting devices  1 . 
     Hereinafter, the spectral analyzers  32   a ,  32   b , and  32   c  may be simply expressed as a spectral analyzer  32  when they are not needed to distinguish from each other. In the example of Embodiment 1, since three object detecting devices  1  are provided, the signal processing circuit  20  includes three spectral analyzers  32 . When N (N is an integer of equal to or more than 2) object detecting devices  1  are provided, the signal processing circuit  20  includes N spectral analyzers  32 . 
     A search range width setter  33  is implemented by, for example, a semiconductor integrated circuit mounting a CPU, or a search range width setting circuit  53  using a one-chip microprocessor and the like. 
     The search range width setter  33  calculates a distance between the object detecting device  1  and each of the other object detecting devices  1  by using the velocity and position information output from the position and velocity information outputting device  21  and the velocity and position information output from the demultiplexer  24 . The search range width setter  33  sets a search range width for frequency by using the foregoing distance calculated in advance, a velocity indicated by the velocity and position information output from the position and velocity information outputting device  21 , a velocity indicated by the velocity and position information output from the demultiplexer  24 , a detectable distance range of the object  2 , a detectable relative velocity range of the object  2 , and individual frequency deviations of the object detecting devices  1 . 
     A combination target selector  34  is implemented by, for example, a semiconductor integrated circuit mounting a CPU, or a combination target selecting circuit  54  using a one-chip microprocessor and the like. 
     The combination target selector  34  determines, for each spectrum analyzed by the spectral analyzer  32 , a frequency search range having the search range width set by the search range width setter  33 . The determination of the frequency search range includes a process of comparing spectral components of each frequency of each individual spectrum analyzed by the spectral analyzers  32   a ,  32   b , and  32   c , and finding, on a basis of results of the comparison, the center frequency of the search range width set by the search range width setter  33 . 
     In addition, the combination target selector  34  compares the spectral components of each frequency within the determined search range, and selects, as a combination target, a frequency having a relative large spectral component from among the frequencies existing within the search range, for each of the spectra analyzed by the spectral analyzers  32   a ,  32   b , and  32   c.    
     A frequency corrector  35  includes a frequency correcting amount calculator  36  and frequency correction processors  37   a ,  37   b , and  37   c.    
     The frequency correcting amount calculator  36  is implemented by, for example, a semiconductor integrated circuit mounting a CPU, or a frequency correcting amount calculating circuit  55  using a one-chip microprocessor and the like. The frequency correcting amount calculator  36  performs a process of calculating individual frequency correcting amounts by using differences between each of the frequencies of the combination targets selected by the combination target selector  34 . 
     Each of the frequency correction processors  37   a ,  37   b , and  37   c  is implemented by, for example, a semiconductor integrated circuit mounting a CPU, or a frequency correction processing circuit  56  using a one-chip microprocessor and the like. 
     The frequency correction processor  37   a  performs a process of correcting the frequency of the digital beat signal, whose clutter component has been removed by the filter  31 , in accordance with the frequency correcting amount calculated by the frequency correcting amount calculator  36 . 
     On the other hand, each of the frequency correction processors  37   b  and  37   c  performs a process of correcting the frequency of the digital beat signal output from the demultiplexer  24  in accordance with the frequency correcting amount calculated by the frequency correcting amount calculator  36 . That is, each of the frequency correction processors  37   b  and  37   c  corrects the frequency of the digital beat signal whose clutter has been removed by a filter  31  provided in each of the other object detecting devices  1 . 
     Hereinafter, the frequency correction processors  37   a ,  37   b , and  37   c  may be simply expressed as a frequency correction processor  37  when they are not needed to distinguish from each other. In Embodiment 1, since three object detecting devices  1  are assumed, the signal processing circuit  20  includes three frequency correction processors  37 . When N object detecting devices  1  exist, the signal processing circuit  20  includes N frequency correction processors  37 . 
     A coefficient determinator  38  is implemented by, for example, a semiconductor integrated circuit mounting a CPU, or a coefficient determining circuit  57  using a one-chip microprocessor and the like. 
     The coefficient determinator  38  performs a process of determining, by using spectral components of the frequencies of the combination targets selected by the combination target selector  34 , weighting coefficients which are used when combining the digital beat signal whose clutter has been removed by the filter  31  and the digital beat signal output from the demultiplexer  24 . 
     A combiner  39  is implemented by, for example, a semiconductor integrated circuit mounting a CPU, or a combining circuit  58  using a one-chip microprocessor and the like. The combiner  39  performs a process of combining the beat signals whose frequencies are corrected by the frequency correction processors  37   a ,  37   b , and  37   c  by using the weighting coefficients determined by the coefficient determinator  38 . 
     An object detector  40  is implemented by, for example, a semiconductor integrated circuit mounting a CPU, or an object detecting circuit  59  using a one-chip microprocessor and the like. The object detector  40  detects the object  2  by performing a process of Constant False Alarm Rate (CFAR) on a beat signal combined by the combiner  39 . 
     In the process of CFAR, magnitude of noise is estimated by using frequencies obtained by adding a reflected wave and noise together and frequencies of noise alone, and stabilizing probability of detection errors of the noise with a CFAR threshold. 
     A distance and velocity calculator  41  is implemented by, for example, a semiconductor integrated circuit mounting a CPU, or a distance and velocity calculating circuit  60  using a one-chip microprocessor and the like. The distance and velocity calculator  41  performs a process of calculating a distance and a relative velocity between the object detecting device  1  and the object  2  detected by the object detector  40 , and outputting the calculation result to the detected-object information displaying device  25  and the object monitor  3 . 
     In  FIG. 5 , it is assumed that individual components of the signal processing circuit  20 , namely, the filter  31 , the spectral analyzers  32   a ,  32   b , and  32   c , the search range width setter  33 , the combination target selector  34 , the frequency correcting amount calculator  36 , the frequency correction processor  37   a ,  37   b , and  37   c , the coefficient determinator  38 , the combiner  39 , the object detector  40 , and the distance and velocity calculator  41  are realized by dedicated hardware. Alternatively, a computer may be used for realizing the signal processing circuit  20 . 
       FIG. 7  is a hardware structural diagram when the signal processing circuit  20  is realized by a computer. 
     When the signal processing circuit  20  is realized by a computer, a program is stored in a memory  71  of the computer, the program describing processing details of the filter  31 , the spectral analyzers  32   a ,  32   b , and  32   c , the search range width setter  33 , the combination target selector  34 , the frequency correcting amount calculator  36 , the frequency correction processor  37   a ,  37   b , and  37   c , the coefficient determinator  38 , the combiner  39 , the object detector  40 , and the distance and velocity calculator  41 , and a processor  72  of the computer executes the program stored in the memory  71 . As the processor  72  of the computer, a FPGA, a DSP, a CPU, or the like is applicable. 
       FIG. 8  is a flowchart illustrating the processing details of the signal processing circuit  20 . 
     Next, the operation will be described. 
     The transmission signal generating circuit  11  generates a transmission signal by performing frequency modulation that changes frequencies over time, as illustrated in  FIG. 4A . The transmission signal generating circuit  11  outputs the transmission signal to the transmission high-frequency circuit  12  and the beat signal extracting circuit  17 . 
     Upon receiving the transmission signal from the transmission signal generating circuit  11 , the transmission high-frequency circuit  12  performs a process of converting a frequency of the transmission signal into a carrier frequency, a filter process of removing spurious emissions and out-of-band frequencies of the transmission signal, and a process of amplifying power of the transmission signal. The transmission high-frequency circuit  12  outputs, as a transmission wave to the transmission antenna  13 , the transmission signal on which the foregoing processes have been performed. 
     After that, the transmission wave is radiated to the space from the transmission antenna  13 . 
     The reception antenna  15  receives a reflected wave of the transmission wave which reflects off and returns from the object  2  to be detected after the radiation to the space by the transmission antenna  13 . The reception antenna  15  outputs the reception signal of the reflected wave to the reception high-frequency circuit  16 . 
     Upon receiving the reception signal from the reception antenna  15 , the reception high-frequency circuit  16  performs a process of converting a frequency of the reception signal into the IF frequency, a filter process of removing an image frequency during the frequency conversion and an unnecessary frequency included in the reception signal, and a process of amplifying power of the reception signal. The reception high-frequency circuit  16  outputs, to the beat signal extracting circuit  17 , the reception signal on which the foregoing processes have been performed. 
     Upon receiving the reception signal from the reception high-frequency circuit  16 , the beat signal extracting circuit  17  extracts a beat signal from the reception signal by multiplying the reception signal and the transmission signal output from the transmission signal generating circuit  11  together, and outputs the beat signal to the reception signal processor  18 . 
     The beat signal indicates a difference between a frequency modulation component of the transmission signal and a frequency of the reception signal. The absolute value of the beat frequency of the beat signal becomes larger as a delay time becomes longer. 
     In the example of  FIG. 4B , the absolute value of the beat frequency in the section (a) is larger than that of the beat frequency in the section (b). 
     A difference in frequency occurs among the beat frequencies of reflected waves received by the object detecting devices  1   a ,  1   b , and  1   c.    
     That is, the frequency differences among the beat frequencies of the reflected waves occur due to the Doppler shift, a delay time difference caused by space propagation, and frequency deviations inside the object detecting devices  1   a ,  1   b , and  1   c.    
       FIG. 9  is an explanatory diagram for explaining a difference in beat frequency between the object detecting device  1   a  and the object detecting device  1   b  or  1   c . In  FIG. 9 , the object detecting device  1   m  is the object detecting device  1   b  or the object detecting device  1   c.    
     The Doppler shift will be described, which is one of factors affecting the beat frequency. 
     Defining that the Doppler shift caused by the reflected wave received by the object detecting device  1   a  is f d1  and the Doppler shift caused by the reflected wave received by the object detecting device  1   m  is f dm , the Doppler shifts f d1  and f dm  are expressed by formulas (1) and (2) below. 
     
       
         
           
             
               
                 
                   
                     f 
                     
                       d 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                   
                   = 
                   
                     
                       
                         v 
                         
                           r 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       + 
                       
                         v 
                         
                           r 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                     
                     λ 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     f 
                     
                       d 
                       m 
                     
                   
                   = 
                   
                     
                       
                         v 
                         
                           r 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       + 
                       
                         v 
                         rm 
                       
                     
                     λ 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In the formulas (1) and (2), λ is a wavelength of the transmission wave as a carrier wave and a reflected wave thereof, v r1  is a relative velocity between the object detecting device  1   a  and the object  2  to be detected, and v rm  is a relative velocity between the object detecting device  1   m  and the object  2 . 
     Therefore, a difference in Doppler shift Δf dm  between the object detecting device  1   a  and the object detecting device  1   m  is expressed by a formula (3) below. 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       f 
                       dm 
                     
                   
                   = 
                   
                     
                       
                         f 
                         dm 
                       
                       - 
                       
                         f 
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                     
                     = 
                     
                       
                         
                           v 
                           rm 
                         
                         - 
                         
                           v 
                           
                             r 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                       
                       λ 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In addition, a movement vector of the object detecting device  1   a  is denoted as a vector v 1 , a movement vector of the object detecting device  1   m  is denoted as a vector v m , and a movement vector of the object  2  to be detected is denoted as a vector v t . Here, due to the electronic filing of the present application, each vector in the description is expressed by a “vector v” because the symbol “→” for a vector cannot be written above the corresponding character. 
     Defining that |vector v 1 |=v 1 , |vector v m |=v m , and |vector v t |=v t , a relative velocity v r1  between the object detecting device  1   a  and the object  2  is expressed by a formula (4) below, and a relative velocity v rm  between the object detecting device  1   m  and the object  2  is expressed by a formula (5) below. 
     
       
         
           
             
               
                 
                   
                     v 
                     
                       r 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                   
                   = 
                   
                     
                       
                         v 
                         1 
                       
                       ⁢ 
                       
                         cos 
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                   ( 
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                     v 
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                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   
                     β 
                     m 
                   
                   = 
                   
                     
                       
                         θ 
                         1 
                       
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                         m 
                       
                     
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                         θ 
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                           tan 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
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                                 m 
                               
                             
                             
                               
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                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Therefore, the difference in Doppler shift Δf dm  between the object detecting device  1   a  and the object detecting device  1   m  can be expressed by the following parameters.
         the velocity v 1  of the object detecting device  1   a      a direction θ 1  from the object detecting device  1   a  to the object  2  to be detected   the velocity v m  of the object detecting device  1   m      a direction θ m  from the object detecting device  1   m  to the object  2     a distance d m  between the object detecting device  1   a  and the object detecting device  1   m      the velocity v t  of the object  2     an angle δ m  indicating a moving direction of the object  2     the distance R 1  between the object detecting device  1   a  and the object  2         

     Ideally, among those parameters, the distance d m  between the object detecting device  1   a  and the object detecting device  1   m  can be calculated from the position and velocity information output from the position and velocity information outputting device  21  connected to the object detecting device  1   a , and the position and velocity information included in a multiplexed signal transmitted from the object detecting device  1   m.    
     Also, ideally, the velocity v 1  of the object detecting device  1   a  can be calculated from the position and velocity information output from the position and velocity information outputting device  21 . In addition, the velocity v m  of the object detecting device  1   m  can be calculated from the position and velocity information included in the multiplexed signal transmitted from the object detecting device  1   m.    
     When a directional antenna is used as the reception antenna  15  of each of the object detecting devices  1   a  and  1   m , the direction θ 1  from the object detecting device  1   a  to the object  2  and the direction θ m  from the object detecting device  1   m  to the object  2  coincide with a directivity direction of a beam of the reception antenna  15 . For this reason, the reception antenna  15  desirably has high directivity. 
     The velocity v t  of the object  2  that is a parameter depending on the object  2 , the angle δ m  indicating the moving direction of the object  2 , and the distance R 1  between the object detecting device  1   a  and the object  2  are unknown. 
     Next, the delay time will be described, which is one of the factors affecting the beat frequency. 
     As understood by  FIG. 4B , the longer the delay time is, the larger the absolute value of the beat frequency is. 
     Defining that a frequency change amount is ξ [Hz/sec] with respect to time change per a unit time of the frequency modulation component in the transmission signal output from the transmission signal generating circuit  11 , a difference in beat frequency Δf pm  caused by a propagation delay difference ΔR between the object detecting device  1   a  and the object detecting device  1   m  is expressed by a formula (7) below. 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       f 
                       pm 
                     
                   
                   = 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         R 
                       
                       c 
                     
                     ⁢ 
                     ξ 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     In the formula (7), c denotes the velocity of light. 
     The propagation delay difference ΔR in the formula (7) is expressed by a formula (8) below.
 
Δ R=R   m   −R   1 =√{square root over (( R   1  cos(θ 1 )) 2 +( R   1  cos(θ 1 )− d   m ) 2 )}− R   1   (8)
 
     Similarly to a foregoing calculation of the Doppler shift, the distance d m  between the object detecting device  1   a  and the object detecting device  1   m , and the direction θ 1  from the object detecting device  1   a  to the object  2  to be detected are known, whereas the distance R 1  between the object detecting device  1   a  and the object  2  is unknown. 
     The frequency deviation inside the object detecting devices  1   a  and  1   m  will be described. This is one of the factors affecting the beat frequency. 
     The frequency deviation inside the object detecting devices  1   a  and  1   m  is a total of frequency differences of the frequency modulation components in the transmission signal generating circuit  11 , the transmission high-frequency circuit  12 , and the reception high-frequency circuit  16 . Those frequency deviations can be determined by device design. 
     For example, if assuming that the frequency deviation inside the object detecting device  1   a  and the frequency deviation inside the object detecting device  1   m  are the same Δf s , a difference in beat frequency Δf bm  between the object detecting devices  1   a  and  1   m  is expressed as a formula (9) below.
 
Δ f   bm   =Δf   dm   +Δf   pm   +Δf   s   (9)
 
     Accordingly, the difference in beat frequency Δf bm  between the object detecting device  1   a  and the object detecting device  1   m  is determined by the velocity v t  of the object  2 , the angle δ m  indicating the moving direction of the object  2 , and the distance R 1  between the object detecting device  1   a  and the object  2 . 
     Upon receiving the beat signal from the beat signal extractor  14 , the ADC  19  of the reception signal processor  18  converts the beat signal into a digital signal and outputs the digital beat signal to the signal processing circuit  20 . 
     Upon receiving the digital beat signal from the ADC  19 , the filter  31  of the signal processing circuit  20  removes a clutter component from the digital beat signal (step ST 1  in  FIG. 8 ). 
     The digital beat signal, from which the clutter has been removed by the filter  31 , is output to the spectral analyzer  32   a , the frequency correction processor  37   a , and the multiplexer  22 . 
     The position and velocity information outputting device  21  detects the position and velocity of the object detecting device  1 , and outputs the position and velocity information indicating the position and velocity of the object detecting device  1  to the search range width setter  33  and the multiplexer  22 . 
     The multiplexer  22  multiplexes the digital beat signal output from the filter  31  and the position and velocity information output from the position and velocity information outputting device  21 , and outputs the multiplexed signal to the communication device  23 . 
     Upon receiving the multiplexed signal from the multiplexer  22 , the communication device  23  transmits the multiplexed signal to each of the other object detecting devices  1 . 
     Similarly, when the other object detecting device  1  combines a plurality of digital beat signals and detects the object  2  from the combined signal, the multiplexed signal is transmitted to another different object detecting device  1 . However, if only a specific object detecting device  1  is configured to perform the detection process of the object  2  whereas the other object detecting devices  1  are configured not to perform the detection process of the object  2 , the communication device  23  connected to the specific object detecting device  1  does not need to transmit the multiplexed signal to the other object detecting devices  1 . 
     In the above case, the communication device  23  connected to the other object detecting device  1  transmits the multiplexed signal to the specific object detecting device  1  that performs the detection process of the object  2 . 
     In Embodiment 1, for simplicity of explanation, the description will be made by assuming that only the object detecting device  1   a  performs the detection process of the object  2  and the object detecting devices  1   b  and  1   c  do not perform the detection process of the object  2 . 
     The communication device  23  connected to the object detecting device  1   a  receives the multiplexed signals transmitted from the object detecting devices  1   b  and  1   c.    
     Upon receiving the multiplexed signals by the communication device  23 , the demultiplexer  24  de-multiplexes the multiplexed signals, and outputs a digital beat signal included in each of the multiplexed signals to the spectral analyzers  32   b  and  32   c , and the frequency correction processors  37   b  and  37   c.    
     In addition, the demultiplexer  24  outputs the position and velocity information included in the multiplexed signals to the search range width setter  33 . 
     Upon receiving a digital beat signal from the filter  31 , the spectral analyzer  32   a  analyzes a spectrum of the digital beat signal (step ST 2  in  FIG. 8 ). 
     Upon receiving the digital beat signals from the demultiplexer  24 , namely, receiving the digital beat signals, from which clutters have been removed by a filter  31  provided in each of the object detecting devices  1   b  and  1   c , each of the spectral analyzers  32   b  and  32   c  analyzes a spectrum of the corresponding digital beat signal (step ST 2  in  FIG. 8 ). 
     For the foregoing spectral analysis of the digital beat signal, a discrete Fourier transform process or a fast Fourier transform process may be used. A result of the spectral analysis includes amplitude or power information as spectral components for each frequency, and also includes phase information for each frequency. 
     In the example of  FIG. 4B , spectral analysis of the digital beat signal in the section (a) and spectral analysis of the digital beat signal in the section (b) are performed. 
     The search range width setter  33  calculates the distance d m  between the object detecting device  1   a  and the object detecting device  1   m  (m=2, 3) by using the velocity and position information output from the position and velocity information outputting device  21  and the velocity and position information output from the demultiplexer  24 . 
     After calculating the distance d m  between the object detecting device  1   a  and the object detecting device  1   m , the search range width setter  33  sets Δf w   m ×2 that is twice a search range width Δf w   m  for frequency according to a formula (10) below, which uses the distance d m , the velocity v 1  of the object detecting device  1   a  indicated by the velocity and position information output from the position and velocity information outputting device  21 , the velocity v m  of the object detecting device  1   m  indicated by the velocity and position information output from the demultiplexer  24 , the detectable distance range of the object  2 , the detectable relative velocity range of the object  2 , and the frequency deviation Δf s  of the device in the object detecting device  1  and the object detecting device  1   m  (step ST 3  in  FIG. 8 ).
 
Δ f   w   m =max(Δ f   bm ( R   1 ,δ m   ,v   t   ,d   m ,θ 1   ,v   1   ,v   m ))  (10)
 
     Hereinafter, the process of setting the search range width Δf w   m  will be specifically described, which is performed by the search range width setter  33 . 
     The search range width setter  33  assigns, to the formula (10), the distance d m  between the object detecting device  1   a  and the object detecting device  1   m , the velocities v 1  and v m , the frequency deviation Δf s , and the direction θ 1  from the object detecting device  1   a  to the object  2  to be detected, which is the directivity direction of the beam in the reception antenna  15 . The search range width setter  33  calculates a difference in beat frequency Δf bm  between the object detecting device  1   a  and the object detecting device  1   m  in accordance with the formula (9) while varying the velocity v t  of the object  2 , the angle δ m  indicating the moving direction of the object  2 , and the distance R 1  between the object detecting device  1   a  and the object  2 , which are unknown. The search range width setter  33  determines a difference in beat frequency Δf bm  indicating a maximum, and sets the determined difference in beat frequency Δf bm  to a search range width Δf w   m  of frequency. 
     More specifically, in accordance with the formulas (3) to (6), the search range width setter  33  calculates the difference in Doppler shift Δf dm  included in the difference in beat frequency Δf bm . 
     At this time, a velocity in a detectable relative velocity range of the object  2 , which is given from the outside, can be used for the unknown velocity v t  of the object  2 , and an angle in a range of 0° to 360° can be used for the unknown angle δ m  indicating the moving direction of the object  2 . 
     The search range width setter  33  calculates the difference in Doppler shift Δf dm  for the combination of the velocity v t  of the object  2  and the angle δ m  indicating the moving direction of the object  2 . 
     Subsequently, the search range width setter  33  calculates in accordance with the formulas (7) and (8) the difference in beat frequency Δf pm  caused by the propagation delay difference ΔR between the object detecting device  1   a  and the object detecting device  1   m.    
     At this time, a distance in the detectable distance range of the object  2 , which is given from the outside, can be used for the unknown distance R 1  between the object detecting device  1   a  and the object  2 , and the difference in beat frequency Δf pm  is calculated for the number of distances R 1 . 
     After calculating the difference in Doppler shift Δf dm  for the combination of the velocity v t  and the angle δ m , and the differences in beat frequency Δf pm  for the number of distances R 1 , the search range width setter  33  calculates in accordance with the formula (9) differences in beat frequency f bm  between the object detecting device  1   a  and the object detecting device  1   m  by using the frequency deviation Δf s  while making a combination of Δf dm  and Δf pm . 
     After calculating the differences in beat frequency f bm , the search range width setter  33  determines a difference in beat frequency Δf bm  indicating a maximum among the differences in beat frequency s f bm , and sets twice the determined difference in beat frequency Δf bm  as a search range width Δf w   m ×2. 
     In Embodiment 1, there are three object detecting devices  1   a ,  1   b , and  1   c . Therefore, a search range width Δf w   a ×2 for the object detecting device  1   a , a search range width Δf w   b ×2 for the object detecting device  1   b , and a search range width Δf w   c ×2 for the object detecting device  1   c  are set. 
     Note that, the distance d m  between the object detecting device  1   a  and the object detecting device  1   m , and the velocity v 1  of the object detecting device  1   a  and the velocity v m  of the object detecting device  1   m  include a measurement error. In addition, the direction θ 1  from the object detecting device  1   a  to the object  2  and the direction θ m  from the object detecting device  1   m  to the object  2  include an error with respect to an actual direction of the object  2  due to spread of the beam. 
     For the reason above, it is desirable to set Δf w   m ×2 as a search range width of frequency in consideration of the errors of the distance d m , the velocities v 1  and v m , and the directions θ 1  and θ m . 
     The combination target selector  34  determines a frequency search range having the search range width Δf w   m  set by the search range width setter  33  for each spectrum analyzed by the spectral analyzer  32 . 
     After determining the frequency search range having the search range width Δf w   m , the combination target selector  34  compares spectral components of frequencies in the determined search range for each spectrum analyzed by the spectral analyzer  32 , and selects a frequency of a combination target from among the frequencies in the search range (step ST 4  in  FIG. 8 ). 
     Hereinafter, the process of selecting a frequency of a combination target performed by the combination target selector  34  will be specifically described. 
     The combination target selector  34  sets each frequency of the spectrum relating to the object detecting device  1   a  analyzed by the spectral analyzer  32   a  as x, and sets the spectral component of the frequency x as f b   a (x). 
     In addition, the combination target selector  34  sets each frequency of the spectrum relating to the object detecting device  1   b  analyzed by the spectral analyzer  32   b  as x, and sets the spectral component of the frequency x as f b   b (x). 
     Further, the combination target selector  34  sets each frequency of the spectrum relating to the object detecting device  1   c  analyzed by the spectral analyzer  32   c  as x, and sets the spectral component of the frequency x as f b   c (x). 
     The combination target selector  34  calculates each power pow(f b   a (x)) of the spectral component f b   a (x) of the corresponding frequency x, and determines maximum power max(pow(f b   a (x))) among a plurality of the calculated powers pow(f b   a (x)). 
     Similarly, the combination target selector  34  calculates each power pow(f b   b (x)) of the spectral component f b   b (x) of the corresponding frequency x, and determines maximum power max(pow(f b   b (x))) among a plurality of the calculated powers pow(f b   b (x)). 
     Similarly, the combination target selector  34  calculates each power pow(f b   c (x)) of the spectral component f b   c (x) of the corresponding frequency x, and determines maximum power max(pow(f b   c (x))) among a plurality of the calculated powers pow(f b   c (x)). 
     The combination target selector  34  determines the largest maximum power max(pow(f b   m (x))) among the maximum power max(pow(f b   a (x))), the maximum power max(pow(f b   b (x))), and the maximum power max(pow(f b   c (x)), in accordance with a formula (11) below. Here, m=1, 2, and 3. 
     After determining the largest maximum power max(pow(f b   m (x))), the combination target selector  34  determines a frequency x m   max  at which the largest maximum power max(pow(f b   m (x))) is obtained, and the object detecting device  1  corresponding to the largest maximum power max(pow(f b   m (x))).
 
( m,x   m   max )=max(max(pow( f   b   a ( x ))),max(pow( f   b   b ( x ))),max(pow( f   b   c ( x ))))  (11)
 
     For convenience of description, it is assumed that the object detecting device  1  corresponding to the largest maximum power max(pow (f b   m (x))) is the object detecting device  1   a.    
     When the object detecting device  1  corresponding to the largest maximum power max(pow(f b   m (x))) is the object detecting device  1   a , the combination target selector  34  sets the frequency search range for the object detecting device  1   a  such that a frequency x a   max , at which the maximum power max(pow(f b   a (x))) is obtained, becomes a center frequency of the search range width Δf w   a ×2 for the object detecting device  1   a.    
     Specifically, the combination target selector  34  sets the frequency search range for the object detecting device  1   a  in accordance with a formula (12) below.
 
 x   a   max   −Δf   w   a   ≤x≤x   a   max   +Δf   w   a   (12)
 
     The combination target selector  34  sets the frequency search range for the object detecting device  1   b  such that the frequency x a   max , at which the maximum power max(pow(f b   a (x))) is obtained, becomes a center frequency of the search range width Δf w   b ×2 for the object detecting device  1   b.    
     Specifically, the combination target selector  34  sets the frequency search range for the object detecting device  1   b  in accordance with a formula (13) below.
 
 x   a   max   −Δf   w   b   ≤x≤x   a   max   +Δf   w   b   (13)
 
     Similarly, the combination target selector  34  sets the frequency search range for the object detecting device  1   c  such that the frequency x a   max , at which the maximum power max(pow(f b   a (x))) is obtained, becomes a center frequency of the search range width Δf w   c ×2 for the object detecting device  1   c.    
     Specifically, the combination target selector  34  sets the frequency search range for the object detecting device  1   c  in accordance with a formula (14) below.
 
 x   a   max   −Δf   w   c   ≤x≤x   a   max   +Δf   w   c   (14)
 
     The combination target selector  34  calculates each power pow(f b   a (x)) of the spectral component f b   a (x) of the corresponding frequency x in the frequency search range for the object detecting device  1   a , and determines the maximum power max(pow(f b   a (x))) among the calculated powers pow(f b   a (x)). 
     The combination target selector  34  selects a frequency x for the maximum power max(pow(f b   a (x))) as a frequency x a   sel  of the combination target. 
     The combination target selector  34  calculates each power pow(f b   b (x)) of the spectral component f b   b (x) of the corresponding frequency x in the frequency search range for the object detecting device  1   b , and determines the maximum power max(pow(f b   b (x))) among the calculated powers pow(f b   b (x)). 
     The combination target selector  34  selects a frequency x for the maximum power max(pow(f b   b (x))) as a frequency x b   sel  of the combination target. 
     Similarly, the combination target selector  34  calculates each power pow (f b   c (x)) of the spectral component f b   c (x) of the corresponding frequency x in the frequency search range for the object detecting device  1   c , and determines the maximum power max(pow(f b   c (x))) among the calculated powers pow(f b   c (x)). 
     The combination target selector  34  selects a frequency x for the maximum power max(pow(f b   c (x))) as a frequency x c   sel  of the combination target. 
     After selecting the frequency x a   sel  of the combination target for the object detecting device  1   a , the combination target selector  34  extracts amplitude a a =abs(f b   a (x a   sel )) of the frequency x a   sel  and extracts a phase φ a =arg(f b   a (x a   sel )) of the frequency x a   sel , and outputs the amplitude a a  and the phase φ a  of the frequency x a   sel  to the coefficient determinator  38 . 
     Note that, abs(⋅) denotes extraction of an amplitude component, and arg(⋅) denotes extraction of a phase component. 
     After selecting the frequency x b   sel  of the combination target for the object detecting device  1   b , the combination target selector  34  extracts amplitude a b =abs(f b   b (x b   sel )) of the frequency x b   sel  and extracts a phase φ b =arg(f b   b (x b   sel )) of the frequency x b   sel , and outputs the amplitude a b  and the phase φ b  of the frequency x b   sel  to the coefficient determinator  38 . 
     After selecting the frequency x c   sel  of the combination target is selected from the frequency search range for the object detecting device  1   c , the combination target selector  34  extracts amplitude a c =abs(f b   c (x c   sel )) of the frequency x c   sel  and extracts a phase φ c =arg(f b   c (x c   sel )) of the frequency x c   sel , and outputs the amplitude a c  and the phase φ c  of the frequency x c   sel  to the coefficient determinator  38 . 
     After the combination target selector  34  selects frequencies of the combination targets x a   sel , x b   sel , and x c   sel , the frequency correcting amount calculator  36  calculates frequency correcting amounts Δx a , Δx b , and Δx c  for the object detecting devices  1   a ,  1   b , and  1   c , respectively, with reference to the frequency x a   sel  of the combination target for the object detecting device  1   a , as shown in formulas (15) to (17) below, for the purpose of improving the combination gain when the plurality of digital beat signals is combined by the combiner  39  (step ST 7  in  FIG. 8 ).
 
Δ x   a =( x   a   sel   −x   a   sel )  (15)
 
Δ x   b =( x   b   sel   −x   a   sel )  (16)
 
Δ x   c =( x   c   sel   −x   a   sel )  (17)
 
     After the frequency correcting amount Δx a  for the object detecting device  1   a  is calculated by the frequency correcting amount calculator  36 , the frequency correction processor  37   a  corrects the frequency of the digital beat signal, from which the clutter has been removed by the filter  31 , in accordance with the frequency correcting amount Δx a , as shown in a formula (18) below (step ST 8  in  FIG. 8 ).
 
 s   a   c ( t )= s   a ( t )×exp(−Δ x   a ×2×π×DataInterval× t )  (18)
 
     In the formula (18), s a (t) represents a time series signal of the digital beat signal output from the filter  31 , and s a   c (t) represents a time series signal of the digital beat signal after the frequency correction. In addition, DataInterval is a sample time interval of the time series signal, and t is a sample number. The sample number is an integer. 
     After the frequency correcting amount Δx b  for the object detecting device  1   b  is calculated by the frequency correcting amount calculator  36 , the frequency correction processor  37   b  corrects the frequency of the digital beat signal output from the demultiplexer  24 , that is, the frequency of the digital beat signal, from which the clutter has been removed by the filter  31  of the object detecting device  1   b , in accordance with the frequency correcting amount Δx b , as shown in a formula (19) below (step ST 8  in  FIG. 8 ).
 
 s   b   c ( t )= s   b ( t )×exp(−Δ x   b ×2×π×DataInterval× t )  (19)
 
     In the formula (19), s b (t) represents a time series signal of the digital beat signal output from the filter  31  of the object detecting device  1   b , s b   c (t) represents a time series signal of the digital beat signal after the frequency correction. 
     After the frequency correcting amount Δx c  for the object detecting device  1   c  is calculated by the frequency correcting amount calculator  36  calculates, the frequency correction processor  37   c  corrects the frequency of the digital beat signal output from the demultiplexer  24 , that is, the frequency of the digital beat signal, from which the clutter has been removed by the filter  31  of the object detecting device  1   c , in accordance with the frequency correcting amount Δx c , as shown in a formula (20) below (step ST 8  in  FIG. 8 ).
 
 s   c   c ( t )= s   c ( t )×exp(−Δ x   c ×2×π×DataInterval× t )  (20)
 
     In the formula (20), s c (t) represents a time series signal of the digital beat signal output from the filter  31  of the object detecting device  1   c , s c   c (t) represents a time series signal of the digital beat signal after the frequency correction. 
     Upon receiving each of the amplitude a a  and phase φ a  of the frequency x a   sel  of the combination target, the amplitude a b  and phase φ b  of the frequency x b   sel  of the combination target, and the amplitude a c  and phase φ c  of the frequency x c   sel  of the combination target from the combination target selector  34 , the coefficient determinator  38  determines weighting coefficients w a , w b , and w c  used for combining the digital beat signals whose frequencies have been corrected by the frequency correction processors  37   a ,  37   b , and  37   c , in accordance with formulas (21) to (23) below (step ST 9  in  FIG. 8 ). 
     
       
         
           
             
               
                 
                   
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                         c 
                       
                       
                         
                           
                             ∑ 
                             m 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 a 
                                 m 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                     ⁢ 
                     
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                           j 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           ϕ 
                           c 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   23 
                   ) 
                 
               
             
           
         
       
     
     In the formulas (21) to (23), m=1, 2, and 3. 
     After the weighting coefficients w a , w b , and w c  are determined by the coefficient determinator  38 , the combiner  39  combines the time series signals s a   c (t), s b   c (t), and s c   c (t) of the digital beat signals, whose frequencies have been corrected by the frequency correction processors  37   a ,  37   b  and  37   c , by using the weighting coefficients w a , w b , and w c  in accordance with a formula (24) below (step ST 10  in  FIG. 8 ).
 
 y ( t )= w   a   s   a   c ( t )+ w   b   s   b   c ( t )+ w   c   s   c   c ( t )  (24)
 
     The combiner  39  outputs a combined signal y(t) to the object detector  40 . 
     Note that, since the weighting coefficients w a , w b , and w c  determined by the coefficient determinator  38  are the weights of maximum ratio combining, the signal power to noise power ratio is maximized by a combining process according to the formula (24). 
     Upon receiving the combined signal y(t) from the combiner  39 , the object detector  40  detects the object  2  by performing the CFAR processing on the signal y(t) (step ST 11  in  FIG. 8 ). 
     Specifically, the object detector  40  analyzes the spectrum of the combined signal y(t), and determines that the object  2  to be detected exists when there is a spectral component equal to or more than the CFAR threshold, which has been given from the outside, among the spectral components of frequencies of the spectrum. 
     When it is determined that the object  2  to be detected exists, the object detector  40  outputs, as the beat frequency, a frequency corresponding to the spectral component equal to or more than the CFAR threshold to the distance and velocity calculator  41 . 
     Upon receiving the beat frequency from the object detector  40 , the distance and velocity calculator  41  calculates, by using the beat frequency, the distance R 1  from the object detecting device  1   a  to the object  2 , and also calculates the relative velocity v r  between the object detecting device  1   a  and the object  2  (step ST 12  in  FIG. 8 ). 
     For example, in a case where the beat frequency f r   A  in the section (a) and the beat frequency f r   B  in the section (b) illustrated in  FIG. 4B  are obtained, the distance R 1  from the object detecting device  1   a  to the object  2 , and the relative velocity v r  between the object detecting device  1   a  and the object  2  can be calculated by solving equations shown in a formula (25) below. 
     
       
         
           
             
               
                 
                   
                     
                       f 
                       r 
                       A 
                     
                     = 
                     
                       
                         
                           
                             2 
                             ⁢ 
                             
                                 
                             
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                         ⁢ 
                         
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                           A 
                         
                       
                       + 
                       
                         
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                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             v 
                             r 
                           
                         
                         λ 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
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                       r 
                       B 
                     
                     = 
                     
                       
                         
                           
                             2 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               R 
                               1 
                             
                           
                           c 
                         
                         ⁢ 
                         
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                           B 
                         
                       
                       + 
                       
                         
                           2 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             v 
                             r 
                           
                         
                         λ 
                       
                     
                   
                 
               
               
                 
                   ( 
                   25 
                   ) 
                 
               
             
           
         
       
     
     In the formula (25), c is the velocity of light, λ is a wavelength of the carrier wave, ξ A  is a change amount of frequency with respect to the time change per unit time of the frequency modulation component in the section (a), ξ B  is a frequency change amount with respect to the time change per unit time of the frequency modulation component in the section (b). In  FIG. 4B , ξ=ξ A =−ξ B . 
     After calculating the distance R 1  from the object detecting device  1   a  to the object  2 , and the relative velocity v r  between the object detecting device  1   a  and the object  2 , the distance and velocity calculator  41  outputs the distance R 1  and the relative velocity v r  to the detected-object information displaying device  25  and the object monitor  3 . 
     Depending on combinations of the frequencies of the combination targets selected by the combination target selector  34 , the distance R 1  calculated by the distance and velocity calculator  41  may exceed the detectable distance range assumed in the object detecting device  1   a . Similarly, the relative velocity v r  calculated by the distance and velocity calculator  41  may exceed the detectable relative velocity range assumed in the object detecting device  1   a.    
     When the distance R 1  calculated by the distance and velocity calculator  41  exceeds the detectable distance range assumed in the object detecting device  1   a , or when the relative velocity v r  calculated by the distance and velocity calculator  41  exceeds the detectable relative velocity range assumed in the object detecting device  1   a , it is desirable that the distance R 1  and the relative velocity v r  are not output to the detected-object information displaying device  25  and the object monitor  3 . 
     Upon receiving, from the distance and velocity calculator  41 , the distance R 1  from the object detecting device  1   a  to the object  2 , and the relative velocity v r  between the object detecting device  1   a  and the object  2 , the detected-object information displaying device  25  displays the distance R 1  and the relative velocity v r  on the display. 
     Upon receiving, from the distance and velocity calculator  41 , the distance R 1  from the object detecting device  1   a  to the object  2 , and the relative velocity v r  between the object detecting device  1   a  and the object  2 , the object monitor  3  performs a process of monitoring the object  2  by using the distance R 1  and the relative velocity v r . 
     The monitoring processing of the object  2  by the object monitor  3  includes a process of recording a relative velocity v r  and a distance R 1  to the object  2  detected by the object detecting device  1  and monitoring change in the position and velocity of the object  2 , a process of predicting the future position and velocity of the object  2 , and the like. 
     As is apparent from the above, according to Embodiment 1, there is provided: the spectral analyzers  32   a ,  32   b , and  32   c  for analyzing a spectrum of the beat signal extracted by the beat signal extractor  14  and the spectra of the beat signals extracted by the object detecting devices  1   b  and  1   c ; the search range width setter  33  for setting a search range width for frequency; the combination target selector  34  for determining a frequency search range having the search range width set by the search range width setter  33 , comparing spectral components of frequencies in the search range, and selecting, for each spectrum analyzed by the spectral analyzers  32   a ,  32   b , and  32   c , a frequency of a combination target from among the frequencies existing within the search range; and the frequency corrector  35  for calculating frequency correcting amounts from differences between each of the frequencies of the combination targets selected by the combination target selector  34 , and correcting a frequency of the beat signal extracted by the beat signal extractor  14  and frequencies of the beat signals extracted by the object detecting devices  1   b  and  1   c  in accordance with each of the frequency correcting amounts, and the combiner  39  for combining the beat signals whose frequencies have been corrected by the frequency corrector  35 . Therefore, it is capable of bringing an effect of enhancing the detection probability of the object  2  to be detected without acquiring the position information of the object  2  in advance. 
     That is, according to Embodiment 1, by selecting frequencies of combination targets from among frequencies included in the reflected waves received by the object detecting devices  1   a ,  1   b , and  1   c  by the search range width setter  33 , the probability of erroneously combining frequencies, which are not relating to the object  2 , is reduced. 
     In addition, the frequencies of the beat signals extracted by the object detecting devices  1   a ,  1   b , and  1   c  are corrected by the frequency corrector  35 . Therefore, even when the frequencies of the beat signals extracted by the object detecting devices  1   a ,  1   b , and  1   c  are different from each other, it is possible to suppress a decrease in gain caused by combination. 
     Further, the beat signals whose frequencies are corrected by the frequency corrector  35  are combined by the combiner  39 , whereby a combined signal is obtained with an improved signal power to noise power ratio. For this reason, object detection accuracy in the object detector  40  and the calculation accuracy of the distance and velocity in the distance and velocity calculator  41  are improved. 
     In Embodiment 1, when the plurality of object detecting devices  1  is installed apart from each other, the beat signals extracted by the plurality of object detecting devices  1  can be combined without calculating the distances among the object detecting devices  1  in the wavelength order. 
     In a case where the distances among the plurality of object detecting devices  1  are fixed, namely for example, when the plurality of object detecting devices  1  is fixed on the ground, or when the plurality of object detecting devices  1  is installed in one moving platform, the distances do not change over time. Therefore, even if lowering the accuracy of the distances among the object detecting devices  1 , it is possible to obtain a combined signal of the plurality of beat signals capable of enhancing the detection probability of the object  2 . 
     In addition, in a case where the object detecting devices  1  are installed on different moving platforms from each other, and the distances among the object detecting devices  1  are fixed, it is possible to obtain a combined signal of the plurality of beat signals capable of enhancing the detection probability of the object  2  even if lowering the accuracy of the distance between the plurality of object detecting devices  1 . 
     In Embodiment 1, although the example has been described, in which the object detecting device  1  detects the object  2  by the distance measurement system called as the FMCW system, the present invention is not limited thereto. For example, the object  2  may be detected by a distance measurement system called as a frequency modulation interrupted continuous wave (FMICW) system. 
     Hereafter, a difference between the FMCW system and the FMICW system will be briefly described. 
       FIG. 10  is an explanatory diagram illustrating the transmission wave and reflected wave and the beat signal. 
       FIG. 10A  illustrates a transmission signal including a plurality of pulses, and  FIG. 10B  illustrates a reception signal including a plurality of pulses. 
       FIG. 10C  illustrates the transmission wave and reflected wave whose frequencies change over time, and  FIG. 10D  illustrates the beat signal whose beat frequency changes over time. 
     In the FMICW system, a transmission signal generated by the transmission signal generating circuit  11  forms a pulse train as illustrated in  FIG. 10A . A transmission wave illustrated in  FIG. 10C , on which the frequency modulation similar to the FMCW system has been performed, is radiated from the transmission antenna  13 . 
     A reflected wave received by the reception antenna  15  is received as a pulse train with a propagation delay as illustrated in  FIGS. 10B and 10C . 
     Since the Doppler shift similar to the case of the FMCW system occurs, the reflected wave has time delays as illustrated in  FIG. 10C , and the frequency of the reflected wave is frequency-shifted by the Doppler shift. 
     The beat signal extracting circuit  17  takes a difference between the reception signal of the reflected wave and the frequency modulation component of the transmission signal generated by the transmission signal generating circuit  11 . The frequency modulation component is the same as in the case of the FMCW system. 
     As illustrated in  FIG. 10D , the output of the beat signal extracting circuit  17  forms a pulse train having a beat frequency similarly to the case of the FMCW system. 
       FIG. 11  is an explanatory diagram illustrating an output signal of the beat signal extracting circuit  17 . 
       FIG. 11A  illustrates the output signal of the beat signal extracting circuit  17 , and  FIG. 11B  illustrates the spectrum in a case where the Doppler shift does not occur in the reflected wave. 
       FIG. 11C  illustrates the spectrum in a case where the Doppler shift occurs in the reflected wave, and  FIG. 11D  is a diagram obtained by enlarging a vicinity of a frequency of 0 Hz in  FIG. 11C . 
     Assuming that the pulse width of the pulse generated by the transmission signal generating circuit  11  is τ and the pulse repetition interval is T=3τ, the spectrum appears as illustrated in  FIG. 11B  when the Doppler shift does not occur in the reflected wave. 
     The envelope of the spectrum is expressed by |sin(frequency)/frequency|, and the power becomes 0 every n/τ. Note that, n is an arbitrary integer other than zero. 
     On the other hand, if the Doppler shift occurs in the reflected wave, the spectrum of the reception pulse train as the reception signal becomes a spectrum with an envelope, which is repeated every 1/T, as illustrated in  FIGS. 11C and 11D . 
     The reception pulse train, which has the spectrum illustrated in  FIGS. 11C and 11D , is sampled by the ADC  19  and input to the filter  31 . 
     In the case of the FMICW system, the signal processing circuit  20  is configured as illustrated in  FIG. 12 . 
     The filter  31  of the signal processing circuit  20  in  FIG. 12  extracts only the filter extraction portions illustrated in  FIG. 11D  in order to remove the clutter near 0 Hz and also remove the repetitive portions of the spectrum repeated every 1/T. 
     For the purpose of lowering the signal processing speed at the subsequent stage to the filter  31 , a sample data decimator  42  is provided for performing a process of narrowing the representation band of the time series signal of the output signal of the filter  31 . 
     The subsequent processing is similar to the case of the FMCW system. 
     Embodiment 2 
     In the foregoing Embodiment 1, the combination target selector  34  selects, as a frequency of a combination target, a frequency having a relative large spectral component among frequencies existing within the search range. With this configuration, it may be possible to select a frequency of the object  2  to be detected. On the other hand, when the frequency of the object  2  is not actually included in the frequencies in the search range, noise in the reflected wave may be selected as the frequency of the combination target. 
     In Embodiment 2 that will be described hereinafter, in order not to select the noise in the reflected wave as the frequency of the combination target, the combination target selector  34  selects a frequency, whose spectral component is larger than a threshold, among the frequencies existing within the search range. 
     Hereinafter, the selecting process will be specifically described of the frequency of the combination target at the combination target selector  34 . 
     Similarly to Embodiment 1, the combination target selector  34  sets each frequency of the spectrum relating to the object detecting device  1   a  analyzed by the spectral analyzer  32   a  as x, and sets the spectral component of the frequency x as f b   a (x), and calculates each power pow(f b   a (x)) of the spectral component f b   a (x) of the corresponding frequency x. 
     In addition, the combination target selector  34  sets each frequency of the spectrum relating to the object detecting device  1   b  analyzed by the spectral analyzer  32   b  as x, sets the spectral component of the frequency x as f b   b (x), and calculates each power pow(f b   b (x)) of the spectral component f b   b (x) of the corresponding frequency x. 
     Further, the combination target selector  34  sets each frequency of the spectrum relating to the object detecting device  1   c  analyzed by the spectral analyzer  32   c  as x, sets the spectral component of the frequency x as f b   c (x), and calculates each power pow(f b   c (x)) of the spectral component f b   c (x) of the corresponding frequency x. 
     Subsequently, the combination target selector  34  compares each power pow(f b   a (x)) of the spectral component f b   a (x) of the corresponding frequency x with a preset threshold. When there is power pow(f b   a (x)) larger than the threshold among the powers pow(f b   a (x)) of respective frequencies x, the combination target selector  34  selects, as a frequency x a   sel  of a combination target, a frequency of the power pow(f b   a (x)) larger than the threshold. 
     Similarly, the combination target selector  34  compares each power pow(f b   b (x)) of the spectral component f b   b (x) of the corresponding frequency x with a preset threshold. When there is power pow(f b   b (x)) larger than the threshold among the powers pow(f b   b (x)) of respective frequencies x, the combination target selector  34  selects, as a frequency x b   sel  of a combination target, a frequency of the power pow(f b   b (x)) larger than the threshold. 
     Similarly, the combination target selector  34  compares each power pow(f b   c (x)) of the spectral component f b   c (x) of the corresponding frequency x with a preset threshold. When there is power pow(f b   c (x)) larger than the threshold among the powers pow(f b   c (x)) of respective frequencies x, the combination target selector  34  selects, as a frequency x c   sel  of a combination target, a frequency of the power pow(f b   c (x)) larger than the threshold. 
     In the foregoing configuration, the combination target selector  34  compares the power of a spectral component of each frequency x with the threshold. Alternatively, the amplitude of the spectral component of each frequency x may be compared with the threshold. 
     Here, it is assumed that no frequency x is selected as a frequency x m   sel  of a combination target (m=1, 2, and 3) for each object detecting device  1 , or assumed that multiple frequencies x are selected. 
     When no frequency x is selected as the frequency x a   sel  of the combination target for the object detecting device  1   a , the frequency correcting amount calculator  36  sets the frequency correcting amounts Δx a , Δx b , and Δx c  for the object detecting devices  1   a ,  1   b , and  1   c , respectively, to zero. 
     When multiple frequencies x are selected as the frequency x a   sel  of the combination target for the object detecting device  1   a , the frequency correcting amount calculator  36  calculates the frequency correcting amounts Δx a , Δx b , and Δx c  in the following manner. 
     For convenience of description, it is assumed that a couple of frequencies x are individually selected as the frequency x a   sel  of the combination target, and the selected frequencies x are expressed as a frequency x a   sel1  and a frequency x a   sel2 . 
     In addition, it is assumed that a couple of frequencies x are individually selected as the frequency x b   sel  of the combination target, and the selected frequencies x are expressed as a frequency x b   sel1  and a frequency x b   sel2 . 
     Further, it is assumed that a single frequency x is selected as the frequency x c   sel  of the combination target. 
     First, in accordance with formulas (26) to (29) below, the frequency correcting amount calculator  36  calculates the frequency correcting amounts Δx a1 , Δx b1 , Δx b2 , and Δx c1  for the object detecting devices  1   a ,  1   b , and  1   c , with reference to the frequency x a   sel1 .
 
Δ x   a1 =( x   a   sel1   −x   a   sel1 )  (26)
 
Δ x   b1 =( x   b   sel1   −x   a   sel1 )  (27)
 
Δ x   b2 =( x   b   sel2   −x   a   sel1 )  (29)
 
Δ x   c1 =( x   c   sel   −x   a   sel1 )  (29)
 
     In addition, in accordance with formulas (30) to (33) below, the frequency correcting amount calculator  36  calculates the frequency correcting amounts Δx a2 , Δx b3 , Δx b4 , and Δx c2  for the object detecting devices  1   a ,  1   b , and  1   c , with reference to the frequency x a   sel2 .
 
Δ x   a2 =( x   a   sel2   −x   a   sel2 )  (30)
 
Δ x   b3 =( x   b   sel1   −x   a   sel2 )  (31)
 
Δ x   b4 =( x   b   sel2   −x   a   sel2 )  (32)
 
Δ x   c1 =( x   c   sel   −x   a   sel2 )  (33)
 
     After the frequency correcting amount calculator  36  calculates the frequency correcting amounts Δx a1  and Δx a2  for the object detecting device  1   a , the frequency correction processor  37   a  corrects the frequency of the digital beat signal, from which the clutter has been removed by the filter  31 , in accordance with the frequency correcting amount Δx a1  and Δx a2 , as shown in formulas (34) and (35) below.
 
 s   a1   c ( t )= s   a ( t )×exp(−Δ x   a1 ×2×π×DataInterval× t )  (34)
 
 s   a2   c ( t )= s   a ( t )×exp(−Δ x   a2 ×2×π×DataInterval× t )  (35)
 
     In the formulas (34) and (35), s a1   c (t) represents a time series signal of the digital beat signal corrected by the frequency correcting amount Δx a1 , and s a2   c (t) represents a time series signal of the digital beat signal corrected by the frequency correcting amount Δx a2 . 
     After the frequency correcting amount calculator  36  calculates the frequency correcting amounts Δx b1 , Δx b2 , Δx b3 , and Δx b4  for the object detecting device  1   b , the frequency correction processor  37   b  corrects the frequency of the digital beat signal output from the demultiplexer  24 , that is, the frequency of digital beat signal, from which the clutter has been removed by the filter  31  of the object detecting device  1   b , in accordance with the frequency correcting amounts Δx b1 , Δx b2 , Δx b3 , and Δx b4 , as shown in formulas (36) to (39) below.
 
 s   b1   c ( t )= s   b ( t )×exp(−Δ x   b1 ×2×π×DataInterval× t )  (36)
 
 s   b2   c ( t )= s   b ( t )×exp(−Δ x   b2 ×2×π×DataInterval× t )  (37)
 
 s   b3   c ( t )= s   b ( t )×exp(−Δ x   b3 ×2×π×DataInterval× t )  (38)
 
 s   b4   c ( t )= s   b ( t )×exp(−Δ x   b4 ×2×π×DataInterval× t )  (39)
 
     In the formulas (36) to (39), s b1   c (t) represents a time series signal of the digital beat signal corrected by the frequency correcting amount Δx b1 , s b2   c (t) represents a time series signal of the digital beat signal corrected by the frequency correcting amount Δx b2 , s b3   c (t) represents a time series signal of the digital beat signal corrected by the frequency correcting amount Δx b3 , and s b4   c (t) represents a time series signal of the digital beat signal corrected by the frequency correcting amount Δx b4 . 
     After the frequency correcting amount calculator  36  calculates the frequency correcting amounts Δx c1  and Δx c2  for the object detecting device  1   c , the frequency correction processor  37   c  corrects the frequency of the digital beat signal output from the demultiplexer  24 , that is, the frequency of the digital beat signal, from which the clutter has been removed by the filter  31  of the object detecting device  1   c , in accordance with the frequency correcting amounts Δx c1  and Δx c2 , as shown in formulas (40) to (41) below.
 
 s   c1   c ( t )= s   c ( t )×exp(−Δ x   c1 ×2×π×DataInterval× t )  (40)
 
 s   c2   c ( t )= s   c ( t )×exp(−Δ x   c2 ×2×π×DataInterval× t )  (41)
 
     In the formulas (40) and (41), s c1   c (t) represents a time series signal of the digital beat signal corrected by the frequency correcting amount Δx c1 , and s c2   c (t) represents a time series signal of the digital beat signal corrected by the frequency correcting amount Δx c2 . 
     Upon receiving each of amplitude a a1  and phase φ a1  of the frequency x a   sel1  of the combination target, amplitude a a2  and phase φ a2  of the frequency x a   sel2 , amplitude a b1  and phase φ b1  of the frequency x b   sel1  of the combination target, amplitude a b2  and phase φ b2  of the frequency x b   sel2 , and the amplitude a c  and phase φ c  of the frequency x c   sel  of the combination target from the combination target selector  34 , the coefficient determinator  38  determines weighting coefficients w 1   a  to w 4   a , w 1   b  to w 4   b , and w 1   c  to w 4   c  used for combining the digital beat signals, whose frequencies have been corrected by the frequency correction processors  37   a ,  37   b  and  37   c , for the combination of the frequencies x a   sel1  and x a   sel2  of the combination target, the frequencies x b   sel1  and x b   sel2  of the combination target, and the frequency x c   sel  of the combination target. 
     [For a Combination of the Frequencies x a   sel1 , x b   sel1 , and x c   sel ] 
     
       
         
           
             
               
                 
                   
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     [For a Combination of Frequencies x a   sel1 , x b   sel2 , and x c   sel ] 
     
       
         
           
             
               
                 
                   
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     [For a Combination of the Frequencies x a   sel1 , x b   sel2 , and x c   sel ] 
     
       
         
           
             
               
                 
                   
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     [For a Combination of Frequencies x a   sel2 , x b   sel2 , and x c   sel ] 
     
       
         
           
             
               
                 
                   
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     After the coefficient determinator  38  determines the weighting coefficients w 1   a  to w 4   a , w 1   b  to w 4   b , and w 1   c  to w 4   c , the combiner  39  combines, by using the weighting coefficients for each combination as shown in formulas (54) to (57) below, the time series signals being the beat signals whose frequency have been corrected by the frequency correction processors  37   a ,  37   b , and  37   c.    
     [For the Combination of the Frequencies x a   sel1 , x b   sel1 , and x c   sel ]
 
 y   1 ( t )= w   1   a   s   a1   c ( t )+ w   1   b   s   b1   c ( t )+ w   1   c   s   c1   c ( t )  (54)
 
     [For the Combination of the Frequencies x a   sel1 , x b   sel2 , and x c   sel ]
 
 y   2 ( t )= w   2   a   s   a1   c ( t )+ w   2   b   s   b2   c ( t )+ w   2   c   s   c1   c ( t )  (55)
 
     [For the Combination of the Frequencies x a   sel2 , x b   sel1 , and x c   sel ]
 
 y   3 ( t )= w   3   a   s   a2   c ( t )+ w   3   b   s   b3   c ( t )+ w   3   c   s   c2   c ( t )  (56)
 
     [For the Combination of the Frequencies x a   sel2 , x b   sel2 , and x c   sel ]
 
 y   4 ( t )= w   4   a   s   a2   c ( t )+ w   4   b   s   b4   c ( t )+ w   4   c   s   c2   c ( t )  (57)
 
     In the foregoing case, four combined signals y 1 (t), y 2 (t), y 3 (t), and y 4 (t) are output from the combiner  39  to the object detector  40 , and the detection process of the object  2  is performed by the object detector  40  for the four combined signals y 1 (t), y 2 (t), y 3 (t), and y 4 (t). 
     According to Embodiment 2, similarly to Embodiment 1, it is capable of bringing an effect of enhancing the detection probability of the object  2  to be detected without acquiring the position information of the object  2  in advance. In addition, it is possible to reduce the probability of erroneously selecting the noise of the reflected wave as a frequency of a combination target. 
     Embodiment 3 
     In the foregoing Embodiment 1, the combination target selector  34  selects a frequency having a relative large spectral component as a frequency of a combination target from among frequencies in the search range. In the foregoing Embodiment 2, the combination target selector  34  selects a frequency having a spectral component larger than a threshold as a frequency of a combination target from among frequencies in the search range. 
     In Embodiment 3 that will be described hereinafter, a frequency, which has a relative large spectral component and whose spectral component is larger than a threshold, is selected as a frequency of a combination target from among frequencies existing within the search range. 
       FIG. 13  is a structural diagram illustrating a signal processing circuit  20  of an object detecting device  1  according to Embodiment 3 of the present invention, and in the figure, since the same reference numerals as those in  FIG. 5  denote the same or corresponding portions, the description thereof will be omitted. 
     A combination target selector  43  is implemented by the combination target selecting circuit  54  illustrated in  FIG. 6 , and performs a process of selecting a frequency of a combination target. 
     A combination target limiter  44   a  compares spectral components of frequencies in spectra analyzed by the spectral analyzer  32   a  with the threshold, and performs processing of selecting a frequency whose spectral component is larger than the threshold. 
     A combination target limiter  44   b  compares spectral components of frequencies in spectra analyzed by the spectral analyzer  32   b  with the threshold, and performs processing for selecting a frequency whose spectral component is larger than the threshold. 
     A combination target limiter  44   c  compares spectral components of frequencies in spectra analyzed by the spectral analyzer  32   c  with the threshold, and performs processing for selecting a frequency whose spectral component is larger than the threshold. 
     A combination target selecting processor  45  selects a frequency having a relative large spectral component from among the frequencies selected by the combination target limiter  44   a  as a frequency of a combination target, also selects a frequency having a relative large spectral component from among the frequencies selected by the combination target limiter  44   b  as a frequency of a combination target, and also selects a frequency having a relative large spectral component from among the frequencies selected by the combination target limiter  44   c  as a frequency of a combination target. 
     In Embodiment 3, an example will be described, in which the combination target selector  43  is applied to the signal processing circuit  20  in  FIG. 5 . Alternatively, the combination target selector  43  may be applied to the signal processing circuit  20  in  FIG. 12 . 
     Next, the operation will be described. 
     The combination target limiter  44   a  sets each frequency of the spectrum relating to the object detecting device  1   a  analyzed by the spectral analyzer  32   a  as x, sets the spectral component of the frequency x as f b   a (x), and calculates each power pow(f b   a (x)) of the spectral component f b   a (x) of the corresponding frequency x. 
     The combination target limiter  44   b  sets each frequency of the spectrum relating to the object detecting device  1   b  analyzed by the spectral analyzer  32   b  as x, sets the spectral component of the frequency x as f b   b (x), and calculates each power pow(f b   b (x)) of the spectral component f b   b (x) of the corresponding frequency x. 
     The combination target limiter  44   c  sets each frequency of the spectrum relating to the object detecting device  1   c  analyzed by the spectral analyzer  32   c  as x, sets the spectral component of the frequency x as f b   c (x), and calculates each power pow(f b   c (x)) of the spectral component f b   c (x) of the corresponding frequency x. 
     Subsequently, the combination target limiter  44   a  compares each power pow(f b   a (x)) of the spectral component f b   a (x) of the corresponding frequency x with a preset threshold. When there is power pow(f b   a (x)) larger than the threshold among the powers pow(f b   a (x)) of respective frequencies x, the combination target limiter  44   a  selects a frequency x of the power pow(f b   a (x)). 
     The combination target limiter  44   b  compares each power pow(f b   b (x)) of the spectral component f b   b (x) of the corresponding frequency x with a preset threshold. When there is power pow(f b   b (x)) larger than the threshold among the powers pow(f b   b (x)) of respective frequencies x, the combination target limiter  44   b  selects a frequency x of the power pow(f b   b (x)). 
     The combination target limiter  44   c  compares each power pow(f b   c (x)) of the spectral component f b   c (x) of the corresponding frequency x with a preset threshold. When there is power pow(f b   c (x)) larger than the threshold among the powers pow(f b   c (x)) of respective frequencies x, the combination target limiter  44   c  selects a frequency x of the power pow(f b   c (x)). 
     In the foregoing configuration, the combination target limiters  44   a ,  44   b , and  44   c  compare the power of the spectral component of each frequency x with the threshold. Alternatively, the amplitude of the spectral component of each frequency x may be compared with the threshold. 
     After the combination target limiter  44   a  selects one or more frequencies x, the combination target selecting processor  45  compares powers pow(f b   a (x)) of the respective spectral component f b   a (x) of one or more frequencies x with each other, and selects a frequency x of the largest power pow(f b   a (x)) as a frequency x a   sel  of a combination target. 
     In addition, after the combination target limiter  44   b  selects one or more frequencies x, the combination target selecting processor  45  compares powers pow(f b   b (x)) of the respective spectral component f b   b (x) of one or more frequencies x with each other, and selects a frequency x of the largest power pow(f b   b (x)) as a frequency x b   sel  of a combination target. 
     Further, after the combination target limiter  44   c  selects one or more frequencies x, the combination target selecting processor  45  compares powers pow(f b   c (x)) of the respective spectral component f b   c (x) of one or more frequencies x with each other, and selects a frequency x of the largest power pow(f b   c (x)) a frequency x c   sel  of a combination target. 
     According to Embodiment 3, similarly to the foregoing Embodiment 1, there are effects that it is possible to enhance the detection probability of the object  2  to be detected without acquiring the position information of the object  2  in advance, and it is possible to reduce the possibility of erroneously selecting the noise included in the reflected wave as a frequency of a combination target. 
     In addition, since the number of frequencies of combination targets for each object detecting device  1  is reduced to only one, there is an effect that processing can be reduced in the frequency corrector  35 , the coefficient determinator  38 , the combiner  39 , and the object detector  40  than in the foregoing Embodiment 2. 
     Note that, in the invention of the present application, within the scope of the invention, free combination of each embodiment, a modification of an arbitrary component of each embodiment, or omission of an arbitrary component in each embodiment is possible. 
     The object detecting device according to the present invention is suitable for high precision detection of an object existing in a space. 
     REFERENCE SIGNS LIST 
       1   a ,  1   b ,  1   c : Object detecting device;  2 : Object to be detected;  3 : Object monitor;  11 : Transmission signal generating circuit;  12 : Transmission high-frequency circuit;  13 : Transmission antenna;  14 : Beat signal extractor;  15 : Reception antenna;  16 : Reception high-frequency circuit;  17 : Beat signal extracting circuit;  18 : Reception signal processor;  19 : ADC;  20 : Signal processing circuit;  21 : Position and velocity information outputting device;  22 : Multiplexer;  23 : Communication device;  24 : Demultiplexer;  25 : Detected-object information displaying device;  31 : Filter,  32   a ,  32   b ,  32   c : Spectral analyzer;  33 : Search range width setter;  34 : Combination target selector;  35 : Frequency corrector;  36 : Frequency correcting amount calculator;  37   a ,  37   b ,  37   c : Frequency correction processor;  38 : Coefficient determinator;  39 : Combiner;  40 : Object detector;  41 : Distance and velocity calculator;  42 : Sample data decimator;  43 : Combination target selector;  44   a ,  44   b ,  44   c : Combination target limiter;  45 : Combination target selecting processor;  51 : Filter circuit;  52 : Spectral analysis circuit;  53 : Search range width setting circuit;  54 : Combination target selecting circuit;  55 : Frequency correcting amount calculating circuit;  56 : Frequency correction processing circuit;  57 : Coefficient determining circuit;  58 : Combining circuit;  59 : Object detecting circuit;  60 : Distance and velocity calculating circuit;  71 : Memory;  72 : Processor