Patent Publication Number: US-9851444-B2

Title: Laser radar device and method of calculating speed of object to be measured

Description:
FIELD OF THE INVENTION 
     The present invention relates to a laser radar device that measures, for example, a wind speed, and a method of calculating the speed of an object to be measured. 
     BACKGROUND OF THE INVENTION 
     A radar device is known as a device that measures the position of an object existing at a remote point. 
     A radar device emits a wave, such as an electromagnetic wave or an acoustic wave, toward space, for example, and receives a wave which is reflected by an object which is a measurement object and returns thereto, and analyzes a received signal of the wave to measure the distance and the angle from the radar device to the object. 
     A weather radar device that sets fine liquid droplets or solid particles (referred to as an “aerosol” from here on) floating in atmospheric air as measurement objects, and that calculates the speed (wind speed) at which the aerosol is moving from the amount of phase rotation of a wave which is reflected by the aerosol and returns thereto is known especially among radar devices. 
     Further, a laser radar device that uses laser light as an electromagnetic wave, especially among weather radar devices, can observe an object with a high angular resolution and is therefore used as a wind direction and wind speed radar device because the divergence of the beam emitted thereby is very small (refer to nonpatent reference 1). 
     A conventional laser radar device emits laser light into atmospheric air, and, after that, receives laser light which is reflected by an aerosol existing in the atmospheric air, and returns thereto (laser light which receives a Doppler frequency shift according to the movement speed of the aerosol as the aerosol moves), and performs heterodyne detection on the laser light and local light, thereby detecting a Doppler signal corresponding to the movement speed of the aerosol (wind speed). 
       FIG. 14  is an explanatory drawing showing the concept of measurement by the conventional laser radar device. 
     The laser radar device performs a process of splitting the laser light (reflected light) reflected by the aerosol in the atmospheric air at each altitude and returns thereto into segments by time. In general, each reflected light split by time is called a “range bin.” 
     The laser radar device performs a coherent integration at very short intervals on each range bin, and performs a Fourier transform within the range bin. 
     After that, in order to improve the signal to noise ratio (referred to as SNR (Signal to Noise Ratio) from here on), the laser radar device performs N incoherent integrations on a pulse for each range bin, as shown in  FIG. 15 . 
     It is generally known that when the N incoherent integrations are performed, the SNR is improved by a factor of √N (refer to patent reference 1). 
       FIG. 16  is an explanatory drawing showing the concepts of a wind speed and a wind speed width which are derived from a reception spectrum. 
     In  FIG. 16 , a spectrum of wind speed which is acquired when a Gaussian beam is emitted is shown, and the peak of this spectrum is defined as a Doppler speed (wind speed). 
     As a method of calculating this Doppler speed, in addition to a peak detection method of calculating a Doppler speed from a frequency corresponding to a peak value of the SNR, there is a centroid calculation method of calculating the center of gravity of one or more peak values in the SNR, and calculating a Doppler speed from a frequency corresponding to the center of gravity (refer to nonpatent reference 1). 
     Further, there is a maximum likelihood estimation method of making variable a parameter of a waveform model of a reception spectrum prepared in advance, searching for a parameter having the highest correlation with a spectrum of a received signal, and calculating the speed of the object to be measured by using the parameter (refer to patent reference 2). 
       FIG. 17  is an explanatory drawing explaining a merit and a demerit of wind measurements using the peak detection method and those of wind measurements using the centroid calculation method. 
     When a sampling frequency is denoted by fs and the number of data points is denoted by p, a frequency resolution Δf is given by fs/p. 
     When a wind measurement is performed by using the peak detection method, there is a case in which no peak of the SNR can be correctly detected, as shown in  FIG. 17( a ) , unless the sampling frequency fs is sufficient. Therefore, the wind speed acquired by using the peak detection method may have an error for its true value. 
     In contrast with this, when a wind measurement is performed by using the centroid calculation method, a wind measurement with accuracy greater than the frequency resolution can be implemented, as shown in  FIG. 17( a ) . 
     However, when a clutter exists and noise caused by rain or the like is mixed in the received signal, as shown in  FIG. 17( b ) , or when there occurs a statistical fluctuation, a clutter component is added to the calculation of the center of gravity, and therefore a larger error is included in the wind speed acquired by using the centroid calculation method. In such a case, the peak detection method can acquire a value closer to the true wind speed. The difference between  FIG. 17( a )  and  FIG. 17( b )  is distinguishable according to the wind speed width acquired. 
     The above-mentioned maximum likelihood estimation method has a merit of being able to determine a wind speed having a high degree of accuracy because it determines a parameter of a waveform model having the highest correlation with the spectrum of the received signal through repetitive calculations. 
     Further, there is provided an advantage of being able to reduce an SNR desirable for the wind measurement to a value smaller than that in the peak detection method (refer to nonpatent reference 2). 
     The maximum likelihood estimation method has another merit of, when a plurality of winds are mixing in the same range bin, being able to measure each wind speed (refer to patent reference 2).  FIG. 18  is an explanatory drawing explaining a wind measurement of a plurality of winds using the maximum likelihood estimation method. 
     However, there is a demerit that because it is necessary to perform repetitive calculations, the speed of the calculation is slower than those in the peak detection method and the centroid calculation method, and the wind measurement rate is slow. 
     RELATED ART DOCUMENT 
     Patent Reference 
     
         
         Patent reference 1: Japanese Unexamined Patent Application Publication No. 2006-284260 
         Patent reference 2: Japanese Unexamined Patent Application Publication No. 2010-271058 
       
    
     Nonpatent Reference 
     
         
         Nonpatent reference 1: “Radar Remote Sensing of Weather and The Atmosphere”, Shoichiro Fukao and Kyosuke Hamazu, Kyoto University Press, Mar. 30, 2005, p. 112-113, ISBN 4-87698-653-3 
         Nonpatent reference 2: Kameyama et al., “Performance of Discrete-Fourier-Transform-Based Velocity Estimators for a Wind-Sensing Coherent Doppler Lidar System in the Kolmogorov Turbulence Regime”, IEEE, VOL. 47, NO. 10-2009. 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     Because conventional laser radar devices are constructed as above, a Doppler speed (wind speed) can be calculated according to a calculation method, such as a peak detection method, a centroid calculation method, or a maximum likelihood estimation method. However, the conventional laser radar devices do not select an appropriate calculation method in consideration of the form of an aerosol in atmospheric air, such as the presence or absence of a clutter, but simply calculates a Doppler speed (wind speed) according to a single calculation method prepared in advance. Therefore, the calculation accuracy of the Doppler speed (wind speed) may degrade dependently upon the form of the aerosol in the atmospheric air. A further problem is that the calculation process of calculating the Doppler speed (wind speed) may take a long time. 
     The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a laser radar device that can calculate the speed of an object to be measured in a short time with a high degree of accuracy, and a method of calculating the speed of an object to be measured in a short time with a high degree of accuracy. 
     Means for Solving the Problem 
     In accordance with the present invention, there is provided a laser radar device including: a laser light transmitter and receiver that emits laser light into atmospheric air, receives laser light which is reflected by an object to be measured existing in the atmospheric air and returns thereto, and outputs a received signal of the laser light; a coherent integrator that performs a coherent integration on the received signal outputted from the laser light transmitter and receiver; a spectrum calculator that performs a Fourier transform on the received signal on which the coherent integration is performed by the coherent integrator and performs an incoherent integration on the received signal after the Fourier transform to calculate a spectrum of the received signal; a signal to noise ratio calculator that calculates a signal to noise ratio from both the spectrum of the received signal which is calculated by the spectrum calculator, and a spectrum of a noise signal; and a peak value detector that detects a peak value of the signal to noise ratio calculated by the signal to noise ratio calculator, in which a speed calculator selects a speed calculation method of calculating the speed of the object to be measured from both the peak value of the signal to noise ratio which is detected by the peak value detector and information showing to which one of a calculation rate of the speed and calculation accuracy of the speed a higher priority is given, and calculates the speed of the object to be measured according to the speed calculation method. 
     Advantages of the Invention 
     Because the laser radar device in accordance with the present invention is constructed in such a way that the speed calculator selects a speed calculation method of calculating the speed of the object to be measured from both the peak value of the signal to noise ratio which is detected by the peak value detector and the information showing to which one of the calculation rate of the speed and the calculation accuracy of the speed a higher priority is given, and calculates the speed of the object to be measured according to the speed calculation method, there is provided an advantage of being able to calculate the speed of the object to be measured in a short time with a high degree of accuracy. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a structural diagram showing a laser radar device in accordance with Embodiment 1 of the present invention; 
         FIG. 2  is a structural diagram showing a signal analyzing unit  6  of the laser radar device in accordance with Embodiment 1 of the present invention; 
         FIG. 3  is a flow chart showing the details of processing (a method of calculating the speed of an object to be measured) performed by the signal analyzing unit  6  of the laser radar device in accordance with Embodiment 1 of the present invention; 
         FIG. 4  is a structural diagram showing a speed calculating unit  16  in the signal analyzing unit  6 ; 
         FIG. 5  is a flowchart showing the details of processing performed by the speed calculating unit  16  when a higher priority is given to the calculation rate of the speed than to the calculation accuracy of the speed; 
         FIG. 6  is a flow chart showing the details of processing performed by the speed calculating unit  16  when a higher priority is given to the calculation accuracy of the speed than to the calculation rate of the speed; 
         FIG. 7  is a structural diagram showing a signal analyzing unit  6  of a laser radar device in accordance with Embodiment 2 of the present invention; 
         FIG. 8  is a structural diagram showing a speed calculating unit  17  in the signal analyzing unit  6 ; 
         FIG. 9  is a flowchart showing the details of processing performed by the speed calculating unit  17  when a higher priority is given to the calculation rate of the speed than to the calculation accuracy of the speed; 
         FIG. 10  is a flow chart showing the details of processing performed by the speed calculating unit  17  when a higher priority is given to the calculation accuracy of the speed than to the calculation rate of the speed; 
         FIG. 11  is an explanatory drawing showing SNR determination in an SNR determining unit  31 ; 
         FIG. 12  is a structural diagram showing a signal analyzing unit  6  of a laser radar device in accordance with Embodiment 3 of the present invention; 
         FIG. 13  is a flow chart showing the details of processing (a method of calculating the speed of an object to be measured) performed by the signal analyzing unit  6  of the laser radar device in accordance with Embodiment 3 of the present invention; 
         FIG. 14  is an explanatory drawing showing the concept of measurement by a conventional laser radar device; 
         FIG. 15  is an explanatory drawing showing the details of signal processing of measuring a Doppler speed which is performed by the laser radar device; 
         FIG. 16  is an explanatory drawing showing the concepts of a wind speed and a wind speed width which are derived from a reception spectrum; 
         FIG. 17  is an explanatory drawing explaining the merits and demerits of a wind measurement using a peak detection method and those of a wind measurement using a centroid calculation method; and 
         FIG. 18  is an explanatory drawing explaining a wind measurement of a plurality of winds using a maximum likelihood estimation method. 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     Hereafter, in order to explain this invention in greater detail, the preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     Embodiment 1 
       FIG. 1  is a structural diagram showing a laser radar device in accordance with Embodiment 1 of the present invention. 
     Referring to  FIG. 1 , a light transmitting and receiving unit  1  is comprised of an optical oscillation unit  2 , an optical system  3 , and a laser light detecting unit  4 , and performs a process of emitting laser light into atmospheric air, receiving laser light which is reflected by an aerosol (object to be measured) existing in the atmospheric air and returns thereto, and outputting a received signal of the laser light to a signal processing unit  5 . The light transmitting and receiving unit  1  constructs a laser light transmitter and receiver. 
     The light transmitting and receiving unit has only to be able to emit laser light which is an electromagnetic wave into atmospheric air, and receive laser light which is reflected by an aerosol and returns thereto, and the light transmitting and receiving unit is not limited to the light transmitting and receiving unit  1  having the structure shown in  FIG. 1 . Therefore, for example, the light transmission and reception unit  1  can have a structure disclosed by patent reference 1. 
     The optical oscillation unit  2  of the light transmitting and receiving unit  1  is a light source that oscillates laser light. 
     The optical system  3  of the light transmitting and receiving unit  1  is comprised of one or more lenses and so on, and is an optical element that focuses the laser light which is reflected by the aerosol and returns thereto onto the laser light detecting unit  4 . 
     The laser light detecting unit  4  of the light transmitting and receiving unit  1  performs a process of receiving the laser light focused by the optical system  3 , converting the waveform of the laser light into an electric signal, and outputting the electric signal to the signal processing unit  5  as a received signal. 
     The signal processing unit  5  is comprised of a signal analyzing unit  6  and a display and storage unit  7 , and performs a process of calculating the speed (wind speed) of the aerosol by analyzing the received signal outputted from the light transmitting and receiving unit  1 , and displaying the wind speed which is the calculation result. 
     The signal analyzing unit  6  of the signal processing unit  5  is comprised of, for example, a semiconductor integrated circuit equipped with a CPU, or a one chip microcomputer, and performs a process of calculating the speed (wind speed) of the aerosol by analyzing the received signal outputted from the light transmitting and receiving unit  1 . 
     In this embodiment, although it is assumed that each of the components in the signal analyzing unit  6  consists of hardware for exclusive use (refer to  FIG. 2 ), the signal analyzing unit  6  can alternatively consist of a computer. 
     In the case in which the signal analyzing unit  6  consists of a computer, a noise signal storage unit  13  which is a component of the signal analyzing unit  6  can be constructed in a memory of the computer and a program in which the details of processing performed by a coherent integration processing unit  11 , a spectrum calculating unit  12 , an SNR calculating unit  14 , a peak SNR detecting unit  15 , and a speed calculating unit  16 , which are components of the signal analyzing unit  6 , are described can be stored in the memory of the computer, and the CPU of the computer can be made to execute the program stored in the memory. 
     The display and storage unit  7  of the signal processing unit  5  is comprised of, for example, a storage unit, such as a RAM or a hard disk, and a display unit, such as a liquid crystal display, and performs a process of displaying the speed (wind speed) of the aerosol calculated by the signal analyzing unit  6 , and also storing the speed (wind speed) of the aerosol. 
       FIG. 2  is a structural diagram showing the signal analyzing unit  6  of the laser radar device in accordance with Embodiment 1 of the present invention. 
     Referring to  FIG. 2 , the coherent integration processing unit  11  performs a process of performing a coherent integration on the received signal outputted from the light transmitting and receiving unit  1 , and outputting the received signal after the coherent integration to the spectrum calculating unit  12 . The coherent integration processing unit  11  constructs a coherent integrator. 
     The spectrum calculating unit  12  performs a process of performing a Fourier transform on the received signal on which the coherent integration is performed by the coherent integration processing unit  11 , and performing an incoherent integration on the received signal after the Fourier transform to calculate a spectrum of the received signal. The spectrum calculating unit  12  constructs a spectrum calculator. 
     The noise signal storage unit  13  stores a spectrum of a noise signal (a signal including only noise in which no signal is mixed) in advance. A spectrum of the noise signal should just be acquired in advance. For example, coherent integration can be performed on a noise signal acquired in real time and a Fourier transform can be performed on this noise signal, and incoherent integration can be then performed on the noise signal after the Fourier transform to calculate a spectrum of the noise signal. 
     The SNR calculating unit  14  performs a process of calculating an SNR (Signal to Noise Ratio) which is a signal to noise ratio by dividing the spectrum of the received signal calculated by the spectrum calculating unit  12  by the spectrum of the noise signal stored in the noise signal storage unit  13 . 
     A signal to noise ratio calculator is comprised of the noise signal storage unit  13  and the SNR calculating unit  14 . 
     The peak SNR detecting unit  15  performs a process of detecting a peak value of the SNR calculated by the SNR calculating unit  14 . The peak SNR detecting unit  15  constructs a peak value detector. 
     The speed calculating unit  16  performs a process of selecting a speed calculation method corresponding to the peak value of the SNR detected by the peak SNR detecting unit  15  from among a plurality of speed calculation methods of calculating the speed (wind speed) of an aerosol (e.g., a peak detection method, a centroid calculation method, and a maximum likelihood estimation method), and calculating the speed (wind speed) of the aerosol according to the speed calculation method. The speed calculating unit  16  constructs a speed calculator. 
       FIG. 3  is a flow chart showing the details of processing (a method of calculating the speed of an object to be measured) performed by the signal analyzing unit  6  of the laser radar device in accordance with Embodiment 1 of the present invention 
       FIG. 4  is a structural diagram showing the speed calculating unit  16  in the signal analyzing unit  6 . 
     Referring to  FIG. 4 , when receiving a command for giving a higher priority to the calculation rate of the speed than to the calculation accuracy of the speed from a user, an SNR determining unit  21  performs a process of selecting the “peak detection method” as the speed calculation method if a peak value (referred to as a “peak SNR” from here on) of the SNR detected by the peak SNR detecting unit  15  is higher than a reference peak value P REF  (peak SNR&gt;P REF ), or selecting the “maximum likelihood estimation method” as the speed calculation method if the peak SNR is equal to or lower than the reference peak value P REF  (peak SNR≦P REF ). 
     In contrast, when receiving a command for giving a higher priority to the calculation accuracy of the speed than to the calculation rate of the speed from the user, the SNR determining unit  21  performs a process of selecting the “centroid calculation method” as the speed calculation method if the peak SNR is higher than the reference peak value P REF  (peak SNR&gt;P REF ), or selecting the “maximum likelihood estimation method” as the speed calculation method if the peak SNR is equal to or lower than the reference peak value P REF  (peak SNR≦P REF ). 
     A peak detection processing unit  22  performs a process of calculating the speed (wind speed) of the aerosol from a frequency corresponding to a peak SNR detected by the peak SNR detecting unit  15  when the “peak detection method” is selected by the SNR determining unit  21  as the speed calculation method. 
     A centroid calculation processing unit  23  performs a process of calculating the center of gravity of one or more peak SNRs detected by the peak SNR detecting unit  15  to calculate the speed (wind speed) of the aerosol from a frequency corresponding to the center of gravity when the “centroid calculation method” is selected by SNR determining unit  21  as the speed calculation method. 
     A maximum likelihood estimation processing unit  24  performs a process of making variable a parameter of a waveform model of a reception spectrum prepared in advance, searching for a parameter with the highest correlation with the spectrum of the received signal calculated by the spectrum calculating unit  12 , and calculating the speed (wind speed) of the aerosol by using the parameter. 
       FIG. 5  is a flow chart showing the details of processing performed by the speed calculating unit  16  when a higher priority is given to the calculation rate of the speed than to the calculation accuracy of the speed. 
       FIG. 6  is a flow chart showing the details of processing performed by the speed calculating unit  16  when a higher priority is given to the calculation accuracy of the speed than to the calculation rate of the speed. 
     Next, the operation of the laser radar device will be explained. 
     First, when the optical oscillation unit  2  of the light transmitting and receiving unit  1  oscillates laser light, the laser light is emitted into atmospheric air by way of the optical system  3 . 
     The laser light emitted into the atmospheric air is reflected by an aerosol existing in the atmospheric air, and a reflected part of the laser light is focused by the optical system  3  of the light transmitting and receiving unit  1 . 
     The laser light detecting unit  4  of the light transmitting and receiving unit  1  receives the laser light focused by the optical system  3 , converts the waveform of the laser light into an electric signal, and outputs the electric signal to the signal processing unit  5  as a received signal. 
     When receiving the received signal from the light transmitting and receiving unit  1 , the signal analyzing unit  6  of the signal processing unit  5  calculates the speed (wind speed) of the aerosol by analyzing the received signal, and displays the wind speed which is the calculation result on the display and storage unit  7 . 
     Hereafter, the details of processing performed by the signal analyzing unit  6  will be explained concretely. 
     When receiving the received signal from the light transmitting and receiving unit  1 , the coherent integration processing unit  11  of the signal analyzing unit  6  performs a coherent integration on the received signal for each range bin i (i=1, 2, . . . , M), as shown in  FIG. 14 , and outputs the received signal after the coherent integration to the spectrum calculating unit  12  (step ST 1  of  FIG. 3 ). 
     For example, when the distance from the laser radar device to the aerosol is 1 km and the distance width of each range bin is 100 m, the number M of range bins is 10. 
     As an alternative, the user can determine the number M of range bins. For example, when the distance from the laser radar device to the aerosol is 1 km, and the user determines the number M of range bins as 20, the distance width of each range bin is 50 m. 
     When receiving the received signal after the coherent integration from the coherent integration processing unit  11 , the spectrum calculating unit  12  performs a Fourier transform on the received signal after the coherent integration for each range bin i. 
     The spectrum calculating unit  12  then performs an incoherent integration on the received signal after the Fourier transform N integration times, N being set up in advance, for each range bin i (refer to  FIG. 15 ) to calculate a spectrum SPC i  of the received signal (step ST 2 ). 
     After the spectrum calculating unit  12  calculates the spectrum SPC i  of the received signal, the SNR calculating unit  14  calculates an SNR which is a signal to noise ratio for each range bin i by dividing the spectrum SPC i  of the received signal by the spectrum of the noise signal stored in the noise signal storage unit  13  (step ST 3 ). 
     After the SNR calculating unit  14  calculates the SNR, the peak SNR detecting unit  15  detects a peak SNR which is a peak value of the SNR for each range bin i (step ST 4 ). 
     After the peak SNR detecting unit  15  detects a peak SNR, the speed calculating unit  16  selects a speed calculation method corresponding to the peak SNR for each range bin i from among the plurality of speed calculation methods of calculating the speed (wind speed) of an aerosol (e.g., the peak detection method, the centroid calculation method, and the maximum likelihood estimation method) (step ST 5 ), and calculates the speed (wind speed) of the aerosol according to the speed calculation method (step ST 6 ). 
     Hereafter, the details of processing performed by the speed calculating unit  16  will be explained concretely. 
     When receiving a command for giving a higher priority to the calculation rate of the speed than to the calculation accuracy of the speed from the user, the speed calculating unit  16  performs the process shown in  FIG. 5  to calculate the speed (wind speed) of the aerosol. 
     In contrast, when receiving a command for giving a higher priority to the calculation accuracy of the speed than to the calculation rate of the speed from the user, the speed calculating unit performs the process shown in  FIG. 6  to calculate the speed (wind speed) of the aerosol. 
     First, the details of processing at the time when receiving a command for giving a higher priority to the calculation rate of the speed than to the calculation accuracy of the speed from the user will be explained. 
     When receiving the command for giving a higher priority to the calculation rate of the speed than to the calculation accuracy of the speed from the user, the SNR determining unit  21  of the speed calculating unit  16  compares a peak SNR detected by the peak SNR detecting unit  15  with the reference peak value P REF  for each range bin i (step ST 11  of  FIG. 5 ). 
     If the peak SNR is higher than the reference peak value P REF  (peak SNR&gt;P REF ), the SNR determining unit  21  selects, as the speed calculation method, the “peak detection method” whose calculation time is shorter than that of the maximum likelihood estimation method because the SNR of the received signal is sufficiently high and the speed (wind speed) of the aerosol can be calculated with a high degree of accuracy even if the “maximum likelihood estimation method” is not selected as the speed calculation method (step ST 12 ). 
     In contrast, if the peak SNR is equal to or lower than the reference peak value P REF  (peak SNR≦P REF ), the SNR determining unit selects the “maximum likelihood estimation method” as the speed calculation method (step ST 13 ). More specifically, because even when a higher priority is given to the calculation rate of the speed, the wind speed may not be able to be calculated with a high degree of accuracy if the “peak detection method” is selected as the speed calculation method under circumstances where the peak SNR is low, the “maximum likelihood estimation method” is selected as the speed calculation method. 
     After the SNR determining unit  21  selects the “peak detection method” as the speed calculation method for the range bin i in question, the peak detection processing unit  22  calculates the wind speed v d  which is the speed of the aerosol from a frequency f d  corresponding to the peak SNR detected by the peak SNR detecting unit  15  (step ST 14 ).
 
 v   d   =f   d λ  (1)
 
     In this equation (1), A denotes the wavelength. 
     After the SNR determining unit  21  selects the “maximum likelihood estimation method” as the speed calculation method for the range bin i in question, the maximum likelihood estimation processing unit  24  makes variable the parameter of the waveform model of the reception spectrum prepared in advance, and searches for a parameter with the highest correlation with the spectrum of the received signal calculated by the spectrum calculating unit  12 . 
     For example, if it is assumed that the transmission waveform of the laser light has a normal distribution and the received waveform of the laser light also has a normal distribution, when the amplitude is denoted by A, the average Doppler speed is denoted by μ d , and the wind speed width is denoted by σ d , the model of the spectrum of the received signal is expressed by the following equation (2). 
     
       
         
           
             
               
                 
                   
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     In the equation (2), the amplitude A, the average Doppler speed μ d , and the wind speed width σ d  which are parameters of the model are made to be variable, and a least square error L (equivalent to a likelihood) shown by the following equation (3) is calculated.
 
 L=Σ{S ( f )− p ( f,A,μ   d ,σ d )} 2   (3)
 
     In this equation (3), the parameters (the amplitude A, the average Doppler speed μ d , and the wind speed width σ d ) at the time when the least square error L is a minimum are the ones which can imitate the received signal most likely. 
     In this embodiment, the value range of the wind speed width σ d  and that of the average Doppler speed μ d  are as follows.
 
1/ω≦σ d   ≦K ×(1/ω)
 
ω denotes a transmission pulse width, and K denotes the number of air quantities which are assumed to be included in the range bin.
 
0&lt;μ d   ≦f   w  
 
     f w  denotes a Doppler frequency which is determined by an assumed wind speed, and is determined by the user according to an environment where the device is installed and performs measurements. 
     After specifying the parameters (the amplitude A, the average Doppler speed μ d , and the wind speed width σ d ) which can imitate the received signal most likely, the maximum likelihood estimation processing unit  24  calculates the wind speed v d  which is the speed of the aerosol by substituting, as the frequency f d , a frequency corresponding to the average Doppler speed μ d  into the above-mentioned equation (1) (step ST 15 ). 
     Although the example of also making variable the amplitude A is shown above, when it is seemed that no clutter exists (e.g., at a time of fine weather), for example, the spectrum SPC i  of the received signal can be normalized by using the largest spectrum of the received signal, and a maximum likelihood estimation can be performed on the normalized spectrum SPC i , thereby being able to reduce the calculator cost. 
     Next, the details of processing at the time when receiving a command for giving a higher priority to the calculation accuracy of the speed than to the calculation rate of the speed from the user will be explained. 
     When receiving the command for giving a higher priority to the calculation accuracy of the speed than to the calculation rate of the speed from the user, the SNR determining unit  21  of the speed calculating unit  16  compares a peak SNR detected by the peak SNR detecting unit  15  with the reference peak value P REF  for each range bin i (step ST 21  of  FIG. 6 ). 
     If the peak SNR is higher than the reference peak value P REF  (peak SNR&gt;P REF ), the SNR determining unit  21  selects, as the speed calculation method, the “centroid calculation method” whose calculation time is shorter than that of the maximum likelihood estimation method because the SNR of the received signal is sufficiently high and the speed (wind speed) of the aerosol can be calculated with a high degree of accuracy even if the “maximum likelihood estimation method” is not selected as the speed calculation method (step ST 22 ). 
     In contrast, if the peak SNR is equal to or lower than the reference peak value P REF  (peak SNR≦P REF ), the SNR determining unit selects the “maximum likelihood estimation method” as the speed calculation method (step ST 23 ). More specifically, because even when a higher priority is given to the calculation rate of the speed, the wind speed may not be able to be calculated with a high degree of accuracy if the “maximum likelihood estimation method” is selected as the speed calculation method under circumstances where the peak SNR is low, the “centroid calculation method” or the “peak detection method” is selected as the speed calculation method. 
     After the SNR determining unit  21  selects the “centroid calculation method” as the speed calculation method for the range bin i in question, the centroid calculation processing unit  23  calculates the center of gravity of one or more peaks SNR detected by the peak SNR detecting unit  15  (in the example of  FIG. 17( b ) , because a peak SNR for wind components and a peak SNR for clutter components are calculated by the peak SNR detecting unit  15 , the center of gravity of the two peak SNRs is calculated). 
     After calculating the center of gravity of one or more peak SNRs, the centroid calculation processing unit  23  calculates the wind speed v d  which is the speed of the aerosol by substituting, as the frequency f d , a frequency corresponding to the center of gravity into the above-mentioned equation (1) (step ST 24 ). 
     The frequency f d  corresponding to the center of gravity is shown as in the following equation (4). 
     
       
         
           
             
               
                 
                   
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                         ⁡ 
                         
                           ( 
                           f 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In this equation (4), S(f) denotes the spectrum SPC i  of the received signal and f denotes the frequency corresponding to the spectrum SPC i , and the spectrum SPC i  is weighted with the frequency f. 
     After the SNR determining unit  21  selects the “maximum likelihood estimation method” as the speed calculation method for the range bin i in question, the maximum likelihood estimation processing unit  24  calculates the wind speed v d  which is the speed of the aerosol, like in the case of giving a higher priority to the calculation rate of the speed (step ST 25 ). 
     As can be seen from the above description, because the laser radar device in accordance with this Embodiment 1 is constructed in such a way that the speed calculating unit  16  selects a speed calculation method of calculating the speed (wind speed) of an aerosol from both a peak SNR detected by the peak SNR detecting unit  15  and information showing to which one of the calculation rate of the speed and the calculation accuracy of the speed a higher priority is given from among the plurality of speed calculation methods of calculating the speed (wind speed) of an aerosol, and calculates the speed (wind speed) of an aerosol according to the speed calculation method, there is provided an advantage of being able to calculate the speed (wind speed) of an aerosol in a short time with a high degree of accuracy. 
     More specifically, because the “peak detection method” or the “centroid calculation method” whose processing time is short is selected as the speed calculation method under circumstances where a peak SNR detected by the peak SNR detecting unit  15  is high, and therefore the speed (wind speed) of an aerosol can be calculated with a high degree of accuracy even if the “maximum likelihood estimation method” is not selected as the speed calculation method, the speed (wind speed) of the aerosol can be calculated in a short time with a high degree of accuracy. 
     In contrast, because the “maximum likelihood estimation method” is selected as the speed calculation method under circumstances where a peak SNR detected by the peak SNR detecting unit  15  is low, and therefore the speed (wind speed) of an aerosol cannot be calculated with a high degree of accuracy if the “maximum likelihood estimation method” is not selected as the speed calculation method, the speed (wind speed) of the aerosol can be calculated with a high degree of accuracy even if the peak SNR is low. 
     Although in this Embodiment 1 the case of receiving a command for giving a higher priority to the calculation rate of the speed than to the calculation accuracy of the speed from the user, and the case of receiving a command for giving a higher priority to the calculation accuracy of the speed than to the calculation rate of the speed from the user are shown, a speed calculation method can be selected in response to another command (e.g., a command for giving a higher priority to a distance resolution) from the user. 
     Embodiment 2 
       FIG. 7  is a structural diagram showing a signal analyzing unit  6  of a laser radar device in accordance with Embodiment 2 of the present invention. In the figure, because the same reference numerals as those shown in  FIG. 2  denote the same components or like components, the explanation of the components will be omitted hereafter. 
     A speed calculating unit  17  performs a process of calculating a wind speed width S w  which is a speed width of an aerosol from a spectrum SPC i  of a received signal which is calculated by a spectrum calculating unit  12 , selecting a speed calculation method corresponding to both a peak SNR detected by a peak SNR detecting unit  15  and the wind speed width S w  from among a plurality of speed calculation methods of calculating the speed (wind speed) of the aerosol (e.g., a peak detection method, a centroid calculation method, and a maximum likelihood estimation method), and calculating the speed (wind speed) of the aerosol according to the speed calculation method. The speed calculating unit  17  constructs a speed calculator. 
       FIG. 8  is a structural diagram showing the speed calculating unit  17  in the signal analyzing unit  6 . In the figure, because the same reference numerals as those shown in  FIG. 4  denote the same components or like components, the explanation of the components will be omitted hereafter. 
     A threshold T SNR2  is set as a reference peak value in addition to a threshold T SNR1  being set up in advance (T SNR1 &gt;T SNR2 ) When receiving a command for giving a higher priority to the calculation rate of the speed than to the calculation accuracy of the speed from a user, an SNR determining unit  31  performs a process of selecting the “centroid calculation method” as the speed calculation method if a peak SNR detected by the peak SNR detecting unit  15  is higher than the threshold T SNR2  and is equal to or lower than the threshold T SNR1  (T SNR2 &lt;peak SNR≦T SNR1 ) or selecting the “maximum likelihood estimation method” as the speed calculation method if the peak SNR is equal to or lower than the threshold T SNR2  (peak SNR≦T SNR2 ). 
     In contrast, when receiving a command for giving a higher priority to the calculation accuracy of the speed than to the calculation rate of the speed from the user, the SNR determining unit  31  performs a process of selecting the “maximum likelihood estimation method” as the speed calculation method if the peak SNR is equal to or lower than the threshold T SNR1  (peak SNR≦T SNR1 ). 
     A speed width determining unit  32  performs a process of calculating a wind speed width S w  from the spectrum SPC i  of the received signal calculated by the spectrum calculating unit  12 . 
     Further, a threshold Tw min  is set as a reference speed width in addition to a threshold Tw max  being set up in advance (Tw max &gt;Tw min ). When receiving a command for giving a higher priority to the calculation rate of the speed than to the calculation accuracy of the speed from the user, the speed width determining unit  32  performs a process of, when a peak SNR detected by the peak SNR detecting unit  15  is higher than the threshold T SNR1 , selecting the “peak detection method” as the speed calculation method if the wind speed width S w  is wider than the threshold Tw max  (S w &gt;Tw max ), or selecting the “centroid calculation method” as the speed calculation method if the wind speed width S w  is equal to or narrower than the threshold Tw max  (Tw min ≦S w ≦Tw max ). 
     In contrast, when receiving a command for giving a higher priority to the calculation accuracy of the speed than to the calculation rate of the speed from the user, the speed width determining unit  32  performs a process of, when a peak SNR detected by the peak SNR detecting unit  15  is higher than the threshold T SNR1 , selecting the “maximum likelihood estimation method” as the speed calculation method if the wind speed width S w  is wider than the threshold Tw max  (S w &gt;Tw max ), or selecting the “centroid calculation method” as the speed calculation method if the wind speed width S w  is equal to or narrower than the threshold Tw max  (Tw min ≦S w ≦Tw max ). 
     The thresholds T SNR1  and T SNR2  in the SNR determining unit  31  and the thresholds Tw max  and Tw min  in the speed width determining unit  32  can be predetermined by, for example, the user, or values based on a model calculation disclosed by above-mentioned nonpatent reference 2 can be used as the thresholds. 
       FIG. 9  is a flow chart showing a part of the details of processing performed by the speed calculating unit  17  when a higher priority is given to the calculation rate of the speed than to the calculation accuracy of the speed. 
       FIG. 10  is a flow chart showing a part of the details of processing performed by the speed calculating unit  17  when a higher priority is given to the calculation accuracy of the speed than to the calculation rate of the speed. 
     Next, the operation of the laser radar device will be explained. 
     First, when an optical oscillation unit  2  of a light transmitting and receiving unit  1  oscillates laser light, the laser light is emitted into atmospheric air by way of an optical system  3 . 
     The laser light emitted into the atmospheric air is reflected by an aerosol existing in the atmospheric air, and a reflected part of the laser light is focused by the optical system  3  of the light transmitting and receiving unit  1 . 
     A laser light detecting unit  4  of the light transmitting and receiving unit  1  receives the laser light focused by the optical system  3 , converts the waveform of the laser light into an electric signal, and outputs the electric signal to a signal processing unit  5  as a received signal. 
     When receiving the received signal from the light transmitting and receiving unit  1 , the signal analyzing unit  6  of the signal processing unit  5  calculates the speed (wind speed) of the aerosol by analyzing the received signal, and displays the wind speed which is the calculation result on a display and storage unit  7 . 
     Hereafter, the details of processing performed by the signal analyzing unit  6  will be explained concretely. 
     When receiving the received signal from the light transmitting and receiving unit  1 , a coherent integration processing unit  11  of the signal analyzing unit  6  performs a coherent integration on the received signal for each range bin i (i=1, 2, . . . , M), and outputs the received signal after the coherent integration to a spectrum calculating unit  12  (refer to  FIG. 14 ), like that according to above-mentioned Embodiment 1. 
     When receiving the received signal after the coherent integration from the coherent integration processing unit  11 , the spectrum calculating unit  12  performs a Fourier transform on the received signal after the coherent integration for each range bin i, like that according to above-mentioned Embodiment 1. 
     The spectrum calculating unit  12  then performs an incoherent integration on the received signal after the Fourier transform N integration times, N being set up in advance, for each range bin i (refer to  FIG. 15 ) to calculate a spectrum SPC i  of the received signal. 
     After the spectrum calculating unit  12  calculates the spectrum SPC i  of the received signal, an SNR calculating unit  14  calculates an SNR which is a signal to noise ratio for each range bin i by dividing the spectrum SPC i  of the received signal by a spectrum of a noise signal stored in a noise signal storage unit  13 , like that according to above-mentioned Embodiment 1. 
     After the SNR calculating unit  14  calculates the SNR, the peak SNR detecting unit  15  detects a peak SNR which is a peak value of the SNR for each range bin i, like that according to above-mentioned Embodiment 1. 
     After the spectrum calculating unit  12  calculates the spectrum SPC i  of the received signal and the peak SNR detecting unit  15  detects a peak SNR, the speed calculating unit  17  calculates a wind speed width S w  which is a speed width of the aerosol from the spectrum SPC i  of the received signal for each range bin i. 
     After calculating the wind speed width S w , the speed calculating unit  17  selects a speed calculation method corresponding to both the peak SNR and the wind speed width S w  from among the plurality of speed calculation methods of calculating the speed (wind speed) of an aerosol (e.g., the peak detection method, the centroid calculation method, and the maximum likelihood estimation method), and calculates the speed (wind speed) of the aerosol according to the speed calculation method. 
     Hereafter, the details of processing performed by the speed calculating unit  17  will be explained concretely. 
     When receiving a command for giving a higher priority to the calculation rate of the speed than to the calculation accuracy of the speed from the user, the speed calculating unit  17  performs the process shown in  FIG. 9  to calculate the speed (wind speed) of the aerosol. 
     In contrast, when receiving a command for giving a higher priority to the calculation accuracy of the speed than to the calculation rate of the speed from the user, the speed calculating unit performs the process shown in  FIG. 10  to calculate the speed (wind speed) of the aerosol. 
     First, the details of processing at the time when receiving a command for giving a higher priority to the calculation rate of the speed than to the calculation accuracy of the speed from the user will be explained. 
     When receiving the command for giving a higher priority to the calculation rate of the speed than to the calculation accuracy of the speed from the user, the SNR determining unit  31  of the speed calculating unit  17  compares a peak SNR detected by the peak SNR detecting unit  15  with the threshold value T SNR1  for each range bin i (step ST 31  of  FIG. 9 ). 
       FIG. 11  is an explanatory drawing showing the SNR determination in the SNR determining unit  31 . 
     When the peak SNR is higher than the threshold T SNR1  (peak SNR&gt;T SNR1 ), the SNR determining unit  31  commands the speed width determining unit  32  to select the “peak detection method” or the “centroid calculation method” whose calculation time is shorter than that of the maximum likelihood estimation method because the SNR of the received signal is sufficiently high and the speed (wind speed) of the aerosol can be calculated with a high degree of accuracy even if the “maximum likelihood estimation method” is not selected as the speed calculation method, as shown in  FIG. 11 . 
     When the peak SNR is equal to or lower than the threshold T SNR1  (peak SNR≦T SNR1 ), SNR determining unit  31  compares the peak SNR with the threshold T SNR2  (step ST 32 ). 
     When the peak SNR is higher than the threshold T SNR2  (T SNR2 &lt;peak SNR≦T SNR1 ), the SNR determining unit  31  selects the “centroid calculation method” as the speed calculation method because the SNR of the received signal has a middle value and there is a possibility that the speed (wind speed) of the aerosol can be calculated with a high degree of accuracy even if the “maximum likelihood estimation method” is not selected as the speed calculation method, but the speed (wind speed) of the aerosol cannot be calculated with a high degree of accuracy if the “peak detection method” is selected as the speed calculation method, as shown in  FIG. 11  (step ST 33 ). 
     In contrast, when the peak SNR is equal to or lower than the threshold T SNR2  (peak SNR≦T SNR2 ), the SNR determining unit selects the “maximum likelihood estimation method” as the speed calculation method because the SNR of the received signal is low and there is a possibility that the speed (wind speed) of the aerosol cannot be calculated with a high degree of accuracy unless the “maximum likelihood estimation method” having a high degree of calculation accuracy is selected as the speed calculation method, as shown in  FIG. 11  (step ST 34 ). 
     When receiving a selection command for selecting the “peak detection method” or the “centroid calculation method” from the SNR determining unit  31 , the speed width determining unit  32  calculates a wind speed width S w  from the spectrum SPC i  of the received signal calculated by the spectrum calculating unit  12  for each range bin i, as shown in the following equation (5) (step ST 35 ). The equation (5) is disclosed in, for example, nonpatent reference 1. 
     
       
         
           
             
               
                 
                   
                     S 
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                       ∫ 
                       
                         
                           S 
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                             ( 
                             f 
                             ) 
                           
                         
                         ⁢ 
                         
                           f 
                           2 
                         
                         ⁢ 
                         
                           ⅆ 
                           f 
                         
                       
                     
                     
                       ∫ 
                       
                         
                           S 
                           ⁡ 
                           
                             ( 
                             f 
                             ) 
                           
                         
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                           f 
                         
                       
                     
                   
                 
               
               
                 
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                   5 
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     In this equation (5), S(f) denotes the spectrum SPC i  of the received signal, and f denotes a frequency corresponding to the spectrum SPC i . 
     In the case in which the SNR is high, the wind speed width S w  can be derived by using a width at a point where the SNR has a lower value than the peak value by one-half of this peak value. In this case, the wind speed width can be derived with a smaller amount of calculations. 
     After calculating the wind speed width S w , the speed width determining unit  32  compares the wind speed width S w  with the threshold Tw max  (step ST 36 ). 
     If the wind speed width S w  is wider than the threshold Tw max  (S w &gt;Tw max ) the speed width determining unit  32  selects the “peak detection method” as the speed calculation method (step ST 37 ). 
     If the wind speed width S w  is equal to or narrower than the threshold Tw max  (S w ≦Tw max ) the speed width determining unit  32  compares the wind speed width S w  with the threshold Tw min  (step ST 38 ). 
     If the wind speed width S w  is equal to or narrower than the threshold Tw max , but is equal to or wider than the threshold Tw min (Tw min ≦S w ≦Tw max ), the speed width determining unit  32  selects the “centroid calculation method” as the speed calculation method (step ST 39 ). 
     Because there is a possibility that the speed (wind speed) of the aerosol can be calculated with a high degree of accuracy even if any speed calculation method is selected if the wind speed width S w  is narrower than the threshold Tw min  (S w &lt;Tw min ), the speed width determining unit ends the process of calculating the speed (wind speed) of the aerosol without selecting any speed calculation method (step ST 40 ). 
     In this embodiment, although the thresholds Tw max  and Tw min  in the speed width determining unit  32  can be determined in advance by the user, as mentioned above, the thresholds Tw max  and Tw min  can be set with respect to 1/ω, where a transmission pulse width is denoted by ω, because the spectrum SPC i  of an ideal received signal is 1/ω. 
     For example, while the wind speed width S w  is equal to or greater than Tw max =2/ω when two or more types of winds are mixed, it can be determined that the reflected light is not the one from an aerosol, but is a clutter if the wind speed width S w  is equal to or less than 2/ω. 
     When the thresholds Tw max  and Tw min  are set this way, the “peak detection method” whose calculation accuracy is not influenced greatly by the existence of a clutter is selected as the speed calculation method under circumstances where a clutter exists, while the “centroid calculation method” whose calculation accuracy is higher than that of the peak detection method is selected under circumstances where no clutter exists. 
     After the speed width determining unit  32  selects the “peak detection method” as the speed calculation method for the range bin i in question, a peak detection processing unit  22  calculates the wind speed v d  which is the speed of the aerosol, like that according to above-mentioned Embodiment 1. 
     After the SNR determining unit  31  or the speed width determining unit  32  selects the “centroid calculation method” as the speed calculation method for the range bin i in question, a centroid calculation processing unit  23  calculates the wind speed v d  which is the speed of the aerosol, like that according to above-mentioned Embodiment 1. 
     After the SNR determining unit  31  selects the “maximum likelihood estimation method” as the speed calculation method for the range bin i in question, a maximum likelihood estimation processing unit  24  calculates the wind speed v d  which is the speed of the aerosol, like that according to above-mentioned Embodiment 1. 
     Next, the details of processing at the time of receiving a command for giving a higher priority to the calculation accuracy of the speed than to the calculation rate of the speed from the user will be explained. 
     When receiving the command for giving a higher priority to the calculation accuracy of the speed than to the calculation rate of the speed from the user, the SNR determining unit  31  of the speed calculating unit  17  compares a peak SNR detected by the peak SNR detecting unit  15  with the threshold T SNR1  for each range bin i (step ST 51  of  FIG. 10 ). 
     When the peak SNR is equal to or lower than the threshold T SNR1  (peak SNR≦T SNR1 ), the SNR determining unit  31  selects the “maximum likelihood estimation method” as the speed calculation method because there is a possibility that the speed (wind speed) of the aerosol cannot be calculated with a high degree of accuracy unless the “maximum likelihood estimation method” having a high degree of calculation accuracy is selected (step ST 52 ). 
     In contrast, when the peak SNR is higher than the threshold T SNR1  (peak SNR&gt;T SNR1 ), the SNR determining unit commands the speed width determining unit  32  to select the “centroid calculation method” or the “maximum likelihood estimation method.” 
     When receiving a selection command for selecting the “centroid calculation method” or the “maximum likelihood estimation method” from the SNR determining unit  31 , the speed width determining unit  32  calculates a wind speed width S w  from the spectrum SPC i  of the received signal calculated by the spectrum calculating unit  12  for each range bin i, like in the case of giving a higher priority to the calculation rate of the speed (step ST 53 ). 
     After calculating the wind speed width S w , the speed width determining unit  32  compares the wind speed width S w  with the threshold Tw max  (step ST 54 ). 
     If the wind speed width S w  is wider than the threshold Tw max  (S w &gt;Tw max ), the speed width determining unit  32  selects the “maximum likelihood estimation method” as the speed calculation method because there is a possibility that the speed (wind speed) of the aerosol cannot be calculated with a high degree of accuracy unless the “maximum likelihood estimation method” having a high degree of calculation accuracy is selected (step ST 55 ). 
     If the wind speed width S w  is equal to or narrower than the threshold Tw max  (S w ≦Tw max ) the speed width determining unit  32  compares the wind speed width S w  with the threshold Tw min  (step ST 56 ). 
     If the wind speed width S w  is equal to or narrower than the threshold Tw max , but is equal to or wider than the threshold Tw min  (Tw min ≦S w ≦Tw max ) the speed width determining unit  32  selects the “centroid calculation method” whose calculation time is shorter than that of the maximum likelihood estimation method because the speed (wind speed) of the aerosol can be calculated with a high degree of accuracy even if the “maximum likelihood estimation method” is not selected as the speed calculation method (step ST 57 ). 
     If the wind speed width S w  is narrower than the threshold Tw min  (S w &lt;Tw min ), the speed calculating unit ends the process of calculating the speed (wind speed) of the aerosol without selecting any speed calculation method because there is a low possibility that the speed (wind speed) of the aerosol can be calculated with a high degree of accuracy even if any speed calculation method is selected (step ST 58 ). 
     After the speed width determining unit  32  selects the “centroid calculation method” as the speed calculation method for the range bin i in question, the centroid calculation processing unit  23  calculates the wind speed v d  which is the speed of the aerosol, like that according to above-mentioned Embodiment 1. 
     After the SNR determining unit  31  or the speed width determining unit  32  selects the “maximum likelihood estimation method” as the speed calculation method for the range bin i in question, the maximum likelihood estimation processing unit  24  calculates the wind speed v d  which is the speed of the aerosol, like that according to above-mentioned Embodiment 1. 
     As can be seen from the above description, because the speed calculating unit  17  in accordance with this Embodiment 2 is constructed in such a way as to calculate a wind speed width S w  which is a speed width of an aerosol from a spectrum SPC i  of a received signal which is calculated by the spectrum calculating unit  12 , selecting a speed calculation method corresponding to both a peak SNR and the wind speed width S w  from among the plurality of speed calculation methods of calculating the speed (wind speed) of an aerosol (e.g., the peak detection method, the centroid calculation method, and the maximum likelihood estimation method), and calculate the speed (wind speed) of the aerosol according to the speed calculation method, there is provided an advantage of being able to select an optimal speed calculation method more strictly compared with above-mentioned Embodiment 1. 
     Embodiment 3 
     Although the example of selecting the “maximum likelihood estimation method” as the speed calculation method when a peak SNR is low is shown in above-mentioned Embodiments 1 and 2, the speed (wind speed) of an aerosol cannot be calculated with a high degree of accuracy when the SNR does not reach a desired SNR even if the “maximum likelihood estimation method” is selected as the speed calculation method. 
     To solve this problem, in accordance with this Embodiment 3, a laser radar device is enabled to calculate the speed (wind speed) of an aerosol with a high degree of accuracy even when the SNR does not reach a desired SNR. 
     More specifically, in accordance with this Embodiment 3, when the SNR does not reach a desired SNR, the number of integrations of an incoherent integration performed by a spectrum calculating unit  12  is increased so as to improve the accuracy of wind speed calculation. 
       FIG. 12  is a structural diagram showing a signal analyzing unit  6  of the laser radar device in accordance with Embodiment 3 of the present invention. In the figure, because the same reference numerals as those shown in  FIG. 7  denote the same components or like components, the explanation of the components will be omitted hereafter. 
     An number-of-integrations setting unit  18  performs a process of setting up the number N of integrations of the incoherent integration performed by the spectrum calculating unit  12  according to an SNR calculated by an SNR calculating unit  14 . The number-of-integrations setting unit  18  constructs a number-of-integrations setter. 
       FIG. 13  is a flowchart showing the details of processing (a method of calculating the speed of an object to be measured) performed by the signal analyzing unit  6  of the laser radar device in accordance with Embodiment 3 of the present invention. 
     In above-mentioned Embodiments 1 and 2, the spectrum calculating unit  12  calculates a spectrum SPC i  of a received signal by performing an incoherent integration on the received signal after a Fourier transform N integration times, N being set up in advance, for each range bin i. The spectrum calculating unit  12  in accordance with this Embodiment 3 differs from that in accordance with any one of above-mentioned Embodiments 1 and 2 in that the spectrum calculating unit calculates a spectrum SPC i  of the received signal by performing an incoherent integration on the received signal after the Fourier transform N integration times, N being set up by the number-of-integrations setting unit  18 . 
     Hereafter, the details of processing performed by the number-of-integrations setting unit  18  and the spectrum calculating unit  12  will be explained concretely. 
     However, because components other than the number-of-integrations setting unit  18  and the spectrum calculating unit  12  are the same as those according to any one of above-mentioned Embodiments 1 and 2, a detailed explanation of the components will be omitted hereafter. 
     When receiving the received signal after the coherent integration from a coherent integration processing unit  11 , the spectrum calculating unit  12  performs a Fourier transform on the received signal after the coherent integration for each range bin i, like that according to any one of above-mentioned Embodiments 1 and 2. 
     Because no SNR is calculated by an SNR calculating unit  14  in a stage where the spectrum calculating unit  12  calculates the spectrum SPC i  of the received signal for the first time, and the number-of-integrations setting unit  18  cannot set up the number N of integrations, the spectrum calculating unit  12  calculates the spectrum SPC i  of the received signal by performing an incoherent integration on the received signal after the Fourier transform N integration times, N being set up in advance, for each range bin i (refer to  FIG. 15 ) (step ST 2  of  FIG. 13 ). 
     After the spectrum calculating unit  12  calculates the spectrum SPC i  of the received signal, the SNR calculating unit  14  calculates an SNR which is a signal to noise ratio for each range bin i by dividing the spectrum SPC i  of the received signal by a spectrum of a noise signal stored in a noise signal storage unit  13 , like that according to any one of above-mentioned Embodiments 1 and 2 (step ST 3 ). 
     After the SNR calculating unit  14  calculates the SNR, the number-of-integrations setting unit  18  updates the number N of integrations which is set to the spectrum calculating unit  12  in advance (step ST 62 ) when the SNR is lower than a threshold T SNR3  which is required in order that a desired degree of calculation accuracy can be satisfied (e.g., T SNR3 &lt;T SNR2 &lt;T SNR1 ) (step ST 61 ). 
     More specifically, when the SNR calculated by the SNR calculating unit  14  is lower than the threshold T SNR3  (SNR&lt;T SNR3 ), the number-of-integrations setting unit  18  sets up the number N of integrations of the incoherent integration performed by the spectrum calculating unit  12  according to the SNR. 
     Concretely, the number-of-integrations setting unit  18  prepares a table in which the number N of integrations corresponding to the SNR is described in advance, refers to the table to read the number N of integrations corresponding to the SNR calculated by the SNR calculating unit  14 , and updates the number of integrations of the incoherent integration performed by the spectrum calculating unit  12  to the number N of integrations. 
     As an alternative, the number-of-integrations setting unit  18  can update the number N of integrations by, for example, performing an arithmetic operation shown by the following equation (6). However, the arithmetic operation shown by the equation (6) is an example and another arithmetic operation can be alternatively performed.
 
 N=N +( T   SNR1   /SNR ) 2   (6)
 
     Further, when a higher priority is given to the calculation accuracy of the speed, the number of integrations can be updated to the number N of integrations which falls within a data acquisition time period which is determined by (a processing time equal to or less than a desired wind speed calculation rate defined by the user+the number of integrations), and which is expected to provide the highest degree of wind speed calculation accuracy. 
     After the number-of-integrations setting unit  18  updates the number N of integrations of the incoherent integration, the spectrum calculating unit  12  performs an incoherent integration on the received signal after the Fourier transform N integration times, N being updated, for each range bin i to calculate a spectrum SPC i  of the received signal again (step ST 2 ). 
     After the spectrum calculating unit  12  calculates a spectrum SPC i  of the received signal again, the SNR calculating unit  14  calculates an SNR which is a signal to noise ratio for each range bin i by dividing the spectrum SPC i  of the received signal by the spectrum of the noise signal stored in the noise signal storage unit  13  (step ST 3 ), like that according to any one of above-mentioned Embodiments 1 and 2. 
     When the SNR calculated by the SNR calculating unit  14  is equal to or higher than the threshold T SNR3  (SNR≦T SNR3 ) the peak SNR detecting unit  15  detects a peak SNR which is a peak value of the SNR for each range bin i (step ST 4 ), like that according to any one of above-mentioned Embodiments 1 and 2. 
     After the spectrum calculating unit  12  calculates a spectrum SPC i  of the received signal and the peak SNR detecting unit  15  detects a peak SNR, the speed calculating unit  17  calculates a wind speed width S w  which is a speed width of the aerosol from the spectrum SPC i  of the received signal for each range bin i, like that according to above-mentioned Embodiment 2. 
     After calculating the wind speed width S w , the speed calculating unit  17  selects a speed calculation method corresponding to both the peak SNR and the wind speed width S w  for each range bin i from the plurality of speed calculation methods of calculating the speed (wind speed) of an aerosol (e.g., the peak detection method, the centroid calculation method, and the maximum likelihood estimation method) (step ST 5 ), and calculates the speed (wind speed) of the aerosol according to the speed calculation method (step ST 5 ). 
     As can be seen from the above description, because the number-of-integrations setting unit  18  in accordance with this Embodiment 3 is constructed in such away as to set up the number N of integrations of the incoherent integration performed by the spectrum calculating unit  12  according to the SNR calculated by the SNR calculating unit  14 , there is provided an advantage of being able to calculate the speed (wind speed) of an aerosol with a high degree of accuracy even when the SNR does not reach a desired SNR. 
     As a result, while a measurement of the wind speed is carried out with stability, the accuracy of calculation of the wind speed for all the range bins can be equalized to hold variations in the data constant. 
     Accordingly, when, for example, detection of a gust of wind or a turbulence is carried out by means of matching using a statistic, the processing time required to perform the matching process can be reduced and erroneous detection which is caused due to a shortage of the statistic or instability can also be reduced when the SNR is constant at all times. 
     Although the example in which the number-of-integrations setting unit  18  sets up the number N of integrations of the incoherent integration performed by the spectrum calculating unit  12  according to the SNR calculated by the SNR calculating unit  14  is shown in this Embodiment 3, the number of integrations of the coherent integration in the coherent integration processing unit  11  can be sets up and the coherent integration processing unit  11  can perform the coherent integration on the received signal the number of integrations which is set up by the number-of-integrations setting unit  18 . 
     Further, although the example in which in a step in which the SNR calculating unit  14  calculates the SNR, the number-of-integrations setting unit  18  sets up the number N of integrations of the incoherent integration performed by the spectrum calculating unit  12  according to the SNR if the SNR is lower than the threshold T SNR3  is shown in this Embodiment 3, the time that the number-of-integrations setting unit  18  sets up the number N of integrations is not limited to the step in which the SNR calculating unit  14  calculates the SNR. For example, in a step in which the speed calculating unit  17  selects a speed calculation method or in a step in which the speed calculating unit  17  calculates the speed (wind speed) of an aerosol, the number-of-integrations setting unit  18  can set up the number N of integrations. 
     While the invention has been described in its preferred embodiments, it is to be understood that an arbitrary combination of two or more of the above-mentioned embodiments can be made, various changes can be made in an arbitrary component in accordance with any one of the above-mentioned embodiments, and an arbitrary component in accordance with any one of the above-mentioned embodiments can be omitted within the scope of the invention. 
     INDUSTRIAL APPLICABILITY 
     Because in the laser radar device in accordance with the present invention, the speed calculator is constructed in such away as to select a speed calculation method corresponding to a peak value of a signal to noise ratio detected by a peak value detector from a plurality of speed calculation methods of calculating the speed of an object to be measured, and calculate the speed of the object to be measured according to the speed calculation method, and can calculate the speed of the object to be measured in a short time with a degree of high accuracy, the laser radar device is suitable for use as a weather radar system or a wind direction wind speed radar device. 
     EXPLANATIONS OF REFERENCE NUMERALS 
       1  light transmitting and receiving unit (laser light transmitter and receiver),  2  optical oscillation unit,  3  optical system,  4  laser light detecting unit,  5  signal processing unit,  6  signal analyzing unit,  7  display and storage unit,  11  coherent integration processing unit (coherent integrator),  12  spectrum calculating unit (spectrum calculator),  13  noise signal storage device (signal to noise ratio calculator),  14  SNR calculating unit (signal to noise ratio calculator),  15  peak SNR detecting unit (peak value detector),  16  and  17  speed calculating unit (speed calculator),  18  number-of-integrations setting unit (number-of-integrations setter),  21  SNR determining unit,  22  peak detection processing unit,  23  centroid calculation processing unit,  24  maximum likelihood estimation processing unit,  31  SNR determining unit,  32  speed width determining unit.