Patent ID: 11867846
Assignee: HEFEI INSTITUTES OF PHYSICAL SCIENCE, CHINESE ACADEMY OF SCIENCES
Field: Measurement (Instruments)
Classification: CPC G | IPC G

Claim 3:
4. The method for radiation calibration of airborne hyperspectral imaging LiDAR system according to claim 1, wherein the step S2 is specified as follows:
(a). According to the operating mode of “single emitting and multiple receiving” of the radar system, the return signal power PR (λ, z) in the multi-channel hyperspectral LiDAR equation can be expressed as follows:, P
          R
         
         (
         
          λ
          ,
          z
         
         )
        
        =
        
         
          Σ
          0
         
         ⁢
         
          η
          ⁡
          (
          λ
          )
         
         ⁢
         
          
           Δλβ
           G
          
          (
          λ
          )
         
         ⁢
         
          
           D
           R
           2
          
          
           8
           ⁢
           
            z
            2
           
          
         
         ⁢
         
          
           
            ε
            ⁡
            (
            λ
            )
           
           [
           
            
             T
             atm
            
            (
            
             λ
             ,
             O
             ,
             z
            
            )
           
           ]
          
          2
         
        
       
      
     
     
      
       
        =
        
         
          c
          ⁡
          (
          λ
          )
         
         ⁢
         
          
           β
           G
          
          (
          λ
          )
         
         ⁢
         
          
           
            
             D
             R
             2
            
            
             8
             ⁢
             
              z
              2
             
            
           
           [
           
            
             T
             atm
            
            (
            
             λ
             ,
             O
             ,
             z
            
            )
           
           ]
          
          
           2
          
         
        
       
      
     
     
      
       
        =
        
         
          I
          ⁡
          (
          λ
          )
         
         /
         R
        
       
      
     
    
   
   
    
     (
     1
     )
    
   
  
 

where, λ is the central wavelength λNCW of each channel obtained during spectrum calibration, PR (λ, z) is the optical power of the return signal received by the LiDAR system channel with the central wavelength of λ, in the unit of W; ρ0 is the average spectral power density output by the laser, in the unit of W/nm; η (λ) is a normalized function of the power density spectrum of the average spectral power density of the laser; Δλ is the corresponding spectral bandwidth in one channel, in the unit of nm; βG (λ) is the reflectivity of the ground object; DR is the effective clear aperture of the receiving telescope, in the unit of m; z is the distance between the LiDAR and the measured ground surface, in the unit of m, z can be measured in real time through the ranging channel; ε (λ) is the optical efficiency of the LiDAR system; Tatm (λ, O, z) is the transmittance of the atmosphere between the LiDAR and the measured ground surface at wavelength of λ; I (λ) is the actual signal intensity of the ground object; R is the responsivity of the detector in the corresponding channel; c(λ)=ρ0η(λ)Δλε(λ), c(λ) is the power intensity of the light pulse energy emitted by the laser into the detector in the radar system;
(b). During calibration under experimental conditions, the optical power PRef (λ) received by the target surface of the detector in the radar system can be obtained as the following formula:

PRef(λ)=C(λ)R(λ)=IRef(λ)/R  (2)

where, R (λ) is the coupling efficiency of the separated laser light entering the radar system, and IRef (λ) is the current output by each channel of the detector in the radar system that can be detected;
(c). The effective clear aperture DR of the receiving telescope is obtained in the radar system; a standard white diffuse reflection board is taken as the ground object target, onto which a laser pulse is emitted by the radar system; the distance between the radar system and the white board is known, so it is possible to obtain the effective clear aperture DR of the receiving telescope in the system and further to obtain the calibration parameters Ccal (λ) of each channel in the system:, C
       Cal
      
      (
      λ
      )
     
     =
     
      
       R
       ⁡
       (
       λ
       )
      
      
       D
       R
       2
      
     
    
   
   
    
     (
     10
     )
    
   
  
 

(d). A corner reflector is adopted in the detection area of the radar system to obtain the laser power output by the target surface of the detector in each channel of the radar system:, P
       Cor
      
      (
      λ
      )
     
     =
     
      
       
        ρ
        0
       
       ⁢
       
        η
        ⁡
        (
        λ
        )
       
       ⁢
       
        Δλε
        ⁡
        (
        λ
        )
       
       ⁢
       
        Q
        ⁡
        (
        λ
        )
       
      
      =
      
       
        
         I
         Cor
        
        (
        λ
        )
       
       R
      
     
    
   
   
    
     (
     3
     )
    
   
  
 

Q (λ) is the optical efficiency reflected by the corner reflector; ICor (λ) is the current signal value of the laser pulse signal reflected by the corner reflector in each channel; Q (λ) and ICor (λ) can be accurately detected; Formula (2) is divided by Formula (3) to obtain:, P
        Ref
       
       (
       λ
       )
      
      
       
        P
        Cor
       
       (
       λ
       )
      
     
     =
     
      
       
        R
        ⁡
        (
        λ
        )
       
       
        Q
        ⁡
        (
        λ
        )
       
      
      =
      
       
        
         I
         Ref
        
        (
        λ
        )
       
       
        
         I
         Cor
        
        (
        λ
        )
       
      
     
    
   
   
    
     (
     4
     )
    
   
  
 

and R (λ) is obtained;
(e). In the repetition frequency of one pulse, the capture card of the radar system needs to capture the signal intensity for three times: the intensity of light separated from the laser IRef (λ), the intensity of return signal reflected by the ground object I′ (λ) and the background noise of the system IBG (λ); when there is no laser pulse signal in the radar system, the intensity data output by the detector in the radar system is the background noise of the system; the intensity of the return signal I′ (λ) includes the actual signal intensity of the ground object I (λ) and the background noise of the system IBG (λ); that is, when the radar system is operating an airborne flying experiment, the signal I′ (λ) output by the detector in each channel of the receiving system is:

I′(λ)=I(λ)+IBG(λ)  (7)

finally, the actual reflection spectrum of the ground object βG (λ) during airborne flying process of the radar system is obtained by substituting Formulas (2), (7) and (10) and R (λ) into Formula (1):, β
       G
      
      (
      λ
      )
     
     =
     
      
       
        
         
          [
          
           
            
             I
             ′
            
            (
            λ
            )
           
           -
           
            
             I
             BG
            
            (
            λ
            )
           
          
          ]
         
         ·
         8
        
        ⁢
        
         
          z
          2
         
         ·
         
          
           C
           Cal
          
          (
          λ
          )
         
        
       
       
        
         
          [
          
           
            T
            atm
           
           (
           
            λ
            ,
            0
            ,
            z
           
           )
          
          ]
         
         2
        
        ⁢
        
         
          I
          Ref
         
         (
         λ
         )
        
       
      
      .
     
    
   
   
    
     (
     11
     )