WO2021103715A1 - 一种机载高光谱成像激光雷达系统的辐射标定方法 - Google Patents

一种机载高光谱成像激光雷达系统的辐射标定方法 Download PDF

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WO2021103715A1
WO2021103715A1 PCT/CN2020/112130 CN2020112130W WO2021103715A1 WO 2021103715 A1 WO2021103715 A1 WO 2021103715A1 CN 2020112130 W CN2020112130 W CN 2020112130W WO 2021103715 A1 WO2021103715 A1 WO 2021103715A1
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radar system
channel
detector
hyperspectral imaging
lidar
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PCT/CN2020/112130
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French (fr)
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刘�东
钱立勇
吴德成
周晓军
钟刘军
魏巍
王文举
王英俭
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中国科学院合肥物质科学研究院
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Priority to US17/577,263 priority Critical patent/US11867846B2/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/88—Lidar systems specially adapted for specific applications
    • G01S17/89—Lidar systems specially adapted for specific applications for mapping or imaging
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/497—Means for monitoring or calibrating
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/481—Constructional features, e.g. arrangements of optical elements
    • G01S7/4814—Constructional features, e.g. arrangements of optical elements of transmitters alone
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/481—Constructional features, e.g. arrangements of optical elements
    • G01S7/4817—Constructional features, e.g. arrangements of optical elements relating to scanning

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  • the invention relates to the technical field of radar detection, in particular to a radiation calibration method of an airborne hyperspectral imaging lidar system.
  • Hyperspectral lidar is a brand-new earth observation technology. Compared with traditional single-wavelength lidar, an important measurement target of hyperspectral lidar is to obtain the back reflection spectrum information of the surface. However, before applying the spectral information measured by the hyperspectral lidar system to tasks such as ground object classification, the system needs to be calibrated. The calibration of the system is very important for the application of hyperspectral lidar data.
  • the existing radiation calibration methods are mainly aimed at ground-based multispectral imaging lidar systems, or passive hyperspectral imaging systems. There is no need for airborne hyperspectral imaging systems. Research on Radiation Calibration Method of Imaging Lidar.
  • the present invention provides a radiation calibration method of an airborne hyperspectral imaging lidar system.
  • the following technical solutions are specifically adopted:
  • An airborne hyperspectral imaging lidar system radiation calibration method includes the following steps:
  • the monochromator in the calibration system emits light signals with different spectral values, scans the radar system separately, and determines the bandwidth and center wavelength of each channel in the radar system;
  • the optical power P Ref ( ⁇ ) received by the detector target surface in the radar system in the experimental state using the diffuse reflection whiteboard as the ground object Target, use the ranging channel to measure the flying height of the radar system, and finally obtain the parameters in the echo signal power P R ( ⁇ ,z), and obtain the actual ground object reflection spectrum ⁇ G ( ⁇ ) measured by the hyperspectral imaging lidar system ).
  • step S1 is specifically as follows:
  • S11 Set up a spectrum calibration mechanism, which includes a monochromator and a beam splitter arranged in sequence.
  • the beam splitter splits two light paths, one light path directly enters the first detector, and the first detector Output data to the first data acquisition system.
  • the data processed in the first data acquisition system is used as reference data.
  • the other optical path of the beam splitter enters the airborne hyperspectral imaging lidar system to be calibrated through the scanning rotating mirror.
  • the second detector in the radar system outputs data to the second data acquisition system;
  • the monochromator emits light signals of multiple wavelengths covering the spectral range of the radar system with the set wavelength accuracy, and the first data acquisition system and the second data acquisition system obtain certain a first current corresponding to the signal value of the light signal of wavelength ⁇ n and I n a second current signal value I 'n, while the channel is determined when the optical signal detector in response to the first input;
  • the monochromator is also provided with a halogen light source and a power source for powering the halogen light source.
  • step S13 the specific steps for obtaining the center wavelength ⁇ NCW in step S13 are:
  • the synchronization signal measured current value I n a first and a second current signal value I 'n compared to afford the wavelength [lambda] n of the n-channel signal corresponding to the ratio of the current I' n / I n;
  • step S2 is as follows:
  • ⁇ is the center wavelength ⁇ NCW of each channel obtained during spectral calibration
  • R( ⁇ ) represents the coupling efficiency of the separated laser into the radar system
  • I Ref ( ⁇ ) is the current value output by each channel of the detector in the radar system, which can be measured
  • Q( ⁇ ) is the light efficiency reflected by the corner reflector
  • I Cor ( ⁇ ) is the current signal value corresponding to each channel of the laser pulse signal reflected by the corner reflector
  • both Q( ⁇ ) and I Cor ( ⁇ ) can be used For accurate measurement, divide the left and right equations of (2) and (3) to get:
  • the acquisition card of the radar system needs to collect the signal intensity three times, the light intensity separated from the laser I Ref ( ⁇ ), and the echo signal intensity I′( ⁇ ), the background noise of the system itself I BG ( ⁇ ); when the radar system has no laser pulse signal, the intensity information output by the detector in the receiving system is the background noise of the system; the echo signal intensity I'( ⁇ ) includes the real ground The object signal intensity I ( ⁇ ) and the background noise of the system I BG ( ⁇ ); that is, in the actual airborne flight experiment of the radar system, the actual output signal I′( ⁇ ) of the detectors in each channel of the receiving system is:
  • the present invention first determines the bandwidth and center wavelength of each channel in the radar system through the spectrum calibration system to achieve spectrum calibration, and then obtains the actual ground object reflection spectrum ⁇ G ( ⁇ ) measured by the hyperspectral imaging lidar system to achieve radiation Calibration. This method solves the radiation calibration of the airborne hyperspectral imaging lidar system.
  • the present invention determines the radiation calibration coefficient of each channel of the radar system according to the hyperspectral lidar equation, so that the hyperspectral lidar can obtain real-time ground features based on the intensity information output by the detectors of each channel during the airborne flight. Reflectance spectrum information, to achieve high-precision acquisition of terrain and ultra-fine classification of the surface.
  • the spectrum calibration adopts a beam splitter synchronous measurement method, which reduces the influence of the instability of the light source on the system radiation calibration, and improves the measurement accuracy and efficiency.
  • Figure 1 is a structural diagram of the airborne hyperspectral imaging lidar system and the spectral calibration system in the present invention.
  • Figure 2 is a schematic diagram of the bandwidth and center wavelength of the spectrum calibration.
  • An airborne hyperspectral imaging lidar system radiation calibration method includes the following steps:
  • the monochromator in the calibration system emits light signals with different spectral values, scans the radar system separately, and determines the bandwidth and center wavelength of each channel in the radar system;
  • the optical power P Ref ( ⁇ ) received by the detector target surface in the radar system in the experimental state using the diffuse reflection whiteboard as the ground target , Use the ranging channel to measure the flying height of the radar system, and finally obtain the parameters of the echo signal power P R ( ⁇ , z), and obtain the actual ground object reflection spectrum ⁇ G ( ⁇ ) measured by the hyperspectral imaging lidar system .
  • step S1 realizes spectrum calibration
  • step S2 realizes radiation calibration, which will be described separately below.
  • the spectrum calibration mechanism includes a power supply, a light source, a monochromator, and a beam splitter arranged in sequence.
  • the beam splitter splits two light paths, and one light path directly enters the second
  • the first detector outputs data to the first data acquisition system
  • the data processed in the first data acquisition system is used as reference data
  • the other optical path of the beam splitter enters the aircraft to be calibrated through the scanning mirror
  • the second detector in the radar system outputs data to the second data acquisition system.
  • the monochromator emits light signals of multiple wavelengths covering the spectral range of the radar system with the set wavelength accuracy, and the first data acquisition system and the second data acquisition system obtain certain a first current corresponding to the signal value of the light signal of wavelength ⁇ n and I n a second current signal value I 'n, while determining the channel response in the first detector when the optical signal input.
  • the wavelength accuracy of the monochromator is that the wavelength accuracy of the monochromator is 0.2nm, and the spectral resolution is also 0.2nm.
  • the spectral range of the radar system covers 400-900nm.
  • the monochromator starts from 400nm and outputs 400nm, 400.2nm, 400.4 in turn. nm...900nm, output optical signal every 0.2nm.
  • the method of synchronous measurement with beam splitter reduces the influence of the instability of the light source on the radiation calibration of the system, and improves the measurement accuracy and efficiency.
  • the light emitted by the monochromator is split by the beam splitter, and a part of it is directly incident on the target surface of the first detector.
  • the first data acquisition system directly measures the value of the first current signal, which is marked as I 400 , I 400.2 , and I 400.4 respectively. ...I 900 ;
  • the other part of the energy enters the radar system through the scanning rotating mirror, and is incident on the target surface of the corresponding detector through the grating beam splitting and subsequent coupling optical path system, and finally the measured current signal value is obtained in the second data acquisition system of the terminal.
  • step S13 The specific steps for obtaining the center wavelength ⁇ NCW in step S13 are:
  • the synchronization signal measured current value I n a first and a second current signal value I 'n compared to afford the wavelength [lambda] n of the n-channel signal corresponding to the ratio of the current I' n / I n;
  • ⁇ is the center wavelength ⁇ NCW of each channel obtained during spectral calibration
  • the optical power received from the target surface of the detector of the radar system is:
  • R( ⁇ ) is the coupling efficiency of the separated laser into the radar system.
  • the corner reflector is used to directly introduce part of the laser into the radar system, and the laser power received by the target surface of the detector in each channel of the radar system is:
  • Q( ⁇ ) is the light efficiency reflected by the corner reflector, which can be accurately measured.
  • a corner reflector is used to directly couple the laser pulse signal passing through the rotating mirror into the radar system.
  • I Cor ( ⁇ ) is the current signal value corresponding to each channel of the laser pulse signal reflected by the corner reflector. Therefore, compare (2) and (3):
  • the background noise of the system itself will also affect the reflection spectrum ⁇ G ( ⁇ ), and the noise of the system needs to be deducted.
  • the acquisition card of the radar system needs to collect the signal intensity three times, the light intensity separated from the laser I Ref ( ⁇ ), the echo signal intensity I′( ⁇ ) after reflection from ground objects, and the system itself The background noise I BG ( ⁇ ).
  • the intensity information output by the detector in the receiving system is the background noise of the system.
  • the actual echo signal intensity I'( ⁇ ) output by each channel of the receiving system including the real ground object signal intensity I( ⁇ ) and the background noise of the system I BG ( ⁇ ).
  • the actual output signal I′( ⁇ ) of the detectors in each channel of the receiving system is:
  • the standard diffuse reflection whiteboard is regarded as the ground object, and the laser pulse emitted by the radar system is incident on the standard diffuse reflection whiteboard.
  • the distance between the radar system and the whiteboard is Knowing, the effective clear aperture D R of the receiving telescope of the system can be obtained, and the calibration coefficient C Cal ( ⁇ ) of each channel of the system can be further obtained as:
  • the atmospheric transmittance is mainly affected by the extinction coefficient of atmospheric molecules and the aerosol extinction coefficient.
  • the extinction coefficient of atmospheric molecules can be calculated based on the standard atmosphere model and ground meteorological observatories, while the extinction coefficient of aerosols can be inverted from the full waveband signal of the hyperspectral lidar.
  • the molecular extinction coefficient of the near-surface atmosphere occupies a small weight in the atmospheric extinction coefficient, and the aerosol extinction coefficient is the main cause of the low-level extinction coefficient.
  • the flying height z is measured in real time by the ranging channel.
  • the actual output signal intensity I'( ⁇ ) of each channel detector, the light intensity separated from the laser I Ref ( ⁇ ), the background noise intensity I BG ( ⁇ ); at the same time, according to the actual The atmospheric transmittance T atm ( ⁇ ,0,z) is obtained by inversion of the echo signal , combined with the calibration coefficient C Cal ( ⁇ ) of each detection channel, and substituted into the formula (11) to obtain the central wavelength ⁇ and
  • the corresponding relationship of ⁇ G ( ⁇ ) covers the reflection spectrum information of ground objects in the wide spectrum range of hyperspectral lidar in real time.

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  • Computer Networks & Wireless Communication (AREA)
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Abstract

本发明涉及雷达探测的技术领域，尤其涉及一种机载高光谱成像激光雷达系统的辐射标定方法。包括如下步骤：S1、使用光谱标定系统，标定系统中的单色仪发出不同光谱值的光信号，对雷达系统分别进行扫描，确定雷达系统中各通道的带宽和中心波长；S2、根据高光谱激光雷达方程的回波信号功率P R(λ,z)，实验状态下雷达系统中探测器靶面接收到的光功率P Ref(λ)，利用漫反射白板作为地物目标、使用测距通道测量雷达系统的飞行高度，最终求取回波信号功率P R(λ,z)中各参数，获得高光谱成像激光雷达系统测出的实际地物反射谱β G(λ)。该方法解决了机载高光谱成像激光雷达系统的辐射标定。

Description

一种机载高光谱成像激光雷达系统的辐射标定方法 技术领域
本发明涉及雷达探测的技术领域，尤其涉及一种机载高光谱成像激光雷达系统的辐射标定方法。
背景技术
目前激光雷达辐射标定的主要技术手段中，主动式激光雷达测距可实现目标高分辨三维信息获取，被动高光谱成像可获得丰富光谱信息，但两种技术手段皆无法实现空间三维-光谱信息同时获取。为将两个技术优势结合在一起，国内外很多研究机构开展了很多尝试，取得了较好的进展，并基本认同了对地观测激光雷达“单波长-多光谱-高光谱”的发展方向。
高光谱激光雷达作为一种全新的对地观测技术，相较于传统的单波长激光雷达，高光谱激光雷达的一个重要测量目标就是获取地表的后向反射光谱信息。然而，将高光谱激光雷达系统测量获得的光谱信息应用于地物分类等任务前，需要对系统进行标定。系统的标定对于高光谱激光雷达数据的应用是非常重要的，现有的辐射标定方法主要针对地基的多光谱成像激光雷达系统，或者是被动的高光谱成像系统，尚不存在对机载高光谱成像激光雷达辐射标定方法的研究。
发明内容
为了解决机载高光谱成像激光雷达辐射标定，本发明提供一种机载高光谱成像激光雷达系统的辐射标定方法。具体采用以下技术方案：
一种机载高光谱成像激光雷达系统的辐射标定方法，包括如下步骤：
S1、使用光谱标定系统，标定系统中的单色仪发出不同光谱值的光信号，对雷达系统分别进行扫描，确定雷达系统中各通道的带宽和中心波长；
S2、根据高光谱激光雷达方程的回波信号功率P R(λ,z)，、实验状态下雷达系统中探测器靶面接收到的光功率P Ref(λ)，利用漫反射白板作为地物目标、使用测距通道测量雷达系统的飞行高度，最终求取回波信号功率P R(λ,z)中各参数， 获得高光谱成像激光雷达系统测出的实际地物反射谱β G(λ)。
具体地说，步骤S1具体如下：
S11、搭建光谱标定机构，所述光谱标定机构包括依次设置的单色仪、分束镜，所述分束镜分出两条光路，一条光路直接进入到第一探测器中，第一探测器输出数据到第一数据采集系统中，第一数据采集系统中处理的数据作为参考数据，分束镜的另一光路经过扫描转镜进入到待标定的机载高光谱成像激光雷达系统中，该雷达系统中的第二探测器输出数据到第二数据采集系统中；
S12、根据机载高光谱成像激光雷达系统的光谱范围，单色仪以设定波长精度发出覆盖雷达系统光谱范围的多个波长的光信号，第一数据采集系统和第二数据采集系统获得某个波长λ n的光信号对应的第一电流信号值I n和第二电流信号值I' n，同时确定该光信号输入时第一探测器中响应的通道；
S13、所有波长的光信号输入完毕后，确定雷达系统中各通道的光谱范围，然后通过电流比值的方法确定每个通道对应的中心波长λ NCW，并根据半高全宽Δλ n定义为第n通道的带宽，即第n通道内的光谱分辨率。
具体地说，单色仪之前还设置有卤灯光源和给卤灯光源供电的电源。
具体地说，步骤S13中求取中心波长λ NCW的具体步骤为：
S131、将同步测量第一电流信号值I n和第二电流信号值I' n相比较，得到第n通道内波长λ n对应的电流信号比值I' n/I n；
S132、求取理想状态下电流信号比值(I' n/I n) max对应波长为该通道的中心波长λ NCW。
具体地说，步骤S2具体如下：
(a)、根据雷达系统单发多收的工作模式，多通道高光谱激光雷达方程的回波信号功率P R(λ,z)，用公式表示如下：
Figure PCTCN2020112130-appb-000001
其中，λ为光谱标定时得到的各个通道的中心波长λ NCW，P R(λ,z)为雷达系统 中心波长为λ的通道接收到的回波信号光功率，单位：W；ρ 0是激光器输出平均光谱功率密度，单位：W/nm；η(λ)是激光器平均光谱功率密度归一化的功率密度光谱分布函数；Δλ是1个通道内对应的光谱带宽，单位：nm；β G(λ)是地物反射率；D R是接收望远镜的有效通光孔径，单位：m；z是激光雷达与被测地表的距离，单位：m，z通过测距通道实时进行测量；ε(λ)是激光雷达系统的光学效率；T atm(λ,0,z)是激光雷达与被测地表之间的大气在波长λ上的透过率，I(λ)表示真实的地物信号强度，R是对应通道内探测器的响应度，c(λ)＝ρ 0η(λ)Δλε(λ)，c(λ)为激光器出射的光脉冲能量入射到雷达系统中探测器功率强度；
(b)、确定在实验室状态下标定时，雷达系统中探测器靶面接收到的光功率P Ref(λ)，公式如下：
P Ref(λ)＝C(λ)R(λ)＝I Ref(λ)/R      (2)
其中R(λ)表示分离出的激光进入雷达系统的耦合效率，I Ref(λ)为雷达系统中探测器的各通道输出的电流值，可测量；
(c)、确定雷达系统中接收望远镜的有效通光孔径D R，将标准的漫反射白板当作地物目标，将雷达系统出射的激光脉冲入射到标准的漫反射白板上，雷达系统与白板之间的距离是已知的，即可得到系统接收望远镜的有效通光孔径D R,进一步得到系统各个通道的标定系数C Cal(λ)为：
Figure PCTCN2020112130-appb-000002
(d)、在雷达系统的探测区使用角反射器，获取雷达系统中各通道探测器靶面输出的激光器功率：
Figure PCTCN2020112130-appb-000003
Q(λ)是利用角反射器反射的光效率，I Cor(λ)是利用角反射器反射的激光脉冲信号在各个通道对应的电流信号值，Q(λ)和I Cor(λ)均可以准确测量，将(2)式和(3)式左右等式相除，得：
Figure PCTCN2020112130-appb-000004
即可求取R(λ)的数值；
(e)、在一个脉冲的重复频率内，雷达系统的采集卡需要采集三次信号强 度，从激光器中分离的光强度I Ref(λ)、经过地物反射后的回波信号强度I′(λ)、系统本身的背景噪声I BG(λ)；雷达系统在没有激光脉冲信号时，接收系统中探测器输出的强度信息为系统的背景噪声；回波信号强度I′(λ)包括真实的地物信号强度I(λ)和系统的背景噪声I BG(λ)；即雷达系统在实际的机载飞行实验时，接收系统各个通道的探测器实际输出的信号I′(λ)为：
I′(λ)＝I(λ)+I BG(λ)      (7)
最后，将公式(2)、(7)、(10)、R(λ)的数值代入公式(1)中，得到雷达系统在机载飞行过程中，实际的地物反射谱β G(λ)为：
Figure PCTCN2020112130-appb-000005
本发明的优点在于：
(1)本发明先通过光谱标定系统确定雷达系统中各通道的带宽和中心波长，实现光谱标定，然后获得高光谱成像激光雷达系统测出的实际地物反射谱β G(λ)，实现辐射标定，该方法解决了机载高光谱成像激光雷达系统的辐射标定。
(2)本发明根据高光谱激光雷达方程，确定雷达系统每一个通道的辐射标定系数，使该高光谱激光雷达在机载飞行过程中，根据各个通道探测器输出的强度信息，实时得到地物的反射光谱信息，实现地形高精度获取和地表超精细分类。
(3)本方法中光谱标定采用分束镜同步测量的方法，降低了光源的非稳定性对系统辐射标定的影响，提高测量精度和效率。
附图说明
图1为本发明中机载高光谱成像激光雷达系统与光谱标定系统配合时的结构图。
图2为光谱标定时带宽、中心波长示意图。
图中标注符号的含义如下：
1-电源 2-卤灯光源 3-单色仪 4-分束镜 51-第一探测器
6-雷达系统 71-第一数据系统 52-第二探测器 72-第二数据系统
具体实施方式
一种机载高光谱成像激光雷达系统的辐射标定方法，包括如下步骤：
S1、使用光谱标定系统，标定系统中的单色仪发出不同光谱值的光信号，对雷达系统分别进行扫描，确定雷达系统中各通道的带宽和中心波长；
S2、根据高光谱激光雷达方程的回波信号功率P R(λ,z)、实验状态下雷达系统中探测器靶面接收到的光功率P Ref(λ)，利用漫反射白板作为地物目标、使用测距通道测量雷达系统的飞行高度，最终求取回波信号功率P R(λ,z)中各参数，获得高光谱成像激光雷达系统测出的实际地物反射谱β G(λ)。
其中步骤S1实现了光谱标定，步骤S2实现了辐射标定，以下分别进行描述。
光谱标定
S11、搭建光谱标定机构；如图1所示，其中光谱标定机构包括依次设置的电源、光源、单色仪、分束镜，所述分束镜分出两条光路，一条光路直接进入到第一探测器中，第一探测器输出数据到第一数据采集系统中，第一数据采集系统中处理的数据作为参考数据，分束镜的另一光路经过扫描转镜进入到待标定的机载高光谱成像激光雷达系统中，该雷达系统中的第二探测器输出数据到第二数据采集系统中。
S12、根据机载高光谱成像激光雷达系统的光谱范围，单色仪以设定波长精度发出覆盖雷达系统光谱范围的多个波长的光信号，第一数据采集系统和第二数据采集系统获得某个波长λ n的光信号对应的第一电流信号值I n和第二电流信号值I' n，同时确定该光信号输入时第一探测器中响应的通道。
其中单色仪的波长精度为单色仪的波长精度为0.2nm，光谱分辨率也是0.2nm，雷达系统的光谱范围覆盖400-900nm，单色仪从400nm开始，依次输出400nm、400.2nm、400.4nm…900nm，每隔0.2nm输出一次光信号。采用分束镜同步测量的方法，降低了光源的非稳定性对系统辐射标定的影响，提高测量精度和效率。单色仪出射的光线经过分束镜分束，一部分直接入射到第一探测器的靶面，第一数据采集系统直接测量出第一电流信号值，分别记作I 400、I 400.2、I 400.4…I 900；另一部分能量通过扫描转镜进入雷达系统，经过光栅分光和后继 耦合光路系统入射到相应探测器的靶面，最终在终端的第二数据采集系统中得到测量的电流信号值，记作I′ 400、I′ 400.2、I′ 400.4…I′ 900。
S13、如图2所示，所有波长的光信号输入完毕后，确定雷达系统中各通道的光谱范围，然后通过电流比值的方法确定每个通道对应的中心波长λ NCW，并根据半高全宽Δλ n定义为第n通道的带宽，即第n通道内的光谱分辨率。
步骤S13中求取中心波长λ NCW的具体步骤为：
S131、将同步测量第一电流信号值I n和第二电流信号值I' n相比较，得到第n通道内波长λ n对应的电流信号比值I' n/I n；
S132、求取理想状态下电流信号比值(I' n/I n) max对应波长为该通道的中心波长λ NCW。
辐射标定
(a)、根据雷达系统单发多收的工作模式，多通道高光谱激光雷达方程的回波信号功率P R(λ,z)，用公式表示如下：
Figure PCTCN2020112130-appb-000006
其中，λ为光谱标定时得到的各个通道的中心波长λ NCW，P R(λ,z)为雷达系统中心波长为λ的通道接收到的回波信号光功率，单位：W；ρ 0是激光器输出平均光谱功率密度，单位：W/nm；η(λ)是激光器平均光谱功率密度归一化的功率密度光谱分布函数；Δλ是1个通道内对应的光谱带宽，单位：nm；β G(λ)是地物反射率；D R是接收望远镜的有效通光孔径，单位：m；z是激光雷达与被测地表的距离，单位：m，z通过测距通道实时进行测量；ε(λ)是激光雷达系统的光学效率；T atm(λ,0,z)是激光雷达与被测地表之间的大气在波长λ上的透过率，I(λ)表示真实的地物信号强度，R是对应通道内探测器的响应度，c(λ)＝ρ 0η(λ)Δλε(λ)，c(λ)为激光器出射的光脉冲能量入射到雷达系统中探测器功率强度；高光谱成像激光雷达辐射标定的最终目的，就是为了得到最真实的地物反射信息，即上式中的β G(λ),因此我们需要将β G(λ)从上述方程中求解出来。
实验室标定式时，从雷达系统的探测器靶面接收到的光功率为；
P Ref(λ)＝ρ 0η(λ)Δλε(λ)R(λ)      (2)
R(λ)是分离出的激光进入雷达系统的耦合效率。雷达系统中各个通道内的探测器输出的电流值记作I Ref(λ),I Ref(λ)可以表示为：I Ref＝P Ref(λ)R。
利用角反射器，将部分激光直接引入到雷达系统中，雷达系统各个通道探测器靶面接收到的激光功率为：
Figure PCTCN2020112130-appb-000007
Q(λ)是利用角反射器反射的光效率，可以准确测量。实验室标定时，利用角反射器将经过转镜的激光脉冲信号，直接耦合到雷达系统中。I Cor(λ)是利用角反射器反射的激光脉冲信号在各个通道对应的电流信号值。因此，将(2)式和(3)式相比较：
Figure PCTCN2020112130-appb-000008
得到从激光器中分离的部分激光能量进入雷达系统的耦合效率R(λ)，带入到(2)式中，得到：
Figure PCTCN2020112130-appb-000009
将(5)式代入到高光谱激光雷达方程，得到地物反射率谱线：
Figure PCTCN2020112130-appb-000010
在实际的地物反射谱线中，系统本身的背景噪声对反射谱线β G(λ)也会造成影响，需要对系统的噪声进行扣除。在一个脉冲的重复频率内，雷达系统的采集卡需要采集三次信号强度，从激光器中分离的光强度I Ref(λ)、经过地物反射后的回波信号强度I′(λ)、系统本身的背景噪声I BG(λ)。
雷达系统在没有激光脉冲信号时，接收系统中探测器输出的强度信息为系统的背景噪声。在一个激光脉冲周期内，接收系统各个通道实际输出的回波信 号强度I′(λ),包括真实的地物信号强度I(λ)和系统的背景噪声I BG(λ)。雷达系统在实际的机载飞行实验时，接收系统各个通道的探测器实际输出的信号I′(λ)为：
I′(λ)＝I(λ)+I BG(λ)    (7)
将(7)式代入到(6)式中，得到：
Figure PCTCN2020112130-appb-000011
实验室标定时，不考虑大气透过率对系统标定的影响。此时，将(8)式进一步简化，得到：
Figure PCTCN2020112130-appb-000012
为了确定接收望远镜的有效通光孔径D R，将标准的漫反射白板当作地物目标，将雷达系统出射的激光脉冲入射到标准的漫反射白板上，雷达系统与白板之间的距离是已知的，即可得到系统接收望远镜的有效通光孔径D R,进一步得到系统各个通道的标定系数C Cal(λ)为：
Figure PCTCN2020112130-appb-000013
将(10)式代入到(8)式中，得到系统在机载飞行的过程中，实际的地物反射谱β G(λ)为：
Figure PCTCN2020112130-appb-000014
雷达系统在实际机载飞行的工作过程中，大气透过率主要受到大气分子消光系数和气溶胶消光系数共同影响。机载飞行过程中，大气分子消光系数可以基于标准大气模型和地面气象观测站计算得到，而气溶胶的消光系数可以通过高光谱激光雷达全波形波段的信号进行反演。实际在计算可见光波段内大气消光系数时，近地层大气分子消光系数在大气消光系数中占据的权重很小，气溶胶的消光系数是引起低层消光系数的主要原因。
机载高光谱激光雷达在空中飞行工作时，飞行高度z由测距通道实时测量得到。依据一个激光脉冲周期时间内，各个通道探测器实际输出的信号强度I′(λ)、 从激光器中分离的光强度I Ref(λ)、背景噪声强度I BG(λ)；同时，依据实际的回波信号反演得到大气透过率T atm(λ,0,z)，结合每一个探测通道的标定系数C Cal(λ)，代入到公式(11)即可得到各个通道的中心波长λ和β G(λ)的对应关系，实时覆盖高光谱激光雷达宽谱段范围内的地物反射光谱信息。
以上仅为本发明创造的较佳实施例而已，并不用以限制本发明创造，凡在本发明创造的精神和原则之内所作的任何修改、等同替换和改进等，均应包含在本发明创造的保护范围之内。

Claims (5)

  1. 一种机载高光谱成像激光雷达系统的辐射标定方法，其特征在于，包括如下步骤：
    S1、使用光谱标定系统，标定系统中的单色仪发出不同光谱值的光信号，对雷达系统分别进行扫描，确定雷达系统中各通道的带宽和中心波长；
    S2、根据高光谱激光雷达方程的回波信号功率P R(λ,z)、实验状态下雷达系统中探测器靶面接收到的光功率P Re f(λ)，利用漫反射白板作为地物目标、使用测距通道测量雷达系统的飞行高度，最终求取回波信号功率P R(λ,z)中各参数，获得高光谱成像激光雷达系统测出的实际地物反射谱β G(λ)。
  2. 根据权利要求1所述的一种机载高光谱成像激光雷达系统的辐射标定方法，其特征在于，步骤S1具体如下：
    S11、搭建光谱标定机构，所述光谱标定机构包括依次设置的单色仪、分束镜，所述分束镜分出两条光路，一条光路直接进入到第一探测器中，第一探测器输出数据到第一数据采集系统中，第一数据采集系统中处理的数据作为参考数据，分束镜的另一光路经过扫描转镜进入到待标定的机载高光谱成像激光雷达系统中，该雷达系统中的第二探测器输出数据到第二数据采集系统中；
    S12、根据机载高光谱成像激光雷达系统的光谱范围，单色仪以设定波长精度发出覆盖雷达系统光谱范围的多个波长的光信号，第一数据采集系统和第二数据采集系统获得某个波长λ n的光信号对应的第一电流信号值I n和第二电流信号值I n'，同时确定该光信号输入时第一探测器中响应的通道；
    S13、所有波长的光信号输入完毕后，确定雷达系统中各通道的光谱范围，然后通过电流比值的方法确定每个通道对应的中心波长λ NCW，并根据半高全宽Δλ n定义为第n通道的带宽，即第n通道内的光谱分辨率。
  3. 根据权利要求2所述的一种机载高光谱成像激光雷达系统的辐射标定方法，其特征在于，单色仪之前还设置有卤灯光源和给卤灯光源供电的电源。
  4. 根据权利要求3所述的一种机载高光谱成像激光雷达系统的辐射标定方法，其特征在于，步骤S13中求取中心波长λ NCW的具体步骤为：
    S131、将同步测量第一电流信号值I n和第二电流信号值I′ n相比较，得到第n 通道内波长λ n对应的电流信号比值I n′/I n；
    S132、求取理想状态下电流信号比值(I' n/I n) max对应波长为该通道的中心波长λ NCW。
  5. 根据权利要求1所述的一种机载高光谱成像激光雷达系统的辐射标定方法，其特征在于，步骤S2具体如下：
    (a)、根据雷达系统单发多收的工作模式，多通道高光谱激光雷达方程的回波信号功率P R(λ,z)，用公式表示如下：
    Figure PCTCN2020112130-appb-100001
    其中，λ为光谱标定时得到的各个通道的中心波长λ NCW，P R(λ,z)为雷达系统中心波长为λ的通道接收到的回波信号光功率，单位：W；ρ 0是激光器输出平均光谱功率密度，单位：W/nm；η(λ)是激光器平均光谱功率密度归一化的功率密度光谱分布函数； Δλ是1个通道内对应的光谱带宽，单位：nm；β G(λ)是地物反射率；D R是接收望远镜的有效通光孔径，单位：m；z是激光雷达与被测地表的距离，单位：m，z通过测距通道实时进行测量；ε(λ)是激光雷达系统的光学效率；T atm(λ,0,z)是激光雷达与被测地表之间的大气在波长λ上的透过率，I(λ)表示真实的地物信号强度，R是对应通道内探测器的响应度，c(λ)＝ρ 0η(λ)Δλε(λ)，c(λ)为激光器出射的光脉冲能量入射到雷达系统中探测器功率强度；
    (b)、确定在实验室状态下标定时，雷达系统中探测器靶面接收到的光功率P Re f(λ)，公式如下：
    P Re f(λ)＝C(λ)R(λ)＝I Re f(λ)/R  (2)
    其中R(λ)表示分离出的激光进入雷达系统的耦合效率，I Re f(λ)为雷达系统中探测器的各通道输出的电流值，可测量；
    (c)、确定雷达系统中接收望远镜的有效通光孔径D R，将标准的漫反射白板当作地物目标，将雷达系统出射的激光脉冲入射到标准的漫反射白板上，雷达系统与白板之间的距离是已知的，即可得到系统接收望远镜的有效通光孔径 D R,进一步得到系统各个通道的标定系数C Cal(λ)为：
    Figure PCTCN2020112130-appb-100002
    (d)、在雷达系统的探测区使用角反射器，获取雷达系统中各通道探测器靶面输出的激光器功率：
    Figure PCTCN2020112130-appb-100003
    Q(λ)是利用角反射器反射的光效率，I Cor(λ)是利用角反射器反射的激光脉冲信号在各个通道对应的电流信号值，Q(λ)和I Cor(λ)均可以准确测量，将(2)式和(3)式左右等式相除，得：
    Figure PCTCN2020112130-appb-100004
    即可求取R(λ)的数值；
    (e)、在一个脉冲的重复频率内，雷达系统的采集卡需要采集三次信号强度，从激光器中分离的光强度I Re f(λ)、经过地物反射后的回波信号强度I′(λ)、系统本身的背景噪声I BG(λ)；雷达系统在没有激光脉冲信号时，接收系统中探测器输出的强度信息为系统的背景噪声；回波信号强度I′(λ)包括真实的地物信号强度I(λ)和系统的背景噪声I BG(λ)；即雷达系统在实际的机载飞行实验时，接收系统各个通道的探测器实际输出的信号I′(λ)为：
    I′(λ)＝I(λ)+I BG(λ)  (7)
    最后，将公式(2)、(7)、(10)、R(λ)的数值代入公式(1)中，得到雷达系统在机载飞行过程中，实际的地物反射谱β G(λ)为：
    Figure PCTCN2020112130-appb-100005
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