1. TanSat/CAPI Calibration and validation
Wu Ronghua
Zhang Peng, Yang Zhongdong, Gu Songyan, Sun Ling, Xu Na
Yan Changxiang, Shao Jianbing,
Hu Yuan, Wang Yacheng, Zhang Ping
National Satellite Meteorological Centre
下午好
下面介绍CAPI的定标和检验情况

2. content General description of CAPI Prelaunch calibration
Solar diffuser calibration
Site validation and data sample
CAPI的基本情况
发射前定标结果
漫射板
漫射板定标结果
图像个例

3. General description of CAPI
TanSat is Chinese Global Carbon Dioxide Monitoring Scientific Experimental Satellite, launched in
CAPI is one of the satellite's payloads, which is the first Cloud and Aerosol Polarization Multispectral Imager on satellite in China.
CAPI provides the necessary information about cloud and aerosol observation for the inversion of CO2 and is useful to conduct research on polarization remote sensing.
Parameters
Specification
Swath
375km±10km
Sampling pixel of each scan
IFOV
Calibration method
Solar diffuser, Lunar
Inter-band Co-registration
<0.25 pixel
Radiometric Calibration accuracy
<5%
Detector consistency within one band
<1%
Band
Center WL
(µm)
WL Range
SNR
Typical radiance (W/m2µmsr)
Polarizing angle
IFOV
(m) 1
0.38
260
28.0 NP
250 2
0.67
160
22.0 0° 3
60° 4
120° 5
0.87
400
25.0 6
1.375
180
6.0
1000 7
1.64
110
7.3 8 9

4. General description of CAPI
Solar diffuser
Instrument coordination
CAPI are divided into VNIR components (0.38μm, 0.67μm, 0.87μm) and SWIR components (1.375μm, 1.64μm band)
Solar diffuser in front of lens, and extended
Incidence angle is 50 degree, and observation angle is 40 degree.

5. Prelaunch calibration
In the simplest case, the total energy changes as polarized light (I, Q, U) passes through the polarizer
In practical application, each device may polarized or depolarized the light
理想状态下，偏振光经过起偏器后，总能量变化如公式
实际光学系统，各器件都会产生或消减偏振，实际公式如下 P1 P2 P3
Theda，Phy，alpha=Polarizer angle，eta=Polarization extinction ratio，epsilon=Polarizer rate
在实际工程应用中，光学成像系统的各视场不同菲涅耳损失、非完美透镜、分光棱镜、滤光片以及偏振片产生的偏振效应都影响测量结果的精度，需要消除偏振相机自身的偏振效应对测量结果影响。
In practical application, each field is different, Fresnel loss of the optical imaging system of non polarization effect, perfect lens, beam splitter, filters and polarizing plate produced will influence the accuracy of measurement results, to eliminate the polarization effect of polarization on the measurement results of the camera's own influence.

6. Prelaunch calibration
spherical integrating source light is non-polarized: Q=0, U=0
Central field of view, device polarized effect can be ignored
积分球为非偏振光，非偏振光入射，视场中心像元，辐射定标公式简化

7. Prelaunch calibration
Pre-launce calibration results
radiation response linearity is good
Calibration for each detector
Band
Slope
Offset
Uncertainty
（%）
correlation coefficient 1
-0.59
0.78 2
-1.60
0.96 3
-0.56
0.60 4
-0.55
1.15 5
-0.40
0.63 6
0.08
0.14 7
0.48
0.17 8
0.24 9
0.52
0.22

8. Solar diffuser calibration
Measuring the reflectivity of diffuse plate by observing geometry under the state of on orbit calibration mode
Measure the spatial distribution of the reflectivity of each band along the cross orbit direction

9. Solar diffuser calibration
Dark current correction
Solar diffuser uniformity correction
Solar zenith correction
Distance correction

10. Solar diffuser calibration
For VIS bands, large difference between pre-launch coefficient and solar diffuser results
For NIR bands, difference is small
图片来自xls表格（定标系数对比）

11. Solar diffuser calibration
Up to , 6solar calibration observations are recorded.
the calibration coefficient is stable, and the relative changes of each channel are less than 0.4%.
Uncertainty(%)
Band 1
0.36
Band 2
0.14
Band 3
0.15
Band 4
0.19
Band 5
0.16
Band 6
0.18
Band 7
Band 8
0.23
Band 9

12. Validation Principal plane Nadir Mode Glint Mode Target Mode
Dunhuang sit validation under the target mode
Principal plane

13. Validation and Sample image
CAPI
敦煌场 比值
380nm
0.174
0.196
0.891
670-P
0.175
0.193
0.907
870nm
0.180
0.199
0.904
1375nm
0.022
0.073
0.314
1640-P
0.227
0.259
0.877
（1）compared with the result of Dunhuang sit, the reflectivity of CAPI is about 10%
（2） it is same that the direction of deviation for each band

14. Validation and Sample image
Example of a CAPI image and degree of polarization (670nm)
RGB picture（R=1640,G=870,B=670)
Distribution of polarization A B C D
Water(A)
Land(B)
Cloud(C)
Cloud(D)
定标后的偏振度，分布是合理的
可以区分不同的地物：水、陆、云

15. Summary
CAPI is a polarization imager. The information of clouds and aerosols is obtained by multichannel polarization bands.
CAPI is calibrated before launching, and its linearity is good.
CAPI using a solar diffuser to achieve on board radiation calibration
There is a difference between the calibration result of the solar diffuser and the pre-launch: the relative differences between the bands 1, 2, 7, 8 and 9 are less than 7%, and the other bands are too large.
The result of the solar diffuse plate calibration is very small during the period of to , and the maximum is not more than 0.4%
Problem:
Compare the Dunhuang sit reflectance, there is about 10% different for every band.

16. The End!
Thank you for your time!
Details