GCOM-C/SGLILunar Calibration Evaluation
Taichiro Hashiguchia, Tomoyuki Urabeb ,Shigemasa Andob,
Yoshihiko Okamurab, Kazuhiro Tanakab, Arata Okuyamac
aRemote Sensing Technology Center of Japan (RESTEC)bJapan Aerospace Exploration Agency (JAXA)
cJapan Meteorological Agency (JMA)

This work is based on a contract with JAXA (JX-PSPC-523224), andcomplies with GIRO usage policy, “Global Satellite Inter-Calibration
System, GIRO and GSICS Lunar Observation Dataset Usage Policy”,
Version 1.0, May 2015, GSICS-RD005.
GCOM-C/SGLI Lunar Calibration Evaluation @ Lunar Calibration Workshop 2020 (Online) : Nov. 17, 2020

1

Contents
1. Introduction
GCOM-C/SGLI overview
SGLI specification

2. SGLI Lunar calibrationSGLI Lunar calibration operation
Analysis method of SGLI lunar calibration
Time-series trend results
Inter-comparison of the onboard calibration

3. Derivation of Calibration CoefficientCorrection of phase angle dependenceValidation using AHI

4. Conclusion

2

GCOM-C overview+Ydeepspace+X
flightdirection+ Z earth
SGLI
VNRIRSSRU
ELU

Second-generation Global Imager
Visible and Near Infrared Radiometer
Infrared Scanning Radiometer
Scanning Radiometer Unit
Electronic Unit

 GCOM-C was successfully launched on December 23, 2017 and is continuing
regular observation operations.
 The various GCOM-C scientific products have been released to public since
December, 2018. [ Data access -->https://gportal.jaxa.jp/ ]3

Second-generation Global Imager (SGLI) Overview
VNR-NP

Non Polarized
Observation Telescopes(24deg FOV x 3)

Solar
Diffuser

EarthView Window

Polarized
Observation
Telescopes(55deg FOVx 2)

Sun Cal.
Window

VNR-PLAbout
1.5m

About
1.4mAbout
1.5m

Visible and Near Infrared Radiometer(SGLI-VNR)
 VNR-NP consists of three 24-degree-FOV
telescopes configured in cross track direction
to realize the wide FOV (70 degrees).
 VNR-PL has the tilting mechanism to observearound ±45 degrees in along track direction.

About
0.7m

Deep
Space
Window

Infrared Scanning Radiometer(SGLI-IRS) The combination of the 45 degrees tilting
scanning mirror and Ritchey-Chretien typetelescope realize the wide 80 degrees FOVobservation swath.
4SGLI Specification
 The SGLI features are 250m (VNR-NP & SW3) and 250/500m (TIR) spatial resolution
and polarization/along-track slant view channels (VNR-PL), which will improve land,250m over the Land or coastal
coastal, and aerosol observations.
area, and 1km over offshore

GCOM-C SGLI characteristics
Orbit
Mission LifeScan

Sun-synchronous(descending local time: 10:30)
Altitude 798km, Inclination 98.6deg
5 years
Push-broom electric scan (VNR)
Wisk-broom mechanical scan (IRS)

1150km cross track (VNR-NP & VNR-PL)
1400km cross track (IRS-SWI & IRS-TIR)VNR-NP
Digitalization 12bitMulti-angle
Polarization 3 polarization angles for VNR-PL
Along track
Nadir for VNR-NP, IRS-SWI and IRS-TIR, obs. for673.5nm anddirection+45 deg and -45 deg for VNR-PL
868.5nm
VNR-NP, VNR-PL: Solar diffuser, LED,Lunar cal. maneuvers, and dark
VNR-PL
current by masked pixels andOn-board
nighttime obs.
calibration
IRS-SWI: Solar diffuser, LED, Lunar, and IRS-SWI
dark current by deep space window
IRS-TIR: Black body and dark current byIRS-TIR
deep space window
Scan width

SGLI channelsLstd
Lmax
SNR at Lstd
IFOV
VNR-NP, VNR-PL,VNR-NP, VNR-PL,VNR-NP, VNR-PL,
CH
IRS-SWIIRS-SWI: nm
IRS-SWI : SNR
m:W/m2/sr/m
IRS-TIR: m
IRS-TIR: NET
IRS-TIR: Kelvin
VN13801060210250
250
VN2412
1075
250
400
250
VN34431064400
300
250
VN44901053120400
250
VN55302041350250
250
VN6565203390400
250
VN7 673.5202362400
250
VN8 673.5
2025210250
250
VN9763
12403501200
250/1000
VN10 868.520830400
250
VN11 868.52030300200
250
P1673.52025250250
1000
P2868.52030300
250
1000
SW1 1050
2057248500
1000
SW2 1380
208103150
1000
SW3 1630
200
35057250
SW4 221050
1.920211
1000
T1
10.8
0.7
300
340
0.2
250/1000
T2
12.0
0.7
300
340
0.2
250/1000


TIR: 500m resolution is also used

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Lunar Calibration Operation
 The lunar observation images are captured by maneuvering GCOM-C attitude around thepitch axis.
 Pitch maneuver rate is 0.15 degree/second with high stability to obtain precise
oversampled lunar image in along-track direction.
 The phase angle(Sun - Moon - Satellite ) is around +7+/-3degree.
• Lunar calibration concept is similar to SeaWiFS.

EarthMoon
Lunar
Observation

Maneuver

VN11

SW036

Analysis Method (VNR)
 Analysis method of SGLI lunar calibration data
[Case of VNR-NP/PL]
NP:500 line
PL:125line

Deep Space Removes dark noise using averaging deep space data per pixel.
 Converts to radiance image , using radiometric parameter.
 To compare with lunar irradiance model, the radiance is converted tointegrated lunar irradiance using following equation.

NP:400 line
PL:100line,

Integrated Area*
NP:160 pixel
PL: 30pixel,

𝐼 : Lunar irradiance (k=ch1~11)
𝑁 : Total number of pixel𝛺 , : Solid angle per pixel include oversampling and sin 𝜃 effect𝜃 : Angle between satellite-moon vector and satellite pitch axis

Deep Space
NP:500 line
PL:125line*Integrated area is defined taking into account stray light.

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Analysis Method (IRS) Analysis method of SGLI lunar calibration data
[Case of IRS-SWIR]

IRS discretely captures the moon because of whisk-broom type radiometer. Therefore, in order toobtain integrated lunar irradiance, it is necessary to round the lunar image.
SW03:400 line
SW01,02,04:100line

ResamplingConverts to radiance image 𝐿 , using radiometric parameter.
The observed pixels of each detector are projected on the AT-CT planein consideration of line-of-sight vector and the pitch maneuver.
 Converts to irradiance image 𝐼 , using the solid angle for each pixel.
 Reconstructs the lunar irradiance image from the weighted average
according to the a field of view of each detector in the resampling grid.
The lunar integrated irradiance 𝐼
is calculated.

IntegratedArea
SW03:90 pixel
SW01,02,04:25 pixel

IRS-SW03

Red line: Resampling grid※Colors show the observed pixel of each scan

Green : Weighted average pixel

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Time-series trend results(VNR)
CH
VN1 VN2 VN3 VN4 VN5 VN6 VN7 VN8 VN9 VN10 VN11WL [nm] 380 412 443 490 530 565 673.5 673.5 763 868.5 868.5

 SGLI/GIRO Time-series trend SGLI/GIRO Time-series trend (Normalized 2018/2/1)

VN06

VN019

Time-series trend results(SWIR)CHWL [nm]

 SGLI/GIRO Time-series trend

SW11050

SW21380

SW31630

SW42210

 SGLI/GIRO Time-series trend (Normalized 2018/2/1)

 These trends indicate fluctuations synchronized with thebehavior of the lunar phase angle
10Inter-comparison of the onboard calibration
 Inter-comparison of the onboard calibration
 Lunar calibration
 Solar calibration (every 8 days)
 Internal light source calibration (every 8 days) These trends are normalized with the first lunar calibration date (February 1, 2018) for comparison.
 In VNIR and SWIR bands, the inter-comparisons between in orbit calibrations are consistent within
1.0%, and these results suggest that the lunar calibration evaluation acquires the degradation
characteristics of the sensor in detail.
11

Derivation of Calibration Coefficient Derivation of calibration coefficient The SGLI/GIRO trends have a feature of phase angle dependence.
 For the construction of the simple study model, the conditions for evaluation are limited tothe following: The roll offset angle is 0° or 1°
 The phase angle range is 5.0°to 11.0°
 Using the simple model shown below, the sensor responsivity degradation and phase angle
dependence were separated by multiple regression analysis.
,𝑓: the SGLI/GIRO trend
𝑔 : the phase angle
𝑑 : the days since launch𝑛 : the number of the lunar calibration𝑎 : the phase angle dependent coefficient𝑏 : the sensor degradation coefficient𝑐 : the constant Evaluation period

1st period

 Confirmation that these characteristics do not dependon the evaluation period.
I.
II.

2018/2-2020/1
2018/10-2020/9

2nd period

12Derivation of Calibration Coefficient Sensor responsivity degradation

 Phase angle dependence
Wavelength [nm]673.52210

 The phase angle dependence at wavelengths longer than VN07(673.5 nm) is statistically significant.
 The dependency increases as the wavelength increases. In particular, the dependence is strong in the SWIR bands.
BandVN01
VN02
VN03
VN04
VN05
VN06
VN07
VN08
VN09
VN10
VN11
SW01
SW02
SW03
SW04

R2

RMSE

0.954
0.982
0.975
0.989
0.973
0.970
0.591
0.625
0.459
0.538
0.550
0.944
0.961
0.917
0.913

0.00310.00180.00200.00110.00160.00120.00160.00150.00170.00210.00200.00250.00310.00190.0027

1st evaluation period
Sensor degradation
dependence [1/day]slope
p-value-6.78E-05
2.54E-12-6.64E-05
3.19E-16-6.24E-05
3.31E-15-5.51E-05
2.23E-18-4.63E-05
7.48E-15-3.29E-05
2.29E-14-5.32E-06
2.05E-02-4.61E-06
3.04E-02
1.67E-07
9.42E-01
1.60E-06
5.34E-01
1.70E-06
4.81E-01-4.54E-05
7.26E-11-6.40E-05
1.12E-11-5.24E-06
6.18E-02
2.80E-06
4.67E-01

Phase angle dependence[1/degree]
slope
p-value
1.08E-03
7.67E-02
5.28E-04
1.32E-01-2.54E-04
4.89E-01-1.82E-04
3.96E-01-4.65E-05
8.70E-01-1.83E-05
9.32E-01
8.58E-04
8.84E-03
9.55E-04
2.64E-03
1.20E-03
1.39E-03
1.63E-03
2.18E-04
1.56E-03
1.65E-04
1.75E-03
1.55E-03
3.62E-03
8.90E-06
4.30E-03
7.53E-10
6.76E-03
1.68E-10

R2RMSE

0.962
0.962
0.985
0.991
0.9820.971
0.656
0.509
0.548
0.615
0.601
0.925
0.935
0.883
0.884

0.00280.00250.00140.00100.00130.00110.00150.00220.00200.00200.00200.00240.00340.00230.0034

2nd evaluation period
Sensor degradation
dependence [1/day]slope
p-value-7.30E-05
4.33E-13-6.59E-05
3.98E-13-5.91E-05
2.23E-16-5.38E-05
2.86E-18-4.86E-05
9.43E-16-3.42E-05
4.31E-14-9.96E-06
7.96E-05-1.07E-05
1.13E-03-7.76E-06
5.54E-03-8.84E-06
2.65E-03-4.85E-06
6.32E-02-3.89E-05
1.05E-10-5.55E-05
1.13E-10-1.05E-05
1.24E-03-1.10E-05
1.35E-02

Phase angle dependence[1/degree]
slopep-value
5.46E-04
2.93E-01-7.39E-05
8.72E-01-7.49E-05
7.69E-01-1.83E-04
3.06E-01-1.81E-04
4.35E-01-1.54E-04
4.60E-01
6.06E-04
4.24E-02
6.48E-04
1.18E-01
1.12E-03
5.58E-03
1.34E-03
1.85E-03
1.65E-03
2.48E-04
1.73E-03
6.68E-04
3.97E-03
4.08E-06
4.25E-03
6.89E-09
6.61E-03
3.01E-0913

Derivation of Calibration Coefficient SGLI/GIRO residual from multiple regression equation

 The residuals of the regression equation tend to annual variation.
• This may suggest the effect of libration.

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Validation using AHI
 Inter-comparison of SGLI and Himawari8-AHI using GIRO
 Inter-comparison of AHI/GIRO and SGLI/GIRO of almost the same observation
conditions (obs. time and geometry) The following differences are corrected by GIRO••Geometric conditions (phase angle, libration, sun-moon / moon-satellite distance)AHI
B01 B02 B03 B04 B05 B06
Spectral response function of AHI and SGLI
Wavelength [nm]470 510 640 860 1600 2300

 Comparison of SGLI/GIRO and AHI/GIRO

SGLI
VN04 VN05 VN08 VN11 SW03 SW04
Wavelength [nm]490 530 673.5 868.5 1630 2210
Ratio SGLI to AHI of lunar irradiance* 1.011 0.983 1.025 1.014 1.037 0.927

 2 cases with almost the same phase angle

Casedatephase_angle

SGLI
AHI
SGLI
②AHI

2019/2/20 3:16
2019/2/20 3:40
2019/4/20 0:27
2019/4/20 3:00

8.255
8.262
9.342
9.792①

selenographic
latitude of satellite-4.796-3.579-6.602-7.067selenographic
longitude of satellite
1.928
2.260
5.118
4.061

selenographic
latitude of sun-0.914-0.914-1.525-1.524selenographic
longitude of sun-5.368-5.567-2.746-4.038

dist_sat_moon

dist_sun_moon

351544398634364414412584

0.991
0.991
1.007
1.007

AHI
AHI/SGLI

SGLI

B01

11%

B02

7%

B03

11%

B04

12%

B05

B06

21%

5%

• AHI results are about 5-20% larger than SGLI.
 These results may be include calculation
errors of the solid angle and oversampling
factor etc.
15

Validation using AHI
 Validation of phase angle dependence correction using AHI/GIRO trend
AHI
WL
[nm]

 AHI/GIRO trend (2019)

B01

B02

B03

B04

B05

B06

470

510

640

860

1600

2300

Phase angle range =5-10°
SGLI observation range

B01

B06

2019/2 data have bias in B01-B04.
Earth shine?

 B04-06 trends have phase angle dependence

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Validation using AHI
 Validation of phase angle dependence correction using AHI/GIRO trend Apply himawari-8/AHI data to SGLI simple model𝑓 𝑏𝑎𝑛𝑑, 𝑔, 𝑑𝑎𝑦 = 𝑎 𝑏𝑎𝑛𝑑 ∗ 𝑔 + 𝑏 𝑏𝑎𝑛𝑑 ∗ 𝑑𝑎𝑦 + 𝑐 𝑏𝑎𝑛𝑑

AHI/GIRO (Phase angle range =5-10)

Correction of AHI/SGLI ratio The phase angle dependence coefficient increases withincreasing wavelength, and AHI and SGLI have similartendencies.
••

It suggests that the dependencies are thecharacteristic of GIRO.
In the case of phase angles 5 and 10, without thisdependence correction, it becomes the error
factors of about 3% {=0.006 x (10-5) @ SW04} inthe SWIR.

17Conclusion
 Conclusion

 GCOM-C/SGLI continuously observes the moon since the launch , we evaluate the lunar calibration
trend using GIRO.
• These trends show similar trends to other onboard calibrations, suggest that the lunar
calibration evaluation acquires the degradation characteristics of the sensor in detail.
• Similar to the heritage instruments, the OBS / GIRO trend shows phase angle dependence,
especially at SWIR band.
 A simple model was constructed to extract the sensor responsivity degradation from OBS/GIROtrend.
• As a result, it was confirmed that the phase angle dependence increases as the wavelength
increases.
• These phase angle dependences were verified using AHI/GIRO trend.
• These results suggest that phase angle dependence corrections are useful for lunar calibration
evaluation using GIRO of other sensors observing at the phase angle range of 5-10 degrees.

 Future plan

 These results will be periodically reflected as the radiometric calibration coefficients for the groundprocessing system.

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