the first irradiance
TRANSCRIPT
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The first irradiance
300 nm
N
S500 nm 300 nm
N Spectral calibration
Absolute radiometric
calibration
Spatial pattern
Stray light
…..and others
Calibration
Bad pixel Linearity Smear Gain PRNU DC Stray light Spectral ….
300 nm
N
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Spectral calibration
FGEMS is nadir measurement (spatial index is 1024)
Spectral calibration
Clear indication of spectral mismatch between measured and reference, asking for
an update of spectral calibration
The algorithm based on the non-linear least square fitting (Kang et al., 2020) is
applied to full spectrum for each spatial index using a reference solar spectrum
It also derives parameters of analytical spectral response function (SRF) for GEMS
w/o SPECAL w/ SPECAL
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Spectral calibration
Updated results
Update of spectral calibration reduces the high frequency variation significantly at all
spatial positions
However, there are systematic differences of irradiance as much as about 10%
w/o SPECALw/ SPECAL
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Spectral calibration
Thermistor dataset vs. spectral shift
Spectral shifts are closely related with several GEMS thermistors
FPA temperature, and telescope temperature
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Spectral calibration
Spectral response function
SRF of GEMS is also characterized using the SRF parameters of a best-matched
analytical function (Asymmetric Super Gaussian function) derived during the spectral
calibration process
In-flight SRFs retrieved from the first GEMS irradiance are similar to prelaunch ones
No significant change occur during the harsh launch process
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Spectral calibration
Monitoring of in-flight SRF
Continuous monitoring of the GEMS SRF for the whole mission lifetime is necessary
The temporal variations of width over from April 23, 2020 to March 21, 2021
indicate quite a stable variation given the preliminary nature of the daily GEMS
irradiance
Variations of 𝑤 (half width at 1/e intensity) are smaller than 0.006 nm and 0.004 nm for
330.0 nm and 390.0 nm, respectively ,
In-flight spectral performance and characteristics of GEMS are similar to those investigated
from prelaunch
Prelaunch SRFs In-flight SRFs
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Relative irradiance
Variation of relative irradiance
During the year, irradiance data show a large variation along both spatial and
spectral direction
The variation pattern differs for different wavelength and locations
Such a spatial variation is not clear in the radiance data, which is one of reason the angular
effect of BTDF variation with the incident sun geometry
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Relative irradiance
Variation of relative irradiance
During the year, irradiance data show a large variation along both spatial and
spectral direction
The variation pattern differs for different wavelength and locations
Such a spatial variation is not clear in the radiance data, which is one of reason the angular
effect of BTDF variation with the incident sun geometry
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BTDF update (preliminary result)
In-flight calibration of BTDF
Update the ground BTDF map using solar reference spectrum and calibrate using azimuth
and relative irradiance (goniometric correction) (Dobber et al., 2004; Kleipool et al., 2020)
Spatial index: 100, 1000, 2000
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BTDF update (preliminary result)
In-flight calibration of BTDF
Systematic biases and clear spatial inhomogeneity are improved
FBATC
FGEMS (November 17, 2020)
FEWHA FEWHA
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BTDF update (preliminary result)
Inter-comparison with OMPS and TROPOMI
On deep convective cloud (the brightest scene) [every 5 days, November 2020 to January 2021]
F I F/I
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BTDF update (preliminary result)
Inter-comparison with OMPS and TROPOMI
Clear sky (the darkness scene) [every 5 days, November 2020 to January 2021]
F I F/I
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Ongoing works (LED, Dark, RSD, radiance)
Validate the ground calibration tables & trend monitoring
Non-linearity, gain, offsets, saturation threshold, bad pixels, etc.
Median TrailingMean LeadingMean Trailing
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Summary
Irradiance show expected characteristics with a few exceptions
There are spatial patterns in both raw digital count and calibrated solar irradiance
which also shows a temporal variation
Irradiance values are smaller than OMPS and similar to TROPOMI
Near future activities
Resolve issues in the diffuser BTDF appeared in updated process
Improve stray light correction, especially at the shorter end of spectrum
Validate the calibration coefficients (linearity, gain...)
Monitor the variation of the in-flight measurements (offsets, dark, LED)
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Thank you!