1. Cal & VAL for Greenhouse gases observation spectrometers
CEOS WGCV-42 Working Group on Calibration & Validation
May 28, 2017, Sioux Falls, SD
Cal & VAL for Greenhouse gases observation spectrometers

2. TANSO is the only FTS for GHG measurements from space
SCIAMACHY
( )
GOSAT
( )
OCO-2 (2014-)
OCO-3 (2018)
GHGsat (2016-)
TanSat (2016-)
TROPOMI (2017-)
GOSAT-2 (2018-)
MERLIN (2020)
GeoCARB
MicroCarb (2020)
ASCENDS (2021)
CarbonSat (2022)
GOSAT-3
G3E
Lab model test 2

3. International effort to demonstrate the effectiveness of satellite GHG observation.
Japanese GOSAT and US OCO-2 have different observation strategies.
TANSO-FTS onboard GOSAT has wide spectral coverage from SWIR to TIR and an agile pointing system at the expense of spatial context, while OCO-2 targets CO2 with higher spatial resolution using imaging grating spectrometers.
Since the early phase of the two projects, both teams have worked in calibration and validation to demonstrate the effectiveness of satellite greenhouse gases observation.

4. Inter-comparison between GOSAT and OCO-2
Radiometric calibration
Prelaunch
X-CAL
Annual Vicarious Calibration at the desert playa in Nevada
CO2 & CH4 profile
In situ CO2 and CH4 on AJAX
XCO2 & XCH4
Dec, 2008＠TKSC
Difference 1.59%,1.1%, 1.4%
GOSAT
OCO-2
Column with EM-27 FTS
Coincident Target
Retrieved Parameter
Comparison over match up points
Calibrated GOSAT and OCO-2 radiance spectra agrees within 5% for all bands.
Retreived XCO2 bias is much less than 0.5ppm over match up observations points. 4

5. Path 36 Path 37 from East from West
Calibration and Validation at RRV, Nevada
Every June near summer solstice (9th in 2017)
Path 36
from East
Path 37
from West
33.0deg
25deg
19deg
19.9deg
TOA Spectral radiance
High altitude
Horizontal
CO2　 CH4
Vertical
CO2　 CH4 O3　
Surface Thermal radiation
Surface and Profile
of Pressure, Temperature, Humidity
Aerosol Optical Depth
Surface
CO2　CH4 CO O 3 Wind speed
Surface Spectral Reflectance
UV-SWIR
BRDF　
Variability 5 1

6. Calibration, Retrieval, and Validation Common standards
Should be calibrated and validated in each level.
H2O
CO2
CH4
Measurement
Level 1: spectra
Level 2: retrieved CO2
GOSAT/TANSO-FTS and OCO-2 observe sunlight reflected from the Earth’s surface and retrieve atmospheric carbon dioxide (CO2) with different observing geometries and ground track repeat cycles.
Most measured spectral radiances (Level 1) agree within 5% for all bands shared by the two instruments
Retrieved CO2 densities also agree within +/- 4ppm (1%).
Level 1
Level 2

7. Level 1: Cross calibration of Radiance spectra
FTS-based GOSAT and Grating base OCO: other than Railroad Valley
Average Difference after BRDF correction
O2A 0.76 μm : 4% <%
Weak CO2 @1.6 μm : 2% <%
Strong μm : 3%
The above level differences do not affect XCO2 retrieval as CO2 is retrieved with differential absorption

8. UV imager vicarious calibration (2017 campaign)
V+UV
V+SWIR UV
V+UV
V+SWIR
Sand-blasted Aluminum
Lower reflectivity
Spectrally flat
Robust
Spectralon
Reflectance degraded in UV when contaminated.
Diffusive (Ideal Lambertian)
Assuming sand-blasted Aluminum BRDF has low spectral dependence.
Larger than 5%

9. Leve 2: CO2 cross-validation GOSAT/ACOS –OCO2 Level 2 XCO2
FTS
⊿XCO2 (ACOS-OCO2) error bar: ⊿XCO2 deviation within GOSAT-FOV
● Temporal : GOSAT – OCO2 time difference : +/- 1 hour ● Spatial : Select OCO2 data within
10.5 km GOSAT footprint
Sep2014
Feb2016
Grating
[Data period] 2014/09～2016/02 ; Level2 matchup : 715 points
Agreement: <0.14ppm over Ocean

10. Geometry Dependency (forward vs backward)
Related retrieved parameters: aerosol optical depth
Uniform within GOSAT 10.5 km footprint
ACOS B7.3 sometimes miss small cloud fractions
Forward is weaker but less geometry sensitive
Correlation of forward cased is much better
Path 36
Forward
Path 37
backward 10
B7.3 AOD vs. Aeronet located at the center of the playa

11. XCO2, XCH4, and XCO Validation with mobile FTSs
Direct solar light observation for column-averaged dry air mole fractions of CO2 and CH4
Just before the campaign
The campaign at Railroad Valley
EM27-NIR
TCCON
EM-27
@Caltech
2 InGaAs detectors
O2, CO2, and CH4
Weak CO and CH4 at 2.3 μm
Thermoelectric cooler
From sunrise to sunset
Intercomparison with high spectral resolution FTS and moderate res. FTS.
The Total Carbon Column Observing Network (TCCON)
EM27-MIR
Single InSb detector
Extended to 4.7 μm for strong CO band
Mechanical cooler

12. (not matured yet, but one of our goals)
Flux estimation
(not matured yet, but one of our goals)
Satellite data alone cannot provide flux information.
Surface wind speed information is needed.
Simplest case:
No influx
Point source
Column averaged
10.5 km diameter
GOSAT can target center of the emission source (accuracy of 0.5 km)
Emission t/hour=△XCH4(ppb)*0.495t*3600s*V(m/s)/5000m

13. Common Standards and Data for Calibrations and Retrieval
Common standards and data
Notes
Calibration
Prelaunch
Cross-calibrate radiometers
Viewing common radiometric standard
Onboard
Solar data
Vicarious
CEOS site (RRV etc.)
Annual campaign
Retrieval
Algorithm
Parameters
Molecular spectroscopy
A priori model (Aerosol)
Input
Calibrated spectra at 0.76, 1.6, 2.0μm
Observation geometry and condition
Level 1
Output
Retrieved column amount GHG
Simultaneously retrieved parameters
(aerosol optical depth, Solar-induced plant chlorophyll fluorescence (SIF))
Level 2
Validation
Ground Site
High resolution spectrometer
mobile spectrometer
TCCON
Air plane
In situ measured data
CONTRAIL etc.
Match up
Land type and Ocean
Viewing Common sites

14. Data products Different satellite data are available from the same site

15. Future plan
9th GOSAT –OCO-2 joint vicarious calibration campaign at RRV, NV (June, 2017)
On June 28 both GOSAT and OCO-2 target form the east viewing toward the sun. Coincident spiral flight wit NASA Ames AJAX to measure in-situ CO2, CH4 and O3 density.
(2) CH4 intercomparison with airborne LIDAR
Coincident flight with airborne model CHARM of MERLIN
August 2017 Small campaign, Spring 2018, CoMET HALO
(3) CAL-VAL method and data
Open web site in JAXA EORC GOSAT web page (2017)
“The cross-calibration of spectral radiances and cross-validation of CO2 estimates from GOSAT and OCO-2.” for MDPI remote sensing special issue for satellite (2017)
(4) Estimated Flux comparison