Calibration and Validation
"Calibration" is a process to characterize the accuracy and precision of Level 1 products mostly from radiometric, spectroscopic, and geometric aspects. Before the launch, TANSO-3 will be calibrated radiometrically and spectrally using laboratory facilities such as integrating spheres and lasers.
After the launch, TANSO-3 calibration will be conducted by multiple methods: calibration using an onboard solar diffuser, lunar calibration, and vicarious calibration. All of the methods have been applied to GOSAT and GOSAT-2 for more than a decade.
TANSO-3 will have a white panel called "solar diffuser" to reflect sunlight. The calibration using a solar diffuser can be done very frequently although the long-term stability of the panel's spectral property should be monitored independently.
The Moon has a more stable reflectance property than the solar diffuser and can be used as a calibration light source for a very long time. For the lunar calibration, the spacecraft will be maneuvered to direct TANSO-3 to the Moon. As the age of the Moon among the lunar calibration observations should be same, its frequency will be once a month at maximum.
In vicarious calibration, simultaneous observation of specific targets on Earth by TANSO-3 and ground-based instruments will be conducted. Railroad Valley Playa, a large dry lake in the US, has been used for the GOSAT Series vicarious calibration. This is truly an independent calibration, but its frequency is limited to about once a year due to weather and seasonal conditions.
"Validation" is a process to characterize the accuracy and precision of physical parameters in higher-level products. The validation of CO2 and CH4 is very important for the GOSAT Series, as the very high precision is required for GOSAT products such as 2 ppm or 0.5 % for CO2. In the GOSAT Series, CO2 and CH4 validation using data from the Total Carbon Column Observation Network (TCCON), a network of ground-based high-resolution Fourier transform spectrometers, has been conducted for more than a decade. The same methodology and the TCCON data will be used in GOSAT-GW CO2 and CH4 validation. In addition, data from other networks such as the COllaborative Carbon Column Observing Network (COCCON) for CO2 and CH4 validation and the Pandonia Global Network for NO2 validation will be used.
Airborne data are also important for CO2 and CH4 validation in the GOSAT Series. Especially, the data from commercial airliners such as the Comprehensive Observation Network for TRace gases by AIrLiner (CONTRAIL) have a long-term and global coverage suitable for satellite data validation. These data will also be used in GOSAT-GW validation.
Pandonia Global Network: https://www.pandonia-global-network.org/