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.

TCCON: https://tccondata.org/
COCCON: https://www.imk-asf.kit.edu/english/COCCON.php
Pandonia Global Network: https://www.pandonia-global-network.org/
CONTRAIL: https://www.cger.nies.go.jp/contrail/