On the zero-level offset in the GOSAT TANSO-FTS O 2 A band and the quality of solar-induced chlorophyll fluorescence (SIF): comparison of SIF between GOSAT and OCO-2
Abstract. Satellite remote sensing of solar-induced chlorophyll
fluorescence (SIF) has attracted attention as a method for improving the
estimation accuracy of the photosynthetic production of terrestrial
vegetation in recent years. The Greenhouse gases Observing Satellite (GOSAT)
has the ability to observe both SIF and the concentrations of CO2 and
 CH4 and thus is expected to contribute to the understanding of the global
carbon budget. Evaluating artefact signals (e.g. zero-level offset caused
by non-linearity in the analogue circuit in the case of GOSAT) is effective
for inferring the instrument status and important for retrieving SIF from
satellite measurements. Here we investigate the characteristics of the
zero-level offset and the consistency of satellite-derived SIFs by comparing
the derived SIF with the Orbiting Carbon Observatory-2 (OCO-2) SIF at
multiple spatial scales (footprint to global). The zero-level offset was
evaluated using filling-in signals over bare soil while investigating the
criteria for identifying barren areas. An analysis of the temporal variation
of the zero-level offset over a period of 9 years suggests that the
radiometric sensitivity of the GOSAT spectrometer changed after switching
the optics path selector in January 2015. The GOSAT SIF was highly
consistent with the OCO-2 SIF, with a bias within 0.1 mW m −2  nm −1  sr −1 for most months and an inter-region bias of about 0.2 mW m −2  nm −1  sr −1 . Our results agree with the previous comparisons and
support the consistency among the present satellite SIF data, which is
important for the utilization of those data.