New Inflight Calibration of OCO-3’s A-Band for Version 11 Products

The Orbiting Carbon Observatory-3 (OCO-3) instrument, operated from the International Space Station, was launched on May 4th, 2019. Its primary objective is to provide high precision estimates of the column-averaged dry-air mole fraction of carbon dioxide (XCO2) and solar-induced chlorophyll fluorescence. It consists of three long-slit imaging spectrometers that provide high resolution spectra in three infrared bands. The shortest wavelength one, named A-Band, is centered at 0.765 μm and characterized by numerous absorption lines of molecular oxygen. Throughout the mission, this band has undergone important changes in instrument response related to contamination of the focal plane array. One way contamination buildup affects OCO-3 data is by imparting an S-like shape to the observed spectra, which cannot be characterized and corrected through analysis of data from the on-board calibrator alone. In Version 10, this artifact manifested itself as a slow drift of XCO2, from February 2020 to January 2021, and required an additional post-retrieval bias correction. In the latest collection, Version 11, OCO-3’s inflight calibration algorithm for the A-Band was augmented to correct this artifact using clear ocean scenes to derive the spectral shape of changes in instrument response. This drastically improved the shape of spectral residuals between observed and forward modeled radiances and also significantly reduced the slow XCO2 drift observed in Version 10.