Orbiting Carbon Observatory-3: Preflight Spectral Calibration and Performance Evaluation

The Orbiting Carbon Observatory-3 (OCO-3) payload was installed on the International Space Station (ISS) in May 2019. OCO-3 is providing the measurements needed to quantify the column average carbon dioxide dry air mole fraction (XCO2) near the ISS orbit track. The instrument line shape (ILS) and spectral dispersion were calibrated during pre-launch payload thermal vacuum testing in April-May, 2018, employing the tunable laser sources and calibration methods developed for OCO and OCO-2. Specific changes to the calibration process for OCO-3 include correcting for a fringing pattern and stray light signature. The ILS and dispersion were then further optimized and validated by comparing solar spectra recorded simultaneously on the ground by the OCO-3 flight instrument and a collocated, high-resolution, Fourier transform spectrometer (FTS) from the Total Carbon Column Observing Network (TCCON). In addition, these instrument parameters were input, along with the calibrated spectral radiances, into the ACOS Level-2 algorithm to retrieve an XCO2 time series that showed good agreement ( 1 ppm) with the TCCON measurements. This paper describes the pre-launch spectral calibration and retrieval analyses.