Stability Assessment of OCO-2 Radiometric Calibration Using Aqua MODIS as a Reference

Abstract

With three imaging grating spectrometers, the Orbiting Carbon Observatory-2 (OCO-2) measures high spectral resolution spectra ( $\lambda /\Delta \lambda \approx$ 19,000) of reflected solar radiation within the molecular oxygen (O ${}_2$ ) A-band at 0.765 $\mathsf{\mu }$ m and two carbon dioxide (CO ${}_2$ ) bands at 1.61 and 2.06 $\mathsf{\mu }$ m. OCO-2 uses onboard lamps with a reflective diffuser, solar observations through a transmissive diffuser, lunar measurements, and surface targets for radiometric calibration and validation. Separating calibrator aging from instrument degradation poses a challenge to OCO-2. Here we present a methodology for trending the OCO-2 Build 8R radiometric calibration using OCO-2 nadir observations over eight desert sites and nearly simultaneous observations from Moderate Resolution Imaging Spectroradiometer (MODIS) with sensor viewing zenith angles of 15 ± 0.5 ${}^{\circ }$ . For the O ${}_2$ A-band, this methodology is able to quantify a drift of −0.8 ± 0.1% per year and capture a small error in correcting the aging of the solar calibrator. For the other two OCO-2 bands, no measurable changes were seen, indicating less than 0.1% and less than 0.3% per year drift in the radiometric calibration of Band 2 and Band 3, respectively.