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)2 measures high resolution spectra (l/Dl 19000) of reflected solar radiation within the O2 A-band at3 0.76 mm and two CO2 bands at 1.61 and 2.06 mm. OCO-2 uses onboard lamps with a reflective4 diffuser, solar observations through a transmissive diffuser, lunar measurements, and surface5 targets for radiometric calibration and validation. Separating calibrator aging from instrument6 degradation poses a challenge to OCO-2. Here we present a methodology for trending the OCO-27 Build 8R radiometric calibration using OCO-2 nadir observations over eight desert sites and nearly8 simultaneous MODIS observations with sensor viewing zenith angles of 150.5o. For the O2 A-band,9 this methodology is able to quantify a drift of -0.80.1% per year and capture a small error in10 correcting the aging of the solar calibrator. For the other two OCO-2 bands, no measurable changes11 were seen, indicating less than 0.1% and less than 0.3% per year drift in the radiometric calibration of12 Band 2 and Band 3, respectively.