Abstract
With a three-channel hyperspectral imaging grating spectrometer, the Orbiting Carbon Observatory-2 (OCO-2) measures high resolution spectra (resolving power of ~17 000) of reflected solar radiation within the O2 A-band at 0.76 micron and two CO2 bands at 1.61 and 2.06 micron. OCO-2 was inserted at the front of the 705 km Afternoon Constellation (A-Train) on August 3, 2014, and flies about 6 minutes ahead of the Aqua spacecraft. The OCO-2 spectrometer collected its first-light spectra on August 6, 2014, and has been returning science data since September 6, 2014. OCO-2 uses onboard lamps with a reflective diffuser, solar observations through a transmissive diffuser, and lunar measurements as well as surface targets for radiometric calibration. Separating the lamp output or lamp or solar diffuser aging from the instrument degradation poses a challenge to OCO-2. Here, we present our methodology for trending the OCO-2 Build 8 radiometric calibration through comparisons with MODIS half-km radiances collocated to the OCO-2 footprints. The data over nine Sahara/ Arabian desert sites have been used to quantify the drift of the OCO-2 instrument throughput. For the 0.76 micron band, a drift of -0.67±0.04 % per year was determined by combining data from all nine desert sites; data from a single desert site gave precisions on the order of ±0.2% per year. For the other two bands, no measurable changes were seen, indicating a drift of less than 0.2% per year. This same methodology is expected to play an important role in the radiometric calibration of the Orbiting Carbon Observatory-3 (OCO-3), currently scheduled to be installed on the International Space Station (ISS) in the spring of 2019. OCO-3 is equipped with onboard lamps, but will take very limited lunar measurements (~three times per year), and no solar measurements due to its configuration aboard the ISS.
Radiometric Comparison of 0.76, 1.6 and 2.0-μm Bands of OCO-2 with Aqua MODIS over Sahara/Arabian Desert Sites
With a three-channel hyperspectral imaging grating spectrometer, the Orbiting Carbon Observatory-2 (OCO-2) measures high resolution spectra (resolving power of ~17 000) of reflected solar radiation within the O2 A-band at 0.76 micron and two CO2 bands at 1.61 and 2.06 micron. OCO-2 was inserted at the front of the 705 km Afternoon Constellation (A-Train) on August 3, 2014, and flies about 6 minutes ahead of the Aqua spacecraft. The OCO-2 spectrometer collected its first-light spectra on August 6, 2014, and has been returning science data since September 6, 2014. OCO-2 uses onboard lamps with a reflective diffuser, solar observations through a transmissive diffuser, and lunar measurements as well as surface targets for radiometric calibration. Separating the lamp output or lamp or solar diffuser aging from the instrument degradation poses a challenge to OCO-2. Here, we present our methodology for trending the OCO-2 Build 8 radiometric calibration through comparisons with MODIS half-km radiances collocated to the OCO-2 footprints. The data over nine Sahara/ Arabian desert sites have been used to quantify the drift of the OCO-2 instrument throughput. For the 0.76 micron band, a drift of -0.67±0.04 % per year was determined by combining data from all nine desert sites; data from a single desert site gave precisions on the order of ±0.2% per year. For the other two bands, no measurable changes were seen, indicating a drift of less than 0.2% per year. This same methodology is expected to play an important role in the radiometric calibration of the Orbiting Carbon Observatory-3 (OCO-3), currently scheduled to be installed on the International Space Station (ISS) in the spring of 2019. OCO-3 is equipped with onboard lamps, but will take very limited lunar measurements (~three times per year), and no solar measurements due to its configuration aboard the ISS.