Abstract
The Orbiting Carbon Observatory 3 payload was installed on the International Space Station (ISS) on May 10, 2019 and completed in-orbit checkout on August 6, 2019. To enable precise retrievals of the column-average carbon dioxide dry air mole fraction, high resolution spectra of reflected sunlight are collected in three infrared channels. Band 1 (“ABO2”) from 757.6 to 772.7 nm is used to measure molecular oxygen and solar-induced chlorophyll fluorescence, and Bands 2 and 3 measure weak and strong carbon dioxide absorption features from 1591.2 to 1622.7 nm (“WCO2”) and 2042.0 to 2082.8 nm (“SCO2”). The three individual grating spectrometers share a common entrance telescope with a 1.8˚ field of regard, divided into eight along-slit footprints. Spectra are acquired at 3 Hz from an altitude of 400-420 km, with each footprint covering roughly 5 km2. OCO-3 employs an agile two-axis Pointing Mirror Assembly to view Earth in nadir over land, near the glint spot over water, and observe in target and area map modes over locations of interest. The PMA also points into an onboard calibrator to view one of three tungsten halogen lamps or to collect dark measurements. On rare occasions OCO-3 has viewed the Moon but cannot safely view the Sun.
The current Level 1B product (Version 10.3) contains a number of imperfections, especially from June 2020 to January 2021 when the instrument was most contaminated. For Version 11, due for public release in 2024, changes were made to ABO2 radiometric calibration, both to the spectrally flat absolute level and the in-band spectral shape. A drift of roughly 1 % per year in Lamp 1, which is assumed to have constant output in Version 10, was corrected. The ratios between preflight and in-orbit checkout were also revised and made footprint-dependent. For spectral shape, data shortly after decontamination is given the spectral shape of Lamp 2 instead of Lamp 1. For later orbits, the spectral shape is based on ratios of ocean spectra to earlier in the cycle. Additional refinements were made to the instrument line shape, spatial response function, and signal to noise coefficients.
OCO-3 was scheduled for a three-year prime mission, which concluded successfully in September 2022. Originally, the payload was to be uninstalled to make space for another mission. More recently, an extension was approved for continued operations through the lifetime of ISS in 2029. When the next payload arrives, currently expected in December 2023, OCO-3 will be temporarily stowed for approximately 6 months instead of being disposed. After reinstallation and another 90-day in-orbit checkout, nominal science operations will resume with no further planned interruptions.
OCO-3 Version 11: Better Data and More Data
The Orbiting Carbon Observatory 3 payload was installed on the International Space Station (ISS) on May 10, 2019 and completed in-orbit checkout on August 6, 2019. To enable precise retrievals of the column-average carbon dioxide dry air mole fraction, high resolution spectra of reflected sunlight are collected in three infrared channels. Band 1 (“ABO2”) from 757.6 to 772.7 nm is used to measure molecular oxygen and solar-induced chlorophyll fluorescence, and Bands 2 and 3 measure weak and strong carbon dioxide absorption features from 1591.2 to 1622.7 nm (“WCO2”) and 2042.0 to 2082.8 nm (“SCO2”). The three individual grating spectrometers share a common entrance telescope with a 1.8˚ field of regard, divided into eight along-slit footprints. Spectra are acquired at 3 Hz from an altitude of 400-420 km, with each footprint covering roughly 5 km2. OCO-3 employs an agile two-axis Pointing Mirror Assembly to view Earth in nadir over land, near the glint spot over water, and observe in target and area map modes over locations of interest. The PMA also points into an onboard calibrator to view one of three tungsten halogen lamps or to collect dark measurements. On rare occasions OCO-3 has viewed the Moon but cannot safely view the Sun.
The current Level 1B product (Version 10.3) contains a number of imperfections, especially from June 2020 to January 2021 when the instrument was most contaminated. For Version 11, due for public release in 2024, changes were made to ABO2 radiometric calibration, both to the spectrally flat absolute level and the in-band spectral shape. A drift of roughly 1 % per year in Lamp 1, which is assumed to have constant output in Version 10, was corrected. The ratios between preflight and in-orbit checkout were also revised and made footprint-dependent. For spectral shape, data shortly after decontamination is given the spectral shape of Lamp 2 instead of Lamp 1. For later orbits, the spectral shape is based on ratios of ocean spectra to earlier in the cycle. Additional refinements were made to the instrument line shape, spatial response function, and signal to noise coefficients.
OCO-3 was scheduled for a three-year prime mission, which concluded successfully in September 2022. Originally, the payload was to be uninstalled to make space for another mission. More recently, an extension was approved for continued operations through the lifetime of ISS in 2029. When the next payload arrives, currently expected in December 2023, OCO-3 will be temporarily stowed for approximately 6 months instead of being disposed. After reinstallation and another 90-day in-orbit checkout, nominal science operations will resume with no further planned interruptions.