Abstract
The Orbiting Carbon Observatory 3 (OCO-3) has been collecting high-resolution spectra of sunlight reflected by Earth in three near-infrared spectral channels since 2019. An external payload on the International Space Station, it uses the spare instrument from the OCO-2 mission, which continues to collect data as a dedicated satellite. The three individual long-slit imaging grating spectrometers have their own focal plane arrays and share a common entrance telescope. The OCO-3 telescope was modified to have a 1.8˚ field of view to yield a similar ground footprint size from the 400-420 km altitude ISS orbit as OCO-2 obtains with its 0.8˚ field of view from its 705km altitude orbit. During preflight testing, the OCO-3 instrument was illuminated by a collimator with slit and pinhole targets in front of a quartz tungsten halogen lamp. The slits and pinholes were scanned across the field of view using a two-axis stage to determine the field of regard, coalignment of the three spectrometers, and the 2D spatial response of each footprint. The initial Ancillary Geometric Product, containing the centers and widths of each footprint, was calculated using an average of columns in the spectral dimension. Subsequently, a defocus was identified that varies significantly in the spectral dimension. As a result, the widths of the along-slit footprints are much larger at one end of the spectral band than at the other. The effect is strongest for the “Strong CO2” channel with a center wavelength of 2.06 microns. An improved characterization of the column-dependent spatial response is key for absolute calibration using the Moon, and important for determining which science scenes are sufficiently uniform to allow good quality atmospheric retrievals. Here, we will summarize recent progress in characterization and its impacts on the OCO-3 data products.
Refined Spatial Response Functions for OCO-3
The Orbiting Carbon Observatory 3 (OCO-3) has been collecting high-resolution spectra of sunlight reflected by Earth in three near-infrared spectral channels since 2019. An external payload on the International Space Station, it uses the spare instrument from the OCO-2 mission, which continues to collect data as a dedicated satellite. The three individual long-slit imaging grating spectrometers have their own focal plane arrays and share a common entrance telescope. The OCO-3 telescope was modified to have a 1.8˚ field of view to yield a similar ground footprint size from the 400-420 km altitude ISS orbit as OCO-2 obtains with its 0.8˚ field of view from its 705km altitude orbit. During preflight testing, the OCO-3 instrument was illuminated by a collimator with slit and pinhole targets in front of a quartz tungsten halogen lamp. The slits and pinholes were scanned across the field of view using a two-axis stage to determine the field of regard, coalignment of the three spectrometers, and the 2D spatial response of each footprint. The initial Ancillary Geometric Product, containing the centers and widths of each footprint, was calculated using an average of columns in the spectral dimension. Subsequently, a defocus was identified that varies significantly in the spectral dimension. As a result, the widths of the along-slit footprints are much larger at one end of the spectral band than at the other. The effect is strongest for the “Strong CO2” channel with a center wavelength of 2.06 microns. An improved characterization of the column-dependent spatial response is key for absolute calibration using the Moon, and important for determining which science scenes are sufficiently uniform to allow good quality atmospheric retrievals. Here, we will summarize recent progress in characterization and its impacts on the OCO-3 data products.