The on-orbit performance of the Orbiting Carbon Observatory-2 (OCO-2) instrument and its radiometrically calibrated products

摘要

The Orbiting Carbon Observatory-2 (OCO-2) carries and points a three-channel imaging grating spectrometer designed to collect high-resolution, co-boresighted spectra of reflected sunlight within the molecular oxygen (Osub2/sub) A-band at 0.765 microns and the carbon dioxide (COsub2/sub) bands at 1.61 and 2.06 microns. These measurements are calibrated and then combined into soundings that are analyzed to retrieve spatially resolved estimates of the column-averaged COsub2/sub dry-air mole fraction, XCOsub2/sub. Variations of XCOsub2/sub in space and time are then analyzed in the context of the atmospheric transport to quantify surface sources and sinks of COsub2/sub. This is a particularly challenging remote-sensing observation because all but the largest emission sources and natural absorbers produce only small (&a?ˉ0.25a?ˉ%) changes in the background XCOsub2/sub field. High measurement precision is therefore essential to resolve these small variations, and high accuracy is needed because small biases in the retrieved XCOsub2/sub distribution could be misinterpreted as evidence for COsub2/sub fluxes. brbr To meet its demanding measurement requirements, each OCO-2 spectrometer channel collects 24a?ˉspectraa?ˉssupa??1/sup across a narrow (&a?ˉ10a?ˉkm) swath as the observatory flies over the sunlit hemisphere, yielding almost 1??million soundings each day. On monthly timescales, between 7 and 12a?ˉ% of these soundings pass the cloud screens and other data quality filters to yield full-column estimates of XCOsub2/sub. Each of these soundings has an unprecedented combination of spatial resolution (&a?ˉ3a?ˉkmsup2/sup/sounding), spectral resolving power (i??/ia????i??/ia?ˉ&a?ˉ17a?ˉ000), dynamic range (a??a?ˉ10sup4/sup), and sensitivity (continuum signal-to-noise ratioa?ˉ&a?ˉ400). brbr The OCO-2 instrument performance was extensively characterized and calibrated prior to launch. In general, the instrument has performed as expected during its first 18 months in orbit. However, ongoing calibration and science analysis activities have revealed a number of subtle radiometric and spectroscopic challenges that affect the yield and quality of the OCO-2 data products. These issues include increased numbers of bad pixels, transient artifacts introduced by cosmic rays, radiance discontinuities for spatially non-uniform scenes, a misunderstanding of the instrument polarization orientation, and time-dependent changes in the throughput of the oxygen A-band channel. Here, we describe the OCO-2 instrument, its data products, and its on-orbit performance. We then summarize calibration challenges encountered during its first 18 months in orbit and the methods used to mitigate their impact on the calibrated radiance spectra distributed to the science community.