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The Orbiting Carbon Observatory-2 (OCO-2) carries and points a three-channelimaging grating spectrometer designed to collect high-resolution,co-boresighted spectra of reflected sunlight within the molecular oxygen(O2) A-band at 0.765 microns and the carbon dioxide (CO2) bands at1.61 and 2.06 microns. These measurements are calibrated and then combinedinto soundings that are analyzed to retrieve spatially resolved estimates ofthe column-averaged CO2 dry-air mole fraction, XCO2. Variations ofXCO2 in space and time are then analyzed in the context of theatmospheric transport to quantify surface sources and sinks of CO2. Thisis a particularly challenging remote-sensing observation because all but thelargest emission sources and natural absorbers produce only small(< 0.25 %) changes in the background XCO2 field. Highmeasurement precision is therefore essential to resolve these smallvariations, and high accuracy is needed because small biases in the retrievedXCO2 distribution could be misinterpreted as evidence for CO2fluxes. To meet its demanding measurement requirements, each OCO-2 spectrometerchannel collects 24 spectra s−1 across a narrow (< 10 km)swath as the observatory flies over the sunlit hemisphere, yielding almost1 million soundings each day. On monthly timescales, between 7 and12 % of these soundings pass the cloud screens and other data qualityfilters to yield full-column estimates of XCO2. Each of these soundingshas an unprecedented combination of spatial resolution(< 3 km2/sounding), spectral resolving power(λ∕Δλ > 17 000), dynamic range(∼ 104), and sensitivity (continuum signal-to-noiseratio > 400). The OCO-2 instrument performance was extensively characterized andcalibrated prior to launch. In general, the instrument has performed asexpected during its first 18 months in orbit. However, ongoing calibrationand science analysis activities have revealed a number of subtle radiometricand spectroscopic challenges that affect the yield and quality of the OCO-2data products. These issues include increased numbers of bad pixels,transient artifacts introduced by cosmic rays, radiance discontinuities forspatially non-uniform scenes, a misunderstanding of the instrumentpolarization orientation, and time-dependent changes in the throughput ofthe oxygen A-band channel. Here, we describe the OCO-2 instrument, its dataproducts, and its on-orbit performance. We then summarize calibrationchallenges encountered during its first 18 months in orbit and the methodsused to mitigate their impact on the calibrated radiance spectra distributedto the science community

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