OCO-3 (Orbiting Carbon Observatory 3)

OCO-3 (Orbiting Carbon Observatory 3) is a NASA carbon-monitoring instrument deployed on the International Space Station in May 2019, attached to the Japanese Experiment Module Exposed Facility (JEM-EF).^[1][2]

OCO-3 shares the same three-channel grating spectrometer design as OCO-2, covering the oxygen A-band, the weak CO2 band, and the strong CO2 band. The spacecraft carries a Pointing Mirror Assembly (PMA) that enables two observing modes beyond the nadir-only geometry of OCO-2: Target Mode, which tracks a specific surface site across the overpass, and Snapshot Area Map (SAM) mode, which collects hundreds to thousands of SIF retrievals across a 80x80 km area at biologically and ecologically diverse sites in a single pass.^[1]

Because OCO-3 orbits on the ISS at 408 km altitude and 51.6-degree inclination, it observes at varying times of day rather than at the fixed sun-synchronous crossing time of OCO-2. This variable local solar time sampling provides a different temporal context for CO2 and SIF measurements compared to sun-synchronous orbits with fixed crossing times. Sun-induced fluorescence retrieval at 757 nm and 740 nm has been demonstrated from OCO-3 data.^[2]