Overview of NASA's Ocean Color Instrument solar calibration architecture, pre-launch tests, and preliminary on-orbit results

Launched in February 2024, the PACE mission represents NASA’s next investment in ocean biology, clouds, and aerosol data records. A key feature of PACE is the inclusion of an advanced satellite radiometer known as the Ocean Color Instrument (OCI), a global mapping radiometer that combines multispectral and hyperspectral remote sensing. The OCI flight-unit was built at NASA’s Goddard Space Flight Center. At the time of this writing, PACE/OCI has launched and completed on-orbit commissioning activities and four months of normal science operations. A key aspect of the OCI architecture is the capability to trend absolute and relative calibration changes over the course of mission life with solar calibration. Every 24 hours a quartz Quasi-Volume Diffuser (QVD), mounted at 90deg from the nadir position of the OCI spinning aperture, is oriented towards the sun via a mechanism as the spacecraft ground-track nears the North Pole. By knowing the irradiance of the sun and the reflectivity of the target, the absolute radiance at the input to the OCI aperture can be determined. The allowable absolute uncertainty budget for each solar calibration measurement is 1.6% 1-σ below 900nm at beginning of life (BOL) and the allowable relative uncertainty budget is ~0.26% 1-σ. The Solar Calibration Assembly (SCA) consists of two quartz QVD targets, one Acktar Fractal Black target, a baffle, and a mechanism which selects targets and opens a door. This paper provides an overview of driving solar calibration requirements, SCA design and orbital maneuver, pre-launch tests and preliminary on-orbit results. Certain pre-launch measurements and analyses are covered in detail including diffuser Bidirectional Reflectance Distribution Function (BRDF) measurements, metrology measurements, and optical measurements of the SCA including stray light evaluation.