Key Differences in OCO-2 and OCO-3 Calibration

Key Differences in OCO-2 and OCO-3 Calibration

Robert Rosenberg, Lars Chapsky, David Crisp, Graziela Keller, Richard Lee, Yuliya Marchetti, Gary Spiers, Shanshan Yu, Annmarie Eldering

Jet Propulsion Laboratory, California Institute of Technology

The Orbiting Carbon Observatory 3 payload was launched to the International Space Station (ISS) on May 4, 2019 and acquired its first science spectra on June 26, 2019. Its three-channel grating spectrometer was originally built as the flight spare for Orbiting Carbon Observatory 2, which launched to the A-Train constellation on July 2, 2014 and acquired its first science spectra on August 6, 2014. Several modifications were made to accommodate the change from a free flying satellite to an ISS payload in a fixed orientation at a lower altitude. These included a new two-axis pointing mirror assembly, new entrance optics, and a new onboard calibrator. While the preflight test program was very similar for both instruments, some test equipment was modified to accommodate OCO-3's larger field of view. In flight, the most significant difference is that OCO-3 is unable to perform solar calibration. This increases the reliance on ground targets, lunar calibration, and onboard lamps. We will discuss the implications of these differences on our methodology for spectral and radiometric calibration, and on the quality of the Level 1B radiance product.