Acquisition of Lunar Spectra to Assist OCO-2 and OCO-3 Calibration

The Orbiting Carbon Observatory (OCO) -2 and -3 instruments, launched in 2014 and 2019 respectively, require accurate and precise calibration to achieve science goals. While onboard lamps and solar diffusers have proven very useful to track changes over short time scales, these calibration sources have themselves exhibited substantial degradation over the extended mission life. OCO-2 has relied on lunar measurements to estimate "slow" irreversible changes in radiometric response, and OCO-3 is attempting to do the same. This work describes how each instrument observes the Moon and highlights key differences in how these datasets are acquired. Several near-coincident observations also present the opportunity to join the two records, which will benefit both missions.