## 1 Introduction
### Objectives of On-Orbit Cross-Comparisons  
Following the OCO-2 launch, two independent measurements of CO\({}_{2}\) are now being made from space. GOSAT TANSO-FTS provides wide spectral coverage, extending from 0.75 to 14.3 \(\upmu\)m, and a simple, uniform instrument line-shape function that can be monitored on-orbit. It has an agile pointing system, but routinely collects data in isolated footprints with no spatial context. The OCO-2 imaging-grating spectrometers can measure CO\({}_{2}\) with higher spatial resolution in contiguous samples, providing spatial context, but the instrument has more numerous and complicated instrument line-shape functions that are more challenging to calibrate [11; 12]. In spite of these differences, CO\({}_{2}\) can be derived from the measurements collected by both instruments using a common retrieval algorithm.  
During the integration and final prelaunch test, GOSAT and OCO-2 were independently calibrated with individual radiometric calibration standards. We have confirmed that GOSAT and OCO-2 calibration standards agree well, within 5%, in two prelaunch cross-calibration campaigns at TKSC (Tsukuba Space Center) and JPL (Jet Propulsion Laboratory) [13]. After launch, GOSAT and OCO-2 were radiometrically calibrated using their own onboard calibration sources. The stability of these on-board calibration systems has been validated by annual vicarious calibration campaigns. As reported in [PERSON] et al. [14], the vicarious calibration error is less than 7%. However, the dynamic  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Satellite** & & **GOSAT** & **OCO-2** \\ \hline
**Spectrometer Type** & & Michelson Fourier transform & Grating \\ \hline \multirow{3}{*}{**Band 1 (ABO2)**} & Spectral Range & 758–775 nm & 758–772 nm \\  & Sampling Interval & 0.2 cm\({}^{-1}\)/0.011–0.012 nm & 0.015 nm \\  & FWHM & 0.02 nm & 0.04 nm \\ \hline \multirow{3}{*}{**Band 2 (WCO2)**} & Spectral Range & 1560–1720 nm & 1594–1619 nm \\  & Sampling Interval & 0.2 cm\({}^{-1}\)/0.045–0.064 nm & 0.031 nm \\  & FWHM & 0.06 nm & 0.08 nm \\ \hline \multirow{3}{*}{**Band 3 (SCO2)**} & Spectral Range & 1920–2080 nm & 2042–2082 nm \\  & Sampling Interval & 0.2 cm\({}^{-1}\)/0.069–0.090 nm & 0.04 nm \\ \cline{1-1}  & FWHM & 0.10 nm & 0.10 nm \\ \hline \multicolumn{2}{c}{**Polarization**} & Two linear (P and S) & One linear \\ \hline \hline \end{tabular}
\end{table}
Table 1: The spectral information of GOSAT and OCO-2.  
range of the input light level using these solar diffuser and bright desert sources is limited. Furthermore, the geometries of the onboard and vicarious calibrations are always the same, as the satellites pass over the calibration targets in predetermined orbits, and the campaigns are always conducted near the summer solstice.  
While these calibration campaigns provide a valuable check on instrument performance, other calibration activities are needed with different input levels and geometries to confirm the spectral radiance linearity through the wide dynamic range that spans both dark ocean and bright desert. It is also useful to compare XCO\({}_{2}\) estimates retrieved from the two instruments since this provides an end-to-end validation of the instrument and data processing systems. In this paper, we describe an effective method for performing an on-orbit inter-comparison of spectral radiances from September 2014 to March 2017 and XCO\({}_{2}\) estimates from September 2014 to May 2016, which are currently available. This dataset includes a large number of scene match-ups compiled over more than two years of simultaneous GOSAT and OCO-2 operations.