## 2 Analytical Method  
### Spectral Radiance Data and Retrieved XCO\({}_{2}\) for the Inter-Comparison  
We use the most recent version of the GOSAT Level 1B V201.201 (L1B V201) spectral radiance. The GOSAT Level 1A (L1A V201.201: raw interferogram) and the Level 1B V201 products are processed by JAXA and distributed from the National Institute for Environmental Studies (NIES) GOSAT Data Archive Service (GDAS) [15]. The XCO\({}_{2}\) from both GOSAT and OCO-2 products, as well as the OCO-2 Level 1B radiances, are downloadable from the NASA Goddard Earth Sciences Data and Information Services Center (GES DISC) [16]. The GOSAT L1B V201 product contains the raw and best estimated radiance spectra, observation geometry, and related parameters [17]. It also contains the context camera image within the FTS field-of-view, used to identify the pointing location and check cloud contamination. Spectral radiances are created by taking an inverse Fourier transform of the raw interferograms, using the radiance conversion table prepared from the GOSAT pre-launch calibration data shown in [PERSON] et al. [8], and radiance degradation factors (RDF) described in detail in [PERSON] et al. [1; 17]. The RDFs are estimated from annual vicarious calibration campaigns in Railroad Valley (RRV), Nevada, USA by the joint GOSAT and OCO-2 teams, and monthly on-orbit solar diffuser calibrations. GOSAT sensitivity experienced rapid degradation in the first two years on-orbit, especially at shorter wavelengths [18]. The degradation has slowed in subsequent years. [PERSON] et al. [14], also note that the degradation rate is slightly higher for the S polarization than for the P polarization in Band 1, because of the FTS beam splitter efficiency. There are two remaining issues concerning the L1B V201 product. The first is spectral response within the short-wavelength infrared (SWIR) bands. Spectral radiances from the GOSAT integrating sphere were calibrated with 5 nm steps for Band 1, and 20 nm steps for Bands 2 and 3. This was done under ambient laboratory conditions. The coarse spectral step, combined with absorption by oxygen (O\({}_{2}\)), CO\({}_{2}\), and water vapor (H\({}_{2}\)O) during the prelaunch calibration, creates an uncertainty on the sphere spectral output, and thus the preflight radiometric calibration. The other issue is Band 1 nonlinearity. GOSAT has both high and medium gain signal chains for the detectors, which can be selected by command to provide the dynamic range needed to measure both dark ocean and bright desert scenes. The FTS Band 1, with its high spectral resolution, needs a large gain, potentially resulting in nonlinearity [17; 19]. Correction parameters, selected by minimizing out-of-band artifacts, still have not fully corrected this issue. In this paper, we filtered out the bright Band 1 scenes that were collected in high gain when the spectral radiance was greater than 140 W/m\({}^{2}\)/\(\upmu\)m/str at a baseline wavelength of 0.759 \(\upmu\)m. About 22% of matched-up spectral radiance data were screened out with this criterion. For the XCO\({}_{2}\) comparison, we filtered out scenes with retrieved albedo greater than 0.3 in GOSAT Band 1, as they might contain imperfect nonlinearity correction. About 30% of matched-up XCO\({}_{2}\) data were screened out with this criterion. [PERSON] et al. [10]and [PERSON] et al. (2017) describe the OCO-2 instrument performance and radiometrically calibrated products and retrieved XCO\({}_{2}\). In general, the OCO-2 instrument performed as expected during its first 18 months. Its absolute radiometric uncertainty was about 4% at 18 months into the mission. Since launch, the OCO-2 instrument has proven to have two degradation contributions: \"fast\" and \"slow\" degradation. The former is thought to be due to ice crystals forming on the ABO2 focal plane assembly (FPA) that degrade the performance of its anti-reflection coating. The FPA has periodically been warmed, and thus decontaminated, as shown in [PERSON] et al. (2018).