## 1 Introduction  
### CO\({}_{2}\) Monitoring from Space  
The Greenhouse gases Observing SATellite (GOSAT) was the first instrument with high spectral resolution and wide spectral coverage designed for measuring greenhouse gases from space. GOSAT was launched on 23 January 2009 and has been returning data for more than eight years. The Orbiting Carbon Observatory 2 (OCO-2) was launched on 2 July 2014 and has been returning data since September 2014. Both instruments are designed to measure the global distribution of atmospheric carbon dioxide (CO\({}_{2}\)) from space.  
The column-averaged dry air mole fraction of CO\({}_{2}\) (XCO\({}_{2}\)) in the atmosphere can be measured remotely using differential optical absorption spectroscopy. Both GOSAT and OCO-2 measure sunlight reflected by the Earth, including contributions from the surface, clouds and aerosols, at 0.76, 1.60 and 2.06 \(\upmu\)m. To estimate the surface and atmospheric geophysical state, absolute instrument radiometric accuracy must be maintained to within 10% [1]. To investigate the uncertainties and biases between the respective data products, the OCO-2 and GOSAT teams have executed a number of cross-comparison studies both prior to launch and on-orbit. These exercises have facilitated the interpretation of the data from both missions.  
In December 2016, the Chinese TanSat was launched [2], adding a third greenhouse gas observing satellite to the international fleet. The Copernicus Sentinel-5 Precursor (S5p) carrying the TROPOspheric Monitoring Instrument (TROPOMI) [3], GOSAT-2 [4] and OCO-3 [5] are also scheduled to be launched in the near future. As the number of distinct greenhouse gas monitoring satellites increases, each with a unique instrument design, the inter-comparison of spectral radiances and cross-validation of atmospheric composition are crucial to confirm post-launch mission success.  
[PERSON] et al. [6] compared retrieved XCO\({}_{2}\) from the GOSAT spectra radiance product with several different algorithms. The single measurement precision requirement has been met for GOSAT XCO\({}_{2}\) (<3 ppm) among several retrieval algorithms, but the uncertainties of the individual algorithms were presented. [PERSON] et al. [7] applied the same Bremen Optimal Estimation DOAS (BESD) algorithm to different satellite instruments and compared XCO\({}_{2}\) from the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on-board the European Space Agency's (ESA) Environmental Satellite (ENVISAT) and GOSAT. They presented long-term consistency between SCIAMACHY, GOSAT and ground based Total Carbon Column Observing Network (TCCON) data.  
This paper describes the inter-comparison of both spectral radiance and retrieved XCO\({}_{2}\) with the two independent spectrometers aboard the GOSAT and OCO-2 satellites within temporally coincident and spatially collocated points using the same XCO\({}_{2}\) retrieval algorithm. The latter part of Section 1 shows instrument specifications and observation methods of GOSAT and OCO-2, and the results of the pre-launch cross-calibration. Section 2 gives a brief summary of the GOSAT and OCO-2 spectral radiance and retrieved XCO\({}_{2}\) products, and the selection methodology used in this inter-comparison. Section 3 presents the spectral radiance comparison, and Section 4 describes the retrieved XCO\({}_{2}\) comparison together with retrieved parameters with the same algorithm. The comparison results are discussed in Section 5.