## 1 July 2015
### Remaining Issues and Possible Solutions  
The current GOSAT and OCO-2 spectral radiances agree within 5% for all bands, except for slightly larger deviations at shorter wavelengths in the ABO2 band. Even though XCO\({}_{2}\) is retrieved by using differential absorption spectra, a radiometric accuracy of better than 10% in the entire mission is needed to distinguish the reflection from the surface and scattering from the atmosphere for accurate light propagation estimation. Scattering by aerosol and thin clouds in the real atmosphere is not negligible when attempting to detect small spatial and temporal variation of atmospheric XCO\({}_{2}\). The present GOSAT V201 products have a nonlinearity over-correction issue in the Band 1 bright data. Regarding these spectral issues, a new GOSAT radiometric conversion table will be created based on the pre-launch data but processed with the new V210 algorithm, improved atmospheric absorption corrections for pre-launch data acquired in the ambient laboratory. This table shall remove the spectral artifacts within the GOSAT Band 1 and Band 3. Fast degradation of the OCO-2 ABO2 band has become predictable and the interval of decontamination activity has increased. Much improved radiometric corrections have been developed and implemented in the Version 8 product.  
The mean biases of retrieved XCO\({}_{2}\) between GOSAT and OCO-2 with the Version 7 retrieval algorithm are \(-0.57\), \(-0.17\) and \(-0.19\) ppm over land with high gain, over ocean with high gain, and over land with medium gain, respectively. [PERSON] et al. (2016) describes how the XCO\({}_{2}\) precision requirements were determined by evaluating the variability of spatial and temporal gradients in XCO\({}_{2}\), the relationship between XCO\({}_{2}\) precision and inferred surface CO\({}_{2}\) flux uncertainties, and the sampling strategy. XCO\({}_{2}\) precisions of 1-2 ppm are needed on regional scales to improve our knowledge of carbon cycle phenomena, but the geographic XCO\({}_{2}\) biases at regional to continental scales will have the largest impact on the inferred CO\({}_{2}\) surface fluxes. [PERSON] et al. (2016) mentioned that the satellite-TCCON differences may exceed the systematic error requirements of less than 0.5 ppm for XCO\({}_{2}\). It has been identified that more research is needed for understanding, such as regional CO\({}_{2}\) surface flux (source/sink) applications. A number of efforts are underway to further improve the XCO\({}_{2}\) products. The Version 8 retrieval algorithm also employs a non-Lambertian BRDF over land. This change is expected to yield more consistent retrievals over land. Finally, there are good prospects for validating the GOSAT medium gain products over bright surfaces. For example, the Moderate spectral resolution EM27 open path FTS with a sun tracker demonstrated accurate and precise XCO\({}_{2}\) retrievals on the ground ([PERSON] et al., 2016), and the EM27 is also portable and traceable with TCCON. We installed one at the bright desert site at Railroad Valley, Nevada temporarily and XCO\({}_{2}\) data with medium gain were validated. By collecting a set of bright surface data at more sites, we can determine and apply a bias correction valued for GOSAT medium gain.  
Figure 12: The OCO-2 and GOSAT retrieved XCO\({}_{2}\) against satellite azimuth and zenith angles during RRV target observations on (**a**) 29 June 2015 and (**b**) 1 July 2015.