We are using Forecast Sensitivity to Observations (FSO) setup that utilizes WRFDA v3.8. We are currently using RTTOV v11.3 and the CRTM version that is included with WRFDA v3.8. Since maturing the FSO capability, RTTOV is now on version 13 and the CRTM version has also advanced. Our understanding is that CRTM generates backwards compatible coefficients for new sensors that we can use with the CRTM version within WRFDA v3.8. It is not clear that new coefficient files are generated to be backwards compatible for RTTOV. If anyone has experience with this and could share insights, it would be greatly appreciated? We are evaluating whether or not it will be necessary to update to WRFDA v4.4 in order to use FSO to evaluate new sensors when they are in space collecting data or continue to utilize v3.8 with older versions of the RTMs.