Revisiting Ray-Tracing Algorithm for Future Limb-Scanning Missions

In the forward model for limb-scanning instruments, ray tracing must be accounted for because variations in air refractivity cause the lines of sight to bend from straight paths into curves. The tangent point of a line of sight defined as the minimum height, depends on both the instrument’s nadir angle and the atmospheric state. To achieve reliable tangent point determination, the off-nadir angles must be calibrated to account for Earth’s ellipsoidal geometry and realistic atmospheric conditions sampled by these lines of sight paths. In this study, we improve the ray-tracing algorithm originally developed for MIPAS operational retrievals. The algorithm is applied to more accurately assess the impact of atmospheric variability on the relationship between off-nadir angles and tangent point localization. It also allows us to estimate the smoothing error associated with the coarse horizontal sampling of the MIPAS observation pattern. We used a configuration representative of the proposed CAIRT mission instrument as a case scenario for future instruments such as the STRIVE mission by NASA.