Infrared limb sounding with Cassini CIRS: Optimal viewing strategy using horizon nodes

In this paper we investigate a question of science optimization during Cassini flybys of Titan. The Composite Infrared Spectrometer (CIRS) makes limb observations - along an atmospheric path above surface - during the closest approach period when the visible horizon circle is moving swiftly across the planet. We have sought to discover if any points on the horizon are preferred for limb sounding due to having minimum movement relative to the surface. By numerical calculation, backed by geometric analysis, we find that two limited regions on the horizon are continuously visible during the entire encounter. We term these `limb nodes' and show how they may be employed by CIRS to optimize science by minimizing the source of systematic error due to spatial smear. These conclusions are applicable to many similar scenarios of spacecraft limb sounding during hyperbolic flyby encounters.