I m not clear with 6S atmospheric correction. which image we use to run 6S atmosphere as an input image to calculate coefficient xa,xb and xc top of atmosphere radiance or top of atmosphere reflectance image?
When using 6S, your input image should be top-of-atmosphere radiance. The reason for this is that it is a physical value, rather than a unit-less value (like reflectance is). This allows the physical model behind the 6S-code to calculate the correct parameters.