Skyline tool surface options greyed out

[AndrewFrost1]

The surface options in the skyline tool are greyed out. The tool runs fine, and I get the option of refraction coefficient and earth curvature with the view shed tool, so it cant be a permissions issue. Has anyone else had this issue?

[DanPatterson_Retired]

From the how skyline works link, it appears that the available options depend on what you have in the data frame. Does this apply?

When an Input Surface is provided, the Virtual Surface options are ignored. If no surface is provided and the Virtual Surface Radius is set to 0, a virtual surface whose elevation is slightly less than the lowest Z-value in the input features will be used, and the Virtual Surface Radius will be set to the average of all distance measurements from the observer point to each feature's envelope.

[AndrewFrost1]

The input surface used makes no difference.