At elevation angles above about 10 or 15 degrees, a C band signal usually travels in nearly a straight line. In space, waves propagate in straight lines (except near large mass) because there is uniform dielectric permittivty and magnetic permeability.
speed_of_light = sqrt(permeability / permittivity)
The permittivity of the atmosphere (or glass) is larger than that of space. C band signals and other electromagnetic radiation, including light, travel slower through materials denser than the vacum of space. Thus, we may see a rainbow from a glass prism or sometimes even from the atmosphere.
The refractive index is the speed of light (a constant) divided by the actual speed of propagation. The refractive index of the atmosphere varies with density. Density of the atmosphere is a function of temperature, pressure and amount of water present. Since temperature and pressure usually decrease as altitude increases, the atmosphere might be thought of as spherically stratified. Thus, at low elevation angles the path of the signal is a curve instead of a straight line.
In addition, when there is significant moisture the curve will be different. When there is a temperature inversion, the curve will be different. A temperature inversion has been known to cause C band radar signals aimed above horizontal to curve back to earth. (Doppler Radar and Weather Observations, Dovniak and Zrnic, Acedemic Press, 1993, pg 27).
I speculate that this is why C band satellites near to the horizon seem to change location, depending on the weather.