Earth's itself represents the second (after Sun) most powerfull natural source of e.m. radiation for EO. Its average emittance can be approximated by that of a blackbody at about 290 K. Even if very less powerfull than Sun such a source is available for EO day and nigth. The maximum of its emission falls in the thermal infrared (around 10 micron) being Earth's emission trascurable in the VIS-SWIR range.
The Rayleigh roughness criterion is a widely used means to estimate the degree of roughness of a considered surface. Considering the phase difference between two rays scattered from separate points of the surface, this depends on the roughness height, the incident angle and, inversely, on the radiation wavelenght. The Rayleight criterion states that a surface can be considered as smooth if the phase difference is less than pigreco/2 radians.