Atmospheric refraction comes into play because of the presence of atmospheric irregularities. Due to refraction, the radio wave encounters the variations in atmospheric refractive index along its trajectory that causes the ray path to curve. These types of path distortion while traversing through the atmosphere are very important for many applications like space geodesy. For this purpose, some preliminary discussions have already been made in Section 1.6 of this book. It is to be mentioned that this refraction introduces some delay of the radio path, which is discussed in Section 1.6.5.