“…The k-factor can be calculated from the rate of change of the refractive index with height and the actual Earth's radius, a, using Snell's law in spherical geometry, knowing that N = (n-1) × 10 6 , where a is given by unit of nmi (a = 6371 km = 3440 nmi). The k-factor value must be multiplied by the actual Earth's radius, a, in order to plot the propagation paths as straight lines [1,3].…”