On the effects of the atmospheric refractive index on airborne SAR

In airborne synthetic aperture radar we usually assume that the signal speed is constant on the atmosphere. However, this is not true. The refractive index of air, denoted by n, depends on temperature, atmospheric pressure and water vapour pressure [1]. Since in the long term these factors depend on the altitude, we may assume that the refractive index is a function of the altitude only, n = n(h). Since n is barely greater than one, it is usually described in term of the refractivity N, defined by N = (n - 1) × 10⁶. The altitude dependence of the atmospheric refractive index can be safely ignored for low to medium SAR imaging resolution. However, the development of airborne SAR systems with ultra-high resolutions [2] poses the question of the influence of the varying atmospheric refractive index on the image quality of such systems.