Atmospheric loss considerations for synthetic aperture radar design and operation

A significant loss factor for Synthetic Aperture Radar (SAR) is the atmosphere, particularly under conditions of adverse weather. Dominating these losses are 1) atmospheric clear-air attenuation due to a) oxygen absorption, and b) water vapor absorption., 2) cloud liquid water absorption.. and 3) attenuation due to rain. These losses are very nonlinear with respect to radar operating frequency, and their relative significance obviously depends on the nature of the weather being modeled. Attenuation is also very operating geometry dependent.. and typically increases with range and shallower depression angles. The net effect is that while airborne radar operation at short ranges often favors higher frequencies owing to higher antenna gain for a given real aperture antenna, radar operation at longer ranges often favors lower frequencies in spite of this. In fact.. at any particular range.. an optimal SAR operating frequency that maximizes SNR exists for a given weather condition.. transmitter power, and antenna aperture area. This paper discusses loss rates due to various loss contributors, and then quantifies expected atmospheric loss rates and their frequency dependence for typical adverse weather specifications for SAR systems. Furthermore, this paper describes airborne SAR operating geometries for which various radar frequency bands are optimal.