The effect of ionospheric scintillation on phase gradient autofocus processing of synthetic aperture radar

Depending on the transmission frequency and signal bandwidth, space based synthetic aperture radar (SAR) may be impacted by disturbances caused by small-scale structured ionization. This paper considers the effects of scintillation (variations in amplitude and phase) on a narrow band SAR that utilizes the phase-gradient autofocus (PGA) method that attempts to compensate for the effects of phase errors across the synthetic aperture. We develop a simulation of SAR/PGA processing that is applied to several simple “scenes” consisting of scatterers that are combinations of straight lines and circles. The multiple phase screen (MPS) technique is used to model the effects of amplitude and phase scintillation for a SAR operating in the equatorial region. Here we assume that the Fresnel zone in the ionosphere is on the same order or larger than the maximum separation of lines-of-sight from any point on the aperture to all points on the scene to be imaged (this is referred to as the small target assumption). It is well known that the degradation in SAR imaging performance is a function of the ratio of the synthetic aperture length to the decorrelation distance of the ionospheric propagation channel. In general if the decorrelation distance is small relative to the required SAR aperture length, scintillation will degrade SAR imaging performance. We use Monte-Carlo simulations involving many MPS calculations with similar statistics to quantify this relationship and to investigate the effect of realistic combined amplitude and phase scintillation on autofocus compensation. Examples are presented of the performance of spotlight-mode SAR/PGA for values of the decorrelation distance and scintillation index that represent natural ionospheric scintillation at UHF. Quantitative results are given for SAR performance in terms of the rms spread of the SAR image of a hypothetical isolated point target in the scene.