As an active sensor,Synthetic Aperture Radar (SAR) finds many applications in earth observation.However,the signal transmission and reception are persistently disturbed through the ionosphere layer at L-band and below.Ionospheric effect brings phase and amplitude errors to the radio signal traversed therein,and further exerts severe impact on the SAR mage quality.In this paper,we review the spatial-temporal ionospheric effects on the imaging performance of SAR systems that operate at different orbital altitudes.Three kinds of effects are investigated:the Faraday rotation,the background ionospheric effects and ionospheric scintillation.Compensation approaches for these effects are then summarized.Finally,problematic issues associated with the ionospheric effects are outlined in the context of SAR theory and applications.