Spaceborne SAR imaging at lower frequencies such as P/UHF/VHF band is severely affected by ionospheric irregularities induced signal scintillation. Mutual coherence function of signals penetrating through the ionosphere was deduced. Electron density in the ionosphere was modeled by Rino's power density function. Patterns were studied that how spaceborne P band SAR image performances varies with parameters of ionospheric irregularities. The study shows that irregularities in ionospheric will defocus the image, causing considerable gain loss of the imaging. Increases in irregularity strength parameter CkL, outer scale Lo, and index of the power density function will further degrade the image. The study is validated by numerical simulation based on phase screen technique.