Space-based synthetic aperture radars can be detrimentally affected by electron density variations in the ionosphere, which cause signal phase variations resulting in image defocusing. In order to evaluate the significance of this problem, this paper determines how often the phase perturbations are large enough to cause defocusing. High cadence GPS TEC signals collected on Ascension Island during sunspot minimum and in Papua New Guinea over half a solar cycle have been analyzed. Both locations lie in the equatorial crest region. For a space-based synthetic radar operating at 435 MHz, the average probability of defocusing varies from zero at quiet times, to around 1% between 18:00 and 24:00 at solar minimum, and to 45% between 18:00 and 24:00 at solar maximum. Corresponding values are given for radars operating at other frequencies. A method is developed to estimate such variations from the large database of S4 measurements.