The vertical electron density distribution can have a significant impact on ionospheric measurements of Total Electron Content (TEC) derived from estimates of Faraday rotation (FR) with spaceborne low-frequency SAR data. Using vertical electron density data and the IGRF-12 magnetic field model, simulations of FR under different vertical electron density distributions for an L-band SAR have been performed. The usual approximation that the geometric and magnetic field parameters can be estimated at 400 km when converting FR to TEC is shown to cause significant errors, especially for regions at low latitude and near the zero FR line.