Spaceborne linearly polarimetric synthetic aperture radar (SAR) provides information about surface properties and scatterer types by measuring the covariances between images formed using orthogonally polarized radio waves. However, ionospheric Faraday rotation (FR) and system calibration errors alter the balance in the polarimetric channels and distort the covariance matrix. Since FR angles are greater at lower radio frequencies, this paper focuses on the Biomass satellite, which is the European Space Agency’s 7th Earth Explorer mission and will carry a P-band (435 MHz) polarimetric SAR. Its primary objective is to measure forest biomass density by combining estimates derived from the polarimetric covariance matrix with estimates based on measuring forest height with polarimetric interferometry and exploiting height-biomass allometric relations. The accuracy of four methods for estimating FR from polarimetric SAR is assessed using simulated images of forest with a range of biomass densities, system errors (H/V channel imbalances, antenna cross-talk and noise) and over a large spread of FR angles. All methods have biases dependent on the FR, the relative phases of the cross-talk components and the channel imbalance phase. The best-performing method estimates FR to better than 5° under worst-case system errors at all FR angles, so the accuracy of biomass density estimates should not be significantly affected. However, cross-talk phases are predicted to vary greatly across the Biomass antenna beam and this could potentially cause the FR bias to vary by several degrees across the swath. These effects may occur even for the small values of FR met in L-band data.