In 2024, the European Space Agency will launch an Earth-observation satellite, called Biomass, which will use a polarimetric interferometric synthetic aperture radar to measure the above-ground biomass content in the world’s forests. This will be the first space-based radar to operate at P-band frequencies (~435 MHz), for which the effects of radio wave refraction, scattering, and Faraday rotation in the ionosphere will be much greater than for previous higher-frequency radars. In this talk, I will introduce the Biomass mission and radar system design, and its unique calibration requirements. I’ll describe how ionospheric refraction distorts the radar image; how ionospheric scintillation (scattering) affects image resolution, contrast, and interferometric coherence; and how the accuracy of biomass measurement is affected by Faraday rotation. Methods for correcting these effects will be described and I will explain how the antenna gain pattern and polarisation channel imbalances and cross-talks are to be calibrated using a giant transponder in Australia.