<p><strong>Abstract.</strong> The International Submillimetre Airborne Radiometer (ISMAR) has been developed as an airborne demonstrator for the Ice Cloud Imager (ICI) that will be launched on board the next generation of European polar-orbiting weather satellites in the 2020s. It currently has 15 channels at frequencies between 118 and 664<span class="thinspace"></span>GHz which are sensitive to scattering by cloud ice, and additional channels at 874<span class="thinspace"></span>GHz are being

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... developed. This paper presents an overview of ISMAR and describes the algorithms used for calibration. The main sources of bias in the measurements are evaluated, as well as the radiometric sensitivity in different measurement scenarios. It is shown that for downward views from high altitude, representative of a satellite viewing geometry, the bias in most channels is less than ±1<span class="thinspace"></span>K and the NEΔ<i>T</i> is less than 2<span class="thinspace"></span>K, with many channels having an NEΔ<i>T</i> less than 1<span class="thinspace"></span>K. In-flight calibration accuracy is also evaluated by comparison of high-altitude zenith views with radiative-transfer simulations.</p>