<p>In this study, we will give an overview of lessons learned during the radiometric calibration of the airborne, high-power Ka-band cloud radar on board the German research aircraft HALO. Within this context, a number of flight experiments over Europe and over the tropical and extra-tropical North-Atlantic have been conducted, where the ocean surface backscatter was used as an external reference reflector. Measurements of signal linearity and signal saturation complement this characterization. To validate the external calibration, joint flights of HALO and the French Falcon 20 aircraft, which was equipped with the RASTA cloud radar at 94 GHz and underflights of the spaceborne CloudSat at 94 GHz have been conducted. Finally, the influence of different radar wavelengths was explored with numerical studies.</p>