A helicopter-based Doppler scatterometer (Multi3Scat) is described. It allows simultaneous measurements of the surface radar backscatter at five different frequencies at co- and cross-polarization at incidence angles of 20deg -65deg from an altitude of 30-300 m. Video and infrared (IR) cameras simultaneously sense the surface in the scatterometers' footprint. The Multi 3Scat is calibrated using measurements carried out over corner reflectors. The stability of the Multi3Scat's signal is found to be, on average, better than 0.5 dB. Typical signal-to-noise-ratio values for sigma-0 range between 10 and 20 dB for cross-polarization and between 15 and 25 dB for copolarization over snow and ice surfaces. The potential of the Multi3Scat to acquire multifrequency multipolarization radar backscatter data and coincident video and IR temperature observations at different incidence angles over remote terrain such as the Arctic Ocean or the Alps is demonstrated.