We present the new airborne Doppler radar KuROS (Ku-band Radar for Observation of Surfaces), which provides measurements of the normalized radar cross section σ° and of the Doppler velocity over the sea. The system includes two antennae rotating around a vertical axis, although only the results from the lower incidence (14°) antenna are presented here. We also give first results from observations performed during two field campaigns held in 2013 (HyMeX and PROTEVS campaigns). Sea wave directional spectra computed by the radar from tilt modulation of σ° are consistent with those given by the directional wave rider moored in the Mediterranean basin, both in terms of significant wave height Hs and main features of the wavenumber spectrum. As concerns the azimuthal distribution, two methods are tested to remove the 180° ambiguity of the radar derived directional spectrum. The first method is based on the correlation between the modulations of σ° and Doppler velocity, which reflects the correlation between sea surface slope and orbital velocity. The second method does not use the Doppler velocity, but computes the cross-spectrum between the modulations of σ° between two power profiles separated by some time lag ΔT, from which the phase velocity of sea waves is deduced. Comparing the sea wave directional spectra disambiguated by both methods, with the directional spectrum given by the wave rider, we conclude that the first method (using Doppler velocity) is more efficient to remove the 180° ambiguity, and should be preferred to the second method.