A radar attached laterally to airplane fuselage to detect obstacles on a collision course with a portion of the airplane facing the radar. The radar includes an emission antennal channel and reception antennal channels in the same plane. The radar Establishing in a radar coordinate system a first distance/Doppler map allowing echoes to be separated into distance and Doppler resolution cells in reception antennal channels; Establishing a second distance/Doppler map of smaller size by selecting a subset of distance/Doppler resolution cells corresponding to possible positions of targets liable to collide with the airplane; Establishing new distance/Doppler maps by forming beams computationally from the subsets of distance resolution cells; and Temporal integration, in each beam, of successive distance/Doppler maps.