A proximity warning system for a helicopter (22) comprising at least two radar units (1-3), preferably three radar units (1-3) arranged to transmit microwaves and receive reflections of said microwaves from obstacles (10). The at least two radar units (1-3) are fixed next to a main rotor head(s) (20) of the helicopter (22) for horizontally scanning an entire environment of 360° around the helicopter (22), all of said at least two radar units (1-3) operating essentially at the same frequency.