A passive collision avoidance system, having particular applicability to aircraft collision avoidance, comprising two receivers operating out of antennas which have a differing sensitivity to the polarization of the incoming signals from a ground broadcast facility. The differing sensitivity ensures that the direct and reflected signals will predominate in different receivers and therefore when the outputs of the receivers are combined the resultant fade rate will give an indication of the movement of the intruding aircraft with respect to the equipped aircraft. Interpretation of the fade rate allows the equipped aircraft to change course until a non-collision course is attained.