  Collision avoidance is a key factor in enabling the integration of unmanned
aerial vehicle into real life use, whether it is in military or civil
application. For a long time there have been a large number of works to address
this problem; therefore a comparative summary of them would be desirable. This
paper presents a survey on the major collision avoidance systems developed in
up to date publications. Each collision avoidance system contains two main
parts: sensing and detection, and collision avoidance. Based on their
characteristics each part is divided into different categories; and those
categories are explained, compared and discussed about advantages and
disadvantages in this paper.
