A collision avoidance system comprises a pair of video cameras mounted to a vertical stabilizer of the aircraft, a machine vision processing unit, and a system to inform the pilots of a potential collision. The machine vision processing unit is configured to process image data captured by the video cameras using stereoscopic and structure from motion techniques to detect an obstacle that is near or in the path of the aircraft. Estimates of the range to the object and the rate of change of that range are computed. With the range and range rate, a time to collision can be estimated toward every point of the aircraft. A pilot warning can be sounded based on the nearness of the potential collision. A method of calibrating the video cameras using existing feature points on the top of the aircraft is initiated in response to power being turned on.