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Collision warning systems are used for commercial air traffic to provide pilots an extra layer of situational awareness (as well as avoidance actions), and are of increased importance due to the recent interest in shared-airspace deployment of unmanned aerial vehicles (UAVs). The design for consistency of a passive collision warning system (PCWS) that uses low- resolution infrared (IR) cameras mounted to the airframe is presented. The lack of range information, as well as the unknown measurement noise statistics, make tracking and the decision for collision warning difficult. Of the utmost importance are estimation of the measurement noise variance of the sensor and consistency of the resulting tracks. The proposed PCWS adaptively estimates this noise variance. The resulting system was found to provide consistent errors as well as accurate estimates of the angular velocity of detected targets. The results were verified through a number of test flights that included an aircraft approaching and passing over a stationary camera, and also flying across its field of view at a nearly constant range. Subsequently, the filter was modified for use on a light aircraft in conjunction with an inertial navigation system (INS).