A method of operating a rotorcraft collision avoidance system is provided. The method includes determining a unique characteristic of a detected rotorcraft, determining the actual length of a first rotor based at least in part on the determined unique characteristic of the rotorcraft, locating a major axis of the first rotor of the rotorcraft from a perspective of a home unit, from the perspective of the home unit, determining an angular extent of the major axis of the first rotor, and determining the then current distance from the home unit to the rotorcraft based at least in part on the determined actual length of the first rotor and the corresponding angular extent.