The real success for Unmanned Aircraft Vehicles (UAV) will be when they can autonomously operate to complete their appointed task. Autonomous operation means both operating beyond visual line of sight (BVLOS) and the ability to operate independent of a pilot. Before this can happen, UAVs need to be much smarter about their surroundings. A key to smarter UAVs is on-board collision avoidance sensing, such as radar. An understanding of the math behind collision avoidance helps dictate what technologies provide the best option for collision avoidance.