A Goldman Sachs estimate concludes that the drone market could climb to $100 billion by 2020, expanding across agriculture, entertainment, aviation, military, and hobby industries. But any space that a drone can reach is a space that could require counter-drone measures in order to protect critical infrastructure, airports, sports stadiums, prisons, and other high-value or at-risk facilities. Also, if the sky is visible, you can be visible to a drone. This summary by Security Today breaks drone security plans into three components: one of radar detection, another of physical defense, and one of open-source monitoring. Each is needed to address the unique threats posed by drones in different sectors. 

Source: Understanding the Drone Threat — Security Today